D2.03 Review of Relevant PTO Systems

Marine Renewables Infrastructure Network

Work Package 2: Ocean Energy System Testing – Standardisation and best practice.

Authors: Jamie Grimwade NAREC Dave Hails NAREC Eider Robles TECNALIA Fernando Salcedo TECNALIA Jochen Bard FH-IWES Peter Kracht FH-IWES Jean-Baptiste Richard FH-IWES Dominik Schledde FH-IWES Atle Rygg Årdal SINTEF Jorun Irene Marvik SINTEF Nils Arild Ringheim SINTEF Harald Svendsen SINTEF Marta Molinas NTNU

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ABOUT MARINET MARINET (Marine Renewables Infrastructure Network for Emerging Energy Technologies) is an EC-funded consortium of 29 partners bringing together a network of 42 specialist marine renewable energy testing facilities. MARINET offers periods of free access to these facilities at no cost to research groups and companies. The network also conducts coordinated research to improve testing capabilities, implements common testing standards and provides training and networking opportunities in order to enhance expertise in the industry. The aim of the MARINET initiative is to accelerate the development of marine renewable energy technology.

Companies and research groups who are interested in availing of access to test facilities free of charge can avail of a range of infrastructures to test devices at any scale in areas such as wave energy, tidal energy and offshore-wind energy or to conduct specific tests on cross-cutting areas such as power take-off systems, grid integration, moorings and environmental data. In total, over 700 weeks of access is available to an estimated 300 projects and 800 external users.

MARINET is consists of five main areas of focus or ‘Work Packages’: Management & Administration, Standardisation & Best Practice, Transnational Access & Networking, Research and Training & Dissemination. The initiative runs for four years until 2015. Partners

Ireland Netherlands University College Cork, HMRC (UCC_HMRC) Stichting Tidal Testing Centre (TTC) Coordinator Stichting Energieonderzoek Centrum Nederland Sustainable Energy Authority of Ireland (SEAI_OEDU) (ECNeth)

Denmark Germany Aalborg Universitet (AAU) Fraunhofer-Gesellschaft Zur Foerderung Der Angewandten Forschung E.V (Fh_IWES) Danmarks Tekniske Universitet (RISOE)

Gottfried Wilhelm Leibniz Universität Hannover (LUH)

France Universitaet Stuttgart (USTUTT) Ecole Centrale de Nantes (ECN)

Institut Français de Recherche Pour l'Exploitation de Portugal la Mer (IFREMER) Wave Energy Centre – Centro de Energia das Ondas (WavEC)

United Kingdom

National Renewable Energy Centre Ltd. (NAREC) Italy Università degli Studi di Firenze (UNIFI-CRIACIV) The University of Exeter (UNEXE) Università degli Studi di Firenze (UNIFI-PIN) European Marine Energy Centre Ltd. (EMEC) Università degli Studi della Tuscia (UNI_TUS)

University of Strathclyde (UNI_STRATH) Consiglio Nazionale delle Ricerche (CNR-INSEAN)

The University of Edinburgh (UEDIN) Brazil Queen’s University Belfast (QUB) Instituto de Pesquisas Tecnológicas do Estado de São

Plymouth University(PU) Paulo S.A. (IPT)

Spain Norway Sintef Energi AS (SINTEF) Ente Vasco de la Energía (EVE)

Tecnalia Research & Innovation Foundation Norges Teknisk-Naturvitenskapelige Universitet (TECNALIA) (NTNU)

Belgium 1-Tech (1_TECH)

Acknowledgements The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7) under grant agreement no. 262552. Legal Disclaimer The views expressed, and responsibility for the content of this publication, lie solely with the authors. The European Commission is not liable for any use that may be made of the information contained herein.

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D2.03 Review of Relevant PTO Systems

REVISION HISTORY

Rev. Date Description Author Checked by 01 19th Nov. Final Draft Jean-Baptiste 2012 Richard 02 27th Nov Executive summary updated Jean-Baptiste 2012 Richard 03 29th Nov Final TMcC Cameron Johnstone 2012

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EXECUTIVE SUMMARY

Traditional electricity generation systems generally operate at steady or quasi-steady state. Consequently, their performances description and the subsequent tests of their components focus on non-transient operations. In Marine Renewable Energy (MRE), however, energy converters operate under much more dynamic conditions. Extensive dynamic tests are therefore required for the Power take-off (PTO) components, and dedicated standard procedures have to be established. This “Review of relevant PTO systems” is conducted with this aim in mind. Typical PTO components are described, and their associations with primary energy capture are specified.

The main focus in on wave energy, which by essence involves dynamics. Though there is a vast diversity of designs in this area, three main configuration types are identified: • oscillating water columns, involving pneumatic turbines in reciprocating air flows, • overtoppers, fitted with low-hydro turbines subject to unidirectional water flows, • other oscillating concepts being mostly fitted with high-pressure hydraulics. Rotating electrical generators are generally used in both cases, except for some in the 3rd category (dedicated linear generators).

Offshore wind turbines PTOs are also discussed, with dynamics due to e.g. turbulences and 3p effects. Tidal Energy converters start from this basis but have a wider variety of concepts, with different dynamics. Such examples are cross-flow turbines or simplified mechanisms. Waves also play a role here.

The dynamics of all devices then depend on the control strategies, which are conditioned by the various generator implementations with the power electronics, also described. In particular, this involves frequency converter with different active components.

Further deliverables focusing on the testing properly speaking of PTO components are D2.11, D4.2 and D4.3.

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CONTENTS

1 INTRODUCTION ...... 3

1.1 MOTIVATION AND ORGANISATION OF THIS DOCUMENT ...... 3 1.2 TERMINOLOGY ...... 3 1.3 PRIMARY ENERGY CAPTURE ...... 4 1.4 CLASSIFICATION OF PTO COMPONENTS ...... 32 2 PTO COMPONENTS, PART I: TRANSMISSION OF MECHANICAL ENERGY ...... 33

2.1 AIR TURBINES ...... 33 2.2 HYDRAULIC CONVERTERS ...... 42 2.3 OTHER MECHANICAL TRANSMISSION SYSTEMS ...... 54 3 COMPONENTS, PART II: CONVERSION TO GRID-COMPATIBLE ELECTRICAL ENERGY ...... 57

3.1 ELECTRIC GENERATORS ...... 57 3.2 FREQUENCY CONVERTERS ...... 64 3.3 ENERGY STORAGE SYSTEMS ...... 72 4 CONCLUSIONS: OBSERVED AND FORSEEN COMPATIBILITIES BETWEEN PRIMARY ENERGY CAPTURE APPROACHES AND PTO COMPONENTS ...... 91

4.1 OFFSHORE WIND TURBINES ...... 91 4.2 TIDAL ENERGY CONVERTERS (TEC) ...... 94 4.3 WAVE ENERGY CONVERTERS (WEC) ...... 95 5 REFERENCES ...... 106

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1 INTRODUCTION

1.1 MOTIVATION AND ORGANISATION OF THIS DOCUMENT

This “Review of relevant PTO systems” is conducted with this aim in mind. It is introduced by a description of primary energy capture techniques (§1.3). This describes the different types of active interfaces between the systems considered and the fluid they extract energy from, and consequently presents the corresponding dynamics. Chapters 2 and 3 then constitute the core of the document, depicting the typical components used in the energy conversion chain of MRE systems. Finally, chapter 4 presents as conclusions some examples of converters at commercial or pre-commercial stages, which are technical solutions formed by the association of those typical components.

1.2 TERMINOLOGY Unless expressively stated, the terminology used in this report is the one defined in the IEC standards [1]. As several terms can have different meanings and definitions depending on the context, the reader is made aware for convenience that the main publication considered here is IEC 62600-1 from Technical Committee 114 [2].

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1.3 PRIMARY ENERGY CAPTURE

1.3.1 Offshore Wind This section describes offshore wind turbines, with sub-sections discussing wind resources, the conversion from wind via mechanical to electric energy, turbine design, and turbine control systems. No attempt is being made to provide a historical overview, and the focus is on modern wind turbine concepts that are relevant for offshore applications.

Offshore wind power is still a small industry compared to onshore wind power, and turbines installed offshore so far are essentially well-proven onshore turbine concepts mounted on some bottom-fixed support structure. However, as the industry grows it is likely that more uniquely offshore designs will emerge.

1.3.1.1 Wind resources The wind at a given location varies in multiple ways: There are turbulent fluctuations on a short time-scale; there are variations driven by atmospheric pressure systems; there are daily variations associated with daily weather patterns; and there are seasonal variations – in Europe there is typically more wind in the winter than in the summer.

A common way to characterise the wind is by means of a distribution curve that shows the frequency of different wind speeds. A good approximation to measured wind distribution can usually be achieved using a Rayleigh distribution, or the more general Weibull distribution that has a tuneable shape parameter in addition to the mean wind speed. An example is shown in Figure 1.1. This figure also shows the energy distribution, i.e. how much energy is captured at different wind speeds. The energy distribution is shifted towards higher wind speeds because the power depends non-linearly on the wind speed, see Section 1.3.1.2.

Wind resource assessments covering large areas have been made by many sources and published as wind atlases. However, although the general wind resource for an area is known from a wind atlas, local conditions such as hills and surface roughness are very important. Offshore wind resources are more constant, but lack of experience means that there are still significant uncertainties associated with the actual wind resources for planned offshore wind farms.

Figure 1.1: Distribution of wind speed (red) and energy generated (blue). The histograms show measured data from the Lee Ranch facility in Colorado, while the curves show the Rayleigh model distribution for the same average wind speed and corresponding maximum theoretical energy capture of a 100 meter diameter turbine facing directly into the wind. Source: Wikimedia

In addition to the wind speed distribution, the wind resource is characterised by directional distribution, usually visualised by a so-called wind rose. Such a distribution quantifies how often the wind comes from any given direction, and is important e.g. for deciding a wind farm layout.

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An important additional factor related to the assessment of wind resources is the reduction in turbine power output due to the presence of other wind turbines, the so-called wake effect whereby a turbine leaves a downstream region with decreased wind speed and increased turbulence. A similar effect is relevant on wind farm level when multiple wind farms are in the vicinity of each other.

Wind speeds increase with height above ground, a phenomenon referred to as wind shear. The strength of the wind shear depends on the surface roughness. Over rough surfaces (e.g. an onshore site with vegetation) the wind shear is more prominent than over smooth surfaces such as the sea. It has design implications since less wind shear means that there is less benefit in moving the wind turbine rotor higher up in the air. The vertical profile of wind speed is often approximated by a logarithmic function, U * z u(z) = ln( ), k z0 where u(z) is the wind speed at height z above ground, U* is the friction velocity, k ≈ 0.4 is the von Karman's constant, and z0 is surface roughness length. If wind speed at a reference height zr is known, this expression can be used to find wind speed at a different height z, u(z) ln(z / z ) = 0 . u(zr ) ln(zr / z0 ) Another approximation that is also often used is the power law profile, α u(z) ⎛ z ⎞ = ⎜ ⎟ , u(zr ) ⎝ zr ⎠ where α is called the wind shear exponent.

Turbulent variability of the wind gives highly variable output for a single wind turbine. But for a wind farm consisting of many turbines geographically spread out, the aggregated output is smoothed, correlating highly with the averaged wind speed in the wind farm. This smoothing effect is even more pronounced for larger areas, e.g. for the total wind power production within a country. Predicting the power output from a single wind turbine with high time resolution naturally carries high uncertainty, but due to the smoothing effect, predictions of wind farm output are much more accurate. This is important for the integration of wind power into the electric power system.

1.3.1.2 Wind Energy Capture A general characteristic of all wind turbines is that their output power depends on the wind speed. The kinetic energy of wind passing through an area A is given by its mass and its speed squared. The mass in turn equals density multiplied by volume, where volume is area multiplied by speed and time interval. Thus we have an energy flux proportional to the wind speed cubed:

dE 1 = ρAu 3 . dt 2 Conservation of energy and momentum implies that only a fraction of this energy can be extracted by a wind turbine. The theoretical upper limit of this ratio is usually referred to as the Betz limit, and has a value equal to 16/27 = 0.593.

For an actual turbine this ratio is a blade characteristic that depends on blade tip-speed ratio and pitch angle. The steady-state active power from any horizontal axis wind turbine can therefore be expressed as 1 P = ρAC (λ, β )u 3 2 p where - P is active power output (W) - ρ is air density (1.225 kg/m3 at 15o C and 1013.3 mbar) - A is the swept turbine area, A = πR2 (m2); R is turbine radius (m) - Cp is the turbine efficiency being a function of λ and β - λ is the tip-speed ratio, λ = ωR/u; ω is rotational speed of turbine (rad/s)

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- β is the pitch angle of the turbine blades, including blade twist due to wind force (o) (see Figure 1.5) - u is the wind speed at hub-height of the turbine (m/s)

Power efficiency

The actual Cp data may vary depending on the detailed blade design, though normally not much for modern blades. Figure 1.2 gives one example of Cp data. Note that the turbine efficiency has a maximum at one specific tip-speed ratio and pitch angle = 0o. This means that generation at maximum efficiency is achieved by adjusting the rotational speed of the turbine (ω) so that the tip-speed ratio is kept constant at its optimal value λopt (in this case = 7.8) and keeping the pitch angle constant at zero degrees.

In general, the Cp dependence on tip-speed ratio and pitch angle implies that the active power output can be controlled by adjusting a) the pitch angle and b) the rotational speed of the turbine. Turbine control is discussed more in Section 1.3.1.4.

Note that although not shown in Figure 1.2, when operating off the optimal tip-speed ratio, small adjustments of the pitch angle (plus/minus one or two degrees) around zero degrees may yield slightly improved efficiency. This is utilized in fixed speed wind turbines for optimizing performance at low wind speeds, and can also be applied in variable speed wind turbines during operation at (fixed) minimum rotational speed.

0,50 0,45 Pitch 0,40 angle (°) 0,35 0,00 0,30 5,00 7,00

Cp 0,25 10,00 0,20 14,00 0,15 20,00 0,10 0,05 0,00 0 5 10 15 20 Tip-speed ratio

Figure 1.2: Turbine efficiency Cp as a function of tip-speed ratio and pitch angle. Example for illustration. Source: SINTEF

Power curve Since the power efficiency is a controllable variable, the relationship between wind speed and power output clearly depends on the control system implementation. Instead of specifying in detail the turbine control system, it is common to describe the turbine in terms of its power curve.

The power curve of a wind turbine describes the steady state relation between wind speed at hub height and the output power from the wind turbine. Figure 1.3 illustrates this characteristic. The wind turbine starts producing power at cut-in wind speeds, typically around 4-5 m/s, and then the power increases with (approximately) the cube of the wind speed until rated power is reached at rated wind speed, typically around 12-15 m/s. Above rated wind speed, the output power is limited either by natural (passive) aerodynamic stall or by actively controlling the pitch angle of the blades.

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120

100

60 Power (%) Power 40

0 0 5 10 15 20 25 30 Wind speed (m/s)

Figure 1.3: Power curve of wind turbine with fixed speed and passive stall (dotted line) or pitch control in combination with fixed or variable speed operation (full line). The graph is for illustration only. Source: SINTEF

Stall is a phenomenon that occurs when the angle of attack of the effective wind speed seen by the blade becomes too high (typically above 10 degrees), and the lift creating the driving torque on the blade is lost. As the effective wind speed is given as the vector sum of the undisturbed wind speed and the speed of the blade element (rotational speed times the distance from centre of rotation), this means that stall (at the outer parts of the blades) in practice appears at high wind speeds only.

3p effects As described in the previous chapter, the wind speed generally increases with the height. This means that the torque and thrust force acting on a blade will vary and have the highest values when it points upwards. For a three-bladed turbine this results in torque and thrust variations with a frequency three times the rotational speed. This effect is usually referred to as the shear effect. Similarly, when a blade passes in front of the tower, there is a sharp fall in the torque and thrust due to reduced wind speed. This is referred to as the tower shadow effect. Those two phenomena constitute the more important part of the so-called “3p effect”, appearing at thrice the turbine rotation frequency.

The 3p variations in thrust forces give rise to periodic variations in tower bending moments that are important for structural design of the tower.

Short term power fluctuations Short-term variations in the wind speed give rise to short-term power fluctuations that impact on fatigue lifetime and are important for the power export into the electrical grid. Such fluctuations are here illustrated through measurements from a 500 kW fixed speed, stall controlled wind turbine. Although such a turbine is not considered suitable for offshore wind power production, it is sufficient for the present purpose of indicating orders of magnitude to highlight the usefulness of power spectral density characterisation.

Measurements of incoming wind speed at hub height and turbine power output are shown on the left side in Figure 1.4, for a time interval of 600 seconds. The mean wind speed is around 8 m/s, and the mean power output is around 200 kW. The right side plot in the same figure shows the corresponding spectral density, derived from the time series, of wind speed and power output. The power density reveals significant power fluctuations around 1.35 Hz, which corresponds to three times the rotor speed, i.e. the 3p effect discussed above. Harmonics of the 3p frequency at 2.7 Hz and 4 Hz are also identifiable in the figure.

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Figure 1.4: Short-term wind speed and power fluctuations for a 500 kW wind turbine. Left: Time series of wind speed and power output. Right: Spectral density of wind speed and power output

1.3.1.3 Turbine design A major distinction concerning wind turbine design is whether it has a horizontal rotor axis or a vertical rotor axis. Vertical-axis wind turbines (VAWT) attracted significant attention in the 80s and early 90s before they lost ground to horizontal-axis wind turbines (HAWT) that are by far the dominant type today. Only horizontal-axis turbines are therefore discussed here.

The typical wind turbine today has three blades, although two-bladed turbines also exist in significant numbers. Three-bladed turbines are perceived to have a better visual impact and experience smoother dynamic loads that are benefits that offsets the added cost of an extra blade. A schematic of a modern wind turbine is shown in Figure 1.5. Offshore turbines consist of more or less standard wind turbines mounted on a substructure. Different substructures are in use. According to EWEA [3], operational offshore wind turbines at the end of 2011 were found with the following substructures, with numbers indicated in brackets: monopile (1021), gravity-based foundation (286), jacket (30), tripile (23), and floating foundation (3). The choice of substructure depends largely on water depth and seabed conditions.

Figure 1.5: Schematic diagram of a modern horizontal-axis, three-bladed wind turbine. Source: Wikimedia

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Offshore turbine market According to EWEA [3], the following manufacturers have supplied wind turbines for offshore installations: Siemens (51%), Vestas (39%), Repower (3%), WinWind (1%), BARD (1%), GE (1%), Areva (<1%). The numbers in brackets indicate the market share in terms of cumulative installed units at the end of 2011.

Offshore wind turbines are exposed to a harsh environment where costs of installation, maintenance and of repairs are much higher than for land-based wind turbines. This altered cost picture has implications for design choices, where reliability is now being more important. One example of this is a move by some manufacturers towards direct-drive solutions that avoid the mechanical gearbox.

By an economy of scale logic, offshore wind power installations tend to favour large turbine units. Most wind turbine models for the offshore market currently being developed have a rated power in the range 5–10 MW.

Floating wind turbines A number of different concepts for floating wind turbines exist and are in development. Two full-scale prototypes are currently being tested: Hywind and WindFloat. Hywind is a spar-buoy with a 2.3 MW turbine mounted on top, whereas WindFloat is a semi-submersible platform with a 2 MW turbine mounted on top. The actual wind turbine in these and most other concepts is a standard turbine that could also be mounted on a bottom-fixed structure. However, there are also several well-advanced concepts that include a genuine floating wind turbine design, such as the down-stream Sway concept.

Pitch vs. stall regulated turbine At wind speed above the rated wind speed, the turbine speed and power output has to be limited in order not to exceed maximum values. In general, power regulation of wind turbines at high wind speed can be achieved by one of three different strategies: 1. Classic pitch control 2. Active stall control (pitch to stall) 3. Passive stall control

The first alternative is the predominant solution, and the one applied in all modern variable speed wind turbines. It implies power limitation by pitching the blades into the wind, which reduces the lift force and hence the aerodynamic torque on the rotor axis. It also reduces the thrust force. The angle of attack is kept small (out of stall) and continuously adjusted in order to maintain constant power at varying wind speeds. In practise this means that the pitch angle is increased as a function of the wind speed. This type of pitch control is commonly used in combination with variable speed wind turbines, but is also in use with fixed speed wind turbines.

The second alternative also involves pitching of the blades, but instead of limiting power by pitching into the wind, it pitches the blades the other direction, i.e. such that stall occurs. Active stall control is used in some fixed-speed wind turbines; the advantage being that it may give less output power fluctuations compared to fixed speed wind turbines with "classic" pitch control. The drawback of active stall control is that operating in the stall region means operating with increased drag hence increased thrust forces on the construction.

The third alternative is to limit turbine power output at high wind speed by means of a blade design that is such that the turbine stalls by itself, without any control action. This is a common solution for very small wind turbines, and is historically important, but not applied in modern, large wind turbines.

The difference between alternative 1 and 2 is illustrated in Figure 1.6, which shows how the steady state pitch angles vary as function of wind speed when following the constant 1500 kW curve.

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25 25

20 20

15 15

10 3000 kW 10 3000 kW 2500 kW 2500 kW 5 2000 kW 5 2000 kW 1500 kW 1500 kW 0 0 1000 kW 1000 kW -5 500 kW -5 500 kW Pitch angle [degrees] angle Pitch 0 kW [degrees] angle Pitch 0 kW -10 -10 Optimisation Power limitation Optimisation Power limitation -15 -15 0 5 10 15 20 25 0 5 10 15 20 25 Windspeed [m/s] Windspeed [m/s]

Classic pitch Active stall

Figure 1.6: Constant-power curves to illustrate steady state pitch angles (black line) as a function of wind speed for a "classic" pitch regulated wind turbine (left) and for an active stall-regulated wind turbine (right)

At cut-out wind speed, commonly 25 m/s, the wind turbine is stopped. This is because the mechanical stress on the structure is rapidly increasing with the wind speed, and as such high wind speeds generally occur seldom, the loss in annual power generation due to such stopping is anyhow modest. Indeed, an optimum design of a wind turbine for a high wind speed site could yield stopping the wind turbine at a higher wind speed; this would depend on the actual wind distribution and the cost of reinforcing the turbine for operation at higher wind speeds. Gradually decreasing the output power at high wind speeds is an alternative option that is being used by some wind turbine manufactures.

1.3.1.4 Control Systems Modern, large wind turbines require an advanced control system that addresses multiple control objectives. These objectives and various control strategies that satisfy them are discussed in this section.

Control objectives The control objective is generally to achieve the lowest possible lifetime cost per kWh, still respecting any external requirements with regards to operation (e.g. emission of acoustic noise or power system limitations). This can be formulated as:

1. Generation at maximum efficiency up to rated power, thereby maximizing generation 2. Minimizing mechanical loading, thereby reducing costs and maximizing life-time 3. Satisfying any external condition.

Objective 1 and 2 are conflicting, and hence, an optimal control strategy should balance these two. A common assumption is however to give weight to maximizing efficiency for wind speeds up to rated power, and for higher wind speeds keep the output power at the rated value. This results in a power curve as illustrated in Figure 1.3. Control strategies can also include systems for minimizing mechanical loading - mainly avoiding operation at critical natural frequencies and trying to do so with a minimum loss of energy efficiency.

The actual control strategies implemented by wind turbine manufacturers are considered of high competitive value and kept secret. Judging from literature a common approach is detailed e.g. in references [4],[5]. Alternative control structures have been suggested and may be in use, but the brief treatment here focuses on what is considered the most common approach, keeping the structure fairly simple and applying straightforward PI control.

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Speed control As indicated by Figure 1.2, the wind turbine efficiency depends on the tip-speed ratio. This implies that the rotational speed that gives optimal turbine efficiency is a function of the wind speed. The control is based on continuous measurements of power output and rotational speed. From measured rotational speed, an optimal, maximum efficiency, power output is derived from known turbine characteristics. This value is compared to the measured power output, and the deviation is fed into a PI controller that essentially adjusts the electrical counter- torque such that the turbine speed is adjusted towards the optimal point.

Assuming a concept with full-scale converter, the generator torque is determined by stator voltage and current, which in turn is controlled by the generator-side converter. In other words, the turbine speed is controlled via the generator side converter. The controller is outlined in Figure 1.7.

Converter interface & control

P ω opt LP f(ω) PI PI - - MId P id

Figure 1.7: Outline of speed control structure using measured rotational speed as input for determining an optimum power set-point signal.

The measured rotational speed used as input for the control should be low-pass filtered (indicated by the LP block in Figure 1.7), i.e. damping any speed ripple and high frequency signal noise. A speed ripple may appear as the drive train forms a multi-mass swing system with relatively low damping; hence wind variations may lead to significant torque and speed oscillations. Indeed, the oscillations may be damped by application of a power system stabiliser, see below. The multi-mass swing system is normally described as a two-mass system (turbine and generator).

With a speed range that in practice is limited to around 10 to 20 rpm for a 3 MW wind turbine, operation at the theoretical optimum efficiency is not necessarily achieved all the way from zero to rated power. The function f(ω) 3 may thus in practice look something as shown in Figure 1.7, i.e. f(ω) = kω between ω1 and ω2, and then linear between ωmin and ω1, ω2 and ωn (rated speed) and ωn and ωmax. Here, ωmax denotes the maximum permitted speed, slightly above the rated speed for allowing some dynamic speed variations, but still keeping the output power at rated.

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1,2

0,8

0,6

0,4 Active power output (pu)Active power output

0,2

ω 0 n 0,4 0,5 0,6 0,7 0,8 0,9 ω2 1 ωmax 1,1 Rotational speed (pu) ω1 ωmin

Figure 1.8: Relation between rotational speed and active power for operation at optimum efficiency between ω 1 and ω 2.

It is thinkable to use also the wind speed as a direct input for the control, but it is difficult to obtain a wind measurement that is representative for the swept rotor area. For this reason, using the wind speed as a direct input for the control is not in use by any wind turbine manufacturer. However, the wind speed is measured at the top of the hub, and gives input for when the wind turbine should start and stop at low (cut-in) wind speed and high (cut- out) wind speed.

Power system stabiliser A power system stabiliser (PSS) may be applied for damping torque and speed oscillations in the drive train that may appear as the drive train with turbine, shaft and generator forms a multi-mass swing system with relatively low damping.

The technique of applying a PSS for damping oscillations in torque (or other power system oscillations) are well known from use with other generators such as gas or hydro plants, and the same technique can be applied also for wind turbines. A possible scheme for using a PSS in wind turbine control is shown in Figure 1.9. The measured rotational speed is band-pass filtered (denoted BP in Figure 1.9) so that only the frequency corresponding to the targeted oscillation is kept significant. There after the oscillating signal is fed into a lead-lag filter that phase-shifts the signal so that it is in counter-phase with the original oscillations. Adding this to the reference output power provides for damping of the speed oscillations, but still keeping the wind turbine operating at (around) maximum efficiency.

PSS 2 ⎛1+Tns ⎞ BP ⎜ ⎟ Converter interface & control ⎝1+Td s ⎠

P ω opt LP f(ω) PI PI - - MId P id

Figure 1.9: Outline of control structure for the generator side converter with a PSS for damping torque oscillations.

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Pitch control The pitch control for limiting the rotational speed to the maximum permitted can be implemented as outlined in Figure 1.10.

Pitch actuator

ω βref Rate Angle β PI τpitch limit limit - - ωmax Gain scheduling

Figure 1.10: Outline of pitch control for limiting active power to rated.

Here, the measured rotational speed ω is compared with the maximum permitted speed ωmax, and the difference is fed into the PI controller giving a reference pitch angle βref as output. The limiter on the PI controller ensures that βref= 0 as long as ω < ωmax.

Gain scheduling can be applied to compensate for the non-linear aerodynamic characteristics of the turbine, i.e. at high wind speeds and high pitch angles the system is much more sensitive to changes in the pitch angle than at low wind speeds.

The time constant of the servo τpitch is included in Figure 1.10 to indicate that setting βref does not lead to an instant change of the pitch angle; a realistic value for τpitch is 0.2 s. The rate of change is also limited, typically 5 or 10 degrees per second for normal operation and 10 to 20 degrees per second for emergency braking (in case of grid loss). The angle range is typically 0 to 25 degrees.

The description above assumes collective pitching of the wind turbine blades. Individual pitch control of each blade is an alternative, and most large modern wind turbines have this capability, as each blade is equipped with a separate pitch actuator. The individual pitch control will then assume the same overall control (for limiting the speed), but can in addition be varied individually to reduce fatigue loads on the structure.

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1.3.2 Tidal Energy

In this study, we focus on Tidal Energy Converters (TECs) that extract kinetic energy from the tidal flows without constraining it entirely. In particular, we exclude tidal barrages or lagoons such as the Barrage de la Rance, Brittany, France (1966, 240MW) [7] or the existing (1980, 20MW) and envisaged barrage and lagoons, respectively, in the Bay of Fundy, Nova Scotia, Canada [8]. Such constructions use relatively standard low-hydro turbines as described in §2.2.2.1, are suitable for fewer locations, and have more important environmental impact.

1.3.2.1 General description of the prime movers Remaining devices are generally similar in principle to wind turbines, as the kinematics of the problem is analogous in both cases.

Although the technology is not as mature as in the wind industry, and that consequently the standards are less established, the predominating technology is axial flow turbine here too. Likewise, second type of devices is a cross- flow turbine. Therefore, this chapter focuses on those two families of converters. The “horizontal-axis” versus “vertical-axis” classification is slightly different and here less relevant from a prime mover point of view as, for instance, tidal cross-flow turbines can have an horizontal transversal axis (e.g. [9]).

There are also other concepts being developed and tested, such as oscillating aerofoils [10] or “tidal sails”. But at the present time, they cannot be considered as “the most relevant” technologies and are therefore not treated in this report.

Axial flow turbines This section draws an overview of the different concepts of axial flow tidal turbines. They are sorted by level of similarity respect to modern wind turbines.

On the one hand, some technology developers chose to implement the state of the art of wind technology knowledge. That is to say, three-blade turbines with pitch control as presented in the Offshore Wind sections of this report (§1.3.1.3 & §1.3.1.4). This is for instance the case of the Andritz Hydro Hammerfest [11], and Tidal Generation Limited [12].

As offshore interventions are much more expensive than onshore, an approach is to design devices as simple as possible in the aim to reduce sources of failures, limit and simplify offshore manipulations, but also to curb investment costs. For instance, 2-blade rotors gain relevance here, as they can be transported assembled by ship much more easily than 3-blade one, hence avoiding blade (dis-)assembly offshore. This is has been illustrated by the SeaFlow and then the SeaGen devices. The latter is the first commercial tidal stream power station ever and therefore a reference in the domain. Rated at 1.2MW and connected to the grid since 2008, it has produced over 3GWh so far [13].

In the objective to improve reliability and cost-efficiency by simplicity, some devices developers prefer not to use variable pitch systems. Drawbacks of this approach are to get more important structural loads, and a lower hydrodynamic efficiency.

Cross-flow turbines Unlike in the wind industry, cross-flow turbines attract relevant interest in tidal context. Here are some of their inherent advantages. First, in the case of a vertical axis, it is simple and almost natural to have most of the mechanism and in particular the electrical components situated above the free surface, what is less constraining in terms of design and maintenance. Then, in the case of an horizontal axis cross-flow turbine, the rotor diameter is limited by the water depth as for axial-flow turbines, but the rotor length is only limited by the width of the site of interest, generally much greater. For this reason, the captured area corresponding to one rotor and consequently to one single PTO system can be

Rev. 03, 29-Nov-2012 Page 14 of 115 D2.03 Review of Relevant PTO Systems much greater too. This goes hand in hand with beneficial scaling effects respect to a configuration in which the same cross-section would be captured by several independent TECs. Also, vertical-axis cross-flow turbine present the advantage to be passively omnidirectional, that is to say that they intrinsically harvest energy from flow without regard to its azimuth nor need of a yaw control system. This property can be of interest for sites more complex than a straight channel.

Cross-flow turbines can be divided into two categories, depending on whether they are driven by drag or lift forces. Some common types of them are illustrated on Figure 1.11 bellow.

Figure 1.11: common types of cross-flow turbines, from [14].

Drag type devices use the variability of the drag coefficient of a profile with the angle of attack. If the profiles are fixed on the rotor, this is the principle illustrated by cup anemometers and by Savonius turbines. They can also be articulated [15]. Such turbines necessarily have a tip-speed ratio smaller than unity.

Lift type devices employ hydrofoils whose span tends to be parallel to the axis of rotation, and whose chord tends to be locally tangential to the speed of the rotor. They take advantage of the relative speed between the foils and the stream to create lift forces that, though mainly radial, have a propulsive tangential component in the frame of reference of the rotor. This tangential force turns into a driving torque. This phenomenon occurs at tip-speed ratios greater than unity. Therefore, such devices might require to be spun up at an initial speed for extracting power, as this is the case for Wells pneumatic turbines described in section §2.1.1. Some of them have articulated foils to improve several characteristics and in particular get rid of this constrain [16].

Lift cross-flow turbines reach greater power coefficients (Cp) than drag devices.

1.3.2.2 Resource and dynamic effects on the prime movers

Despite the similarities with wind, there are still some major differences. The first obvious one is the change in density, being around 800 times higher for water than for air. The second one is that the speed of tidal streams is generally lower than the wind speed. Those two phenomena account for a linear and cubic factor in the thrust and in the power density, respectively. At energetic tidal sites, streams are generally not one order of magnitude slower than standard wind speeds. This leads to a higher power density: for the same rated power, a TEC is smaller than a wind turbine -but at the cost of a greater thrust-. Then, compared to wind but also wave, the temporal distribution of the resource is more favourable: as illustrated in Figure 1.12, the whole speed range of devices has a similar rate of occurrence, with its maximum at around 50% of maximum speed. As a consequence, efficiency at partial load is less critical than for the two other technologies, generally operating at around 20% of their rated capacity. This can be observed by comparing Figure 1.12 with Figure 1.1.

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12%

10%

6% distribution 4%

0% 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 velocity bin [m/s]

Figure 1.12: rates of occurrence for the inflow speed for the SeaFlow prototype (blue), and corresponding contributions to the power output (red). Courtesy Fraunhofer IWES. Another difference in the resource is that tidal flows are subject to be constrained. This can be by the environment (tidal channel, free surface), leading to blockage effect that enables to exceed the Belz limit. This can also be quite easily done with ducts, increasing the power density.

Turbulence So far, the site and device characteristics taken into account in the power prediction of TECs are based on time scales of a few minutes. This is small respect to a tidal period1, hence expected to represent adequately the variation of power output over each tidal cycle. But this is still large respect to the time scale of local dynamic effects. These are on the one hand mechanical dynamics of the turbines: natural mechanical frequencies are generally a few Hz, and turbine revolution period a few seconds. On the other hand is the unsteadiness of the flow. One naturally thinks about the wave-induced motions but in addition to this, with characteristic speeds greater than 1 m.s-1 and length scales greater than 10 m, energetic tidal flows can only have Reynolds numbers greater than 107 and are therefore fully turbulent. This can be seen in the following time series from the SeaFlow device:

1 At European tidal sites, it is generally 12h25min, period of the principal lunar semi-diurnal M2 tidal constituent.

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220

Leistung [kW] 200 power kW 3,0 Strömungsgeschwindigkeit [m/s] 180 current velocity

160 2,5 140

120

100 2,0 powe Leistung k w k Leistung

80 current Strömungsgeschwindigkeit [m/s] Strömungsgeschwindigkeit 60 1,5 40

0 1,0 0 300 600 900 1200 Zeit [s]

time Figure 1.13: Time series of the flow speed and power output measured on the SeaFlow device. Courtesy Fraunhofer IWES.

Though the topic has not been as extensively studied so far as it has been for wind, here also the phenomenon is considered in a first approach to be similar, as observed in [17] where “the streamwise turbulence intensity during non-slack flow was found to be approximately 12-13%”, with “ratio of the streamwise turbulence intensity to that of the transverse and vertical intensities […] typically 1:0.67:0.5”. Lower values are measured in [18]: 11% turbulence intensity, corresponding to a streamwise value between this and 6% (assuming isotropy), 8% with the ratio proposed by [17], what is in agreement with Figure 1.13.

As the available power varies with the cube of the speed, those inflow speed fluctuations have a significant impact on the power output: at first order, 10% streamwise turbulence intensity leads to 30% variation in the available power.

Arrays So far, TEC prototypes are generally tested alone. In contrast, production units should eventually be organized in arrays. Such configuration are tested at small scale in basin with simplified models such as porous plates [19], [20], and simulated with different levels of complexity [21], [22]. Mostly, such configurations have an unfavourable impact on the downstream turbines’ behaviour, if any. This topic too is common with wind, but is of greater importance here as, constrained between seabed and free surface, tidal turbine wakes generally dissipate slower than in the case of wind turbines. First of all, upstream turbines cause a global shadow effect reducing the mean input speed. But a phenomena that is more important for the PTO point of view is that flows disturbed by TECs have much more fluctuations than unaffected flows that only present a natural turbulence. An example is the configuration of 2-row staggered array, having a downstream turbine in the accelerated flow between two upstream ones [19]. Here, attention must be paid for the tip of the downstream blades not to experience too much turbulence from the wakes of the upstream turbines, what would generate great dynamic loads [20]. Nevertheless, interesting configuration using such flow dynamics exists, e.g. taking advantage the asymmetry of vertical axis turbine wake [23], [24]. Waves In the category of the inflow fluctuations, the case of wave is another important difference with wind turbines. First of all, for floating devices it obviously induces motions for the whole structure that lead to fluctuations in the relative speed between inflow and prime mover. But even in a more general way, it induces fluctuating speeds in the water column that are not negligible. This obviously influences structural design (e.g. [25], “the out-of-plane bending moments [of the blades] were found to fluctuate by as much as fifty per cent of the mean value” because of this phenomenon). This also has an impact on the power quality. For instance, for the SeaFlow prototype, the typical power output fluctuation due to the wave

Rev. 03, 29-Nov-2012 Page 17 of 115 D2.03 Review of Relevant PTO Systems was of 5%. This corresponds to a clear peak in the spectrum of the electrical power at the wave frequency, greater than the 2p peak, as can be seen in Figure 1.14:

Figure 1.14: Spectral analysis of power output for the SeaFlow Device.

Also, it has been shown that at energetic tidal sites, dedicated wave-current interaction (WCI) models are required to simulate reliable dynamic flow data, as illustrated in Figure 1.15.

Figure 1.15: Simulations of wave-current interaction in shallow water with different models. In black is the mean velocity profile across the water column. Coloured are the velocity profiles at trough and crest of wave (courtesy Fraunhofer IWES).

Particular case of axial flow turbines As in the case of wind, 3-bladed axial flow turbines suffer from 3p effects, and 2-bladed turbines from greater 2p effects. In addition with the velocity shear across the water column that is a common problem for all devices, shadow effect can be more important because in a number of cases, the rotor is places downstream of its support

Rev. 03, 29-Nov-2012 Page 18 of 115 D2.03 Review of Relevant PTO Systems structure. This can be for self-orientation purpose [26] or in the case of fixed devices that present this characteristic during ebb or flood only [13].

At high flow speeds, stall-regulated devices have the drawback to experience greater loads. Therefore, the simplicity gained in the mechanism by the absence of pitch system goes hand in hand with the need for more robust components and structure. Consequently, the advantage of this approach regarding reliability and costs is not necessarily acquired.

Particular case of cross-flow turbines Unlike for axial flow turbines, shear in the velocity profile does not induce dynamic loads in vertical axis turbines. Nevertheless, the torque ripples during rotor revolution are important for cross-flow turbines [27]. First obvious reason for this is that each blade passes through the wakes of the other ones, plus its own for lift devices. But in a more general way, the cyclic loads on each blade due to the continual changes in relative speed magnitude and orientation have no reason to equilibrate and be constant in time when summed over the whole rotor. With a fixed- pitch, axial-span turbine as reference, those dynamic effects are smoothed in the case of helical blades and can be smoothed by pitch control, but coupling the two is technically challenging.

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1.3.3 Wave Energy

1.3.3.1 Wave energy resource

While over 70 per cent of the earth's surface is covered with water, the energy contained within waves has the potential to produce up to 80,000TWh of electricity per year, sufficient to meet our global energy demand five times over [28].

The waves can be characterized as sinusoids and the main characteristics defining them are: height (H), wavelength (L) and depth (h). These three parameters are independent of each other and the all the rest are related in some way to them. That is, the period is dependent of the wavelength and depth, and the wave amplitude is defined to be half the wave height. The wave height is independent of h and L except in terms of stability.

h L

Figure 1.16. Main characteristics of a wave

The wavelength increases when increasing period. A frequent assumption of “infinite depth” is applied when the depth is greater than half the wave period. This hypothesis of neglecting the seabed interface simplifies a lot the nonlinearities of the mathematical formulations. Other simplifications are applicable for relative “shallow water”, when h/L <20.

Waves are created when wind blows over the sea surface and are a combination of potential energy (energy due to displacement of the free surface with respect to the rest state) and kinetic energy (due to motion of the particles). The energy flux in indefinite depths is approximately proportional to their period and to the square of its amplitude. The used form to indicate the wave energy is kW/m. Across the water column, the energy flux decays exponentially from the top to the bottom.

In real-waves there is a range of heights, periods and directions. Thus, a sea state is the result of the superposition of a large number of monochromatic waves with different amplitudes, wave periods and phases. This means that the ocean surface can be modelled by adding a number of sinusoidal monochromatic waves with different amplitudes, wave periods, directions and phases. The sea state can be presented by the spectral distribution that describes how much energy is present on the different frequencies. Each sea state can be defined by a representative height (usually significant wave height Hs) and a representative period

Figure 1.17. Spectral distribution of a sea state [29]

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As it can be seen in Figure 1.18 a real sea state has a great variability with high amplitude variations of displacements, accelerations and forces in a very short time. This variability implies that Wave Energy Converter (WEC) PTO components may be subject to different dynamic load patterns and thus consideration of other dominant failure modes should be made. For example, the design has to consider the resonance with fatigue loads.

Figure 1.18. Variability of the waves [30]

The sea states that take place in a site along the year represent the wave climate and its knowledge is fundamental for the design and power performance assessment of wave energy converters. Wave climates are more energetic on offshore deep sea, although they are the most difficult to extract due to the maintenance costs and the electrical facilities needed to connect the WEC to the grid.

Figure 1.19. Global and European offshore wave power spectral distribution [31]

1.3.3.2 WEC power performance assessment To evaluate the expected annual output of a WEC, it is necessary the occurrence matrix or scatter diagram of location when the device will be installed. The scatter diagram gives the probability of occurrence of a sea state in the chosen location.

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Figure 1.20. Scatter diagram [30]

The matrix has axes of height and period and the number of occurrences of each combination is shown in the table cells corresponding to each height and period.

The steps for evaluating the energy production of a WEC in a selected location are the followings:

- Generation of the power matrix of the device. This matrix can be elaborated by coupling each sea state defined the device conversion model, thus calculating the power produced in a particular sea state with defined conditions. - Balancing through one of the typical wave spectra of the selected location. - Estimated annual average electrical power across the matrix of occurrence of the sea state with the matrix of electric power obtained from the combination of the above. The sum of the resulting values in each cell represents the average annual power.

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Figure 1.21. Power performance assessment illustration [30]

1.3.3.3 Scalability When designing a prototype to test in a wave flume the physical parameters have to be calculated following the law of Froude [29]. The scale ratios defined according this law are presented in Table 1.1.

Parameter Model Full scale Length 1 r Area 1 r2 Volume/Mass/Force 1 r3 Time 1 r0.5 Speed (linear) 1 r0.5 Power 1 r3.5 Table 1.1. Conversion of parameters in model scale to full scale with scale ratio 1:r [29]

The scale ratio will be limited by the basin dimensions with its wave generating abilities and the physical phenomena to be analysed. In the case of the basin limits, the wave conditions in full scale need to be scaled down to the waves that can be generated in the basin. But sometimes not all the basin parameters suit, like when scaling the water depth in the scaled prototype given that the basin water depth is rarely modified.

Regarding physical phenomena analysed, measured parameters like motion, forces and power production can be converted from the model to full scale as indicated in Table 1.1. Here the scale limits appear when considering the construction of all the components of the scaled prototype. For example, if the prototype has a scaled PTO, the viability of supplying all the scaled components (for example small hydraulic accumulators) will limit the scale of the

Rev. 03, 29-Nov-2012 Page 23 of 115 D2.03 Review of Relevant PTO Systems test. Moreover, it has to be pointed the fact that the power is scaled with an exponent of 3.5, which leads to a very small powered scaled components.

1.3.3.4 Classification of WEC primary energy capture Early work was targeted primarily at floating devices and typically classified a given device as being a Point Absorber, a Terminator or an Attenuator and the descriptors are still used to a certain extent. They are intended to describe the principle of operation and provide information on the geometry of the device.

In an attempt to group and classify ocean energy devices, the primary energy capture technique is typically used as a demarcation between device classes. Often, the same or similar prime movers and generators can be employed in very different devices, and so it is reasonable to classify devices according to the dynamics of the primary energy capture method.

For the structure and contents of this classification the reference [32] has been used.

Oscillating Water Column (OWC) The Oscillating Water Column (OWC) device [33] converts wave motion into pneumatic energy within an enclosed chamber. Then the air passes through a turbine that is connected to a generator. The primary power capture converts wave energy into pneumatic energy. An illustration of a typical system is shown below:

Figure 1.22 Oscillating Water Column When the input of the wave compresses the air in the chamber it goes out through a turbine, and when the wave retires, the air flows into the chamber driving the turbine again. The turbine needs a special design to be able to rotate in the same direction with this bidirectional flux. This fact leads to a lower yield than in a conventional turbine. It is possible to use hydraulic PTOs instead of pneumatic PTOs in OWCs; however, the vast majority of OWCs use pneumatic PTOs. The OWC systems can be installed onshore and offshore. The onshore ones require the conjunction of a number of natural features on the site, and cost of civil works is high. Due to its better accessibility the first OWC installations were onshore ([34],[35]), nevertheless there are several offshore OWC installations with some developers working on them like [36] and [37]. The dimensions are quite big due to the needs of the air chamber, hence the high prices of each device. A prominent characteristic of OWC devices is that it usually does not have moving devices in contact with water what reduces the effects of fatigue and biochemical impact. A representative OWS installation is Pico OWC Plant in Azores Islands.

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Figure 1.23. Pico OWC Plant [34] Following a list with the most well known OWC devices based mainly on the global status report [38] is presented.

Technology / Plant Company Country Primary Energy PTO System Name Capture AWS III AWS Ocean Energy United Kingdom OWC Floating Air Turbine Ltd. Limpet WaveGen United Kingdom OWC onshore Air Turbine Mutriku WaveGen / EVE Spain OWC onshore Air Turbine Pico OWC Wave Energy Centre Portugal OWC onshore Air Turbine Port Kembla Oceanlinx Australia OWC Nearshore Air Turbine OE Buoy Ocean Energy Ltd. Ireland OWC Floating Air Turbine OWEL Offshore Wave United Kingdom OWC Floating Air Turbine Energy Ltd. Table 1.2. Main OWC devices

Attenuators Attenuators [39] are floating long devices that operate parallel to the wave direction. These devices capture energy from the relative motion between two or more bodies caused when waves pass or from the inertial motion induced.

Figure 1.24 Attenuator

The devices that capture energy from a relative motion are typically long multi-segment structures. The device motion follows the motion of the waves. Each segment, or pontoon, follows oncoming waves from crest to trough. The floating pontoons are usually located either side of some form of power converting module. Passing waves create a relative motion between each pontoon. This relative motion can then be converted to mechanical power in the power module, through either a hydraulic circuit (most common) or some form of mechanical gear train.

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The most typical example of an attenuator is the Pelamis WEC [40]

Figure 1.25. Pelamis The most well-known attenuator devices are listed in the following [38].

Technology / Plant Company Country Primary Energy PTO System Name Capture McCabe wave Pump Hydam Technology Ireland Attenuator Hydraulic circuit Ltd. Pelamis Pelamis Wave Power United Kingdom Attenuator Hydraulic circuit Ltd. Sea Power Platform Sea Power Ltd. Ireland Attenuator Hydraulic circuit Wello Wello Finland Attenuator Hydraulic circuit Table 1.3. Main attenuator devices

Point Absorbers Point absorber devices [41] are generally axi-symmetric about a vertical axis and therefore, indifferent to the incident wave direction. They are small in comparison to the incident wavelength, and usually a number of point absorbers are installed forming a line. Point Absorber devices usually consist of two main components – a displacer which is a buoyant body that moves with wave motion, and a stationary or slow moving reactor. Energy can be extracted through the relative motion between the displacer and the reactor. This can be accomplished using electromechanical or hydraulic energy converters.

Figure 1.26 Point Absorber The linear generator is the most direct method for harvesting the energy from a point absorber since it converts the linear motions between the buoy and its reference directly into electricity. For a great number of wave energy devices, pressurized hydraulics is the most popular method of power take-off.

The point absorbers are often based on buoy type systems such as PowerBuoy U.S. company Ocean Power Technologies.

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Figure 1.27. PowerBuoy [42] Following a list with the most well known point absorber devices is presented [38].

Technology / Plant Company Country Primary Energy PTO System Name Capture Power Buoy Ocean Power USA Point Absorber Hydraulic circuit Technologies Seabased Seabased AB Sweden Point Absorber Linear Generator Wavebob Wavebob AB Ireland Point absorber Hydraulic circuit AquabuOY Finavera Canada Point absorber Hydraulic circuit (Aquaenergy) CETO III Carnegie Wave Ireland Point absorber Hydraulic circuit Energy Limited Wave Star Wave Star Energy Denmark Point absorber Hydraulic circuit (Multiple points) Table 1.4. Main point absorber devices

Overtopping devices Overtopping devices [43] extract energy from the sea by housing the waves when these impinge on a structure such that forces water to run up a ramp over that structure thus raising its potential energy. The water can then be stored in some form of a reservoir. The potential energy of the water is converted to kinetic energy using a conventional hydro turbine. After exiting the turbine, the water is then returned to the sea.

Figure 1.28 Overtopping

Overtopping devices generally use reflector arms in order to focus the waves to the reservoir. These devices are fundamentally low-head hydro power plants, except the source of water is from the sea rather than rivers or lakes. They tend to be typically much larger than other devices as significant volumes of water capture are necessary.

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These devices have one clear advantage over other wave energy devices - the inclusion of a reservoir allows for inherent energy storage. This can be used to produce a more consistent level of power supplied to an electrical grid. Onshore overtopping devices are not very common as the site requires the combination of a number of natural features, and the cost of civil works is high.

A representative system of an overtopping device is the Wave Dragon developed in Denmark by the company of the same name.

Figure 1.29. Wave Dragon [44] The main point overtopping devices developed or being developed is shown in the following table [38].

Wave Dragon Wave Dragon Ltd. Denmark Overtopping Hydro Turbine TAPCHAN (*) Norwave A.S. Norway Overtopping Hydro Turbine Seawave Slot-Cone WAVEnergy AS Norway Overtopping Hydro Turbine Generator Floating Wave Power Sea Power Sweden Overtopping Hydro Turbine Vessel Table 1.5. Main overtopping devices (*) Actually is stopped

Oscillating Wave Surge The Oscillating Wave Surge devices [45] extract the energy caused by wave surges and the movement of water particles with them. At the seabed, on or near the shore, the water particle motion becomes a back and forth motion. It is from this oscillating surge motion that the device extracts energy. These devices can be secured to the seabed, on or near the shore.

Figure 1.30 Oscillating Panels

Energy is typically extracted using hydraulic converters secured to a reactor. A representative nearshore system is the Oyster blade developed by Scottish company Aquamarine.

The most well known oscillating wave surge devices are listed in the following table [38].

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Figure 1.31. Oyster [28] Technology / Plant Company Country Primary Energy PTO System Name Capture Wave Roller AW-Energy Oy Finland Oscillating Wave Hydraulic circuit Surge C-Wave C-Wave Ltd. United Kingdom Oscillating Wave Hydraulic circuit Surge Oyster AquaMarine Power United Kingdom Oscillating Wave Hydraulic circuit / Ltd. Surge Hydro Turbine Table 1.6. Main oscillating wave surge devices

Submerged Pressure Differential This system is based on the static pressure fluctuation caused by the oscillation of the water level to the passage of the wave [46]. Passing waves cause the sea surface elevation above the device to rise and fall. A pressure differential is created above the device as waves pass. The system can contain a closed air chamber which volume will vary depending on the pressure at which it is subjected. The bottom of the chamber (reactor) is fixed to the seabed, while the upper part (displacer) can move vertically. The air chamber behaves like a spring whose stiffness may be modified by pumping air into or out of it (thereby changing the volume of the chamber).

Figure 1.32. Submerged Pressure Differential The PTO for this system device can be a linear generator or a hydraulic circuit. These devices are submerged some meters under the water or directly fixed to the seabed that makes them less vulnerable to storms. An example of this technology is the Archimedes Wave Swing device originally developed by the Dutch company Teamwork Technology and operated by the Scottish company AWS Ocean Energy [46].

Following a list with the most well known submerged pressure differential devices is presented [38].

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Technology / Plant Company Country Primary Energy PTO System Name Capture AWS I (*) TeamWork Portugal Submerged pressure Linear generator differential CETO I (*) Seapower Pacific Pty Australia Submerged pressure Hydro Turbine Ltd differential PYSIS PIPO Systems SL Spain Submerged pressure Mechanic differential / Point absorber Table 1.7. Main submerged pressure differential devices (*) Actually stopped

1.3.3.5 Control at primary energy capture level

The optimum energy absorption is obtained when the WEC natural frequency of oscillation is close to the incident wave frequency—this situation is called resonance. If the WEC natural frequency is not tuned to the incident waves the energy absorption decays rapidly. So it is necessary to achieve phase control in order to maximise the power absorbed.

Since the wave frequency changes over time, the WEC natural frequency must also change in order to find the resonance continuously. Changing a device natural frequency is known as tuning, and may involve adjusting its size, shape, mass, stiffness or damping, or a combination of these. Tuning can be considered in three contexts [47], depending on when and how quickly it is done.

Fixed tuning is considered when WEC properties are impossible or at least impractical to change once it is constructed. It should be set during the design process so that the device frequency response is a good overall match to the wave spectrum at the intended sea location. This is a basic way of optimisation that does not involve any kind of control. However, this type of tuning needs to be considered in every project independently of the device control capability. Slow tuning involves adjusting some device properties to match the current wave climate over a period of several minutes to hours (i.e. from sea state to sea state). An example of slow tuning is pumping water in/out of a tank to change the WEC buoyancy and therefore its mass and stiffness. Usually slow tuning is a process based on searching the peak wave energy flux. Slow tuning allows more energy to be captured in the longer term. Finally, an ideal tuning system would be able to know the height and period of incoming waves before they reach the WEC and adjust its properties in advance to extract the maximum possible energy from Wave waves. This is called fast tuning or wave-to-wave tuning. In practice, fast tuning is difficult to implement, partly because it is difficult to predict wave characteristics and also since it is difficult to change some of WEC properties quickly. However, fast tuning may allow more energy to be captured than slow tuning. A WEC control system should combine aspects of slow tuning and fast tuning.

It has been established that optimum energy absorption from a single mode of oscillation unconstrained WEC in regular seas can be achieved when the following two conditions are fulfilled [48]: i) the device’s oscillating velocity must be in phase with the wave excitation force (this condition is automatically satisfied without control if the body is in resonance with the wave); ii) the amplitude of the oscillation must be adjusted to the optimum value where absorbed power equals the power radiated into the sea by the oscillating device. The consequence of optimum control is that some energy has to be returned into the sea during some small fractions of each oscillation cycle. This is why optimum control is also known as reactive control. In practice, optimum control leads to reversible and very complex PTO mechanisms. Therefore, the second condition is rarely addressed.

Several suboptimal control strategies have been proposed to avoid this problem. The most interesting one is the so- called latching control [49]. The aim of this strategy is to achieve approximate optimum phase control by locking the WEC in a fixed position at the instant when the device’s oscillating velocity becomes zero (i.e. the extreme

Rev. 03, 29-Nov-2012 Page 30 of 115 D2.03 Review of Relevant PTO Systems excursion) and then releasing it after a certain time period that needs to be determined. It is a simple and discrete control strategy where the single control parameter is the latching time interval. Compared to optimum control, latching presents moderate instantaneous power absorption, but it still increases considerably absorbed power with reference to passive control.

For real seas, which are irregular, the preceding optimum conditions can be still applicable to every harmonic wave component if linearity is assumed. However, these conditions are non-causal, which means that it is required the prediction of the incoming irregular waves some time into the future. The better the waves can be predicted, the closer the converted power may approach the theoretical maximum in regular seas.

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1.4 CLASSIFICATION OF PTO COMPONENTS

As observed in §1.3, the primary energy capture in offshore renewable can have various forms, with the greater diversity occurring in the wave sector. A similar scheme occurs for the PTO systems.

As the very wide majority of Marine energy converters produces useful energy in an electrical form, this report does not focus on exceptions to this rule, and the PTO components are here categorised into two parts.

First, the components used upstream of the conversion to electrical energy are described in Chapter 2. Most of the ones used in wave energy are summarised in Figure 1.33 below:

Figure 1.33 - Block Diagram of Alternative Wave Motion to Electrical Generation Methods

Then, Chapter 3 presents the constituents of the energy chain dealing with this conversion, and with downstream electrical power treatment. The scope covered by this latter section might therefore be considered slightly wider than the IEC definition of PTO (“mechanism that converts the motion of the prime mover into a useful form of energy such as electricity”), by including electrical power treatment devices that are not a mechanism strictly speaking. Nevertheless, such component can be very important regarding the dynamics of the whole energy conversion chain, and are necessary for the produced electricity to be useful. Hence their inclusion in this review.

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2 PTO COMPONENTS, PART I: TRANSMISSION OF MECHANICAL ENERGY

2.1 AIR TURBINES The following chapters deal with the characteristics and behaviour of air-turbines for oscillating water column WEC. As described in §1.3.3.4, the airflow in the duct of an OWC is oscillating due to the oscillating water level in the OWC chamber. As this is not common for other applications where air-turbines are used – e.g. wind energy conversion or gas power plants – special turbines are required, which transfer the oscillating airflow to a unidirectional torque at the turbine shaft. In the 1970s the first turbine providing this behaviour was invented and named after its inventor, A.A. Wells. Throughout the last decades several other turbines for the use in OWCs were proposed, developed and tested. As they share the common feature of delivering a uni-directional torque in a bi-directional airflow, these turbines are subsumed under the term self-rectifying turbines. In the following three different types of self- rectifying turbines are introduced. These are the:

- Wells turbine, - Impulse turbine and the - Dennis-Auld turbine.

Firstly the working principle of the turbines is described. Based on this the important characteristics are derived and compared among the different types of turbines and finally examples of prototype turbine, which have been used in real applications, are given.

Following the chapters about the different turbines some considerations of the dynamic behaviour of the turbines are given and finally the effects of up-scaling of the turbines are discussed, which is assumed to be an important factor due to the fact that currently mainly down-scaled prototypes of OWCs are tested, which need to be up-scaled in order to reach a commercial feasible level.

2.1.1 Wells turbines

As wind turbines, the working principle of Wells turbine is based on radial aerofoils. On each of them, the incident airflow generates a lift L and a drag D force. The resulting force on the rotor blade can be split up into a tangential force, which is used to extract the power, and an axial force. In Figure 2.1 this effect is depicted on a rotor blade cross-section of a wind energy converter. See for instance [50] for further theoretical background.

Figure 2.1 (source Bundesverband WindEnergie)

In order to achieve the self-rectifying behaviour, the Wells turbines use symmetrical rotor blades with their chord in the rotation plane, hence a symmetrical rotor as illustrated in Figure 2.2. The airflow itself is axial, and therefore

Rev. 03, 29-Nov-2012 Page 33 of 115 D2.03 Review of Relevant PTO Systems perpendicular to the blades. Nevertheless, the rotational speed of the turbine implies an apparent flow angle, i.e. angle of attack, decreasing with the tip-speed ratio. For λ = 1 this angle is 45°, and at tip-speed ratio of a few units the angle of attack is small enough for the blades to perform without stall. In this configuration, the aerodynamics of the aerofoil creates a lift force perpendicular to the apparent flow direction. The tangential component of the lift is greater than the drag, and therefore drives the turbine. Due to the symmetry of the system, when the flow is reversed the turbine torque is still positive and thus an oscillating airflow is converted into a rectified output (unidirectional torque). One drawback of such turbines is that they are not self-starting: they must first be accelerated by an external source of energy (e.g. generator used as motor) to reach high tip-speed ratio. In [51] an overview of the technology of Wells turbines is given.

Figure 2.2: schematic representation of a standard Wells turbine

Obviously one of the characteristics of major interest is the efficiency of the turbine. Another interesting property is the dimensionless pressure drop, which indicates the axial forces on the turbine. For steady-state conditions, those data are generally expressed as a function of the flow ratio, which is the inverse of the tip-speed ratio. In Figure 2.3 corresponding curves for different Wells turbine are depicted.

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Figure 2.3: Steady-state behaviour of different Wells turbines as function of flow ratio (source: [51]).

The Wells turbine can be considered the quasi-standard turbine used for this application at the moment. In most of the OWC devices built until today Wells turbines were applied. This is despite the fact that Wells turbines suffers from several shortcomings ([52]):

- Narrow flow range at which the turbine operates at useful efficiencies, - Poor starting characteristics, - High operational speed and consequent noise, - High axial thrust.

Two OWCs applying Wells turbines are the onshore WECs Pico [53] and Limpet [35]. Both turbines used differ from the basic layout depicted in [51]. For efficiency optimization the Pico Wells turbine is equipped with pitched blades and the Limpet turbine is a biplane turbine, using two counter rotating Wells turbines (see [51]). Both have a rated power in the range of some hundred Kilowatts. [54] shows some key figures of the Pico Wells turbine. In the Limpet WEC the counter rotating Wells turbines drive two 250 kW generators. The turbines are operated with variable speed in the range of 700 to 1,500 rpm ([55]).

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Figure 2.4: Pico OWC Wells turbine (source [54])

Rated electrical power 400kW Blade length 0.25 Nominal speed 1475 rpm Solidity 0.66 Tip diameter 1.7 m Idling loss 6.7 kW Blade pitch range -40° to 40° Design airflow rate 120 m3.s-1 Number of blades 15 Design pressure drop 5900 Pa Blade tip speed 131 m.s-1 Damping coefficient 50 Pa. m3.s Hub ratio 0.71 Efficiency at full flow 58% Blade chord at root 0.20

Table 2.1: Characteristics of the Pico Wells turbine (source [54])

2.1.2 Impulse turbines In order to overcome the shortcomings of the Wells turbine the so-called Impulse air turbine was proposed and developed by Kim, Setoguchi et all ([56]).

In opposition with the Wells turbines using almost axial lift forces, in Impulse air turbines the airflow is deviated tangentially by guide vanes so that a tangential force on the rotor blades is directly produced by transferring the kinetic energy of the airflow to the rotor. In Figure 2.5 an example layout of the guide vane/rotor blade design is depicted.

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Figure 2.5 Example Guide-vane and rotor layout for a pneumatic Impulse turbine. Source: [56]. (Same orientation as Figure 2.2: the rotor axis is vertical)

Along with the working principle go some advantages of this turbine compared to the Wells turbine ([52]):

- High starting torques/Good starting characteristics, - Low operational speed and - Wide range of flow coefficients at which the turbine operates under reasonable efficiency.

Actually the impulse turbine can be subdivided into three types of turbines, which share the working principle/shape of the rotor blades, but differ in terms of the guide vanes. The guide vanes can either be fixed or pitched, whereas the pitching mechanism can either be controlled by the airflow (self pitch controlled/ the guide vanes flip under the action of aerodynamic moment due to the change of air flow direction) or by another active mechanism like a hydraulic ram. Obviously the pitched guide vanes mean an increased number of moving parts and thus reduced reliability, increased maintenance and investment costs etc., but on the other hand the turbines using pitched guide vanes show a efficiency well above the efficiency of the un-pitched ones [56].

Comparing the efficiency of Wells and Impulse turbines one has to distinguish between steady flow condition and the operation of the turbine under oscillating airflow and real sea conditions. In Figure 2.6 the efficiency of an impulse turbine with self pitch controlled guide vanes under steady airflow conditions is depicted. A comparison with Figure 2.3 shows that the maximum efficiency is in the same range as the maximum efficiency of a Wells turbine. So an impulse turbine with un-pitched guide vanes can be assumed to suffer from a lower maximum efficiency than a Wells turbine. This situation changes if the efficiency in reciprocating air-flow/irregular sea states is addressed. Due to the smoother efficiency curve of the impulse turbine the mean efficiency in this operation circumstances is well above the mean efficiency of the Wells turbine (see figure Figure 2.7).

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Figure 2.6: Impulse turbine characteristics in steady flow conditions as a function of tip-speed ratio. Source: [56].

Figure 2.7: Efficiency of different self-rectifying air-turbines in irregular sea-states. Source [52].

The one major drawback of the impulse turbine with movable guide vanes is the significantly increased number of moving parts (see for instance [57]) that have to withstand a large number of oscillations per day. An alternative design is an impulse turbine with fixed guide vanes, which might be a good compromise between efficiency and mechanical robustness (see Figure 2.7).

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Figure 2.8: The Vizhinjam wave energy plant. Source [58].

One example of an OWC equipped with an impulse turbine is the wave energy plant at Vizhinjam, Kerala, India. The plant was built and commissioned 1990-1991 and originally operated with a Wells turbine (see [59]). In order to improve the efficiency throughout the years several different generator and turbine sets have been tested. Among these in 1997 an impulse turbine with linked guide vanes rated at 55kW, operating at a speed of 750RPM was installed in the device and successfully tested.

2.1.3 Dennis-Auld Turbine The so-called Dennis-Auld turbine was developed in a cooperation between the University of Sydney and the Australian company Energetech, now OceanLinx ([60]). As the Wells turbine the Dennis-Auld turbine uses aerofoil blades. In the contrary to Wells turbines the blades are symmetrical respect to the maximum thickness line rather than to the chord line (see Figure 2.9). In order to achieve an optimal angle of attack the blades are pitchable around their neutral position. In Figure 2.10 the different positions of the blades over one wave cycle are depicted.

Figure 2.9: Blade design for a Dennis-Auld turbine. Source: [61].

Figure 2.10: Blade pitching sequence in oscillating flow for a Dennis-Auld turbine. Source: [61].

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As the impulse turbine the Dennis-Auld turbine is designed for low rotational speeds / high torques and a wide range of high efficiency (see Figure 2.11). As the impulse turbine with movable guide vanes the Dennis-Auld turbines has the disadvantage of a large number of movable parts.

Figure 2.11: Efficiency curves for a Dennis-Auld turbine, as function of flow ratio. Source: [61].

In 2005 OceanLynx did a first commissioning of the MK1 OWC full-scale prototype, which applied a Dennis-Auld turbine. In [62] some key figures of the plant and turbine are given. The rated speed of the turbine is in the range of approx. 500RPM. The turbine was operated by a regenerative frequency converter allowing for 500kW peak and 440kW average power. Throughout the sea trials the plant was not grid connected. Thus the produced power was dissipated in dump loads, due to the maximum power of the dump loads the max power of the turbine was limited to 150kW.

2.1.4 Dynamic behaviour

As described in chapter 1.3.3.1, the airflow and thus the power input to the turbine is highly fluctuating due to the nature of the power resource. Obviously the airflow fluctuation affects the behaviour and performance of the turbine and thus the behaviour of the overall WEC. Aspects that have to be addressed in this context are for instance:

- Turbine performance under irregular/oscillating air-flow - Advanced controls to optimize the power output. - Simulation of dynamic aspects - Grid connection / Grid codes - Energy storage systems to smooth the power output - Use of the turbine inertia to smooth the turbine power output - Generator rating - etc.

In the course of developing wave energy conversion towards a commercial feasible technology a lot of these aspects have been addressed by researchers and by the industry. Nonetheless still a lot of research is required to fully understand all the different effects, thus helping wave energy harvesting to become a mature technology. In the following some examples of research work carried out and according results are described. Here mainly the aspects directly concerning OWCs and air turbines are discussed.

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Turbine performance validation: Comparing the reports on turbine development and calculation in older literature to more recent reports it can be seen, that the turbine behaviour under irregular flow conditions moves more and more in the focus. In [51], published 1995, it’s reported that test-rigs used for validation of the performance, calculated by radial equilibrium approach and actuator disk method, provided only uni-directional airflow. In the contrary in [52], published in 2001, a test-rig providing an oscillatory airflow is described, giving the chance of validating results like depicted in Figure 2.7.

Control strategies: As described above, the turbine efficiency is a function of the tip-speed ratio. Thus by constantly adapting the turbine speed to the flow speed, the power output of the OWC is increased. This effect is well known from other application like wind energy conversion. Nevertheless special control schemes are required to deal with the continuously occurring airflow zero crossings and other unique aspects like the guide vane switching of an impulse turbine. In [63] a detailed description of the control scheme for a Dennis-Auld turbine is given, showing that based on an appropriate set of sensors the power output can be significantly increased. An option for further control improvement is the application of short-term forecasting methods in the control schemes allowing in theory for an optimal control, limited only to the accuracy of the forecasting methods. Even though short-term wave forecasting has been discussed in several scientific papers [64], [65]; so far no forecasting-based control has been tested in real sea trials.

Dynamic simulation of overall WEC: Throughout the design process of an OWC a dynamic simulation of the system is required. In [63] the design of the control schemes based on such a dynamic simulation is described. In [66] a study on the application of ultracapacitors for power smoothing of a 570kW OWC is described, besides the various aspects of the ultra-capacitors also valuable information like peak-to-average power of the generator are given, which can be used for the design of the generator system.

Turbine inertia as energy storage device for power smoothing: Similar to a flywheel the rotating mass of the turbine can be used to store energy and thus to smooth the power output of an OWC (see for example [63] and [32]). If this effect shall be exploited to smooth the power output of an OWC to a large extend, it should be taken into account that the power optimization of the turbine by speed adoption over wave cycles will lead to contradictory requirements on the turbine speed control.

2.1.5 Effects of scaling

Currently different device developers test or have tested downscaled WECs in real sea trials. An example of a downscaled prototype is the OEBuoy ([57]) rated at 15 kW. In order to produce electricity at reasonable costs, these devices have to be up-scaled to full-sized devices, designed for installation at sites like the Irish Atlantic coast (average wave power: 75 kW/m) or the Danish North Sea coast (average wave power: 21 kW/m) ([67]). Based on the results from sea trials and simulations, several developers propose and advertise full-scale version of their devices. In Table 2.2 some data of some proposed full-scale devices are listed. The rated power of these devices ranges from 1 to 2.5MW. The more important figure with respect to the generator system is the rated power of the turbines applied. As can be seen below, the turbine power is in the range of around 0.5 to 1 MW, which is a reasonable assumption to be a good indicator also for the power range of turbines applied in other projects (short term). A reasonable estimation of the maximum size of future turbines for OWC WECs – upper limit of 2 MW - is given in [68] (p.110).

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blueWave greenWave

Developer OceanLinx Ltd. OceanLinx Ltd. Structure Floating Fixed Primary energy OWC OWC capture

Turbine type airWave (Dennis-Auld) airWave (Dennis-Auld) Rated power of Approx. 2.5 MW Approx. 1 MW device Rated power per 420 kW 1 MW turbine Number of 6 1 turbines Table 2.2: Example of proposed full-scaled devices (sources [69], [70])

From the data above two important figures for the generator systems can be derived: Firstly the rated power of the single generators can be expected to be in the range from some hundred kilowatts to ca. 1MW and maybe up to 2 MW in the long term. There are also exceptions from this power range like in the case of the Mutriku breakwater OWC WEC ([71]), which is equipped with 16 turbines rated at 18.5kW each and is said to be the first commercial wave energy plant by the developers. Nonetheless as the specific costs (€/kW) of these small-scale turbine- generator systems can be assumed to be well above the specific costs of turbine-generator systems in the range of some 100 kW, this is unlikely to be a competitive design for future offshore WECs. Secondly a rough estimation of the rated speeds of future turbines can be derived: On the one hand the more recent air-turbine designs – impulse- and Dennis-Auld turbine [61] – have rather low operational speeds and on the other hand up-scaling the rated power of turbines normally goes along with reduced speed due to limitations in the maximum tip speed. So the rated speeds of future turbines can be assumed to be in a range of some hundred rpm – as mentioned above this should be considered as a rough estimation!

2.2 HYDRAULIC CONVERTERS

One of the major challenges of wave energy converters is concerned with how to drive electrical generators. Heaving and nodding-type devices are not directly compatible with conventional rotary electrical machines, and a transmission system is therefore required to interface the wave energy converter with the electrical generator.

Hydraulic converters are probably the most obvious avenue to investigate when dealing with energy capture from ocean waves, but this can be in a number of forms using either seawater as the medium, or some other form of fluid in a sealed system such as fresh water or mineral based oil. The sections that follow look at the different types of hydraulic systems employed to date.

The power available to any hydraulic system is proportional to the head pressure and flow rate of fluid, and can be expressed as [72]:

� = �� where

� is the mechanical power � is the hydraulic efficiency of the system � is the density of the fluid � is the acceleration due to gravity (9.81m/s²)

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� is the volume flow rate passing through the turbine (m³/s) � is the effective pressure head of fluid across the system (m)

2.2.1 Hydraulic Circuit

Hydraulics are used in a number of wave energy converter devices, because of their flexibility and ease of turning hydraulic energy into mechanical rotation, and hence electrical power generation. In many hydraulic systems, a pump is driven by a rotating machine, and the actuator translates this energy into linear motion, whereas in a wave energy system the opposite is usually true. An example of hydraulic technology in wave energy is the Pelamis device, which has been in development for some years, and has also been generating power to the grid in a number of locations, both as part of the device development, and more recently as a commercial device. The Pelamis device is a semi-submerged, articulated structure composed of cylindrical sections linked by hinged joints. Its design power output is 750kW, and a picture of the device in operation can be seen in Figure 2.12 below;

Figure 2.12 - Pelamis Wave Energy Converter in Operation (courtesy Pelamis)

The wave-induced motion of each of the hinged joints is resisted by sets of hydraulic rams configured as pumps [73]. The wave motion on the device drives these rams back and forth, which pumps hydraulic oil around the power generation system. The oil flows through smoothing accumulators which then drain at a constant rate through to hydraulic motors, driving electrical generators to produce electricity. Simplistically the hydraulic motors can be considered as the inverse of a hydraulic pump i.e. in a ‘normal’ application, an electric motor would turn the hydraulic pump to generate hydraulic force elsewhere in the system. Here the principle is reversed, with the hydraulic force from the rams turning the hydraulic pump (termed motor in this case), which in turn rotates an electrical generator. The accumulators are sized to allow continuous, smooth output across wave groups, and equally are used within the hydraulic circuit as a means to decouple the hydraulic pumps from the hydraulic motors. A simple oil-to-water heat exchanger is included to dump excess power in large seas and to provide the necessary thermal load in the event of loss of the grid. Each individual section of the Pelamis device contains its own power take off system complete with power generation as described above, and therefore inherently has a certain amount of built in redundancy [73] i.e. if one power take off section fails, the remainder may well continue to operate.

Pelamis is a sealed system utilising oil as the fluid to drive the hydraulic motors. Containment is therefore a significant issue in such a system, and it is understood that two forms of ingress/egress protection are employed in the form of flexible rubber bellows, which would need to fail to permit any leakage to the outside environment [73]. It is also understood that the hydraulic fluid in use is a biodegradable transformer oil, so that should a leakage occur the oil will biodegrade within the marine environment.

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2.2.1.1 Accumulator

An accumulator is a device for storing energy in the form of fluid under pressure. Due to its fast response time, a gas-loaded bladder type accumulator is most appropriate for a wave energy system [74]. Its function is to remove or damp (as much as possible) the pulsatile nature of the fluid energy captured by the hydraulic pumps. Effective sizing of the accumulator means a lot less work for other components in maintaining a constant operating pressure, and also a much steadier fluid flow to the turbine or hydraulic motor. Pre-charge pressure for the accumulator system should normally be in the order of 60% - 80% of the system pressure to be maintained. The decoupling provided by accumulators allows pump torque to be varied so as to capture the most energy from the variable pressure delivery, whilst the generator is driven at a steady rate.

2.2.1.2 Hydraulic Motor

In 2001 the UK government produced a report on the R & D priorities of wave energy [75], which identified the need for hydraulic motors that had low part load losses and high torque pumps i.e. to ensure maximum efficiency from the motor at lower than optimum loads or wave conditions. As a consequence some UK hydraulic manufacturers looked into the development of their hydraulic machines, which in some instances resulted in the substitution of the mechanical flow control of the machine, to electronic control of poppet valves using specifically developed software, resulting in the required efficiency improvements. Part load efficiency is also greatly increased as cylinders can be completely deactivated if required. The advantage of using a hydraulic motor solution is that they offer variable displacement operation to suit the available power, with a high-speed output to drive the electrical generator. Equally a sealed hydraulic circuit also offers the possibility of using multiple hydraulic motors and electrical generators, which can be engaged or disengaged from service depending upon the amount of available hydraulic power, or the demands of the electrical load. For some floating devices, the use of hydraulic motors also offers significant flexibility in terms of location of the motors and generators etc. This is because these items are no longer physically restricted to the mechanical drive positions, and can be located at the ideal location from a weight distribution perspective, and thereafter the hydraulic power connections routed to suit.

In terms of the differing types of hydraulic motor there are three main types that could be considered. These are piston, vane, and gear motors, and are covered in the following sections.

Piston Motor A piston motor can be radial or axial. In a radial piston motor, the pistons are oriented radially from the shaft. For an axial piston motor, illustrated in Figure 2.13 bellow, the pistons are aligned to the rotating shaft and the swashplate angle is fixed (i.e. there is no compensator and control piston). The inlet side of the motor is the high- pressure side and the outlet is low pressure. The pressure difference causes the pump to rotate. Since the swashplate is fixed, the speed of this motor is controlled by either controlling inlet pressure (Δp across the motor) or directly the flow rate.

Like pumps, hydraulic motors tend to have 9 pistons, or possibly 7 (more pistons increase displacement and hence increase output torque). Piston motors provide the best sealing for high input pressures and work best in high torque, low speed applications. Having the best sealing, they are also the most efficient. An axial piston motor with a fixed swashplate is unidirectional (rotate in 1 direction only). To be bi-directional, the swashplate would need to be variable position. Piston motors also tend to have a case drain line to allow piston leakage to flow to return.

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Figure 2.13 - Example of an Axial Piston Motor Vane Motor Vane motors are more suitable for high-speed applications. The vane motor rotates as hydraulic fluid at high pressure flows through the motor. More vanes reduce output torque ripple, but also lead to higher pump friction.

The vanes are attached to the drive shaft and fit closely to the housing ring (or cam ring) to minimize leakage. The vanes are pushed out by hydraulic pressure, centrifugal force or springs.

For vanes, which rely on centrifugal force to extend, the may be attached to the drive shaft (or rotor) via a slot, which allows the vanes to translate in the slot and also slide in the radial direction. As pressure is applied, the vanes will start to rotate the rotor (shaft). As speed is increased, the vanes move outward in the slots and contact the wall, providing a seal on the outer surface. The housing (or cam ring surface) can have a ramp shape to further reduce pressure at the pump outlet. This is possible because the rotor slot allows a vane to move radially to adjust to the housing (or cam ring) shape.

For vane motors that utilize spring loaded vanes, the spring helps to hold the vane against the housing (or cam ring) to ensure sealing at low ΔP across the pump. This helps the motor develop starting torque faster. At higher speeds, centrifugal force helps hold the vanes out.

The vane motor shown in Figure 2.14 has two ports, an inlet and outlet. It is possible to have a four-port vane motor, which splits the flow through two separate paths. A four-port vane motor will have twice the torque, but will operate at ½ the speed as a similar sized two-port vane motor. An advantage of this solution is to balance bearing loads by axial symmetry of the forces.

For vane motors, a valve can control which port has high pressure and which port has low pressure, leading to a bi- directional motor. Those motors become less efficient at high inlet pressures (due to potential for more slippage and leakage past the vanes). They are less noisy than other motor types, and are generally less expensive than piston motors.

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Figure 2.14 - Example of a Vane Hydraulic Motor

Gear Motor A gear motor schematic is shown in Figure 2.15. This system has two spur gears rotating in a common housing. Both gears rotate, although only one gear is connected to the output shaft. Fluid enters where the gears mesh together as shown in Figure 2.15. The gears then rotate in the direction of the arrows, as the greatest pressure drop is around the outside of the housing. Also, by putting the input port where the gears mesh together puts the effective area of 2 gear teeth against the resisting pressure acting on 1 gear tooth. The efficiency of gear motors is lower at low speeds and increases at high speeds.

Gear motors are generally very compact relative to their displacement and are able to operate at high speed. They are also less expensive than a piston or vane motor. However, they can be noisy and are the least efficient of the 3 motor types. Gear motors can also be operated in a reversible (bi-directional) manner.

Figure 2.15 - Example of a Gear Hydraulic Motor

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Hydraulic Motor Efficiencies Hydraulic motor efficiency will vary with speed and load on the system. A typical performance chart for a hydraulic piston motor is shown in Figure 2.16. As can be seen in the top chart, the volumetric efficiency is high and constant over the range of speeds. However, the overall efficiency (which is the product of the volumetric and mechanical efficiencies) drops off at higher speeds, which implies there is an optimal speed range for each particular hydraulic motor. The lower chart shows a linear relationship between flow and speed, where the slope of the line is the displacement. Also shown in the lower chart is the motor output torque, which tends to drop off at higher speeds.

Figure 2.16 - Typical Piston Hydraulic Motor Performance Graphs The overall benefits of hydraulic motors include variable speed control, the ability to withstand stall torque easily, and relatively high power to weight ratios. Motors are self-contained units and simple in operation, which leads to high reliability. The main issues with hydraulic motors in service are excessive leakage, seal failures and noise.

2.2.2 Hydro Turbines Several different types of hydro turbine have been used in wave energy converters, and the technology itself is mature having been used in power generation schemes for many decades. Indeed the technology has developed to the point where very high efficiency devices exist, and so with regard to wave energy it becomes less of an issue of whether power can be generated, and more about how to extract the energy from the wave to deliver to the turbine. Turbines are also divided by their principle way of operating, and can be either classified as hydro impulse turbines, or hydro reaction turbines. A reaction turbine contains a rotating element known as a runner, which is fully immersed in water and is enclosed within a pressure casing. In contrast an impulse turbine runner operates in air, and is driven by jets of water. For impulse turbines a nozzle is used to convert the low velocity water into a high- speed jet, and the runner blades then deflect the jet so as to maximise the change of momentum of the water, and therefore maximise the force on the blades. Reaction turbines typically used in wave energy converters to date include Francis and Kaplan designs, whilst Pelton wheels are the most typical type of impulse turbine. These turbine types are therefore described in the following sections.

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2.2.2.1 Kaplan Turbines (Low Hydro)

The Kaplan turbine is a well-recognised turbine, and visually it looks very similar to a simple propeller as used in airplanes or ships. However the blades of the turbine are adjustable and the turbine itself sits within a tube with adjustable inlet guide vanes controlling the flow of water to the turbine runner. They are now widely used throughout the world in high-flow, low-head power production. The Kaplan turbine is known as an axial flow turbine, or an inward flow reaction turbine, which means that the flow direction does not change as it crosses the rotor, and the fluid itself changes pressure as it moves through the turbine and transfers its energy. Power is therefore recovered from the hydrostatic head as well as from the kinetic energy stored in the flowing water [76]. An example of a simplified Kaplan turbine is illustrated in Figure 2.17 below;

Figure 2.17 - Simplified Kaplan Turbine Layout (Courtesy Renewables First)

As can be seen in Figure 2.17, inlet guide vanes can be varied to regulate the amount of flow through the turbine, which is also referred to as a wicket gate. Ultimately this can be closed off completely to bring the turbine to rest if required. The inlet itself is a scroll-shaped tube that wraps around the turbine, and water is then directed tangentially through the vanes and spirals on to the turbine blades causing it to spin. Depending upon the position of the vanes of the wicket gate they introduce differing amounts of ‘swirl’ to the flow, and ensure that the water hits the rotor at the most efficient angle. The rotor blade pitch is also adjustable, from a flat profile for very low flows to a heavily pitched profile for high flows (see Figure 2.18). This variable geometry of both wicket gate and rotor blades means that the flow operating range is very wide, and the turbine efficiency remains high throughout this range. Typically Kaplan turbines operate at efficiency values of in excess of 90%, but this may be lower in very low head applications. With particular regard to wave energy, the level of head is obviously dependent upon the type of energy capture system employed, and therefore appropriate sizing of the Kaplan turbine would need to be undertaken to give as wide an operating range as possible.

The outlet (or draft tube as shown in Figure 2.17) is specially designed and helps decelerate the water and recover the kinetic energy in the flow by reducing the water pressure at the exit of the rotor. The turbine itself does not need to be at the lowest point of water flow as long as the draft tube remains full of water. However a higher turbine location will increase the suction that is imparted on the turbine blades by the draft tube, and the resulting pressure drop can lead to cavitation and therefore excessive drop in efficiency and lifetime.

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Figure 2.18 - Kaplan Turbine Rotor Blade Positions (Courtesy Renewables First)

Theoretically a Kaplan turbine could technically work across a very wide range of flows and heads, but as they are relatively expensive, and other turbine types can be more effective on higher heads, they tend to be the turbine of choice for lower head sites with high flow rates. Typically they are used on sites with net heads from 1.5m to 20m and peak flow rates from 3 m3/s to 30 m3/s. Although 20m heads are unlikely to be seen by a wave energy capture system, heads of several metres are probable, and flow rate will be dependent upon the type of capture mechanism employed and its scale. An exploded view of a Kaplan turbine, showing turbine blades, wicket gate and generator assembly can be seen in Figure 2.19 below;

Figure 2.19 - Exploded View of Kaplan Turbine [77] Typical cost of a Kaplan turbine obviously varies dependant on the site conditions, and installation costs will vary greatly for the same reason. However an example of a 315mm diameter Kaplan operating with 8.2m head and 0.5m³/s flow is around 18k€ for the turbine alone, which produces a peak of approximately 32kW [78]. A rough estimate of cost for a 300kW Kaplan would be 200k€ based on a 5m head and 7.5m³/s flowrate (2008 values) [79].

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For a wave energy application the Kaplan turbine is suited to overtopping devices, where the wave energy is captured by using wave reflectors to direct water towards and over a ramp. Once behind the ramp the water becomes trapped temporarily in a storage reservoir that is at a higher level than the ocean, and therefore develops a relatively low head of water pressure that is released in a controlled manner via the Kaplan turbine, generating power in the process. An example of this type of wave energy converter is the Wave Dragon, and a system outline of Wave Dragon can be seen in Figure 2.20 below [44]:

Figure 2.20 - Wave Dragon System Outline (Courtesy Wave Dragon)

2.2.2.2 Francis Turbines Another example of the reaction turbine is the Francis turbine, which is not entirely dissimilar to a Kaplan turbine. Physically the turbine has a similar external appearance as a Kaplan turbine, and they can be spiral cased or open flume machines. As with a Kaplan the spiral casing is tapered to distribute water uniformly around the entire perimeter of the runner and guide vanes feed the water into the runner at the correct angle. The runner blades are profiled in a complex manner and direct the water so that it exits axially from the centre of the runner. In doing so, the water imparts most of its pressure energy to the runner before leaving the turbine via a draft tube [80]. Unlike a Kaplan turbine though the runner blades are not adjustable, and therefore when the flow is reduced the efficiency of the turbine falls away. However, where Kaplan turbines are always mounted vertically, Francis turbines can be mounted either vertically or horizontally to suit the process. A picture of a Francis turbine can be seen in Figure 2.21 below;

Figure 2.21 - Francis Turbine Example

2.2.2.3 Pelton Turbines

A Pelton wheel is an impulse turbine that is among the most efficient type of water turbine that has been designed. It extracts energy from the movement of moving water, instead of the weight of the water that would be seen in a traditional water wheel.

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The nature of the geometry of the buckets on the rim of a Pelton wheel is designed so that when the rim runs at ½ the speed of the water jet, the water leaves the wheel with very little speed, extracting almost all of the water’s energy, thereby allowing for a very efficient turbine. An example of a Pelton wheel can be seen in Figure 2.22 below;

Figure 2.22 - Old Pelton Wheel from Walchensee Power Plant, Germany

The Pelton wheel shown in Figure 2.22 above has been removed from its casing. It would normally reside within an enclosure with controllable nozzles or spear valves mounted through the casing to direct the water jet into the buckets, and a gravity outlet in the base for the water return flow. In operation the nozzles (or spear valves) direct forceful streams of water against the spoon-shaped buckets mounted around the edge of the wheel. As the water flows into the bucket, its direction changes to follow its contours, which in turn exerts pressure on the bucket and hence the wheel. As the water travels round the inside shape of the bucket it, is decelerated and then flows out the other side of the bucket at low velocity. In the process, the water's momentum is transferred to the turbine. The water then falls under gravity through the outlet return at the base of the wheel enclosure. Often two buckets are mounted side-by-side, thus splitting the water jet in half (see Figure 2.23).

Figure 2.23 - Pelton Wheel Bucket This balances the side-load forces on the wheel, and helps to ensure smooth, efficient momentum transfer of the fluid jet to the turbine wheel. Also the dual bucket arrangement eliminates the potential of a ‘dead spot’ in the central area of a single bucket, where it may be incapable of deflecting the incoming water jet [80]. The Pelton wheel is most efficient in high head applications; thus, with this technology more power can be extracted from a water source with high-pressure and low-flow than from a source with low-pressure and high-flow, even when the two flows theoretically contain the same power.

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2.2.2.4 Turbine Comparisons

As can be seen from the foregoing sections on Kaplan Turbines, Francis Turbines, and Pelton wheels, they each have their own ideal operating region, where the selection of one type against another becomes more apparent. In the case of the Pelton wheel this is high pressure (or head) with low flow, whilst the Kaplan is better suited to low head and high flow. Whilst the Francis turbine can sit between these two types, it’s also true to say that the efficient operating range of a specific Francis turbine is lower and hence less flexible than the other two types. This can be represented graphically as shown in Figure 2.24 and Figure 2.25 below [72];

Figure 2.24 - Turbine Flow vs. Head Pressure Comparison The operating regions shown in Figure 2.24 would seem to suggest a gap between where a Kaplan turbine operates versus where a Pelton wheel would be ideal. However whilst this region could be filled by a Francis turbine (as shown), the reality is that both a Pelton wheel and Kaplan turbine operate across a very wide range, and therefore even when not in their ideal efficiency range they still operate at very good efficiencies well outside their optimal. This can be illustrated as a flow ratio against the design flow rate as shown in Figure 2.25 below [72];

Figure 2.25 - Efficiency Curves of Turbine Types As can be seen in Figure 2.25, the efficiency of both a Kaplan turbine and Pelton wheel is still above 80% even when below 30% of the optimal hydraulic throughput, whereas a Francis turbine drops below 80% at 60% of its optimal throughput.

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A table showing some advantages and disadvantages of impulse and reaction turbines is given in Table 2.3 below:

Turbine Type Advantages Disadvantages Impulse Turbines Greater tolerance of particles in Require medium to high pressure water head Better access to working parts No pressure seals around shaft Easier to fabricate and maintain Better ‘part flow’ efficiency

Reaction Turbines Suitable for low head high flow Higher purchase cost due to applications precision manufacture Faster speed of rotation, may allow Tolerances and clearances more direct coupling to generator critical More compact package for similar Maintenance more difficult power output Table 2.3 Comparison of Impulse vs. Reaction Turbines

For both technologies, using seawater as the working fluid may be the most obvious and environmentally friendly method, but there are limitations in terms of wear from particles and sand, deposition of solids, biological growth, lubrication, and corrosion. Such issues need to be considered throughout the design of a power take off system in terms of selected materials, cost, maintenance, and useful component life.

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2.3 OTHER MECHANICAL TRANSMISSION SYSTEMS Another method of capturing the wave motion and converting it into electrical power output is by direct mechanical mechanisms. Such systems usually employ some form of mooring to fix the position of the device, and thereafter translate the wave motion into mechanical rotation, in order to drive an electrical generator. Such systems include the Manchester Bobber, and the Ocean Harvester, and are discussed in the following paragraphs.

Transmission of the wave motion energy to standard electrical generator can also be made by purely mechanical systems. Such mechanisms convert the wave motion into mechanical rotation. Two examples are the Manchester Bobber, and the Ocean Harvester, and are discussed in the following paragraphs.

The Manchester Bobber is a point absorber array device comprising a grid of floatation buoys, which are coupled to generators via cables. The array itself is mounted to the seabed therefore the generators and pulleys etc. are mounted on a fixed stable platform above the ocean surface. The wave energy converter is therefore a pulley-based system, with the buoy at one end of the cable, and a counterweight at the other. Each cable itself runs around a pulley that in turn drives a generator via a clutch, gearbox and flywheel arrangement. As each float descends, the pulley speed attempts to exceed the output speed causing the clutch to engage and accelerating the entire shaft system. At maximum speed the clutch disengages, allowing the output shaft to continue its forward rotation whilst the pulley decelerates and reverses during ascent of the float. Whilst the clutch is disengaged, the output shaft continues to rotate due to the inertia of the flywheel but decelerates due to energy extraction (i.e. the power output). A gearbox is used to increase the output shaft speed hence reducing the size of flywheel and generator required to produce a given output power.

There are a number of advantageous design features to this concept;

a. The vulnerable mechanical and electrical components are housed in a protected environment well above sea level, which makes for ease of accessibility.

b. All mechanical and electrical components are standard ‘off the shelf’ items, and are readily available, resulting in high reliability compared to other devices, with a large number of more sophisticated components.

c. The Manchester Bobber will respond to waves from any direction without requiring adjustment.

d. The ability to maintain and repair specific 'Bobber' generators (independent of others in a linked group) means that generation supply to the network can continue uninterrupted.

A graphical representation of the Manchester Bobber can be seen in Figure 2.26 below;

Figure 2.26 - The Manchester Bobber (Courtesy ODE Website)

Another mechanically driven system of wave energy power generation is the Ocean Harvester.

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Like the Manchester Bobber the Ocean Harvester is a point absorber which utilises a cable and pulley system to drive an electrical generator, but in this instance rather than a having float coupled to the drive cable, the device itself is floating and so one end of the cable is fixed to the seabed, with the other end again fastened to a counterweight. This can be seen in Figure 2.27 below;

Figure 2.27 - The Ocean Harvester (Courtesy Ocean Harvesting)

According to the manufacturers website ’The slow and highly fluctuating wave motion is converted into a unidirectional rotation with smooth torque and speed utilising the cable and winch system, suitable for a conventional high speed generator’ [82]. Additionally ’the counterweight in the Ocean Harvester provides high storage capacity and a nearly constant torque through the PTO to the generator.’ As well as the cable tether employed to drive the pulleys and generator, the system is also moored to ensure that the device remains within given bounds thereby ensuring that the cable and counterweight system never reaches the end of its travel, and stops producing power. The mooring representation can be seen in Figure 2.28 below;

Figure 2.28 - Ocean Harvester with Mooring Lines

The manufacturers own website gives the following as technology advantages;

• Efficient energy capture: with the use of a winch system. Energy is absorbed throughout the full rise of each wave (end limits of winch drums never reached). Self-adjusting to changing tide levels. • Power smoothing: using mechanical power take-off (PTO) with a counterweight enables peak energy from the waves to be intermediately stored and utilized without the need to oversize components in PTO and power electronics. Mechanical components are much more efficient in terms of processing the highly fluctuating energy from ocean waves compared to hydraulic components.

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• Power generation: with the use of a conventional generator running at a continuous and high speed. Power smoothing in the PTO reduces the working range of the generator that greatly improves efficiency and utilization rate. Compliant to the grid requirements on electrical quality. • Survivability: Winch systems reduce structural strain. Counterweight limits the mechanical peak loads/torque in the system in all wave conditions. PTO can be disengaged to put the wave energy converter in standby/failsafe mode to survive the roughest sea states. Position moorings (secondary moorings) are used to secure station keeping.

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3 COMPONENTS, PART II: CONVERSION TO GRID-COMPATIBLE ELECTRICAL ENERGY

3.1 ELECTRIC GENERATORS

3.1.1 Rotating Generators When converting mechanical energy into electrical energy, mostly rotating motion is used. Therefore, generators in conventional power stations (coal, gas oil, nuclear), hydro power stations, wind turbines or vehicles use rotating generators. A rotating generator is composed of two parts: The stator (fixed) is cylindrical and in high speed machines is large comparing to the diameter; and the rotor, usually inserted in the stator cavity, is the rotating part of the machine. Although this is the generic topology of rotating generators, new turbine designs are leading to new generator concepts.

3.1.1.1 Asynchronous (Induction) Generators They are called induction machines since the rotor magnetic field is provided via electromagnetic induction from the stator magnetic field. The name of asynchronous machines is because the rotor rotating speed is not exactly the synchronous speed imposed by the grid. They are robust and simple, have little maintenance (no brushes) and have been for long very extended in the industry. In renewable energy converters (especially wind energy), squirrel cage induction generators have been very used. The reasons for its popularity were mainly its simplicity, low cost, and low maintenance requirements, usually restricted to bearing lubrication only. The induction generator has reactive power consumption, which among others factors, decreases its efficiency. To compensate for it, a capacitor bank can be inserted in parallel with the generator in order to obtain about unity power factor. Since the induction generator is directly coupled to the supply grid, the energy converter has a high impact on it because of the necessity to obtain the excitation current from it. The main limitation of this generator is the lack of speed regulation capacity. The generator, as described above, is essentially a fixed speed solution with a speed range of only within about 5% of synchronous speed. This wouldn’t allow the prime mover to work at the optimal operation point, thus, achieving very little efficiency. This fact is solved when connected to grid through power electronics. In this case, the speed of the generator can be allowed to vary not being imposed by the electrical grid.

Besides the squirrel cage induction generator, the rotor can be not short-circuited with access to the rotor windings. This option has been very extended in wind turbines where the rotor winding is connected to grid through a bidirectional power converter (doubly-fed induction generator topology). The variable-speed operation depends on the converter size that usually is about the 30-40% of the rated power. This is an advantage because it reduces the size and cost of the power electronics and allows active and reactive power control. The rotor side converter and the grid-side converter can be controlled independently, allowing the control of the power factor at the grid side. On the other hand, it needs slip-rings, what increases the maintenance of the system.

One last alternative for variable speed operation is using a squirrel cage induction generator with a full-power bidirectional converter. In this case, there is no need for reactive power compensation elements and no slip-rings. However, this alternative does not bring many advantages to the use of a synchronous generator as shown in the following section.

3.1.1.2 Synchronous Generators It is the most extended topology in conventional power plants and more and more used in renewable energy generators. They have no slip and their speed is completely synchronized with the grid frequency when the armature windings located in the stator are directly connected to the electrical grid.

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The magnetic field is generated by an exciter that provides DC current in the generator winding, or by a permanent magnet rotor. In both cases, since the speed of the synchronous generator is fixed to the grid frequency, the rotational speed of the prime mover is essentially fixed which presents several drawbacks in any energy conversion device (these drawbacks are inherent to fixed speed operation with both, synchronous or asynchronous generators): - Energy capture: the power extraction of the ocean devices is a nonlinear function. To extract the maximum power in each situation, the control will determine the optimum speed, which will differ from the fixed speed. - Mechanical stress: since the speed of the turbine is fixed by the grid frequency, the resource power fluctuations (wind, wave…) are transmitted directly as torque pulsations, causing mechanical stress. Drive train and gearbox must be able to withstand the absolute peak loading conditions. - Power quality: The resource variations are not only transmitted to the drive train, they are also directly transmitted to the grid. Nowadays, it is not common the direct connection of the synchronous generators to grid, and power electronic converters are used. This allows complete variable-speed operation, and active and reactive power control. This solution, comparing to the induction generator also with power electronics presents several advantages. - They can operate at slower speeds, allowing the direct-drive train without gearbox. - They have higher efficiency, as they do not have reactive power consumption. - They can operate without being grid connected.

Permanent Magnet Synchronous Generators The most extended alternative for the use of synchronous generators are the Permanent Magnet Synchronous Generators (PMSG). The use of permanent magnets avoids the slip-rings and thus, the rotor copper losses disappear reducing the need for refrigeration. In PMSG the pole count can be increased allowing the construction of slower machines (in asynchronous generators a high pole count implies a too big magnetizing current [83]). This permits the construction of gearless devices that, due to the low operational speed of the prime movers, need a slow machine for direct-drive operation.

Permanent magnet direct-drive technology consists of a wind/wave-driven turbine rotor turning a permanent magnet synchronous generator, which does not require a gearbox to operate. The generator produces alternating current that is delivered to the grid via AC-DC-AC conversion by a full-power converter. These generators are now recognized in the wind industry as a reference technology, in particular to meet the requirements of the offshore wind market, and are a widely studied alternative in the new ocean devices. In this kind of generator, the rotor has no winding excitation, so it is not necessary to inject field magnetizing currents (not consumption of reactive power). To get the magnetic field, the rotor is formed by permanent magnets. Rotor shaft can be attached directly to the prime mover, without needing a gearbox. Thus, the speed of rotation of the blades is transmitted to the permanent magnets creating a rotating magnetic field. This avoids the need to feed the rotor current.

However, for direct-drive operation the PMSG needs: - A high pole count to get suitable frequency at low speed - A big rotor diameter for the high torque (and high pole count)

The advantages of this topology are:

- The alternating current is taken directly from fixed terminals, not needing brushes continuously subjected to friction. - It saves the rotor winding and the current source that feeds it. No winding losses. - Being self-excited allows its operation isolated from the network. - High power generating efficiency and a better power curve: the direct drive PMSG avoids all rotor winding and mechanical energy losses associated with gearboxes and couplings. With full power converter it provides the flexibility to optimize rotational speed for maximum energy capture, which reduces transmission loss and allows higher generation levels, especially at low wind velocities.

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- Lower maintenance costs and less downtime: as a result of a low component count and the absence of a gearbox, the reliability and maintenance costs that are the lowest in its class. As a result, the use of cranes is less. Besides of eliminating the gearbox (it is not necessary to control the level of lubricating oil), the absence of brushes avoids the need of replacing the generator due to wear of the brushes.

Disadvantages of a direct drive PMSG:

- Permanent magnets tend to be demagnetized to work in high temperatures inside the generator. - The supply of these powerful magnets, generally made from rare earth metals such as Neodymium and only available in a few countries, is influenced by existing political and economic factors that could lead to a shortage of strategic minerals and rare earth elements. - As the speed of the machine is imposed by the resource (in wind turbines imposed by the speed of the wind nominally about 10 m/s for a good harvest) and also for a given power, the size of the turbine increases with decreasing the speed of rotation (it requires a large number of poles which increases the size). The more powerful direct-drive turbines require large and heavy machines.

In recent years, the performance of permanent magnets is improving while the cost of power electronics is decreasing. Therefore these trends make permanent magnet machines with a full-scale power converter more attractive for the direct-drive wind generators.

3.1.2 Linear Generators

As described in §1.3.1 and 3.1.1, developers have been designing so called “direct-drive” wind turbines, without gearbox but rather an adapted generator directly coupled to the rotor. This has the potential to provide a simpler system requiring fewer moving parts, lower maintenance requirements and higher efficiency. In comparison with standard technology, such generators are larger and handle greater torque. Similar principle of a special generator directly coupled to the prime mover also occurs for wave energy capture, with linear generators. The basic concept of a linear generator is to have on the one hand a translator (equivalent of the rotor in a rotary machine) on which magnets are mounted with alternating polarity, and on the other hand the support structure containing windings (equivalent to the stator in a rotary machine). This kind of generator produces electricity from a translation motion, and has almost no application except wave energy. There, the translator can be coupled to an oscillating prime mover, with the support structure being part of the reciprocator. A simple outline of a device using this principle is shown in Figure 3.1 below;

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Figure 3.1 – Example Outline Schematic of Linear Generator Wave Energy Converter

As can be seen from Figure 1.33 previous, direct drive linear generator is the most direct technology for generating electrical power from waves, as directly translating wave motion into electrical output, with no intermediate stages or conversion devices with associated losses.

However, such systems are very challenging, for addressing a variety of dynamic issues. First, output voltage is obviously varying both in frequency and amplitude. Mechanical strains are very high, and power factor low. The generators typically have an air gap shear stress in the region of 20-40kN/m2. Hence if the machine rating is 2MW at 2m/s, the force is equal to 1MN, which will require an air gap surface area of 25m² at best. Also, having less intermediate energy forms or rotating parts provides much less possibilities of power storage and smoothing in the energy chain.

3.1.3 Particular aspects of electric generators

Although rotating generators are used in most of the cases due to the maturity of this technology, different concepts are arising in order to adapt to each conversion device.

In the wind industry, a good example is the construction of a generator with the rotor outer to the stator that allows a full integration between the generator and the prime mover. The blades of the turbine can be directly coupled to the generator active rotor. This allows the reduction of the torque path, and thus, the reduction of the stiffness requirements of the rotor and the stator. When coming to wave energy converters, the devices can take advantage of the linear motion of the wave and avoid mechanical losses using a linear generator. The choice between synchronous and asynchronous generators is not so straightforward and it is directly linked to the application and environment. Aspects like power, speed, size, weight, cost, maintenance and control have to be considered. In addition, usually the choice of the complete topology is made, considering the generator and the power electronics for the grid connection. This implies that in offshore devices, also the sea to shore connection can influence the decision.

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3.1.3.1 Scaling One of the challenges for the renewable energy sector is to minimise costs. Concerning marine energies, offshore wind is where the most relevant experience has been gathered. There, the trend in the construction of turbines has shown that the technical considerations for this goal include turbine weight reduction; larger rotors and advanced composite engineering leading to higher yields; and design for offshore installation, operation and maintenance. Many offshore costs do not increase linearly with turbine power. Particularly, if larger devices lead to smaller numbers of units being deployed in farms of similar capacity [84]. O&M costs can be similar for both smaller and larger turbines offshore, as they share high fixed costs of getting access to them [85]. Therefore it is expected larger turbines that will be cheaper and will lower the O&M costs. For the next generation farms, industry is developing new turbines up to 10 MW for fixed as well as floating installations. To achieve such a jump in capacity, new designs of electric generator may be needed. As to the power trains, some manufacturers go for direct drive (gearless) turbines, while others still prefer gearbox designs at least up to the current 5-6 MW power range. Anyway, although the size and weight mean added technical difficulty, it is estimated that 25% of costs are not scalable. This means that, ultimately, the larger turbines, and therefore more powerful, present more advantages than small ones from an economic standpoint.

When considering the potential for weight and size reduction, it is worth exploring the fundamental scaling relationships of an electrical generator. One of the main relations is given by the following equation:

S: Apparent power output for an electrical generator 0 Br : excitation field (T) at no-load 02ω Ks: electrical loading (A/m) SBKrL= rsπ 0 a 0 p with r : average radius of armature winding (m) La: active length (m) ω: angular frequency (rd/s) p: number of pole pairs

This relation means that, to obtain higher power, it is necessary to: work at high speed, increase the machine size 2 0 (πr La:), work with high magnetic fields (Br ) and/or enhance the electrical loading capability (Ks). Ks is limited by the 0 electrical loading (Joule losses, i.e. efficiency) and refrigeration of windings. Br is also limited by the saturation of Fe poles (~2T), and the performance of the excitation conductor (losses in the rotor coils). In the case of permanent 0 magnet (PM) machines, Br is limited by the maximum induction around 1.3 T (for NdFeB magnets). Thus it is clear that the options for scaling up the specific power for a generator are limited. Up scaling issues for wind turbines were analysed in the FP6 UPWIND project.

3.1.3.2 Superconductivity In recent years, superconductive materials have emerged as real alternatives to conventional resistive materials such as copper. Laboratory machines and even first commercial developments demonstrate that superconductivity is already a mature technology. Most applications are large synchronous machines with superconductive rotor windings. Some features of these high temperature superconductive synchronous machines are [86]: - compactness in volume and weight - higher efficiency even including additional power for refrigeration - high over-load capacity and possibility of under-excited operation - stiff operational behaviour - reduced noise and vibrations However, superconductive machines are not always the best choice for a given application. Only those applications where small sizes or weights, slow rotation speeds or large torques are required are economically viable. Wind turbine generators, especially in direct drive configuration, could be a promising application for superconductive machines, as they fulfil the key requirements of high power at reduced weight.

In general terms and considering synchronous generators with iron poles, the use of superconductive materials in rotating machines presents some advantages. Conventional generators use copper both in rotor and stator

Rev. 03, 29-Nov-2012 Page 61 of 115 D2.03 Review of Relevant PTO Systems windings. Therefore, its substitution by superconductive high current density wires allows a very significant reduction of weight and volume (between 30 and 50%).

Moreover, there is a limitation in the use of PMSG in direct drive topology, when the size of the generators grows too much to cope with the important torque of the machine. This limit is in the range of 10 MW and above, where the advantages of reducing mass and weight compensate the extra costs of superconductivity and the associated hardware required (cryostats, cryocoolers, etc.).

3.1.3.3 Thermal behaviour

One of the design requirements of an electric machine is its thermal characteristic, being at least of equal importance as the electromagnetic design [87], [88]. The temperature rise of the machine eventually determines the maximum output power with which the machine is allowed to be constantly loaded, so it is critical to make a good definition of electric insulators as well as a cooling system in order to reduce weights and costs and increase efficiency. Electric insulators are classified in different classes (B, F, H, C), depending on the maximum allowable temperature they withstand, according to IEC 62114. Besides, thermal performance of a rotating electric machine is defined in the International Standard IEC 60034-1 (Rotating electrical machines – Part 1: Rating and performance), concretely in its part 8: Thermal performance and tests.

The problem of temperature rise can be faced in two ways:

- In most generators, adequate heat removal is ensured by convection in air, conduction through the fastening surfaces of the machine and radiation to ambient. In machines with a high power density, direct cooling methods can also be applied. Sometimes, even the winding of the machine is made of copper pipe, through which the coolant flows during operation of the machine. - In addition to the question of heat removal, the distribution of heat in different parts of the machine also has to be considered. This is a problem of heat diffusion, which is a complicated three-dimensional problem involving numerous elements such as the question of heat transfer from the conductors over the insulation to the stator frame. It should be borne in mind that the various empirical equations are to be employed with caution. The distribution of heat in the machine can be calculated when the distribution of losses in different parts of the machine and the heat removal power are exactly known. In transient states, the heat is distributed completely differently than in the stationary ones. For instance, it is possible to overload the motor considerably for a short period of time by storing the excess heat in the heat capacity of the machine.

The lifetime of insulation can be estimated by statistical methods only. However, over a wide temperature range, the lifetime shortens exponentially with the temperature rise of the machine. A rise of 10K cuts the lifetime of the insulation by as much as 50%. The machines may withstand temporary, often-repeated high temperatures depending on the duration and height of the temperature peak. A similar shortening of the lifetime applies also to the bearings of the motor, in which heat-resistant grease can be employed.

The temperature rise of the winding of an electrical machine increases the resistance of the winding. A temperature rise of 50K above ambient (20 ◦C) increases the resistance by 20% and a temperature rise of 135K by 53%. If the current of the machine remains unchanged, the resistive losses increase accordingly. The average temperature of the winding is usually determined by the measurement of the resistance of the winding. At hot spots, the temperature may be 10–20K above the average. In case of a permanent magnet electric generator, there is another critical issue related to temperature. If heating a magnet past its Curie temperature, the molecular motion destroys the alignment of the magnetic domains and the magnetization is lost.

3.1.3.4 Overload capacities Any electric generator must be designed for a set of specified operating conditions and should not be deliberately overloaded continuously because it means overheating, which shortens its life.

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The permitted loading levels are defined for a machine on the basis of the design of the insulation and the cooling of the machine.

However, the generators have the capacity to deliver more power than its rated one during short periods. According to IEC 60034, AC generators having rated outputs not exceeding 1200 MVA shall be capable of withstanding a current equal to 1.5 times the rated current for not less than 30 s. AC generators having rated outputs above 1200 MVA shall be capable of withstanding a current equal to 1,5 times the rated current for a period which shall be agreed, but this period shall be not less than 15 s.

3.1.3.5 Seal failure in submerged machines Electrical generators in wave and tidal power plants are subject to severe environmental conditions. Particularly the seals between the moving shaft and the stationary generator as these are mechanically stressed under pressure of the seawater [89]. A possible solution is flooding the electrical generator and the air gap respectively as this could overcome seal failure [90]. This way, the strains on the seals can be reduced, as they would have to seal non-moving parts against the seawater only. The lifetime of seals could be prolonged with this measure. On the other hand, this will also result in changes to the electrical behaviour of the windings and the electrical machine respectively. Some comments about this option: • A protective coating needs to be applied onto the windings to prevent early outages caused by corrosion • Flooding the air gap might have a positive influence on the thermal behaviour of the machine. It may be necessary to apply forced circulation of the sea water to achieve an improvement compared to air gap operation. • Most critical aspects are the transient and the thermal behaviour of the coils and the insulation system itself.

3.1.3.6 Permanent magnets and corrosion The use of permanent magnets has many advantages as less maintenance, slow operation (necessary for direct- drive operation), good efficiency, etc. However, these magnets cannot be exploited to their maximum extent for some applications due to their poor corrosion resistance in various environments [91]. The corrosion causes surface degradation and thereby affects the magnetic properties significantly. Sometimes, disintegration of materials takes place. In fact, corrosion decides the lifetime of magnets. Generally, the corrosion resistance of the magnetic materials depends mainly on the chemical composition, as well as on the processing technique. It also depends on the nature of different phases present in the magnets. There are different options to improve corrosion resistance, like the addition of alloying elements (which can lead to a loss in the magnetic properties) and coating (with surface engineering). Nevertheless, this is a critical aspect of permanent magnets that should be taken into consideration.

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3.2 FREQUENCY CONVERTERS Frequency converter is a common and often necessary component in PTO systems for offshore energy. The component allows the generator frequency to be completely independent from the fixed frequency of the electric grid. This property is required in all topologies where the electrical generator has to operate at variable speed. For other topologies where it is possible to operate the generator at a fixed rotational speed, the frequency converter is not a strictly required component. However, additional benefits can motivate the inclusion of a frequency converter irrespectively of other elements in the PTO.

This chapter will focus on the standard frequency converter topologies used for offshore energy applications. First, the fundamental technology and operating principles are described. Then, the most common topologies are presented. They are compared with respect to operational principles, control possibilities and dynamic interactions with other components. Finally, three subchapters include specific discussions on wind, tidal and wave power, respectively.

Figure 3.2 shows a frequency converter with ratings applicable for offshore energy.

Figure 3.2: GE MV3000 full power converter (www.ge-energy.com)

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3.2.1 Frequency converter technology A general schematic of a full frequency converter is shown in Figure 3.3. It contains two separate converters, often referred to as bridges. The generator side converter, or rectifier, converts the Alternating Currents (AC) from the generator into Direct Currents (DC). The intermediate DC-link between the two bridges acts as a decoupler that allows independent frequencies in the generator and the grid. The grid side converter, or inverter, transfers the currents back to AC. Moreover, it is synchronized to the constant frequency in the power grid.

The frequency converter topology with two bridges sharing a common DC-link is referred to as back-to-back configuration.

Generator Power grid

Generator side Grid side converter converter (rectifier) (inverter) Figure 3.3: General back-to-back frequency converter with intermediate DC-link

The key technology in all types of AC-to-DC conversion is semiconductor switches. They have the ability to turn the current on and off, and can therefore be used to generate AC from DC and vice versa. The semiconductor components can be grouped in three categories depending on their characteristics:

- Passive - Half-active - Fully active

A comparison between them is presented in Table 4. In general, the fully active components have increasing spread in recent years due to their control-related advantages.

Table 4: Comparison between passive, half-active and fully active semiconductors Component category Advantages Disadvantages Passive Smallest cost No controllability Example: Diodes Robust and reliable Low power losses

Half-active Medium cost Limited controllability (can turn on current) Example: Thyristor Robust and reliable

Fully active High controllability High cost (can turn on and off current) Examples: IGBT, IGCT, GTO High switching frequency Less robust and reliable High power losses

Two fundamental concepts for back-to-back frequency conversion exist. One is based on a constant DC-link voltage, and is denoted Voltage Source Converter (VSC). The other is based on a constant DC-link current, and is denoted Current Source Converter (CSC). The VSC is presently dominating offshore energy applications, but efforts within R&D may increase the penetration of CSC-technology in the future. The remainder of this chapter focuses on the VSC.

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3.2.1.1 Voltage Source Converters The standard configuration for the back-to-back VSC is shown in Figure 3.4. Its major components are: 1. DC-link capacitor. This component is used to stabilize the DC-link voltage. The capacitor acts as a small energy storage unit that enables the decoupling between the generator and the grid. 2. DC-link chopper resistor. This component is an optional part of the converter, and is composed by a switch in series with a resistor bank. This allows for dumping of energy in cases where the power grid is unable to transmit active power, for example during grid faults. This will prevent any harmful overvoltages in the DC- link. The chopper resistor is normally able to receive the converter rated power for a duration in range of several seconds. 3. AC-filters. The semiconductor bridges are not able to produce perfect sinusoidal currents. The output always contains some degree of harmonic distortion. This can be partly removed by filtering equipment. Several types of filters exist, and they are normally composed of inductors, capacitors and resistors. They can be connected both in series or parallel to the converter. Depending on several factors, filters may be required on both the generator and grid side of the converter. However, it is possible to use the leakage reactance in the generator as a filter. On the grid side, it can be sufficient to use a step-up transformer as a filter. 4. Semiconductor bridges. This is the core technology in the converter. A number of semiconductor switches are connected together in order to produce AC from DC and vice versa. The bridge arrangement can be passive, half-active or active according to the definitions in the previous section. It is not required that both bridges belong to the same definition, but at least one of the bridges must have active technology.

Generator 4 4 Power grid 3 1 2 3 G

Generator side Grid side converter converter (rectifier) (inverter) Figure 3.4: Back-to-back VSC-configuration and its associated components (1: DC-link capacitor, 2: DC-link chopper resistor, 3: AC-filters, 4: Semiconductor bridges)

3.2.2 Frequency converter topologies for offshore renewables In this section, three standard frequency converter topologies for wave, wind and tidal are outlined and evaluated. They are all based on voltage source converter technology, but some important differences must be highlighted.

It should be remarked that other topologies are also being used, and a variety of new concepts are presented in research. A more thorough overview can be found in [92]. This reference is specifically focusing on wind power, but the contents have high relevance for both wave and tidal as well.

3.2.2.1 Fully active converter The fully active converter is normally the most flexible choice, but also the most expensive one. It contains two fully active bridges connected back-to-back between the generator and the grid. See Figure 3.5 for a schematic overview.

Generator Power grid G

Active bridge Active bridge Figure 3.5: Fully active bridge topology This topology brings flexibility towards both the generator and the grid. Active bridges are normally equipped with sophisticated control algorithms that are able to precisely and independently control the speed and magnetic flux in

Rev. 03, 29-Nov-2012 Page 66 of 115 D2.03 Review of Relevant PTO Systems the generator. This enables a wide range of optimisation possibilities with respect to energy output, mechanical stress and power losses.

The fully active converter is applicable to all kinds of electrical generators. Since the converter is able to control the generator magnetic flux, there is no need for a separate generator excitation system.

The grid side active bridge features dynamic reactive power control, which can be used to support the grid voltage. This can be utilised for grid power loss minimisation and voltage stabilisation. Some grid codes require reactive support from renewable power plants.

The converter has an important limitation regarding the current capability. Active semiconductors have a current limit which is normally slightly larger than the current delivered at rated power. This is in contrast to standard grid- connected electrical generators which can deliver many times the rated current for a limited duration, e.g. for tripping protection system units during grid faults.

On the other hand, the active bridge has excellent Fault ride-through (FRT). This is a requirement specified in most grid codes forcing all generation units to remain grid-connected for a grid voltage dip with a given magnitude. See section 1.3.1.4 for more details on FRT. Particularly wind power have earned a poor reputation regarding FRT over the last decades due to the large fleet of wind turbines including grid-connected induction generators with limited FRT-capability. Strengthened FRT-capability is therefore an additional motivation factor to include a frequency converter in the PTO system. Table 5: Fully active converter advantages/disadvantages summary Advantages Disadvantages Full-range speed control of generator High cost and power losses Generator flux-control Limited overcurrent-capability (no need for separate excitation) Dynamic reactive grid support Excellent FRT-capability

3.2.2.2 Passive Generator Bridge This topology is close to equal to the fully active converter, except from the passive bridge connected to the generator. See Figure 3.6 for a schematic overview.

Generator Power grid G

Passive bridge Active bridge Figure 3.6: Converter with passive generator bridge The motivation behind replacing the active generator bridge with the passive one is mostly related with the cost. A passive generator bridge is attractive if the requirements regarding rapid flux and speed control are not too strict. Its advantages related with reduced cost and complexity might then outweigh the disadvantages regarding the overall control functionalities.

While the active bridge is able to generate a symmetric set of AC-voltages, the passive bridge requires that the generator produces the AC-voltages. This puts constraints on the types of compatible generators. The induction generator is not applicable, but any synchronous generator can be used.

The topology is able to control the speed of the generator. The grid-side converter can control the power flow through regulation of the DC-link voltage. However, the dynamics in this control system is slower and to some extent less precise than in the fully active bridge. The passive bridge also produces a higher amount of harmonic distortion, which might needs to be compensated by additional filtering.

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Another drawback with the passive bridge is the lack of reactive power control towards the generator. Additional shunt capacitances are normally required between the converter and the generator in order to increase the amount of active power that could be extracted.

The interface towards the grid is still a fully active bridge. The aspects discussed in section 3.2.2.1 regarding reactive power support, limited overcurrent capability and FRT are therefore also valid for this topology.

Table 6: Passive generator bridge converter advantages/disadvantages summary Advantages Disadvantages Full-range speed control of generator Medium cost and power losses Dynamic reactive grid support Limited overcurrent-capability Excellent FRT-capability Requires separate generator excitation (not compatible with induction generator) Mono-directional power flow

3.2.2.3 Doubly fed generator converter The doubly fed converter topology has increased its spread over the last decade. Similar to the passive generator bridge topology it offers a compromise between cost and control flexibility. It is normally combined together with a wound rotor induction generator (IG), and this complete system is referred to as the Doubly Fed Induction Generator (DFIG). The stator of the generator is directly connected to the grid, while the rotor is connected to the converter by means of brushes and slip rings. See Figure 3.7 for a schematic overview. Generator Power grid G

Active bridge Active bridge Figure 3.7: Doubly fed topology, normally connected to induction generators (DFIG) The major advantage behind the DFIG is the reduced rating of the converter, with its associated cost reduction. Typically it is rated at 20-40 % of the generator power.

The DFIG is able to control the speed of the generator even though the stator currents are forced to follow the fixed electric frequency. This is achieved through rotor slip control. Since the converter is not rated at the full current, the speed can only be controlled within a limited band (typically ≈ ± 30 %). Increasing the converter rating would increase the speed control range. However, if the rating is too high, the relative advantages compared with the other topologies are reduced.

The grid side converter in the DFIG could be either passive or active. Due to the reactive power considerations mentioned in section 3.2.2.1, the active bridge is the most common choice. It is important to remark that the reduced converter rating implies that the amount of reactive power support is constrained compared with the fully rated converters. Table 7: Doubly fed converter advantages/disadvantages summary Advantages Disadvantages Limited range speed control of generator Low cost and power losses Dynamic reactive grid support (limited range) Limited overcurrent-capability Good FRT-capability

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3.2.3 Offshore wind power Frequency converters are gaining increasingly popularity in wind turbines, particularly offshore. This is due to the following reasons, elaborated in separate subsections below: • Multipole generators in combination with frequency converters removes the need for a mechanical gearbox • Variable speed operation increases average energy output and reduces mechanical stress. • Frequency converters with active bridges allows dynamic reactive compensation and excellent FRT- capability

3.2.3.1 Variable speed operation and MPPT The control flexibility brought by the frequency converter is explained generally in section 3.2.2. In offshore wind turbines, an important task is to control the rotational speed to follow the maximum aerodynamic efficiency. This is referred to as maximum-power-point-tracking (MPPT). The frequency converter control system can adjust the amount of power that is sent to the grid. This makes it possible to accelerate or decelerate the turbine dynamically.

The turbine inertia is the most important limitation in the speed control dynamic response. For large offshore wind turbines, the dynamic response of the speed control is delayed with a duration in the range of seconds.

More details on how the MPPT-control is achieved are given in section 1.3.1.4.

3.2.3.2 Frequency converter grid interface Most frequency converter topologies in wind turbines include an active bridge connected to the grid. This allows for dynamic reactive power control as explained in section 3.2.2. In fact, several wind turbine manufacturers are now supplying centralized wind farm controllers that are able to control the reactive power of each turbine in a coordinated manner. Seen from the grid, the farm will then behave similar to a conventional large power plant. This is a highly useful feature particularly in power systems where the wind power penetration is reaching high levels, since the wind power then has to comply with grid codes in order to reach sufficient power system stability.

3.2.4 Tidal power

As discussed in chapter 1.3.2, TEC technology has many similarities with wind turbine technology. As for wind turbines, the use of frequency converters can be favourable in tidal current turbines to allow for variable speed operation, and with that increased power extraction. SeaGen, Kobold and Clean Current are examples of tidal current turbine concepts in operation [93]. SeaGen has two horizontal axis turbines with variable speed induction generators, similar to the DFIG shown in Figure 3.7. The Kobold turbine is a vertical axis turbine with permanent magnet synchronous generator (PMSG), with a gear-box between. The generator is connected to a grid via a full power converter. Clean Current is a ducted horizontal axis turbine with a direct drive (gearless), variable-speed PMSG [94]. Also this concept has a grid connection through a full power converter. Details on the converters for Kobold and Clean Current are not public, but can be as the back-to-back converters shown in Figure 3.5 or Figure 3.6, with passive and active generator bridges respectively.

Research is going on within this area. [95] suggests a control scheme for doubly fed induction generators in tidal current turbines. A method to reduce bearing failures on turbines with induction generator and grid side converter is suggested, intended both for wave and tidal current energy. The idea is to control the power converter in order to reduce radial forces on the rotor. Bearing failures, typically caused by misalignment in the drive train, accounts for a significant share of failures in wind turbines, and the same will be true for tidal current turbines that are subject to very large forces from the water. A tidal current generation system with topology similar to the converter with passive generator bridge shown in Figure 3.6, and direct drive PMSG is suggested in [96]. The converter control is intended to increase the power capture and reduce the stress on the turbine. Maximum power point tracking (MPPT) is achieved using a boost converter connected in the DC-link.

3.2.5 Wave power Frequency converters in wave power plants have various objectives and control tasks depending on the type of prime mover and other parts of the PTO system. The frequency converter is always connected to some type of

Rev. 03, 29-Nov-2012 Page 69 of 115 D2.03 Review of Relevant PTO Systems electric generator, but the operation characteristics of the generator can be very different from one wave power concept to another. The requirements for the frequency converter are mainly related with two main factors:

• The extent of generator speed variations. Some systems can let the generator operate at fixed speed, while other need variable speed operation that changes direction in an oscillatory pattern. • The extent of torque variations. Some applications demand rapid variations in generator torque. In the case that bi-directional power flow is required, passive and half-active bridges on the generator side are not applicable.

Figure 3.8 shows an overview of compatibilities between the frequency converter and the different wave PTO systems described in chapter 2. Fixed-speed generator does not require a frequency converter, and is therefore connected directly to the power system. Both hydraulic, pneumatic and turbine based systems are possible to combine with fixed-speed generators. However, variable speed operation brings significant advantages, and should always be evaluated. For example, the average power production can be increased if the turbine devices are operated at optimal speed, analogous to maximum power-point tracking explained in section 3.2.3.1.

Mechanical systems including a more direct interface between the wave motion and electrical generator normally bring more requirements to the electrical parts of the PTO system. The mechanical systems utilize either a linear or rotary electrical generator, both in which operate at variable speed. Furthermore, the frequency converter can be utilized to control the motion of the mechanical devices through electromechanical force and torque.

The type of control applied by the converter greatly influences the motion of the mechanical devices. An important design parameter is the peak-to-average power ratio. How converter control and rating affects the energy extraction is discussed in [97] and [98].

The frequency converter can be integrated together with energy storage devices in order to smooth the power sent to the grid. Typically, they are integrated on the DC-link between the two bridges as shown in Figure 3.8. Example storage devices for such use are electrochemical capacitor or batteries. Energy storage is treated further in chapter 3.3.

The most suitable frequency converter for wave energy is the fully active topology described in section 3.2.2.1. This choice allows for accurate control of motion and forces, in addition to bi-directional power flow.

Fixed-speed Hydraulic/ Rotary Electrical Pneumatic generator Frequency converter

Variable-speed Hydrodynamic Generator Grid side Power system Turbine Rotary Electrical interaction bridge bridge (DC or AC) Generator

Energy storage Linear optional Mechanical (electrochem. capacitor Generator system or battery)

Figure 3.8: Extension of Figure 1.33 focusing on the role of the frequency converter

3.2.6 Summary • A frequency converter is always required if the electrical generator should run at variable speed • Variable speed operation is normally associated with increased average energy output and reduced mechanical stress • The converter can control motion and forces on mechanical parts. Converter control is normally rapid and accurate.

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• Most technology prospects for wind, tidal and waves include a frequency converter and some level of active control. Many of the same for concepts for generator to grid connection are relevant for all three energy sources.

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3.3 ENERGY STORAGE SYSTEMS

It is well recognised that energy storage solutions are of great interest for renewable energies. On the one hand, this is for energy management, due to the inherent intermittency of most of those energy sources. This topic involves large storage capacities. On the other hand, it is also for power quality management, to cope with the dynamics of the power production. This second topic requires less storage capacity, but better cycle life properties. An overview of the most relevant energy storage systems is drawn hereafter, that helps scoping those two challenges.

3.3.1 Pumped hydroelectric storage

This subsection will give a brief description of pumped hydroelectric storage (PHS) system and its main characteristics.

3.3.1.1 Technology description

Pumped hydroelectric storage (PHS) stores potential energy from height differences between water levels in two reservoirs. Compared to ordinary hydroelectric power it has the additional ability to pump water from the lower reservoir to the upper reservoir. [99].

A simplified sketch of a PHS is illustrated in Figure 3.9.

Figure 3.9 Schematic illustration of a pumped hydroelectric storage (PHS) system [100].

A PHS basically consists of two reservoirs at different height levels, a unit to pump water from lower to upper reservoir, and a turbine to generate electricity when water is returning from upper to lower reservoir. The height difference between the two reservoirs and the volume of the stored water decides the amount of stored energy. The pump is used to store potential energy typically during off-peak hours, while the turbine converts potential energy into electricity during peak hours [101].

PHS technology can use existing reservoirs with height differences, but also underground caverns, subsurface reservoirs or open sea as the lower reservoir. Pumped hydroelectric storage can either be realized with a reversible pump turbine (turbining when the attached electric machine acts as a generator, pumping when the attached electric machine acts as a motor) or as two separate aggregates, one for pumping, and one for turbining. [102].

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3.3.1.2 Application areas

Pumped hydroelectric storage (PHS) is a mature technology with large volume, large power capacity, large storage period, high efficiency, long life and relatively low capital cost per unit of energy. Typical rating of PHS is approximately 1000 MW (100 MW – 3000 MW). The rating of PHS is the highest of all available electric energy storages, and typical applications are therefore energy management, frequency control and provision of reserve. [101], [99].

World wide there is approximately 250 PHS plants with a cumulative generation capacity of 120 GW, and the capacity is increasing with approximately 5 GW/year [99].

3.3.1.3 Main characteristics

Typical time Response Charge time Discharge Self discharge time Life time Cycle life constants time time [%/day] [years] [number] PHS s - min 1) - 1 – 24+ h 1) ~0 1) 50-100 1) 2x104 – 5x104 1) 40-60 2)

1) [103], 2) [101] Table 3.8 Typical time constants for pumped hydroelectric storage (PHS) systems.

Characteristics Energy density Power density Efficiency Maintenance Maturity interval [Wh/kg] [Wh/dm3] [W/kg] [W/dm3] [%] PHS 0.5–1.5 1) 2) 0.5-1.5 2) - - 75-85 1) - Available on market

1) [103], 2) [101] Table 3.9 Typical properties for pumped hydroelectric storage (PHS) systems.

Costs figures Costs [€/kW] [€/kWh] PHS 500-3600 1) 60-150 1) 400-1500 2) 4-70 2)

1) [103], 2) [101] . Specifically denotes capital costs. Original figures in $:600-2000 $/kW and 5-100 $/kWh. Table 3.10 Typical cost figures for pumped hydroelectric storage (PHS) systems.

3.3.1.4 Key features

A pumped hydroelectric storage (PHS) system stores energy in the form of potential energy from water in a elevated reservoir and is a long-time energy storage with typical operating range in tens of hours.

Advantages: - Large energy storage capacity - High power capacity - Quick start-up times - Low self-discharge - Long technical life-time - High cycle life - Relatively high efficiency - Proven technology

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Disadvantages: - Reliance of favourably geography (proximity to reservoir) - Environmental and societal influence: areas flooded by reservoirs can be large, and here both flora, habitat, and human activities (if any) are obviously destructed.

3.3.2 Hydrogen energy storage This subsection describes the technology and main properties of hydrogen energy storage systems.

3.3.2.1 Technology description

Hydrogen energy storage systems use the natural reaction forming water from dihydrogen and dioxygen to produce electricity, and the opposite reaction to convert electricity into a storable form. Therefore, such systems are composed of two types of components. On the one hand, conversion devices that handle those chemical reactions; on the other hand, storage components properly speaking.

Energy conversion

The natural reaction between hydrogen and oxygen is highly exothermic and even explosive. It can therefore be conducted in an Internal Combustion Engines (ICE), converting the energy into a mechanical form that can be used to drive a standard rotative electrical generator as those described in section 3.1.1. This approach is somehow similar to the production of electricity via Diesel systems, and can even be achieved by modifying them [99].

Nevertheless, this process suffers from the relatively low efficiency of ICEs. As a consequence, despite investment prices estimated to be up to 5 times higher [104], new solutions are being developed using fuel cell technology. Fuel cells directly produce electricity from external supplies of fuel (anode side) and oxidant (cathode side), which react electrochemically in the presence of an electrolyte. Fuel cells differ from batteries in that they consume reactants, which must be replenished, while batteries store electrical energy chemically in a closed system. Here, the fuel is hydrogen while the oxidant is oxygen taken from the surrounding atmosphere, and transferring the energy via a mechanical form is avoided. There are a number of concepts of hydrogen fuel cells, including Proton Exchange Membrane (PEM) Fuel Cells and Alkaline Fuel Cells [101].

For the opposite conversion process, consuming electricity to generate hydrogen and oxygen from water, two technologies are also available.

First, hydrogen can be produced via a water electrolyzer. Large plants are usually realized by using alkaline electrolyser, which achieve efficiency up to 70% [105]. For renewable energy applications, this component must have a quick response to unstable electrical power input. One drawback of this technology is a necessary downtime of 30 to 60 minutes between turning the component off and on again. Also, operation at partial load is delicate what forces such systems to be shut off bellow a load varying from typically 25-50%, to 5-10% to most advanced products [106]. Though partial load problems can generally be minimized by redundancy of smaller units, the conjunction of those features with a fluctuating input power makes the problem more complex here in the context of marine renewables.

As an alternative, some hydrogen fuel cells are reversible [101], typically PEM technology [107]. This solution also can increases reliability, and to a certain extend balance costs, by reducing the number of components in the system.

Hydrogen storage

Hydrogen can be stored in different states. It can be gaseous, liquefied or stored in physical or chemical bonds. Up to now the storage in the gaseous phase is the most economical way. From a first approach, the storage of hydrogen

Rev. 03, 29-Nov-2012 Page 74 of 115 D2.03 Review of Relevant PTO Systems gas is similar to the case of any other gas, with the security requirements inherent to its explosive nature. One typical technical solution is cylindrical or spherical steel tanks.

The quantities of gas involved require storage at high pressure or in great volumes. For instance, the Utsira demonstration plan in Norway, that feeds autonomously 10 houses by regulating wind power, needs a capacity of 2400 normal cubic meters [106]. Under atmospheric pressure, this corresponds to a sphere of 17m diameter.

Though storages for compressed hydrogen are relatively cheap and available in big capacities, the compression of hydrogen consumes energy. A compression up to 1000bar uses around 15% of the energy content of the hydrogen [105]. Therefore, hydrogen is also stored at lower pressure in salt mine caverns, with examples in the USA and UK [108]. In Germany, the hydrogen infrastructure consists of conventional pressure tanks, pipelines and transportation solutions (ships, trucks and trains). There, the use of hydrogen in the natural gas network up to 5% is allowed, which opens a great window of opportunities [109].

Also, the Power-to-Gas (P2G) process uses hydrogen as an energy source to convert it into substitute natural gas (SNG). The so called Sabatier-process is used to hydrate CO2 with the help of a catalytic converter to methane. Methane is the same matter as natural gas. The P2G process enables the hydrogen to be used with contemporary technology. It can be transported in the gas-grid, burned in internal combustion engines or gas power plants or even in conventional heating systems. This technology is the missing link between the electrical and the gas system. The upscale of the technology is still object of research.

3.3.2.2 Application areas

With high energy density, but low cycle life and quite slow response time, hydrogen systems are not suitable for power quality application but for energy management. Nevertheless, they can hardly take advantage of seasonal fluctuations of wind power so far [106]. This form of energy storage is also very promising for powering vehicles [104].

3.3.2.3 Main characteristics

Typical time Response Charge time Discharge Self discharge time Life time Cycle life constants time time [%/day] [years] [number] Hydrogen Min - - 0.5-2 5-15 >1000 energy storage Table 3.11 Typical time constants for Hydrogen energy storage systems [103].

Characteristics Energy density Power density Efficiency Maintenance Maturity interval [Wh/kg] [Wh/dm3] [W/kg] [W/dm3] [%] - 1 Hydrogen 800- >500 20-502 Available on the energy storage 100001, market / Pre- 600-12002 commercial stage

1) [103], 2) [101] Table 3.12 Typical properties for Hydrogen energy storage systems. Costs figures Costs [€/kW] [€/kWh] 1 Hydrogen 2000-6600 5-162 energy storage

1) [103], for fuel cell systems, 2) [101], original figures in $: 6-20 $/kWh. Table 3.13 Typical cost figures for Hydrogen energy storage systems.

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3.3.2.4 Key features [101]

Advantages: - Low self-discharge [103] - high energy density (0.6–1.2 kWh/kg) - high scalability - independent system charge rate, discharge rate and storage capacity, - modular construction with ability to add further modules and/or re-configuration at a later date, and potential to provide surplus hydrogen off-gas supplies for road transport applications – environmentally benign operating characteristics.

Disadvantages: - Despite the high efficiency of fuel cells in the generator configuration, the overall efficiency of hydrogen storage systems is quite low and stays under 50%

3.3.3 Compressed air energy storage (CAES)

This subsection briefly describes compressed air energy storage (CAES) systems and its main characteristics.

3.3.3.1 Technology description

Compressed air energy storage (CAES) offers a method to store low cost off-peak energy in form of compressed air in a reservoir (underground or aboveground): The electricity is generated by releasing the compressed air from the reservoir, preheat the cool, high-pressure air and direct the preheated air into an expansion turbine driving a generator [110].

A sketch of a plant illustrates the main components, see Figure 3.10.

Figure 3.10 Schematic diagram of CAES [111].

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CAES has five major components [110], [101]: - Motor/generator with clutches on both ends (to engage/disengage it to/from the compressor or turbine trains) - Multi-stage air compressor of with inter- and aftercoolers to achieve economy of compression and reduce moisture content of compressed air - A turbine train consisting of both high-and low-pressure turbines - Underground or aboveground compressed air storage, including piping and fittings - Equipment controls and auxiliary equipment (fuel storage and handling, cooling system, mechanical systems, electrical systems, heat exchanger)

Compression of air creates heat, while air expansion causes cooling. Advances in technology has led to development of new processes for CAES which can be sub-divided into three categories dependent of how the heat and cooling are processed [103]: - Diabatic CAES, where the compressed air is stored and the heat from the compression is lost. Gas turbines are used in order to preheat the air when energy is needed. Efficiency is 40-54 %, but alternative designs with improved efficiency exists (e.g. CAES with recuperated cycle, CAES with combined cycle etc.) - Advanced Adiabatic CAES (AA-CAES), where the heat from the compression process is stored and reused when the gas is released. Heat can be stored in solid, fluid or molten salt solutions (temperatures 50 ̊C – 600 ̊C). The process does not need additional gas co-firing and the efficiency of the energetic process reaches ~70%. - Isothermal compression, which makes use of thermo-dynamically reversible cycle: Temperature is maintained constant during air compression and expansion by allowing continuous heat exchange. Theoretically the process approaches 100 % efficiency.

It is expected to have improvements in CAES operation together with identification of new locations, e.g. compressed air storage in vessels or above ground (CAS or SSCAES, i.e. Small Subsurface CAES). Here the air is stored in small high-pressure vessels and these systems are independent of geology. They are able to hot start in seconds and cold start in minutes [102].

The rest of this subsection will deal only with diabatic CAES.

3.3.3.2 Application areas

The CAES system is a mechanical storage where the energy is stored as compressed air in a reservoir. Its main advantage is large energy storage capacity, high power capacity and relatively high efficiency. CAES are suitable for applications like e.g. load following, peak shaving, price arbitrage, frequency regulation, seasonal fluctuation regulation and voltage control [103].

Two CAES units are in operation in the world today: - Huntorf (Germany) was put into operation in 1978 and utilises two salt caverns with a total of 310000 m3. A 60 MW compressor provides a maximum pressure of 10 MPa and the system can generate 290 MW for two hours. - McIntosh (USA) started up in 1991 and utilises a salt cavern of 500000 m3 with an air pressure of ~7.5 MPa. The system can generate 110 MW for up to 26 hours. Utilizes a recuperator (see Figure 3.10), which reduces the fuel consumption by approximately 25 % compared to the Huntorf plant.

3.3.3.3 Main characteristics Typical time Response Charge time Discharge Self discharge time Life time Cycle life constants time time [%/day] [years] [number] CAES 5 -15 min 1) - 1 – 24 h 1) ~0 1) 25-40 1) 5x103 – 2x104 1)

1) [103] Table 3.14 Typical time constants for CAES systems.

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Characteristics Energy density Power density Efficiency Maintenance Maturity interval [Wh/kg] [Wh/dm3] [W/kg] [W/dm3] [%] - CAES 30-60 1) 2) 3-6 2) - 0.5-2 2) 42-54 1) - Available on market 12 3] 70-89 2)

1) [103], 2) [101], 3) [102] Table 3.15 Typical properties for CAES systems.

Costs figures Costs [€/kW] [€/kWh] CAES 400-1150 1) 10-120 1) 300-600 2) 1.5-35 2)

1) [103], 2) [101]. Specifically denotes capital costs. Original figures in $:400-800 $/kW and 2-50 $/kWh. Table 3.16 Typical cost figures for CAES systems.

3.3.3.4 Key features

A compressed air energy storage (CAES) stores energy in forms of pressurized air in reservoirs and is a long-time energy storage with typical operating range in tens of hours.

Advantages: - High power capacity (50-300 MW) [99] - Large energy storage capacity (2-50+ h) [99] - Quick start-up times (~10 min) [99] - Can respond to load changes (load following): Sustain frequent start-up/shut-down cycles [101] - Significantly lower capital costs than pumped hydro storage [99] - Relatively high efficiency - Charging – discharging decoupled [102] - Proven technology - Low self-discharge

Disadvantages: - Reliance of favourable geography (proximity to underground storage volume) [99] - Conventional CAES: o Environmental influence: Not carbon neutral (CO2 emission from gas combustion in the expansion cycle) o Availability of natural gas (has to be associated with a gas turbine plant) [101]

3.3.4 Electrochemical Batteries

The term battery includes a plurality of technologies. In spite of its differences a battery always consists of two separated areas that are capable of exchanging ions. The battery electrodes store the chemical energy in its material. The reaction surface of the electrodes is fundamental important for the effectiveness of the battery. The electrodes are made of high porous material that has a surface up to a few square meters per gram [112]. When a battery is connected to external load on the negative electrode an oxidation process occurs. This process releases electrons that “flow” through the external electric circuit. The positive electrode absorbs the electrons in its active material (reduction process). The efficiency of this process is exposed to the aging of the battery (internal influences) and other external influences, such as temperature.

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This section presents an overview of the most established battery technologies, as well as some more recent ones that appear promising.

3.3.4.1 Lead-Acid batteries This very well-known technology is about 150 years old. The positive electrode consists of lead dioxide and the negative of lead that is surrounded by diluted sulphuric acid. To prevent short circuits a separator is isolating both areas. While discharging the battery the acid reacts to lead-sulphate that leads to a cell voltage of 2 volts. The reaction equation is: + 2− Pb + PbO2 + 4H + 2SO4 → 2PbSO4 + 2H2O This relatively high voltage leads to some secondary reactions, which are losses in the sense of energy storage. This limits the maximum storage time of this technology. The low price (around 50€/kWh) of lead-acid batteries makes them the most often used technology. Its lifetime varies from 4 to 30 years and it cycle number is between 500 and 2000 cycles. The efficiency is around 80 % and the energy density is given with 35Wh/kg [113].

3.3.4.2 Li-Ion batteries Even though the lithium-ion batteries seem to be the cutting edge technology, they have been present in research, space- and military applications for 40 years. Nevertheless the technology is still undergoing significant changes. This leads to a variety of different materials, types and specifications. The so called “Rocking-Chair” batteries are the most common systems nowadays. The electrodes of such systems are providing a host lattice to the lithium-ions. The lithium-ions are “rocking” back and forth between those electrodes during charging and discharging. The settling process in-between the layer of the electrodes material is called intercalation and has a very high efficiency (90%). The reaction equation for a common type of li-ion battery is as follows: LiC + Li CoO → Li C + Li CoO 6 y 2 1−x 6 x+ y 2 Most of the systems have a cell voltage between 2.5 and 4.2 volts. The energy density is way much higher compared to lead-acid batteries (50 – 200 Wh/kg). The average lifetime is around 5 years and the average number of cycles is 4.000. Since the functional principle has no secondary reactions this technology has almost no self discharge. The relatively high costs (500-800 €/kWh), some safety challenges and rare materials are cutting back the advantages. [113].

3.3.4.3 Sodium-Sulphur-Batteries (NaS) This technology is the only commercial technology that operates at high temperature (290°C -360°C). The negative electrode consists of molten sodium and the positive electrode consists of molten sulphur. These highly reactive materials are separated by a solid electrolyte. During discharge the sodium atoms are moving through the separator after delivering an electron to the current collector. The reaction equation is:

2Na + xS → Na2Sx (x = 5−3) This reaction leads to a cell voltage at around 2 volts with an efficiency of about 90%. The energy density is relatively high (160-230 Wh/kg) with potentially low cost materials. If the thermal losses are excluded there will be no self- discharge. The lifespan of such systems is around 15 years and the cycle number is about 2500. [113].

3.3.4.4 Vanadium-Redox-Flow Batteries This technology has a lot in common with fuel cells. The battery consists of two tanks and a cell. The energy is stored within a liquid inside of the two (plastic) tanks. The power comes from the cell in which the reaction takes place. The two electrolytes contain vanadium-ions in different valance-states (II-V) that flow through the cell and pass by a membrane from each side. During discharge vanadium (II) gives away an electron and is oxidized to vanadium (III). In parallel on the other side of the membrane vanadium (V) is reduced to vanadium (IV). The charge exchange between both liquids is achieved by a stream of hydrogen through the membrane. The reaction equation is: + + 2+ 2+ 3+ VO2 + 2H +V →VO + H2O +V The voltage is around 1.4 volts and the efficiency is around 75%. The gravimetric energy density is comparably low (35Wh/kg) but since the storage itself is extremely cost efficient (plastic tank) this seems to be acceptable. The lifespan is stated with 7 to 15 years with a very high number of cycles (10.000). [113].

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3.3.4.5 Main characteristics

number of Efficiency Specific energy Costs lifetime Level of maturity cycles [%] [Wh/kg] [€/kWh] [years] Lead-acid 500-2,000 80 35 50 4-30 Mass production Li-Ion 4,000 90 50-200 500-800 5 Available on market NaS 2,500 90 160-230 15 Recent Vanadium-Redox-Flow 10,000 75 35 7-15 Recent

Table 3.17 Quantitative comparison of typical battery performances [113]

3.3.5 Flywheels

This subsection describes the technology and main properties of flywheel energy storage (FES) systems.

3.3.5.1 Technology description

Flywheel energy storage (FES) is a mechanical storage system that stores kinetic energy in a rotating mass. The kinetic energy of a flywheel is related to the moment of inertia and angular velocity according to the equation 1 1 � = ��! = ��!�! ! 2 4 where Ek is the kinetic energy [J] I is the moment of inertia of the rotating mass [kg·m2] ω is the angular velocity [rad/s] m is the mass of the rotor [kg] r is the mean radius of the rotor [m]

As regards energy capacity of a flywheel, the equation shows that increasing the rotational speed is more effective than increasing the rotor mass.

A simplified sketch of a flywheel energy storage (FES) system is shown in Figure 3.11.

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Figure 3.11 Sketch of a modern high-speed flywheel energy storage (FES) system. (Illustration of Pentadyne's GTX module.) The electric energy is converted from or into kinetic energy through an electrical machine: A converter system controls the direction and the quantity of power exchanged between the external circuit and the energy storage system. Charge: The electrical machine acts as a motor and energy is supplied to the storage system by increasing the rotor speed. Discharge: The electrical machine acts as a generator and energy is supplied from the storage system while the rotor speed decreases.

Flywheels can be divided into low-speed (up to typ. 10000 rpm) or high-speed systems (up to typ. 50000 rpm). Low- speed systems usually have steel discs, while high-speed systems use composite rotors. Some low-speed systems with heavy rotors may have mechanical clutches in order to adapt to a fixed frequency electrical system [110]. However, most flywheel systems today utilises frequency converters to connect to the external system and the rest of the text will only comprise this option.

Energy losses during operation of a FES transform into heat and raise the operating temperature as well as reduce efficiency. High-speed flywheel rotors must therefore be housed in an evacuated chamber (vacuum or low viscosity containment) in order to avoid severe dynamic heating. Likewise, in order to reduce bearing losses in high-speed FES, magnetic bearings are used.

The bearing losses are an important parameter in order to increase the efficiency of a flywheel. E.g. a high-speed FES manufacturer calculated a loss reduction from 10 % per hour of stored energy to 2 % by moving from mechanical bearing to passive magnetic bearing [114]. Future systems will make use of passive and active magnetic bearings. High temperature superconductive (HTS) materials are the ultimate bearing system: The superconductive bearings are completely passive, while active bearings need active control of the electromagnetic field to provide positional stability of the rotor [115]. However, superconductive bearings need a cryogenic system (additional costs and complexity).

3.3.5.2 Application areas A flywheel energy storage system is a mechanical short-time energy storage (tens of seconds). Its main advantages are high cyclability, high energy efficiency and fast response times (ms) [103]. Typical applications are high power combined with short duration (hundreds of kW within tens of seconds) [101]. Examples are short time support in distributed power systems (power quality sags and surges < 5 s), UPS for outages (< 10 minutes), voltage regulation and support for FACTS (flexible alternating currents transmission systems). [103].

Flywheel systems make use of several parallel connected modules in order to achieve longer run times and higher energy capacities [102]. Typical products are rated 100-250 kWelectric with 3.3 to 25 kWh energy capacity [101].

For ocean energy application FES can typically be used to smooth out short time variation in power production, increase power quality and reinforce fault ride-through capability.

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3.3.5.3 Main characteristics Typical time Response Charge time Discharge Self discharge time Life time Cycle life constants time time [%/day] [years] [number] Flywheel energy s 1) Comparable to 15 s - 15 min 1) 20-100 1) 20+ 1) 105-107 1) storage discharge time

1) [103] Table 3.18 Typical time constants for flywheel energy storage. Characteristics Energy density Power density Efficiency Maintenance Maturity interval [Wh/kg] [Wh/dm3] [W/kg] [W/dm3] [%] - Flywheel energy 5-130 1) 400-1600 1) 1000-2000 2) 85-95 1) Quarterly: Air filter Low-speed: storage Yearly oil change Available on market Bearings every 3 y High-speed: 3) Pre-commercial

1) [103], 2) [101], 3) [110] Table 3.19 Typical properties for flywheel energy storage. Costs figures Costs [€/kW] [€/kWh] Flywheel energy 100-300 1) 1000-3500 1) storage 190-250 2) 700-3500 2)

1) [103], 2) [101]. Specifically denotes capital costs. Original figures in $:250-350 $/kW and 1000-5000 $/kWh. Table 3.20 Typical cost figures for flywheel energy storage.

3.3.5.4 Key features

A flywheel energy storage system is a mechanical short-time energy storage with typical operating range in tens of seconds.

Advantages: - Long life time/cycle life - Fast response capability (decided by the interfacing frequency converter) - Deep discharge capacity, without influencing the lifetime - High round-trip efficiency - Relative fast recharge (some system charge and discharge at the same rate) - Commercial systems exists (although high-speed systems have limited availability) - Almost no environmental influence (e.g. no hazardous materials nor emission)

Disadvantages: - Limited experience: Data proving claims of long life and low cost must be provided. - Safety issues: Potential hazardous failure modes (e.g. break-up of the rotating mass) - Relatively high parasitic and intrinsic losses

3.3.6 Electrochemical capacitors

This subsection describes the technology and main properties of electrochemical capacitors (EC), also referred to as electric double layer capacitors (EDLC), ultracapacitors and supercapacitors.

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3.3.6.1 Technology description

Electrochemical capacitors store energy by means of two solid electrodes immersed in an electrolyte solution with a separator between the electrodes. This is similar to the essential structure of batteries rather than conventional capacitors, which separates the two conducting electrodes by an insulating dielectric material.

Batteries store their energy in the electrode materials and have a charge/-discharge process, which is accompanied by the exchange of electrons between the electrode and the electrolyte (Faradaic charge transfer). The charge process of an electrochemical capacitor, however, ideally does not involve any electron transfer across the electrode interface, and the storage of electric charge and energy is electrostatic: The electrochemical capacitor store the electric energy as a charge separation in the double layer formed at the interface between the solid electrode and the liquid electrolyte in the micropores of the electrodes [116], [113] (see Figure 3.12). This process is highly reversible and therefore electrochemical capacitors can be charged and discharged thousands of times without degradation.

Figure 3.12 Principle of a single-cell electrochemical capacitor and illustration of the voltage drop at the electrode/electrolyte interface. [116] The equations applicable to electrochemical capacitors are the same as for conventional capacitors: The relationship between the double layer capacitance, C, of an electrode immersed in an electrolyte can be estimated to �! �! � � = � where C is the capacitance ε0 is the permittivity of free space εr is the relative permittivity of the electrolyte A is the area of the electrode d is the thickness of the material where the energy is stored (e.g. the double layer)

A high capacitance is achieved when two of these electrodes are combined. The maximum energy stored in an electrochemical capacitor is defined by 1 � = ��! !"# 2 where Emax is the maximum stored electrical energy [J]

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C is the capacitance of the device [F] V cell voltage [V]

Electrochemical capacitors are made with porous electrodes with pore diameters in the nanometre range, which results in large electrode surfaces. In addition the distance between the electrode plates are very small (less than 1 nm). Due to this design the capacitance increases along with the A/d ratio, resulting in energy storage capabilities for electrochemical capacitors that are approximately two orders of magnitude greater than conventional capacitors [101].

Three types of electrochemical capacitors exist: Double-layer capacitors, pseudo-capacitors and hybrid capacitors (where only double layer capacitors are commercial available today). The double-layer capacitors operate as outlined in [116]: Charging and discharging of the interfacial double layer. The charge-discharge mechanism of pseudo-capacitors or redox capacitors involves charge across the double layer. The redox reaction is a reversible process between multiple oxidation states in the electrode material. These charge movements occurs rapidly and with little internal resistance. As a result, the energy storage mechanism in pseudo-capacitors is a very simple process compared to batteries, which involve slow, complex Faradaic processes with irreversible heat production [117]. Consequently, pseudo-capacitors utilize the volume of the materials, not only the surface as with double layer electrochemical capacitors, and the capacitance may increase considerably. The capacitance arises from the relation between the extent of charge acceptance (Δq) and the change of potential (ΔV), e.g. Δq/ΔV or dq/dV, which is equivalent to a voltage dependent pseudo-capacitance C (C=dq/dV). Generally, pseudo capacitors have higher energy densities than EDLCs. Hybrid capacitors can be fabricated with one electrode made from double layer (carbon) material and the other electrode from a pseudo capacitance material (battery like material). It is also possible to use two dissimilar mixed metal oxides as electrodes or doped conducting materials. In general, hybrid capacitors will have higher specific energy than EDLCs but significantly lower power capability. Furthermore, their charge-discharge characteristics (V vs. Q) are very non-linear [113].

The specific capacitance of the electrode materials defines primarily the capacitance of an electrochemical capacitor. On the other hand, the cell voltage (and resistance) is decided by the electrolyte. Three different types of electrolytes are utilized [113]: - Aqueous (KOH or sulphuric acid) with cell voltage typical 1.0 V. Commercial available. - Organic (propylene carbonate or acetonitrile) with cell voltage typical 2.5-3.0 V. Commercial available. - Ionic liquid with cell voltage typically 3.3-4.0 V (high ionic resistance at near-room temperature). Research stage.

Electrochemical capacitors with the best performance today use acetonitrile electrolytes, because they provide the highest cell voltage as well as the lowest ionic resistance. However, safety aspects of acetonitrile include toxicity and flammability and intensive research is made on finding a nontoxic substitute with low resistivity [113].

Manufacturers provide a range of high power EC modules today. These modules are made from series connection of single EC cells. All modules have inherent balancing circuits for each cell in order to avoid cell overvoltage and variations in cell voltage during cycling.

It is not practical to discharge a capacitor to 0 V during operation in an application. Instead an operating window is decided between maximum (Vmax) and minimum (Vmin) operating voltages. For most applications the usable energy (Eu) can be denoted as [102] ! �!"# �! = �!"# 1 − ! �!"# 2 where Emax = ½CV .

A complete electrochemical capacitor energy storage system includes a control system and a power electronic converter in order to interface the energy source and load. Many different topologies are possible. One is depicted in the Frequency Converter section, see Figure 3.8.

3.3.6.2 Application areas

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An electrochemical capacitor energy storage typically has operating times in tens of seconds. Its main advantage is low maintenance needs, very fast charge and discharge times and high cycle life [103]. ECs were introduced to the market as power backup for computer memory (2–5 V/some Farad), while manufacturer today offers EC modules of e.g. tens of volts and hundreds of Farads. Typical applications are high power combined with short duration and many repetitive cycles.

Examples of use are parallel operation with batteries (extend peak power of battery and energy density of capacitor in e.g. hybrid/electric vehicles, UPS), capture and store energy from regenerative breaking (e.g. elevators, diesel- electric traction (cranes, locomotives), hybrid/electric vehicles) and subsequently supply this captured energy for typical peak loads like acceleration. Other promising application areas are power quality issues (VAR support, frequency and voltage regulation, harmonic protection).

For ocean energy application EC storage systems can typically be used to smooth out short time variation in power production, increase power quality as well as fault ride-through capability. Note, however, that commercial EC components are reported to have insufficient cycle life for utilization in power smoothing applications of an oscillating water column wave energy converter, where the expected maintenance interval was 5 years [118]. In wind systems, ECs are already used for wind turbine pitch control systems and voltage regulation and VAR support [119].

3.3.6.3 Main characteristics

Typical time Response Charge time Discharge Self discharge time Life time Cycle life constants time time [%/day] [years] [number] Electrochemical ms 1) Comparable to ms – 1h 1) 2-40 1) 20+ 1) 104 - 108 1) capacitors discharge time

1) [103] Table 3.21 Typical time constants for electrochemical capacitors.

Characteristics Energy density Power density Efficiency Maintenance Maturity interval [Wh/kg] [Wh/dm3] [W/kg] [W/dm3] [%] - Electrochemical 0.1-15 1) - 500-5000 2) 100000+ 2) 85-98 1) - Available on market capacitors

1) [103], 2) [101] Table 3.22 Typical properties for electrochemical capacitors.

Costs figures Costs [€/kW] [€/kWh] Electrochemical 100-400 1) 300-4000 1) 2) 2) capacitors 70-210 210-1500

1) [103], 2) [101]. Specifically denotes capital costs. Original figures in $:100-300 $/kW and 300-2000 $/kWh. Table 3.23 Typical cost figures for electrochemical capacitor energy storage systems.

3.3.6.4 Key features

An electrochemical capacitor energy storage is a short-time energy storage with typical operating time in tens of seconds. Advantages: - High power density - Fast response capability (limited by the interfacing frequency converter)

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- Very fast charge and discharge time (limited by the interfacing frequency converter) - High cycle life - High round-trip efficiency - Deep discharge capacity, without influencing the lifetime - No maintenance and robust to temperature extremes - State-of-charge always known - Modularity with respect to voltage and capacitance - Modest environmental influence (e.g. little hazardous materials and emission. Two exceptions for electrolytes: KOH (modest) and acetonitrile (toxic, flammable))

Disadvantages: - Low energy density - High costs - Except for some EDLC, not proven technology - Cell voltage limited to typical 1-3 V - Relatively high intrinsic losses (self-discharge)

3.3.7 SMES This subsection will present the technology and main properties of Superconductive Magnetic Energy Storage (SMES) systems.

3.3.7.1 Technology description

A superconducting magnetic energy storage (SMES) stores energy in the magnetic field associated with the direct current flowing through superconducting wires in a large magnet (coil). Due to superconductivity, the storage in the magnet becomes virtually lossless. The volumetric energy density E ([J/m3]) associated with the magnetic field is given by [120], [121] �! � = � �� = 2�!�! where � is the magnetic field intensity [T] H is the magnetizing force (B = μ0μrH) μr is the average relative magnetic permeability μ0 is the magnetic permeability of free space

The following equations relates energy E, voltage V, current I and power P in a SMES: 1 � = ��! 2

�� = � ��

�� = = = ��

Charge: A positive voltage is applied across the coil, causing the coil current to increase. Discharge: A negative voltage is applied across the coil, causing the coil current to decrease. By adjusting the voltage level, the coil can be charged/discharged at specified power levels [121]. The product of the applied voltage and the instantaneous current determines the power [110].

A schematic sketch of a simplified SMES system is shown in Figure 3.13

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Figure 3.13. Schematic illustration of a SMES system and its main components. The power conditioning system (PCS) includes a power electronic converter and a control system [122]. Superconducting magnetic energy storages (SMES) were originally envisioned for large-scale load-levelling devices (diurnal load variations). However, due to cost considerations, the focus was moved towards smaller devices, where the prime task was fast response, high power capability for transmission lines, power quality and custom power applications [121].

SMES-devices with limited energy content (typically 1-10 MJ, i.e. 0,28-2,8 kWh) are called micro-SMES. These are the only commercial SMES-devices so far. The applications are power quality improvements at industrial plants or distributed sources in utility transmission systems (increases transfer capability, stability and power quality) [120], [121], [110].

The superconducting magnets today are usually based on low-temperature superconductors (LTS) like niobium- titanium conductors (NbTi), which use liquid helium as coolant (typical temperature 1,8-4,2 K). High-temperature superconductor (HTS) materials exist, which can maintain zero resistance at a much higher temperature (some above 120 K). Reduced cooling means reduced costs, in particular for small SMES systems, where the cryogenic system constitutes a major expense. HTS based SMES systems are emerging [121] and their magnets are used in laboratory and pre-commercial tests today [122], [123]. Liquid nitrogen (typical 77 K) is usually used as coolant to HTS, although their operating temperature may vary between 20-77 K. A sketch of the cryogenic system is illustrated in Figure 3.14.

Figure 3.14. Simplified illustration of the cryostat, coil and refrigerator in a SMES system [124].

SMES energy density is limited to by mechanical stresses to values of ten of kJ/kg (0.003 kWh/kg), due to the Lorentz forces generated by and on the magnet coils [102].

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3.3.7.2 Application areas Superconductive magnetic energy storage (SMES) systems are electric energy storages with operating times in tens of seconds. Its main advantage is very fast response and high energy storage efficiency. (No electrical losses, but heat leaks in the cryogenic system create losses for the overall system).

One of few commercial SMES-devices so far, are the D-SMES (distributed) and PQ-(I)VR SMES (power quality (industrial) voltage regulator) devices from American Superconductor (AMSC). These are micro-SMES devices with 3- phase output. The magnet uses LTS material, while the current leads are made by HTS.

Figure 3.15 A cross-section of the D-SMES from American Superconductors (AMSC): A complete micro-SMES system integrated on a trailer, with an output rating of 3,0 MW at 480 V and an energy storage of 3 MJ (0,83 kWh). Length: Approx. 16 m. Weight: Approx. 25000 kg [125] Approximately 100 MW of SMES units are in operation world wide [101], whereof about 50 MW of the capacity is installed in USA for power quality or uninterruptible power supply applications [99]. Japan has several SMES built, where one of the objectives is to protect sensitive loads from voltage dips. Examples are a 5 MW-7 MJ device (LTS) and a 1MVA SMES (HTS) [102].

3.3.7.3 Main characteristics

Typical time Response Charge time Discharge Self discharge time Life time Cycle life constants time time [%/day] [years] [number] SMES ms 1) (tens of) s ms – 5 min 1) 10-15 1) 20 1) 104 1) ms – 8 s 2)

1) [103], 2) [101] Table 3.24 Typical time constants for SMES systems. Characteristics Energy density Power density Efficiency Maintenance Maturity interval [Wh/kg] [Wh/dm3] [W/kg] [W/dm3] [%] - SMES 0.5- 5 1) 0.2 – 2.5 500-2000 2) 1000-4000 2) 95 1) - µSMES: Available on market, but limited usage

1) [103], 2) [101] Table 3.25 Typical properties for SMES systems.

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Costs figures Costs [€/kW] [€/kWh] SMES 100-400 1) 700-7000 1) 2) 100-700 2)

1) [103], 2) [101] . Specifically denotes capital costs. Original figures in $:150-1000 $/kW and 1000-10000 $/kWh Table 3.26 Typical cost figures for SMES systems.

3.3.7.4 Key features

A SMES storage is a short-time energy storage with high power capability and operating time in tens of seconds.

Advantages: - Very fast response capability (MWs in ms [126], [127], [128]; <0,5 ms for the D-SMES from American Superconductor [125].) - Inherent high efficiency during charge/recharge cycles (90 %), refrigeration losses included. More than 95 % reported [125], [128]. - Deep discharge capacity, without influencing the life-time - Complete recharge in minutes [128] (90 s for the PQ-VR from American Superconductor) - No moving parts (except for refrigeration system) - Almost no environmental influence (e.g. no hazardous materials nor emission, although powerful source for magnetic fields in proximity of magnet: Large SMES require significant shielding to prevent magnetic fields in the surrounding area. [102]) Disadvantages: - High costs - Limited energy storage capacity - Cryogenic (cold temperature technology) hardware required

3.3.8 Comparison tables for Energy storage technologies

Typical time Response Charge time Discharge Self discharge time Life time Cycle life constants time time [%/day] [years] [number] PHS s - min 1 – 24+ h ~0 50-100 2x104 – 5x104 40-60 H2 Min 0.5-2 5-15 >1000 CAES 5 -15 min 1 – 24 h ~0 25-40 5x103 – 2x104 Bat: Lead-acid 4-30 500-2,000 Bat: Li-Ion 5 4,000 Bat: NaS 15 2,500 Bat: Vanadium- 7-15 10,000 Redox-Flow Flywheel energy s Comparable 15 s - 15 min 20-100 20+ 105-107 storage to discharge time Electrochemical ms Comparable ms – 1h 2-40 20+ 104 - 108 capacitors to discharge time SMES ms ( tens of) s ms – 5 min 10-15 20 104 ms – 8 s Table 3.27: Typical time constants for energy storage systems. Sources listed in each section.

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Characteristics Energy density Power density Efficiency Maintenance Maturity interval [Wh/kg] [Wh/dm3] [W/kg] [W/dm3] [%] PHS 0.5–1.5 0.5-1.5 75-85 Available on market H2 800- >500 20-50 Available on the 10000, market / Pre- 600-1200 commercial stage CAES 30-60 3-6 0.5-2 42-54 Available on market 12 70-89 Bat: Lead-acid 35 80 Mass production Bat: Li-Ion 50-200 90 Available on market Bat: NaS 160-230 90 Recent Bat: Vanadium- 35 75 Recent Redox-Flow Flywheel energy 5-130 400-1600 1000-2000 85-95 Quarterly: Air filter Low-speed: storage Yearly oil change Available on market Bearings every 3 y High-speed: Pre-commercial Electrochemical 0.1-15 500-5000 100000+ 85-98 Available on market capacitors SMES 0.5- 5 0.2 – 2.5 500-2000 2) 1000-4000 95 - µSMES: Available on 2) market, but limited usage Table 3.28: Typical properties for energy storage systems. Sources listed in each section.

Costs figures Costs [€/kW] [€/kWh] PHS 500-3600 60-150 400-1500 4-70 H2 1 2000-6600 5-16 CAES 400-1150 10-120 300-600 1.5-35 Bat: Lead-acid 50 Bat: Li-Ion 500-800 Flywheel energy 100-300 1000-3500 storage 190-250 700-3500 Electrochemical 100-400 300-4000 capacitors 70-210 210-1500 SMES 100-400 700-7000 100-700 Table 3.29: Typical cost figures for energy storage systems. Sources listed in each section.

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4 CONCLUSIONS: OBSERVED AND FORSEEN COMPATIBILITIES BETWEEN PRIMARY ENERGY CAPTURE APPROACHES AND PTO COMPONENTS

4.1 OFFSHORE WIND TURBINES

Wind energy conversion concepts used in offshore turbines today and expected to be common in the near future are briefly described below. Variable speed generator with gearbox By far the most common system in modern wind turbines sold in the last decade is variable speed turbines with a gearbox that transforms the low speed rotation of the blades to high speed rotation in the generator. Variable speed (as opposed to fixed speed) is essential in order to extract maximum energy from the wind, as discussed above. The change from fixed-speed to variable-speed turbines has also been motivated by stricter grid connection requirements, i.e. grid codes. Variable speed turbines require to be fitted with a frequency converter, what is described in section 4.1. The typical electrical configurations are described bellow:

Doubly fed induction generator (DFIG) These are the most common generator concepts in large wind turbines sold today. It uses a power electronic converter for feeding the rotor winding and thereby allows operation at variable speed. The power rating of the converter is typically about 30 % of the rated power of the generator. The DFIG offers variable speed operation with a reduced converter cost compared to a full converter solution (see below).

Squirrel cage induction generator (SCIG) Variable speed operation using a standard SCIG can be achieved by inserting a full-scale converter between the generator and the grid. The main drawback of such a system is the cost of the converter. This is a less common solution, but exists in some Siemens designs.

Permanent magnet synchronous generator (PMSG) This system relies on permanent magnets in the generator rotor to set up the rotating magnetic field, and a full scale converter to allow variable speed. The main advantages are lower weight, no electrical excitation system and slip rings (brushless), and better efficiency and simpler fault-ride-through capabilities compared to the DFIG. The disadvantages are higher costs than for wound-rotor synchronous generators, and losses in the converter.

PMSG

Gearless wind turbines The gearbox is considered a troublesome component in wind turbines, and its removal would lead to increased reliability, reduced weight, and reduced maintenance costs. Wind turbines without a gearbox are denoted direct- driven, and they normally feature a permanent magnet synchronous generator (PMSG) connected to a frequency

Rev. 03, 29-Nov-2012 Page 91 of 115 D2.03 Review of Relevant PTO Systems converter. How this solution makes it possible to remove the gearbox is now explained. The same principle applies to tidal turbines.

Wind turbine rotors are typically designed for a maximum rotation frequency of 0.3–2 Hz, depending on the size. Since the electric frequency in the power grid is typically 50 or 60 Hz, the needs to be converted with a factor between 25 and 150. From an electric point of view it is possible to achieve this conversion using the frequency converter only, but this would cause magnetic saturation in the generator iron. In order to avoid the saturation phenomena, the generator electric frequency must be kept above ≈10 Hz. Therefore, all wind turbines are equipped with either a mechanical gearbox, multipole generators, or a combination of both. This allows the blades to rotate at their desired speed between 0.5 – 2 Hz, whilst the generator can operate at an electric frequency in range of 50 Hz.

A multipole generator can be viewed as a generator with an embedded electromagnetic gearbox. An even number of magnetic poles are placed on the rotor, yielding an equivalent gear-ratio equal to the number of pole pairs. For large offshore wind turbines, the number of required pole pairs for a direct-driven turbine lies in the range of 50. In practice, this is difficult to obtain using a wounded synchronous generator, and close to impossible for an induction machine. However, PMSGs can be constructed with the required number of poles.

Even though it is possible to have a grid-connected direct driven PMSG operating at a fixed frequency of 50 Hz, the standard solution is to use a frequency converter between the generator and the grid. This is due to several reasons: The PMSG cannot control its output voltage in contrast to the wounded synchronous generator, and the amount of harmonic distortions can be large. In addition, the frequency converter makes it possible to operate the generator at a frequency below 50 Hz, which translates into a reduced number of required electrical poles. Typical frequencies of a direct driven PMSG lie in the range of 15-40 Hz.

Nacelle weight reduction with hydraulic systems From the expression for turbine power output given in chapter 1.3.1.2, and the relationship between power P and torque T, it follows that P R3u 2 T = ∝ . ω λ That is, the torque scales as the cube of rotor radius. Since nacelle weight is essentially determined by turbine torque rating, it also scales, for a given design, as the cube of rotor radius. On the other hand, nacelle weight is an important variable for wind turbines because of its obvious impact on tower and substructure design. A lighter nacelle puts less demand on the support structure, giving overall cost reductions. This is particularly important for large offshore wind turbines. So it is highly desirable to avoid the cubic weight increase by improving the design. In reality this has also happened, with empirical data showing that weight has scaled roughly as the square of the radius [129]. This, however, has only been possible through innovation and adaptation of new nacelle designs. Due to the continued demand for ever larger turbines, further innovation is needed and indeed on-going.

One novel idea is to replace the gearbox, converter, and transformer with a hydraulic transmission system and place the generator at the tower base. Another idea is to replace traditional copper in the generator by high-temperature superconducting material, thereby reducing volume and weight by allowing higher current density. A third approach is to eliminate transformer and one converter by using a generator that directly gives high voltage DC output. There are also a number of ideas for improving more traditional generator design such as to achieve weight reductions.

Fault ride-through capability The capability of a wind turbine to remain connected during a short grid-side fault is a key requirement for grid code compliance. Assuming a drive-train topology with full-scale converter, a braking resistor may be used for protecting the converters during voltage dips and other power system transient states, see Figure 4.1. The braking resistor may be connected with a power electronic switch over the DC link allowing for fast switching on and off, possibly many times during a voltage dip. The operation may be as follows: In case of a voltage dip the DC link voltage will increase as the wind induced power will be greater than the power being fed to the grid. As soon as the DC link voltage is increased above a certain threshold value (Uhh), the braking resistor is connected. The power going through the

Rev. 03, 29-Nov-2012 Page 92 of 115 D2.03 Review of Relevant PTO Systems braking resistor together with the power being fed to the grid will now be greater than the wind induced power, and the DC link voltage will decrease. At a certain value (Uhl) the braking resistor will then disconnect. This cycle with hysteresis of connection and disconnection of the braking resistor will continue until the grid voltage is back to normal. This system with braking resistor is also described in reference [6].

See also chapter 3.2 for fault ride-through capability discussed from a frequency converter point of view.

Grid side converter DC link = Udc ~

Udc>Uhh: close Udc

Associated storage solutions

Although offshore wind production is also varying on a seasonal basis, here, most relevant focus regarding storage solutions is on smoothing the power input over days. CAES can be an option for this, but only on shore. Hydrogen systems could represent a solution for remote locations such as the Utsira Island in Norway [106], and research is on-going about the possibilities of generating hydrogen on offshore marine energy platforms [130], as medium for energy storage and carriage. Research is also on-going for adapting pumped-hydro storage to offshore underwater environment [131].

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4.2 TIDAL ENERGY CONVERTERS (TEC)

OpenHydro and Clean Current use an association of technologies that appears relevant and quite distinct from the “wind” solutions. It illustrates well the objective of “reliability by simplicity”: First, what visually appears quite clearly on those turbines is the use of a duct. In addition with an increase in power density, it is expected to protect the rotor. Then, the blades have a fixed pitch, and therefore the prime mover has only one single moving part [132], [94]. On the top of this, the mechanical chain is simplified to a maximum with the generator being direct-drive, permanent magnet, and directly integrated in the prime mover [133], [134]. Due to the large diameter of direct-drive turbine generators, this is only made possible by the presence of the duct: this is where the electrical windings are fitted, while the permanent magnets are fixed in the periphery of the rotor, thus avoiding any electrical contacts between fixed and moving parts.

Nevertheless, most of the advanced tidal concepts still follow the path of modern wind turbine technologies. In particular, pitch control in association with variable speed is necessary to achieve power control as illustrated Figure 4.2. This also presents the advantage to reduce loads by avoiding stall.

Figure 4.2 : Tests of power control for the SeaGen prototype (courtesy Fraunhofer IWES).

Also, the cross-flow turbines now attract more interest in tidal context than they do in wind energy. This is mainly due to the possibility not to submerge the drive train, and to have greater blockage ratios. But the corresponding PTO is expected to experience high dynamic loads due to the torque ripple mentioned in the primary energy capture section.

Associated storage solutions

With a clear power production pattern which main period is shorter than a day, and low peak/average production ratio, TEC require less associated storage capacity than wind or wave energy for coping with intermittency. For instance, it has been shown that an energy storage capable of accumulating even less than 1h of TEC rated power could be highly beneficial for market compliance [135]. However, such storage system would have shorter and more frequent charge/discharge cycles than in the case of wind, namely up to 1400 cycles per year.

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4.3 WAVE ENERGY CONVERTERS (WEC)

At present there is little design consensus for wave energy devices with no industry standard device concept. Due to the diverse nature of the wave resource it appears unlikely that the industry converge to the use of one single device concept, but rather to a small number of device types adapted to different wave climates.

Wave energy devices currently make use of a very wide range of technologies for primary energy conversion. As discussed in §1.3.3.4, WECs can be classified by its primary energy capture as:

- Oscillating Water Column: a column of water moves up and down with the wave motion, acting as a piston, compressing and decompressing the air. This air is ducted through an air turbine. - Attenuator: generally long floating structures aligned in parallel with wave direction, which then absorbs the motion caused by the waves. Its motion can be damped to produce energy. - Point Absorber: floating structure absorbing energy from all directions of wave action due to its small size compared to the wavelength. - Overtopping devices: they are composed of a wave surge/focusing system, and contains a ramp over which waves travel into a raised storage reservoir. - Oscillating Wave Surge Converter: it extracts energy from the surge motion of the waves. They are generally seabed mounted devices located in nearshore sites. - Submerged pressure differential: devices, in which the motion of waves causes the sea level to rise and fall above the device, inducing a pressure differential in the device.

A brief look at the wave energy conversion schemes reveals that, in sharp contrast to wind turbines, there is a wide range of absorbing principles and may require cascaded conversion mechanisms. Different systems operate on different methods of wave-device interaction (such as heave, pitch or surge) and may need pneumatic, hydraulic or mechanical stages.

There are different PTO systems and several of them can be combined with one primary energy capture system. Nevertheless, revising the state of the art of WECs, it can be concluded that there is a relation between some PTO and the primary energy capture, as shown in Table 4.1. Primary energy PTO: Transmission of capture mechanical energy OWC Air turbine Attenuator Hydraulic circuit Point Absorber Hydro/Hydraulic (when not OWC) circuit, Direct drive Overtopping device Low-head hydro turbine Oscillating Wave Hydro/Hydraulic Surge circuit Submerged Pressure Hydraulic circuit / Differential Direct drive Table 4.1. Related primary energy capture system and PTO

This can be illustrated by the following wave energy converter projects and devices:

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Oscillating Water Column projects and devices

One example of OWC using a Wells turbine is LIMPET (Land Installed Marine Powered Energy Transformer), which makes use of land topographical features to construct the chamber and mount the turbine generation system on the shore line. A sectional diagram of LIMPET construction is shown in Figure 4.3 below;

Figure 4.3 - LIMPET Cross Section (Courtesy Voith Hydro Wavegen)

A picture showing an overview of the device can be seen in Figure 4.4 [136]:

Figure 4.4 - LIMPET Construction Other fixed OWCs are the PICO plant, also constructed on a cliff, and more recently the Mutriku plant with 16 air chambers integrated in the construction of an artificial breakwater [71]. In addition, the principle of floating OWC also receives interest, with for instance the following prototypes: Oceanlinx blueWave that is a floating array structure (see Figure 4.5), and the Ocean Energy OE Buoy (see Figure 4.6).

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Figure 4.5 – Oceanlinx blueWave

Figure 4.6 - Ocean Energy OEBuoy

Hydraulic PTO projects and devices One application of a Pelton wheel based power take off system is the Oyster wave energy converter developed by Aquamarine Power, with much of the control system and electrical system design carried out by Narec. An outline of this system is shown in Figure 4.7 below [28]:

Figure 4.7 - Oyster System Outline (Courtesy Aquamarine Power)

In the case of the power take off system for the Oyster wave energy converter, the initial hydraulic pressure is created via hydraulic cylinders that are pumped as a result of wave action (or wave surge) on an oscillating flap [137].

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As can be seen in Figure 4.7 above, the base of the wave energy converter is anchored to the sea bed, with one end of each hydraulic cylinder attached to the base. The oscillator flap is constructed as a series of hollow steel tubes, which can be flooded with water to sink the flap to the seabed for maintenance or survival purposes. In operation the oscillator tubes are full of air, and therefore are naturally buoyant, and so as the base of the flap is fixed and pivoting on the seabed mounted base, the oscillator flap is subsequently pushed back and forth by wave motion. As the opposite ends of the hydraulic cylinders are attached to the oscillator flap, the cylinders draw water into themselves when extending, and discharge when compressing into the hydraulic flow line at considerable pressure. The resulting hydraulic pressure is then fed to the spear valves mounted on the Pelton wheel casing, which then produce the water jets onto the buckets of the Pelton wheel. The spear valves are controlled by a computerised system to maintain an optimum system pressure for the generator torque. A simple block diagram of a typical Pelton wheel power take off system is shown in Figure 4.8 below;

Figure 4.8 - Block Diagram of Pelton Wheel Power Take Off System

The features and benefits of such a system are [28]: • The offshore device is mechanical • Minimal underwater moving parts • No off-shore control system, gearbox, or shut-down mode • No complex offshore electronics • Nearshore location • Can operate in storm conditions • Keeps electricity production out of the water • Minimal ecological impact • Easy to access

The spear valve directs a jet of water onto the buckets of the Pelton wheel, which is designed to operate at rotational speeds of up to 3000 rpm. The speed of the water jet is controlled by changing the effective flow area of the spear valve, which in turn controls the system pressure. In order to conserve energy and stabilise the output power of the generator, the Pelton wheel is directly connected to a flywheel on one side, and to the generator itself on the opposite side of the Pelton wheel driveshaft. Figure 4.9 below shows a photograph of this arrangement, with the key components of the system indicated [138].

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Figure 4.9 - Oyster Power Take Off System During Narec Testing

It is a recognised feature of wave power that over a wave cycle the instantaneous power fluctuates significantly. Because of this fluctuating power, the Pelton wheel speed and system operating pressure have significant fluctuations over the wave cycle. Consequently the resulting hydraulic system needs to feature accumulators to reduce the pulsations within the system, and also fast acting control of the spear valves to try and maintain a steady flow to the buckets of the Pelton wheel. The decoupling provided by accumulators allows pump torque to be varied so as to capture the most energy from the variable pressure delivery, whilst the generator is driven at a steady rate.

The primary control of the system is achieved by metering the flow of water through the spear valves onto the Pelton wheel and by controlling the torque applied by the generator. The primary inputs into the control system are therefore the water flow rate, the system pressure, the generator/flywheel rotational speed, and spear valve position. The spear valves are then adjusted continuously to keep the average operating pressure in the system as close as possible to the optimum target pressure for the sea state, while simultaneously keeping the ratio of the spear valve nozzle velocity and the Pelton wheel bucket speed close to its optimal value. The ideal ratio of Pelton wheel periphery speed to the water jet speed is 0.5, and whilst in traditional applications maintaining this ratio is relatively straightforward as the water pressure and wheel speed are constant, clearly the same is not true for a wave energy device due to the pressure fluctuations. One possible outcome is that if the ratio is deviated for significant periods of time, erosion of the Pelton wheel buckets may occur.

One potential advantage that arrays of such devices would have is that the greater the number of hydraulic sources (i.e. wave energy converters) in the array, the less the system will cycle through extremes of pressure and pulsations in the delivery pipework. This could also be beneficial where flow measurement is concerned, as some flow metering devices (for example magnetic flow meters) are intolerant of pressure pulsations, and will likely result in a lower lifespan for the instrument. The final regulation of the variable speed generator output is by certified power electronics as described in §3.2, that provide the necessary full rectification and inversion before a step-up transformer supplies power to the grid, complying with the local grid codes such as the G59 regulations in the UK [81].

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Another pumping device operating on similar principles is the WaveRoller, which is illustrated in Figure 4.10 below;

Figure 4.10 - The AW Energy WaveRoller (Courtesy AW Energy)

The WaveRoller is similar in operating principle to the Oyster wave energy converter in that it is a seabed mounted device with hinged plates which are attached to piston based hydraulic pumps. As the wave surge reaches each of the plates, so the plate oscillates transferring the energy to the pump and creates hydraulic energy through a pipeline back to a generator system [139].

An example of a Pelton wheel system powered by a hydraulic wave driven pump is the SurfPower buoyant wing, as shown in Figure 4.11 below;

Figure 4.11 - SurfPower Buoyant Wing Operation (Courtesy SurfPower)

For the SurfPower system a buoyant wing floats on the ocean surface, and is attached to one end of a piston pump with the other end of the pump anchored to the sea bed [140]. The system is therefore a point absorber and wave action strokes the piston pump, thereby drawing water into the pump that is expelled via a pipeline to a Pelton wheel based generator system onshore.

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Linear Generator projects and devices The concept of direct drive in wave energy has been demonstrated within the Archimedes Wave Swing device, and a picture of the installation of the device can be seen in Figure 4.12 below [141]:

Figure 4.12 - Archimedes Wave Swing Linear Permanent Generator

The device is a seabed mounted Submerged Pressure Differential, and consists of an air-filled chamber with a floater that moves down with the crest of a wave and up with the trough of a wave.

A prototype of just such a machine entitled Snapper was developed as part of an FP7 project, led by Narec with collaboration from The University of Edinburgh, Ecotricity, Meccanotecnica Riesi, SubseaDesign, EM Renewables, Technogama, and Ocean Resource [142]. Snapper works like a typical linear generator in which a set of magnets mounted in a translator are moved up and down inside multiple coils of wire of an armature. However, there is a crucial difference with Snapper: alongside the armature coils is a second set of magnets of alternating polarity. With reference to Figure 3.1, the revised device could therefore be represented as shown in Figure 4.13 below;

Figure 4.13 – Outline of Snapper Arrangement

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These additional armature magnets prevent the translator magnet assembly from moving up and down smoothly in relation to the armature. Instead magnetic forces between the armature and translator repeatedly couple the two sub-assemblies together until the external force is able to overcome it. This results in a series of faster movements (faster relative movements between armature and translator) more suited to classical electrical generation. In order to also store energy, the armature includes spring sets that give an additional amount of oscillation after each magnet decoupling or ‘snap’ recovering further energy. The translator array is longer than the stator to allow for the range of relative movement due to the wave motion and changes in the water depth due to tides. The system itself is arranged in a double-sided configuration to balance the magnetic attraction between the two sets of magnets and guides are provided to maintain the separation.

Figure 4.14 - 3D Representation of Snapper Wave Energy Converter

Figure 4.15 below shows the development of the Snapper prototype on the linear test rig at Narec’s facility in Blyth, UK.

Figure 4.15 - Testing of Snapper Translator on Linear Test Rig

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Figure 4.16 below shows the assembled prototype during transportation to the wave test tank.

Figure 4.16 - Assembled Snapper Prototype A photograph of the resulting prototype under test at Narec’ wave tank test facility can be seen in Figure 4.17 below;

Figure 4.17 - Snapper Prototype under Test at Narec Wave Tank By way of illustration, the test system comprises 80 magnets on each of the two armature arrays. The armature is 4m long and 250mm wide. Each magnet is 20mm thick. The armature magnets are 30mm wide in the direction of travel to provide space for the coils. Additional space may be provided by using shallow slots in the laminated core. The translator magnets are 40mm wide [143]. All the high strength permanent magnets are made of the modern rare-earth material Neodymium-Iron-Boron. This introduces the challenge of high forces between components and demanded that the designed supporting structure be both strong and stiff to maintain the necessary clearances. This directly opposes the need for low mass in the armature. However, the use of double-sided magnet arrangements for both the translator and surrounding armature helps to keep forces in balance and avoid too great a mass penalty.

The Snapper device provides velocity amplification related directly to the acceleration of the armature and so it was important to keep the armature mass to a minimum during design development. Equally the selection of the

Rev. 03, 29-Nov-2012 Page 103 of 115 D2.03 Review of Relevant PTO Systems bearings was also an important consideration during the design phase, as it was important to minimise the coefficient of friction, and thus minimise losses.

The selection of the spring type was also a significant consideration. It is important to have springs that can store the required amount of energy without fatigue over a large number of cycles, yet they must not contribute too much to the mass of the armature. Furthermore, they must be resilient to the marine environment.

Performance of the springs, as well as additional device characteristics relating to the other mechanical challenges will be monitored during the validation phase of the current FP7 project [144].

Concepts such as Seabased’s wave energy converter, and Trident Energy’s PowerPod as shown in Figure 4.18 and Figure 4.19 below also use linear generators, with point absorber and submerged pressure differential prime movers too, respectively.

Figure 4.18 - Seabased Wave Energy Converter

Figure 4.19 - Trident Energy PowerPod

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In the case of the PowerPod shown in Figure 4.19, this representation shows a number of linear generators located around the base of an offshore wind turbine, thereby enabling the merging of these two power generation types, and sharing infrastructure.

Associate storage solutions In addition with seasonal or day variations as in wind, smoothing power to curb peak-to-mean ratio is here a problem of much shorter time scales like the wave period, e.g. seconds. Investigation for this purpose is lead on the use of supercapacitors in the DC link of frequency converters [66], [118]. One of the corresponding issues is the high number of cycles: a few thousands a day depending on wave climate.

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5 REFERENCES

[1] http://std.iec.ch/glossary [2] IEC 62600-1, ed. 1.0 (2011), “Marine Energy – Wave, Tidal and other water converters – Part 1: Terminology” [3] EWEA, "The European offshore wind industry key 2011 trends and statistics", 2012 [4] Hansen, M.H.; Hansen, A.D.; Larsen, T.J.; Øye, S.; Sørensen, P.; Fuglsang, P.; “Control design for a pitch- regulated, variable speed wind turbine”. Risø-R-1500(EN) (2005) 84 p. [5] Hansen, A.D., Iov, F., Sørensen, P., Cutululis, N., Jauch, C., Blaabjerg, F. “Dynamic wind turbine models in power system simulation tool DIgSILENT”. Risø-R-1400(ed.2)(EN) (2007) 190p. [6] James Conroy and Rick Watson (2007) “Torsional Damping Control of Gearless Full-Converter Large Wind Turbine Generators with Permanent Magnet Synchronous Machines”, Wind Engineering, Volume 31, no. 5, 2007 pp 325–340. [7] http://activites.edf.com/production/hydraulique-et-energies-nouvelles/energies-marines/comment-ca- marche-41260.html [8] Hydrodynamic Impacts due to Tidal Power Lagoons in the Upper Bay of Fundy Canada, Cornett et all, EWTEC 2011 [9] McAdam, Houlsby, Oldfield, Structural and Hydrodynamic Model Testing of the Transverse Horizontal Axis Water Turbine, EWTEC 2011 [10] www.pulsetidal.com [11] www.hammerfeststrom.com [12] www.tidalgeneration.co.uk [13] www.marineturbines.com [14] Khan et al, Hydrokinetic energy conversion systems and assessment of horizontal and vertical axis turbines for river and tidal applications: A technology status review, Applied Energy vol 86 (2009) 1823–1835 [15] http://www.atlantisstrom.de [16] Coiro et al, Dynamic Behavior of Novel Vertical Axis Tidal Current Turbine: Numerical and Experimental Investigations, International Offshore and Polar Engineering Conference, 2005 [17] Milne et al, Characteristics of the Onset Flow Turbulence at a Tidal-Stream Power Site, EWTEC 2011 [18] Thomson et al, Quantifying turbulence for tidal power applications, OCEANS 2010 [19] Myers, Bahaj, “An experimental investigation simulating flow effects in first generation marine current energy converter arrays,” Renewable Energy, Vol. 37, No. 1, Jan 2012. [20] Myers, Keogh, Bahaj, Experimental investigation of inter-array wake properties in early tidal turbine arrays, Oceans 2011 [21] Palm et al, The Applicability of Semi-Empirical Wake Models for Tidal Farms, EWTEC 2011 [22] Churchfield et al, A Large-Eddy Simulation Study of Wake Propagation and Power Production in an Array of Tidal-Current Turbines, EWTEC 2011 [23] Li and Calisal, Modeling of twin-turbine systems with vertical axis tidal current turbines Part I – Power output, Ocean Engineering 2010 [24] Li and Calisal, Modeling of twin-turbine systems with vertical axis tidal current turbines Part II – torque fluctuation, Ocean Engineering 2011

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[25] Milne et al, The Role of Waves on Tidal Turbine Unsteady Blade Loading, ICOE 2010 [26] www.oceanflowenergy.com [27] Winchester et al, Torque ripple and power in a variable pitch vertical axis tidal turbine, EWTEC 2011 [28] Aquamarine Power, http://www.aquamarinepower.com/ [29] Development of recommended practices for testing and evaluating ocean energy systems. IEA | Ocean Energy Systems, Annex II Report, 2003. [Online], Available: www.iea-oceans.org. [30] Marine Energy Glossary. The Carbon Trust. 2005 [31] Wave Energy Utilization in Europe Current Status and Perspectives. WaveNet. European Network for Wave Energy. 2002. [32] D. O’ Sullivan, D. Mollaghan, A.Blavette and R.Alcorn (2010). Dynamic characteristics of wave and tidal energy converters & a recommended structure for development of a generic model for grid connection, a report prepared by HMRC-UCC for the OES-IA Annex III. [Online], Available: www.iea-oceans.org. [33] Evans, D. V., "The Oscillating Water Column Wave-energy Device," IMA J Appl Math, vol. 22, pp. 423-433, 1978. [34] Pico power plant, http://www.pico-owc.net/ [35] LIMPET (Land Installed Marine Powered Energy Transformer), www.wavegen.co.uk/what_we_offer_limpet_islay.htm [36] Oceanlinx Ltd., http://www.oceanlinx.com/ [37] Ocean Energy Ltd., http://www.oceanenergy.ie/ [38] J. Khan and G. Bhuyan (2009). Ocean Energy: Global Technology Development Status, Report prepared by Powertech Labs for the IEA-OES. [Online], Available: www.iea-oceans.org [39] Henderson, R., "Design, simulation, and testing of a novel hydraulic power take-off system for the Pelamis wave energy converter," Renewable Energy, vol. 31, pp. 271-283, 2006. [40] Pelamis Wave Power Ltd., http://www.pelamiswave.com/ [41] Ricci, P., Lopez, J., Santos, M., Villate, J. L., Ruiz-Minguela, P., Salcedo, F., and O.Falcão, A. F. d., "Control Strategies for a simple Point-Absorber Connected to a Hydraulic Power Take-off," in European Wave and Tidal Energy Conference, 2009. [42] Ocean Piwer Technologies http://www.oceanpowertechnologies.com/ [43] Jasinski, M., Knapp, W., Faust, M., and Fris-Madsen, E., "The Power Takeoff System of the Multi-MW Wave Dragon Wave Energy Converter," in European Wave and Tidal Energy Conference, 2007. [44] Wave Dragon ApS, http://www.wavedragon.net/ [45] Chaplin, R. V., Rahmati, M. T., Gunura, K., Ma, X., and Aggidis, G. A., "Control systems for Wraspa," in Clean Electrical Power, 2009 International Conference on, 2009, pp. 93-97. [46] Polinder, H., Damen, M. E. C., and Gardner, F., "Linear PM Generator system for wave energy conversion in the AWS," Energy Conversion, IEEE Transactions on, vol. 19, pp. 583-589, 2004. [47] J.P. Ruiz-Minguela, M. Santos, P. Ibañez, J.L. Villate, F. Salcedo; "Control Techniques of Ocean Wave Energy Converters", Internacional Conference on Energy (IADAT-ICE), Bilbao 2008. [48] Falnes, J., Ocean Waves and Oscillating Systems, Cambridge University Press, Cambridge, UK. 2002. [49] Budal, K., Falnes, J., "Interacting Point Absorbers with Controlled Motion", Proceedings of the Conference Power from Sea Waves, Edinburgh, UK, 1979, pp. 129-42. [50] Hansen, Martin O. L. (2008): Aerodynamics of wind turbines. 2. Aufl. London ;, Sterling, VA: Earthscan. [51] Raghunathan, S., The Wells air turbine for wave energy conversion, Progress in Aerospace Sciences, Vol. 31, pp335-386, 1995

Rev. 03, 29-Nov-2012 Page 107 of 115 D2.03 Review of Relevant PTO Systems

[52] Setoguchi, T., Santhakumar, S., Maeda, H., Takao, M., & Kaneko, K. (2001). A review of impulse turbines for wave energy conversion. Renewable Energy, 23(2), 261-292.

[53] Hwww.pico-owc.net [54] Taylor, J.R.M., Caldwell, N.J., Design and Construction of the variable-pit chair turbine for the Azores wave energy plant, Proc. Third European Wave Power Conference, 30 September – 2nd October 1998, Patras Greece [55] Islay Limpet wave power plant, 1 november 1998 to 30 april 2002 report, http://www.wavegen.co.uk/pdf/LIMPET%20publishable%20report.pdf [56] Kim, T.W., Kaneko, K., Setoguchi, T., Inoue, M., Aerodynamic performance of an impulse turbine with self- pitch-controlled guide vanes for wave power generator. Proc 1st KSME-JSME Thermal and Fluid Engg Conf 1988;2:133–7. [57] Thiebaut, F., O’Sullivan, D., Kracht, P., Ceballos, S., Lopez, J., Boake, C., Bard, J., Brinquete, N., Varandas, J., Gato, L.M.C., Alcorn, R., Lewis, A.W., Testing of a floating OWC device with movable guide vane impulse turbine power take-off, 9th European Wave and Tidal Energy Conference, Southampton, England, Sep. 5-9, 2011 [58] Anand, S., Jayashankar, V., Nagata, S., Toyota, K., Takao, M., Setoguchi, T., Turbines for wave energy plants, Proceedings of the 8th International Symposium on Experimental and Computational Aerothermodynamics of Internal Flows, Lyon, July 2007 [59] Ravindran M., Jayashankar V., Jalihal P., Pathak A.G., The Indian Wave Energy Program – an overview, TIDE (Teri Information Digest on energy), Vol.7, No.3, pp. 173-188 (1997). [60] http://www.oceanlinx.com/ [61] Finnigan T., Auld D., Model Testing of a Variable-Pitch Aerodynamic Turbine, Proceedings of The Thirteenth (2003) International Offshore and Polar Engineering Conference, Honolulu, Hawai, USA, May 25–30, 2003 [62] Gareev, A., Analysis of variable pitch air turbines for oscillating water column (OWC) wave energy converters, Doctor of Philosophy thesis, University of Wollongong. School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, 2011. http://ro.uow.edu.au/theses/3418 [63] Alcorn, R.G., Finnigan, T.D., Control Strategy Development for an Inverter Controlled Wave Energy Plant, International Conference on Renewable Energies and Power Quality, 2004, Barcelona. [64] Fusco, F.; Ringwood, J.V.; , Short-Term Wave Forecasting for Real-Time Control of Wave Energy Converters, Sustainable Energy, IEEE Transactions on , vol.1, no.2, pp.99-106, July 2010. doi: 10.1109/TSTE.2010.2047414. [65] Tedd, J., Frigaard, P., Short term wave forecasting, using digital filters, for improved control of wave energy converters, Proc. ISOPE 2007, pp. 388-394 [66] Murray, D.B., Hayes, J.G., O’Sullivan, D.L. Egan, M.G., Supercapacitor Testing for Power Smoothing in a Variable Speed Offshore Wave Energy Converter, Ieee Journal Of Oceanic Engineering, Vol. 37, No. 2, April 2012 [67] Pontes, M.T., Athanassoulis, G.A., Barstow, S., Bertotti, L., Cavaleri, L., Holmes, B., Molisson,D. , Oliveira- Pires, H., The European Wave Energy Resource, 3rd European Wave Energy Conference, Patras, Greece, 1998 [68] Carbon Trust report on “Oscillating Water Column Wave Energy Converter Evaluation” by Arup Energy”, Copy Right The Carbon Trust, 2005 [69] http://www.oceanlinx.com/products/bluewave [70] http://www.oceanlinx.com/products/greenwave [71] Torre-Enciso, Y., Ortubia, I., Lopez de Aguileta, L.I., Marques, J., Mutriku Wave Power Plant: from the thinking out to reality, 8th European Wave and Tidal Energy Conference, Uppsala, Sweden, Sep. 7-10, 2009, Paper Number 214

Rev. 03, 29-Nov-2012 Page 108 of 115 D2.03 Review of Relevant PTO Systems

[72] Mini Hydro Power – Eindhoven University of Technology [73] The OPD Pelamis WEC: Current Status and Onward Programme, RW Yemm, Ocean Power Delivery Ltd; RM Henderson, Ocean Power Delivery Ltd; CAE Taylor, Ocean Power Delivery Ltd [74] Design and Control Considerations for a Wave Energy Converter, Gary Nolan, National University of Ireland; John Ringwood, National University of Ireland [75] DTI Technology Road Map – Wave Energy, DTI & Ove Arup [76] Renewables First Website (www.renewablesfirst.co.uk) [77] US Army Corps of Engineers [78] Small Hydropower Website (www.smallhydropower.com) [79] North West Hydro Resource Model – Engineering Dept of Lancaster University [80] Strategies to Promote Small Scale Hydro Electricity Production in Europe - Zvonimir Guzovic, Branimir Matijaševic [81] Energy Networks Association - Recommendations For The Connection Of Generating Plant To The Distribution Systems Of Licensed Distribution Network Operators [82] Ocean Harvesting Website (www.oceanharvesting.com) [83] A. Binder and T. Schneider, “Permanent magnet synchronous generators for regenerative energy conversion – a survey,” in Proc. EPE’05, Dresden, Germany, Sep. 2005. [84] “The Onshore and Offshore Wind Power Generation Market Outlook”. 2010. Business Insights. [85] 2011 Technology Map of the European Strategic Energy Technology Plan (SET-Plan). JRC Scientific and Technical Reports. [86] J. Frauenhofer et al. “Basic Concepts, Status, Opportunities, and Challenges of Electrical Machines using High-Temperature Superconducting (HTS) Windings,” Journal of Physics: Conference Series 97 (2008) 012189. [87] J. Pyrhönen, T. Jokinen and Hrabovcová, V. ,”Design of Rotating Electrical Machines”, John Wiley & Sons (2008), Ltd, Chichester, UK. doi: 10.1002/9780470740095. [88] D.A. Staton, S. J. Pickering, D. Lampard,“Recent Advancement in the Thermal Design of Electric Motors,” SMMA 2001 Fall Technical Conference “Emerging Technologies for Electric Motion Industry”, Durham, North Carolina, USA, 3-5 Oct, 2001. [89] T. Judendorfer, J. Fletcher, N. Hassanain, M. Mueller and M. Muhr, “Challenges to Machine Windings used in Electrical Generators in Wave and Tidal Power Plants,” IEEE Conference on Electrical Insulation and Dielectric Phenomena. CEIDP '09, pp. 238 – 241, 2009. [90] J. Fletcher, T. Judendorfer, M. Mueller, N. Hassanain and M. Muhr, "Electrical issues associated with seawater immersed windings in electrical generators for wave- and tidal current-driven power generation," Renewable Power Generation, IET, vol. 3; 3, pp. 254-264, 2009. [91] I. Gurrapa, “Corrosion studies related to suitability of permanent magnets for biomedical applications,” Elsevier Materials Characterization 48, pp. 63– 70, 2002. [92] Jamal A. Baroudi, Venkata Dinavahi, Andrew M. Knight, A review of power converter topologies for wind generators, Renewable Energy, Volume 32, Issue 14, November 2007, Pages 2369-2385, ISSN 0960-1481 [93] Khan, J.; Bhuyan, G.; Moshref, A.; Morison, K.; Pease, J.H.; Guerney, J.; "Ocean Wave and Tidal Current Conversion Technologies and their Interaction with Electrical Networks"; 2008 IEEE Power and Energy Society General Meeting – Conversion and Delivery of Electrical Energy in the 21st Century, 8 pages, July 2008 [94] www.cleancurrent.com/technology/design.htm

Rev. 03, 29-Nov-2012 Page 109 of 115 D2.03 Review of Relevant PTO Systems

[95] Shek, J.K.; Dorrel, D.G.; Hsieh, M.; Macpherson, D.E.; Mueller, M.A.; "Reducing bearing wear in induction generators for wave and tidal current energy devices"; IET Conference on Renewable Power Generation, RPG 2011; 6 pages, 2011. [96] Jiang, Y.; Rong, M.F.; Hua, L.Y.; "Variable Speed Constant Frequency Tidal Current Energy Generation and Control Strategy for Maximum Power Point Tracking and Grid Connection"; International Conference on Sustainable Power Generation and Supply, SUPERGEN 2009, 6 pages, Apr. 2009 [97] Molinas, M.; Skjervheim, O.; Andreasen, P.; Undeland, T.; Hals, J.; Moan, T.; Sorby, B.; , "Power electronics as grid interface for actively controlled wave energy converters," Clean Electrical Power, 2007. ICCEP '07. International Conference on , vol., no., pp.188-195, 21-23 May 2007 [98] Tedeschi, E.; Molinas, M.; "Tunable Control Strategy for Wave Energy Converters With Limited Power Takeoff Tating", IEEE Transactions on Industrial Electronics, vol.59, no.10, pp.3838-3846, Oct. 2012 [99] Beaudin M. et al: Energy storage for mitigation the variability of renewable electricity sources: An updated review. Elsevier. Energy for Sustainable Development 12. 2010. pp 302-314. [100] http://www.all-natural-energy.com/pumped-hydro.htm [101] Chen H. et al: Progress in electrical energy storage system: A critical review. Progress in Natural Science 19. 2009. pp 291-312. [102] Alanen R. et al: EERA Joint programme on Smart Grids – Sub-Programme 4: Electrical Storage Technologies: D4.1 Electrical Energy Storage Technology Review. 2011. [103] 2011 Update of the Technology Map for the SET-Plan. 16. Electricity Storage in the Power Sector. pp 133- 146. [104] Chalk, Miller, Key challenges and recent progress in batteries, fuel cells, and hydrogen storage for clean energy systems, Journal of Power Sources 159 (2006) 73–80 [105] Eichlseder, H., & Klell, M. (2010). Wasserstoff in der Fahrzeugtechnik. Springer Fachmedien GmbH, Wiesbaden, Germany. [106] Ulleberg et al, The wind/hydrogen demonstration system at Utsira in Norway: Evaluation of system performance using operational data and updated hydrogen energy system modeling tools, International Journal of Hydrogen Energy 35 (2010) p.1841–1852 [107] Ulleberg, Modeling of advanced alkaline electrolyzers: a system simulation approach, International Journal of Hydrogen Energy 28 (2003) 21 – 33 [108] Crotogino, F., & Hamelmann, R. (2008). Wasserstoff-Speicherung in Salzkavernen zur Glättung des Windstromangebots. KBB Underground Technologies GmbH, Fachhochschule Lübeck. [109] Deutscher Verein des Gas und Wasserfaches e.V. Regelwerk G260 “Gasbeschaffenheit” [110] EPRI-DOE Handbook of Energy Storage for Transmission & Distribution Applications, EPRI, Palo Alto, CA, and the U.S. Department of Energy, Washington, DC: 2003. 1001834. [111] http://climatetechwiki.org/technology/jiqweb-caes [112] Jossen, A., & Weydanz, W. et al (2006). Moderne Akkumulatoren richtig einsetzen. Reichardt editions, Untermeitingen, Germany. [113] Reddy T.B et al: Linden's handbook of batteries, Fourth edition. McGraw Hill. ISBN 978-0-07-162421-3. 2012. [114] Previous information of AFS Trinity Power Corporation web page: www.afstrinitypower.com [115] Hassenzahl W.V.: More application of superconductivity to electric power systems. IEEE Power Engineering Review, June 2000. pp 4-6. [116] Kötz R., Carlen M.: Principles and applications of electrochemical capacitors. Pergamon. Electrochimica Acta 45, 2000. pp2483-2498.

Rev. 03, 29-Nov-2012 Page 110 of 115 D2.03 Review of Relevant PTO Systems

[117] Brandhorst H.W., Chen Z.: Achieving a high pulse power system through engineering the battery-capacitor combination. IEEE The sixteenth annual battery conference. 2001. pp.153-156. [118] Murray D.B. et al: Applications of Supercapacitor Energy Storage for a Wave Energy Converter System. EWTEC 2009. 8th European Wave and Tidal Conference. 2009. pp. 786-795. [119] http://www.maxwell.com/products/ultracapacitors/industries/wind [120] Giese R F: Superconducting energy systems. Argonne National Laboratory. Argonne, USA, 1994. [121] Giese R F: Progress toward high-temperature superconducting magnetic energy storage (SMES) Systems Systems – A second look. Argonne National Laboratorie, Argonne, USA. 1998. [122] Taylor P. et al: Pathways for Energy Storage in the UK. Centre for Low Carbon Futures. 2012. http://www.lowcarbonfutures.org/projects/energy-systems/energy-storage, Factsheet available for download: Superconducting magnetic energy storage. [123] Ali M.H.: An overview of the SMES application in power and energy system. IEEE Trans. on sustainable energy, vol 1, no 1, 2010. pp 38-47. [124] http://www.intechopen.com/books/dynamic-modelling/dynamic-modelling-and-control-design-of- advanced-energy-storage-for-power-system-applications [125] Previous information from the web site http://www.amsc.com/ [126] Luongo C A: Superconducting storage systems: An overview. IEEE Transactions on Magnetics, vol 32, no 4, 1996, pp 2214-2223. [127] Ribeiro P F: SMES for enhanced flexibility of FACTS devices. Presentation on IEEE panel session on FACTS controllers and operational experience, T&D and substations committee, Canada, July 1999. [128] Torre W V, Eckroad S: Improving power delivery through the application of superconducting magnetic energy storage (SMES). Presentation on IEEE power engineering society meeting, January 2001. [129] EWEA, "Wind Energy – The Facts" (chapter 3, Figure 3.25), 2009, available at http://www.wind-energy-the- facts.org. (Accessed 2012-07-04) [130] H2Ocean FP7 project website: www.h2ocean-project.eu [131] Batterien und Speicher, Eigenschaften relevanter Speichertechnologien, Bedarf, Status und Trends, Schledde, D., et al, Kieler Branchenfokus Windindustrie, Kiel, 2012 [132] http://www.openhydro.com/techOCT.html [133] Patent WO2009074347 (A1) - A HYDROELECTRIC TURBINE GENERATOR COMPONENT, http://worldwide.espacenet.com/publicationDetails/biblio?FT=D&date=20090618&DB=EPODOC&locale=en _EP&CC=WO&NR=2009074347A1&KC=A1&ND=5 [134] Patent WO2008014584 (A1) - AXIAL AIR GAP MACHINE HAVING STATOR AND ROTOR DISCS FORMED OF MULTIPLE DETACHABLE SEGMENTS, http://worldwide.espacenet.com/publicationDetails/biblio?FT=D&date=20080207&DB=EPODOC&locale=en _EP&CC=WO&NR=2008014584A1&KC=A1&ND=5 [135] Barbour, E., Bryden, I G., Energy storage in association with tidal current generation systems, Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy, 2011 225:443 [136] The Limpet Wave Power Project – The First Years of Operation, T Whittaker, Queens University Belfast; W Beattie, Queens University Belfast; M Folley, Queens University Belfast; C Boake, Queens University Belfast; A Wright, Queens University Belfast; M Osterried, Queens University Belfast; T Heath Wavegen Ltd [137] The Development of Oyster – A Shallow Water Surging Wave Energy Converter, Trevor Whittaker, Aquamarine Power; David Collier, Aquamarine Power; Matt Foley, Queens University Belfast; Max Osterried, Queens University Belfast; Alan Henry, Queens University Belfast; Michael Crowley, Aquamarine Power [138] “Power Take-Off Case Studies for Marine Renewables”, Paul McKeever, Narec [139] AW Energy Website (www.aw-energy.com)

Rev. 03, 29-Nov-2012 Page 111 of 115 D2.03 Review of Relevant PTO Systems

[140] SurfPower Website (www.surfpower.ca) [141] Current and Novel Electrical Generator Technology for Wave Energy Converters, M A Mueller, University of Edinburgh; Henk Polinder, Delft University of Technology; Nick Baker, University of Lancaster [142] Snapper Website (http://www.snapperfp7.eu) [143] “Snapper”: an efficient and compact direct electric power take-off device for wave energy converters – Prof E Spooner, Durham University UK; Dr J Grimwade, Narec UK [144] “Snapper” Wave Energy Capture – The Mechanical Challenge – Paul McKeever, Narec UK; Dr Chong Ng, Narec UK; Mark Taylor, Narec UK; Prof Ed Spooner, EM Renewables UK; Dr Markus Mueller, University of Edinburgh UK; Richard Crozier, University of Edinburgh UK

Rev. 03, 29-Nov-2012 Page 112 of 115