Reciprocating Engine Power Solutions in the Era of Hybrids: Optimizing Integrated Power Plant Design

Author: Erlend Vaktskjold, Technology Specialist, Bergen Engines AS, Rolls-Royce Power Systems Co-author and presenter: Leif-Arne Skarbø, Chief Technology Officer, Bergen Engines AS, Rolls-Royce Power Systems Co-author: R.M. Bhaskar, Director - System Project Delivery, Bergen Engines AS, Rolls-Royce Power Systems

1. Introduction

Predicting the future is a challenging task, but when it comes to power generation mix, one thing seems certain – the penetration of variable renewable energy sources will continue to grow over time as predicted in many energy outlook reports, most recently DNV-GL’s Energy Transition Outlook 2018.

Figure 1. Historical and predicted world electricity generation by power type. Note the remarkable overall growth in electricity energy demand, and the growth in wind and solar PV: only these technologies show a significant increase in 2050 compared to 2016.

While many technologies exist for harvesting energy from the atmosphere, from running rivers, from the sea and from ground heat, wind mills and solar cells are dominating the currently seen growth in installed power. In many markets, energy from onshore wind and solar cells is competitive without any form of subsidy. As costs continue to fall, these renewable energy sources will become more competitive, in more and more markets. This development will enable not only reduction in cost of electrical energy, but also a reduction in fossil fuel usage, even in DNV-GL’s forecast of worldwide power demand increasing by 160% within 2050 [1].

Rolls-Royce sees this as both a natural and necessary development for the world’s economy: lower cost of energy with less impact on the natural environment is generally good for business. We recognise the need to drastically reduce the emission of climate gases, with the ultimate goal of zero net emissions. However, between now and some happy future with abundance in renewable energy supply and storage solutions sufficiently large to enable 100% renewable despatch-on-demand power, there will be a power mix consisting of mainly thermal, nuclear, hydro, wind and solar photo-voltaic (PV). In this paper we address the role of flexible thermal power plants in the era of de-carbonization. In particular, we shall discuss medium speed reciprocating power plants, and how these can be effectively erected and provide the necessary support for increased penetration of variable renewable energy sources.

Definition of terms

AC Alternating current ancillary services stable transmission of electrical energy require a variety of operations and capabilities beyond generation and transmission lines. Ancillary services is used as a collective term to cover these capacity factor ratio of annually average power production from a power plant to its peak installed power

CHP Combined heat and power – term used to describe a plant where the waste heat energy is used for heating purposes curtailment term used to describe the un-used (actually not produced) energy which could be produced if there was a simultaneous demand dispatchable term used for power plants capable of controlled dispatch, i.e. deliver power to the grid whenever requested genset Short for generating set, in this context an assembly consisting of reciprocating engine driving a synchronous electrical machine for the purpose of generating electrical power kWe Unit of active power, with emphasis that the measure is electrical power. Used here to distinguish the electrical power output from a genset from the mechanical output from the engine.

LTSA Long term service agreement – a business model for providing service

PV Shorthand for solar photo-voltaic generation of electricity. Another common terms are solar cells or solar panels

PWh petawatt-hour – unit of energy 1015Wh. 10PWh = 10.000TWh = 0.86Mtoe (million tonnes of oil equivalents)

RoCoF Rate of change of frequency [Hz/s] – number indicating how rapid grid frequency is changing

SC Short circuit

VAr Unit for reactive power. Used as shorthand for reactive power

VRE Variable Renewable Energy – any form of renewable energy resource which in its nature has a variable power output. Typical examples are solar PV and wind turbines whose power output depend on local and immediate weather conditions

2. Renewable Power Problem

Ever deepening penetration of variable renewable energy (VRE) resources in a power grid is presently introducing grid stability problems. While this is well described in literature (see e.g.[2] and [3]), a brief summary is provided here for completeness.

2.1. Power Balance

The first part of the problem relates to frequency stability. Frequency is a global variable, common across an entire interconnected grid. Any imbalance in demand and generation power will result in a change of frequency. This follows directly from Newton’s second law, relating the sum of torque Σ푇 applied to a rotating mass with inertia 퐽 and angular velocity 휔̇

∑ 푇 = 퐽휔̇ (1)

Traditional power generation technologies use electrical machines who directly relate the speed of rotation 휔̇ of the machines to the frequency 푓 of the oscillating voltage and current on the AC grid. The same holds for a large number of power consumers. Thus in the above equation, the inertia of all connected machines, both generators and consumers, can be added to represent a total mass inertia 퐽 of machines rotating synchronous with the grid frequency. To see how the grid frequency responds to imbalance in generation and demand, equation 1 is rearranged as follows

휔 1 ∑ 푇 1 푃푔푒푛푒푟푎푡푖표푛 − 푃푑푒푚푎푛푑 = ∫ 푑푡 ≈ ∫ 푑푡 (2) 휔푏푎푠푒 2퐻 푇푏푎푠푒 2퐻 푆푏푎푠푒

2 퐽휔푏푎푠푒 where the inertia constant 퐻 of the grid is defined by 퐻 = and 푆푏푎푠푒 is the sum of the base power 2푆푏푎푠푒 (normally the VA rating) of the connected generator capacity [4]. We can see directly the role of the inertia constant. Any power imbalance need to be corrected to maintain a stable frequency, and the time available to react depends on the inertia constant.

Imbalances arise continuously due to changes in demand, and occasional trips in generation or transmission. Demand changes can reasonably well be predicted days and hours in advance, based on past experience. Thus, traditional generation control is based on ahead planning of predicted load, with minor generation capacities reserved for primary control responding immediately to frequency deviation, and secondary control reserves to restore frequency to nominal value and also restore primary control reserve. Tertiary control reserves are then activated to restore the secondary control reserves. Figure 2 show typical time scales for application and withdrawal of generation under primary, secondary and tertiary control.

Figure 2. Idealised power-time chart indicating the activation time scales for primary, secondary and tertiary control reserves. Also shown is inertia providing instant resistance to frequency change.

Large synchronous areas, with many plants and multiple interconnections, can operate with very stable frequency with as little as 0.5% primary control reserve. This value increase with smaller synchronous area, but also with increased the relative size of the larges plant, or largest transmission segment, in order to sustain an outage of these large units..

Introducing intermittent renewable generation will result in additional imbalance, as error in predicted sun and wind adds to the error in demand prediction. As a result, more primary, and secondary control reserves must be available. While good weather forecasting is necessary to reduce the uncertainty to a minimum, deep VRE penetration requires substantial tertiary control reserves available for those windless, but cloudy, days.

2.2. Inertia

Closely related to the power balance is the role of inertia. As shown in equation (2) above, the total mass inertia of machines rotating synchronous with the grid frequency directly determine the rate-of- change-of-frequency (RoCoF) and peak frequency deviation resulting from instantaneous and the time- integral of power imbalance respectively. More rotating mass means a stiffer grid in terms of its frequency. While conventional power generation are tied to the grid through synchronous generators, PV generation has no rotating mass, and most wind turbines are de-coupled from grid frequency through power-electronic interfaces. Thus deep VRE penetration not only increase unpredictable and predictable power imbalance, but also reduce the inertia constant of the grid. For this reason, the concept of virtual inertia may become indispensable. An increasing amount of study and practical implementation of virtual inertia can be found in literature, e.g. [5]

2.3. Voltage stability

While frequency is a global variable within a synchronous area, voltage is a local variable –it’s relative value between 3-phases, voltage with respect to earth, and phase angles relative to phase currents. Conventional generators with synchronous machines have the capacity drive reactive loads, and to instantly and automatically generate large currents in case of low voltage event such, as a short circuit: The peak current may rise to 8 times the rated current. These machines can sustain such high currents for several seconds without sustaining any damage owing to the thermal capacity and low electrical resistance of copper windings. High currents flowing into a fault are reliably measured and is used to isolate the fault, typically within a fraction of a second.

VRE is typically connected through power electronic interfaces, which, unless amply dimensioned, have limited capability to provide reactive power and short circuit currents: Due to the properties of semi- conducting materials, and the small volume where the current switching takes place, ratio of maximum short circuit current to rated current capacity can be as low as 1.0. As a result, cost effective VRE solutions are not sufficient to maintain voltage stability, particularly under transmission/distribution fault conditions. Depending on the grid topology and protection schemes, voltage stability requirements may result in a technical limit to VRE penetration.

2.4. Renewable power support

In this section we have addressed some technical challenges with high penetration of VRE. To summarise, increased VRE penetration will demand  More tertiary capacity – when the wind isn’t blowing and the sun is not shining  More primary capacity – to balance the unpredictable part of VRE  More secondary capacity – to restore primary capacity  More inertia: spinning machines or virtual inertia  More reactive power and short circuit capacity

Today, 28% annual penetration of VRE on the island of Ireland already represents technical challenges with respect to grid stability. Here, the so-called System Non-Synchronous Penetration (SNSP), a real- time ratio of non-synchronous power generation to total demand, is limited to 65% [6]. Other countries, such as Germany and Denmark have reported periods of time where the entire national electricity demand is met by renewables. This, however, has been possible only because both Denmark and Germany is part of the continental European synchronous area. VRE penetration within this area is still relatively low. Thus we see here a significant gap between possible VRE penetration today, and the situation emerging in the late 2020s and beyond. In [7], simulation of the predicted power mix during a sunny and windy week has been presented, see figure 3 below.

Figure 3. Generation and demand profiles predicted for Germany in 2035. A high renewable week is shown. Considering only power balance and least cost of energy, complete shutdown of all thermal plants is predicted during daytime. Wind and solar alone is able to produce more than 200% of the demand during daytime, but still dispatchable power plants are activated every night. In Bloomberg’s simulation, batteries start to play a role in the power spinning multiple hours.

While the simulation is performed for Germany, it represents a situation which must be both possible, and common, should the forecast for 2050 in figure 1 above hold: at times, 100% power within whole synchronous areas are based on VRE. This is a technical challenge yet to find its solution on a large scale, in particular relating to the inertia and short circuit capacity. The large amount of curtailment, however, may come to use, either by driving dedicated flywheels, or providing virtual inertia and short circuit reserves. In any case, today’s practice of VRE plants operating at the maximum power point cannot be used exclusively: active control in relation to grid frequency and voltage must apply also to VRE plants in this scenario.

It is also important to consider how the situation looks during a low renewable week, see figure 4. During a low renewable week, substantial capacity of dispatchable generation must be activated to balance the demand. With currently available technologies, dispatchable here will mean thermal plants running on fossil fuels.

Figure 4. Generation and demand profiles predicted for Germany in 2035. A low renewable week is shown. Considerable power and energy from dispatch able power plant is required.

Finally, it is important to note that the technologies deployed to support VRE should expect to run at low capacity factors. Thus, the operational cost of thermal plants will no longer be dominated by fuel cost and rated power efficiency. Investment, stand-by cost and part-load fuel efficiency will become increasingly important. In the next sections we will show how medium speed reciprocating power plants are ideal for supporting renewable power in a cost- and energy effective manner.

3. Reciprocating engine technology

Reciprocating internal combustion engines are available across a very wide range of size and power. Rolls-Royce Power Systems design and manufacture 4-stroke generating sets for electrical power generation in the range 27-3.250kWe using high speed engines as prime movers (1500-1800rpm), and in the range 1.400-11.700kWe using medium speed engines (720-1000rpm). Engines are available for running on natural gas, diesel, residual fuel oil, and a range of biogas and other fuels on request. The applications in power generation range from small industrial stand-by diesel generators, continuous operation isolated power plants (including marine), combined heat and power, and dedicated grid- connected power plants. Grid parallel operation range from baseload to peaking/scheduled load following (tertiary control) and backup with fast start-up (secondary control). Primary control is (still) rarely used in traditional generation mix due to the relative small power rating of reciprocating engine power plants to the larger turbine based plants.

The range of size and applications of reciprocating engines extends of course much further if other manufacturers’ products are considered, and smaller engines (e.g. cars and tools) and larger engines (2- stroke marine) are also included. This range is a testimony of the versatility and flexibility offered by the reciprocating engine technology. In this section we will discuss some of the features of medium speed engines relevant for power plants in the 20 MW to 1 GW range.

3.1. Steady state performance

The lean-burn spark ignited port-fuel-injection gas engine concept has been developed by Rolls-Royce Power Systems Company Bergen-Engines AS since 1984 [8]. Much like a diesel engine, no throttling means low pumping losses and high cycle efficiency at all power levels. Mechanical friction, fairly constant at fixed speed, becomes the main source of variation in the net fuel efficiency of this type of engine. Turbocharging means high power density with little additional friction. The result is electrical efficiency up to 50% and a flat fuel consumption curve, see figure 5 below.

50

40

30

20

Electrical Electrical ISO efficiency fuel [%] 10

0 0 20 40 60 80 100 Electrical power output [%]

Figure 5. Typical electrical efficiency with fuel consumption according to according to ISO 3046-1 (ICFN) and ISO 8528-1. This curve is taken from a generating set with a medium speed, lean-burn, spark ignited, port-fuel-injection type engine. A theoretical efficiency curve assuming constant cycle efficiency and constant friction is shown dotted for comparison.

The waste heat from the engines can further be utilised: combined heat and power (CHP) is used extensively in district heating applications, or industrial applications, where heat is used locally and the electrical power is either partly locally, partly exported to the grid. Typical distribution of available heat in exhaust, cooling and radiation is shown in figure 6 below.

100% 90%

80% 70% 60% LT heat (~50C) 50% Radiation 40% Lube oil heat (~75C) 30% HT heat (fixed 90C) %of energy fuel input 20% 380C 425C 455C Exhaust heat 10% 455C 0% 10% 15% 25% 35% 50% 60% 75% 85% 100% Electrical power output [%]

Figure 6. Heat dissipation as % of fuel energy input. The available outlet temperature is shown. In CHP plants, all sources but radiation is utilised giving total efficiency above 90%. For power plants, the high temperature available in the exhaust gases is used in many applications to raise steam.

3.2. Transient performance

Bergen Engines lean burn gas engines and diesel engines alike is designed to run with air excess ratio around 2.0 and above. This means that from any operating point, combustion is possible with up to twice the amount of fuel. The port fuel injection system makes is possible to increase and decrease fuel amount on gas engines similar to diesel engines. The result is that both types of engines have excellent capability to meet step load increases, essential for island mode operation. Load rejection, e.g. when disconnecting all load from 100% load, is handled without needing to stop the engine, securing continuous operation. See figure 7 for an example of load steps applied to an isolated generator, with associated frequency deviation and recovery.

Figure 7. Frequency deviation and recovery during step load performance testing. Typical limiting values for frequency deviation are shown in dotted red. Overs-peed trip limit is shown solid red. These curves are taken from a marine diesel genset.

In operation parallel to a power grid of some size, the notion of a “step load” is not so relevant. Here, transient performance relates to the ability to follow a load set point, to perform primary regulation and to start from standby and ramp up load in secondary regulation. In particular, duty as secondary control reserve sets the most demanding requirements for transient performance: In this mode of operation, the engine is preheated by means of jacket water circulation whilst in standby, ready to start when called upon. Then, air starters crank the engine before fuel is added, accelerating up to rated speed. Excitation of generator starts during the acceleration, allowing synchronisation toward grid frequency and phase to take place immediately. When the circuit breaker is closed, the load is increased following the agreed trajectory. During this phase, turbocharger acceleration is often the limiting factor, and fuel and ignition must be carefully balanced to manage the load increase without thermal overload. Start times of 3 and 5 minutes from start command to rated power output have been committed to, using the Bergen B35:40V20AG2 genset.

100 1000

80 800

60 600 Power Demand Power output 40 400

Poweroutput [%] Engine Speed Gensetspeed [rpm]

20 200

0 0 -25 0 25 50 75 100 125 150 175 200 225 250 275 300 Time from start request [s]

Figure 8. A typical start sequence for a secondary control reserve application. In this example, the genset is online 75s after start request, and full power is achieved after 235s. Much faster starting times are possible, but this requires that every part of the sequence is optimised., reducing each element’s mean time and variation.

Another operation which may at first glance appear demanding is load following/peak shaving. In such plants, the total load is ramped up and down to meet grid demand profiles. An example of such a plant’s weekly profile is shown below. In this case, the individual unit power output is in the 80-100% range where the fuel efficiency is the highest. Also, operating the plant this way, the engines are accumulating less running hours, i.e. deferred maintenance. Consequently, the plant operator can meet the dispatch demand with fuel and maintenance cost equivalent to running significantly higher capacity ratio.

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20 Plant Plant poweroutput [%] 0 0 1 2 3 4 5 6 7 Days of operation Figure 9. A week’s operation of a multi-unit peak shaving plant. The capacity ratio this particular week, i.e. the average power output on this chart, was 63%.

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20 Gensetpower [%]output 0 0 1 2 3 4 5 6 7 Days of operation Figure 10. Same plant and week as in figure 9 above, showing the load profile of an individual genset. Note that the individual unit is shut down on a daily basis. The average power whilst running was 93%.

3.3. Maintenance and availability

We have demonstrated above how a medium speed reciprocating engine is capable of quick and frequent load changes. It should however be noted that every start and stop represents a thermal cycle. Even if the engine is pre-heated in standby, material around the combustion chamber will see one load cycle every time. In particular, the cylinder head is an exposed component. For Bergen Engines, however, this represents no challenge. The design has proven itself on decades of operation on ships where daily start and stops, and frequent load changes, is the norm rather than the exception. Also the new engine types are based on this design and experience, and in fact, the exact same engine components are used extensively in ship propulsion.

When it comes to wear and maintenance intervals, one of the major difference between reciprocating engines and turbomachinery is related to the sliding movement of pistons with respect to cylinder lines, and operation of air and exhaust valves. For liquid fuelled engines, fuel injection equipment have sliding movement interfaces, and nozzle wear. Spark ignited gas engines will wear out spark plugs. All this lead to certain maintenance intervals, different for different engines, and for different duty. On Bergen medium speed engines running full power, guaranteed availability is 98.5%, taking into account both scheduled and unscheduled maintenance. Until the first major maintenance point at 15.000 running hours, work is completed within a single day shift, without needing additional cool-down period. Major maintenance is performed on the engine where it stands, and takes typically 2 weeks for a V20 engine. Taking advantage of Bergen’s exchange pool, pre-conditioned subassemblies can be put straight onto the engine, reducing the downtime to one week. The exchanged parts are then taken away for overhaul in a dedicated workshop.

Long term service agreement (LTSA) is offered to secure for best performance and predicable maintenance costs [9].

4. Power plant layout and installation

A medium speed power plant can be built anywhere, and tailored to meet the specific requirements in every case due to the versatile building blocks. Bergen Engines offers modular but flexible power plant arrangements developed over many years based on experience from a vast range of operating conditions from very low to very high ambient temperatures (-50C to +50C), and from very dry to tropical conditions.

Options are available from standard to customised layouts, with and without ducted air systems for combustion air, depending on site conditions. Exhaust systems are optimised for low back pressure in order to maximise fuel efficiency. Bespoke design options are offered for specific local conditions or customer requirements. Special consideration is given to requirements for maintenance activities throughout the lifetime of the plant.

In this section, we present a configuration consisting of 12 generating sets of type B36:45V20AG, with a total capacity of 140MW in open cycle option and 150 MW in combined cycle option. The plant then includes steam generation and steam turbines. Full power to grid can be achieved within 5 minutes in open cycle mode and power from secondary cycle, i.e. from the steam turbines, can be added within 60 minutes.

Figure 11. Power plant layout with twelve B36:45V20PG generating sets with secondary cycle option using superheated exhaust gas boilers and steam generators. The plant include all required equipment from gas conditioning to HV switchyard, and is self-sustained with necessary maintenance access, warehouse, workshops and office areas are included within the perimeter. Approximately 16.100m2 ground area is needed for the power plant, 23.150m2 including secondary cycle option with exhaust gas boilers, steam turbines and HV switch yard. Area requirement can be further optimized based on customer requirement.

Figure 12. Section through the genset building showing the internal layout with combustion air intake and exhaust to the left. Main ventilation with main inlet from the right and outlet through the roof top is carefully designed to ensure adequate cooling of all equipment inside the building. Pipe connections to the engine is on the left, leading directly toward the radiator coolers on the far left. The generator itself is on the right, securing the coldest ventilation air and short cable stretch to the switchboard building on the far right.

4.1. Ease of Construction

The power plant design include single to multi-engine installations using standard layouts. This allows that auxiliary equipment, so-called balance of plant, to be completed prior to the arrival of gensets at site. This enables simple construction as well as reduce overall construction period of projects.

Figure 13. Bird’s perspective of the power plant The power plant concept can is further easily scalable in terms of varying sizes of genset, from single to multi engines plants, and covering a wide range of scope from basic genset supply up to a full package including, for example, exhaust system (with options for boilers and SCR units), combustion air system, low temperature and high temperature cooling systems, station control, fuel systems and ventilation, depending on customer preferences. As an additional advantage, the modular design of power plant allows for future expansion.

Figure 14. Configuration for a 360MW power plant. Using the same sets of building blocks as shown here, a power plant can be configured anywhere in the range 20MW – 1GW. Note how the modularization allow for a stepwise construction.

A typical project of 360MW Plant can be completed within 12 to 16 months from contract signing to plant hand-over depends on plant location. This is achieved due to implementation of modular concept.

The above plant with multiple identical Modules. Each Module Consist of self-contained design with multiple B36:45V20AG Gas engines and all auxiliary equipment.

The example shows 6 blocks of 60 MW each with a total plant capacity of 360 MW. A block of 60 MW each with 6 x B36:45V20AG Gas engines are built with modularization concept. This can be easily expanded to e.g. 720 MW with 12 such blocks arranged along with all other related equipment. All the blocks are identical to each other. This is easy to install and easy to expand based on customer’s need.

5. Hybrid power plant

Hybrid, according to the Oxford dictionary defines something “of mixed character; composed of different elements”. In a hybrid power plant, power is generated from different sources. Here we choose to define that the different sources include at least some form of VRE resources and some form of thermal units. Energy storage may or may not be included. Electrical energy is flowing unidirectional into the grid through a point of common coupling. In this respect, a hybrid power plant may seem similar to a micro grid, but there are some distinct differences: 1. Own consumption behind the meter is limited to auxiliary equipment required to run the power plant 2. As a consequence, energy flow is mainly unidirectional into the grid.

As for a microgrid, operation of VRE, thermal units and storage is synchronised by a local controller such that the plant can be considered as a single entity when viewed from the grid.

5.1. Case study: A Gas-PV hybrid power plant

As a starting point, we will consider a gas/PV hybrid power plant and a simplified dispatch profile with 100MW delivered 24/7, with +/-5MW reserved for primary control, or so-called frequency dependent load regulation. Based on weather data for a particular geolocation, hourly average irradiation and temperature is given through a full year. From this data series, PV and thermal generation has been simulated. The power plant consist of generating sets of type B36:45V20AG, each delivering 11.7MW electrical power. In this case, 12 units are installed to allow spare capacity for scheduled maintenance during plant operation, and one backup unit for unscheduled maintenance The solar plant has single- axis tracking system. Own power consumption of the plant is modelled based on the generation mix and cooling demand, taking ambient temperature into account.

In figure 15, power profiles over two weeks for a particular configuration is shown. In addition, fuel consumption and other performance metrics are calculated for a full year’s operation with this configuration. A range of different size PV plants are simulated to see how different configurations will perform. The results are presented in figure 17 below. We see that until 150MWp installed PV, all solar power is dispatched, and annual fuel consumption drop steadily until below 70% of the fuel consumption with no PV installation. With further increased PV capacity, curtailment starts to take place, and the reduction in fuel consumption for every added PV panel is not so steep. 120 100 80 60 40 20 0

Generatedpower[MW] 01.mar 08.mar 15.mar

Genset power PV power PV Curtailment

Figure 15. Two weeks of gas/PV hybrid power plant operation simulated with PV capacity of 160MWp. The internally generated AC power shown is around 100-103MW to deliver fixed 100MW to the grid. We can see some PV curtailment taking place on sunny days.

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Figure 16. Genset operation through the two weeks operation shown in figure 15 above. The units are started and stopped according to PV generation. Note how all units are stopped for a period of time around noon on most days, while other days, most units are running through the day.

100 500 90 450 PV capacity factor 80 400 PV curtailment factor

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% 50 250 factor 40 200 # of # starts Avgerage unit power 30 150 20 100 Fuel consumption 10 50 0 0 Starts per unit 0 50 100 150 200 250 300 Installed PV capacity [MWp]

Figure 17. Annual performance as function of installed PV capacity.

In this section we have shown a simplified simulation of flexible gas generators combined with a PV plant of variable size. The simulation was based on hourly average irradiation values. As we shall see in the next section, short term fluctuations in irradiation may have severe impact on the dispatch accuracy. The gas/PV plant configuration discussed in this section will have further deficits in terms of ancillary services to the grid 1. Inertial response: as the plant cycles between 10 and zero synchronous generators, the inertia of the grid will be influenced 2. Reactive power capacity and short circuit capacity will similarly change as the generation move to and from PV

These potential shortcomings will also be discussed in the next section.

5.2. Gas-PV hybrid power plant with energy storage

As the photo-voltaic effect generate a flow of electrons immediately and proportional to the number of photons penetrating the active material of a solar cell, the DC electrical power from the panels follows directly from the sun intensity, with negligible time delay or smoothing. Without any additional energy storage, also the AC power from inverters will follow the sun, so to speak. This pose challenges to a hybrid power plant whose target is to form a reliable and stable power source. In figure 8 below, the irradiation onto a single point sensor is compared to that onto a large field 750m x 750m.

Figure 18. Example of single point and field average solar irradiation over 1500 seconds. The field irradiation is estimated using 19 single-point measurements scattered over an area covering approximately 750m x 750m. The extreme rapid fluctuations on the single point is somewhat smoothened out over the entire field. Data is from [10 ], while diagrams are from [11]. As an example within this example, the 100% step in point irradiation is reduced to a 60% ramp over 50s.

We see that size matters – the larger the field, the slower the fluctuation. Note however that the amplitude of the largest fluctuations is not reduced with increasing size: A transition from clear sky to cloudy will, some days, eventually be effective over entire fields, countries and synchronous areas.

A simple example can be useful to explain this: Consider a large dense cloud moving with 30m/s on an otherwise clear day. On a single point, irradiation will drop from high to low during the time it takes for the cloud edge to cover the sun, i.e. in a few seconds. The output power from a field of PV panels, however, will gradually drop over the time it takes the cloud to cast its shadow over the entire field. Considering the example in the section above with 160MWp installed over a square 220Ha area. The shortest length of the field is then approximately 1500m. In such a case, the sudden step in point irradiation will result in a ramp in power output over 1500m / 30m/s = 50s. Consider further that all gas generators was stopped during the sunny hours before the cloud’s appearance. Starting the engines and ramping up the load from this point would leave a significant drop in the combined production profile as shown in figure 23. In this example, a capacity of 55MW would be required, while the energy storage to fill this gap is (only) 0.8MWh + losses.

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PV power

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0 -10 10 30 50 70 90 Seconds Figure 19. Hypothetical power profile during a complete shading of a 160MWp solar field 1500m x 1500m with flexible gas generators, in this case online 30 seconds after initial reduction in PV output, ramping up to balance the load after further 70 seconds. Required power profile from energy storage is also shown.

The example illustrate some important features for a hybrid power plant 1. Size matters – speed of fluctuations depend on PV field area (in addition to cloud shape, density and speed) 2. Size alone will not smooth PV power variation amplitude, i.e. significant power capacity from energy storage system is required to secure stable supply 3. Fast activation time of gas generators can limit the required power requirement from the energy storage system 4. The energy requirement may be surprisingly small when combined with flexible gas generators.

Table 2 list a set of configurations which can be considered for PV-gas hybrid power plant. Table 2 – Some configurations for a 100MW PV-gas hybrid power plant Case Configuration Comments on power Ancillary services 0 Reference case: Will usually meet hourly power No inertial response during 160MWp installed PV. Stop dispatch, but with significant sunshine hours; limited SC and engines when possible. underproduction with suddenly VAr capacity No energy storage appearing clouds 1 160MWp PV. Increases annual fuel Full inertial response; full SC Run 10 gas generators consumption by 15% compared and VAr capacity 24/7, but with variable load to reference case. Gas down to 0% depending on generators must be able to PV production. follow PV production. Rare No energy storage events of under production 2 160MWp PV Typical discharge time of 60s Virtual inertia response Energy storage with fits the rapid shading scenario, possible with energy from supercapacitors but can only balance one or energy storage. two such events. Frequent start SC and VAr capacity increase and stop of gas generators will with additional DC/AC be required. Fuel consumption inverters will increase

3 160MWp PV Typical discharge time of 30 Virtual inertia response with Energy storage with minutes is sufficient to balance energy from energy storage. Batteries most short term variation in PV SC and VAr capacity increase power. Simulation with hourly with additional DC/AC irradiation will be valid with inverters some further reduction in fuel consumption due to less curtailment.

For cases 2 and 3, the questions are further: what power rating should the energy storage system have, and how much energy must be stored? To perform a proper design meeting power profile with minimum fuel consumption, a full-year simulation based on high temporal resolution (~1Hz) of an entire solar field production would be required. We have seen that both weather data, giving hourly average values, as well as high temporal resolution point measurements, are both inadequate, for different reasons.

An indirect approach to this sizing problem is to consider the requirements for ancillary services. We will first consider virtual inertia. In order to simulate the effect of inertia, power to the grid must be quickly and automatically delivered or drawn from the grid depending on the rate-of-change-of frequency. Assuming virtual inertia should be provided until 2.5Hz/s, from equation 2, the change in power would 2.5Hz 푃 be = 2 ∙ 퐻 ∙ s = 10% for virtual inertia 퐻 = 1.0s. The extent of virtual inertial support is in the 푆푏푎푠푒 50Hz order of seconds, so, to summarize, 10% of plant VA rating spare inverter capacity supported by super- capacitors would suffice.

Considering reactive power capability, operation at cos 휙 = 0.8 is common practice, while some grid codes or plants require steady state operation down to cos 휙 = 0.6 and transient further down. Thus, 1/0.8= 125% of nominal active power would be the minimum current rating of inverters.

However, when it comes to short circuit capacity, the actual requirement may be very different depending on network topology and protection scheme. We will not attempt to provide a general guidance here other than that energy requirement is negligible, while the current rating may have to be many times the plant VA rating.

5.3. Gas-PV hybrid power plant with energy storage and spinning machines

The configuration proposed in this section will form a fully despatch-able power plant, capable of unit commitment in terms of inertial response, active power regulation (primary, secondary and tertiary) as well as reactive power and short circuit capacity. The motivation would be to secure dispatchable energy prices while producing much of that with low-cost and low-carbon VRE. Excess VRE could possibly be delivered as grid-following power at a lower price.

In the hybrid plant described above, the most economical option, case 0, have no inertial response and limited VAr and SC capacity during sunshine hours. A slight modification of the genset architecture can eliminate this problem. The alternative configuration shown below allows the engine to be shut down and stopped while the generator and flywheel remains rotating synchronous with the grid. This provides continuous supply of inertial response as well as reactive power and short circuit capacity at minimal additional cost: the power to drive the synchronous machine at no load amount bearing and fan losses, well below 1% of the nominal output. This can further be minimised if the cooling fan is arrange to rotate independently of the generator rotor.

Figure 20. Normal genset architecture for a B36:45V20AG, showing engine mounted separately from the generator on elastic elements.

Figure 21. Alternative configuration with flywheel fitted to the generator shaft. A clutch between the elastic coupling and flywheel enables disconnection of the engine from the generator.

A further advantage of this configuration is reduced start-up time: With reduced inertia on the engine side, acceleration can be achieved considerably faster, with time to reach rated speed in the range 8-12 seconds for diesel engines, and 12-20 seconds for gas engines. Furthermore, time spent to achieve speed and phase synchronisation with the grid can be skipped altogether, as the clutch, requiring no phase matching, is simply engaged to enable a smooth torque transfer between engine and the already synchronous generator.

Meeting all technical requirement for dispatchable power and ancillary services with minimum fuel consumption, this configuration combined with a large PV field and a relatively small battery can indeed be part of ever greener power production.

6. Solutions for micro-grid

According to US Department of Energy, a microgrid is a “group of interconnected loads and distributed energy resources within clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid. A microgrid can connect and disconnect from the grid to enable it to operate in both grid-connected or island-mode”. As such, a microgrid can both be thought of as an extension of the hybrid plant concept discussed above, as well a mini-version of the traditional wide area synchronous grid.

Figure 22. Example of a microgrid setup with its main components: VRE resources, generating sets, energy storage, and controllable load. The microgrid controller controls both generation and consumption through data communication with all the internal devices. Import/export to the grid, in agreement with grid operator is also controlled by the master controller.

In grid-connected mode, the flow of power through the electrical boundary may go in either direction. The difference, however, between a microgrid and any radial section of a traditional grid, is that the grid operator may request particular levels of power and ancillary services through the point of common coupling (PoCC). Consequently, a practical solution is to arrange a single communication link to a central controller within in the microgrid. The central microgrid controller in turn controls both generation and consumption within the grid. The task at hand is to comply with the demand from the grid side, whilst serving the need of the internal consumers and secure dispatch of internal generation. This forms a multi-dimensional real-time optimisation problem, with technical constraints and multiple parties who all would like to see their own commercial profit maximised. Even if this is a hard problem to solve, it is likely to yield better solutions than trying to solve the problem for entire synchronous areas. This way, microgrids can represent a range of solutions to the technical and commercial challenges of large penetration of VRE.

Presently, the main motivation for developing microgrids is to secure continuous supply even when the main grid goes down due to extreme weather or other events. Another motivation is to kick-start electrical supply in rural and fast developing areas, where central grid infrastructure cannot be developed quickly enough. In this case, a working microgrid system can be made operational on a short time-scale, and only later connect to the main grid. In both cases, the microgrid must be capable of providing all power and ancillary services required for continuity of supply and stable operation. In island mode, owing to the reduced inertia and short circuit capacity of the isolated grid, protection devices may need a different set of settings, suitable for the isolated system.

Unscheduled transition from grid-connected mode to island mode may result in a black-out depending on the reason for disconnection, the power flow through the and the on-line capability of the grid generators at the time of disconnection. In such cases, even small size energy storage may be sufficient to bridge the gap in generation until standby generators of the microgrid is up and running. If not, black start generators are required to start and energize the dead microgrid. Reciprocating engine generators are ideal to provide the required flexibility, fast start-up time and dynamic load response to continuously balance the variation in generation and demand in an isolated microgrid.

7. Conclusions

In this paper we have presented a selection of recent energy outlooks. Although magnitude and rate-of- change differ between outlooks, and will certainly differ from the actual future, it seems certain that the electrical power generation mix will contain increasing amounts of variable renewable resources, in particular wind and solar PV. We have discussed how this transition forces other power sources to become ever more flexible, capable of quick starting , rapid load changes, and starting and stopping on a daily basis, or even more often. Maintaining profitability under these conditions with part load operation and low capacity factors will be challenging for traditional power generation technologies.

Generating sets based on medium speed reciprocating engines represent a solution to these challenges. Multiple units arranged in modular power plants in the range 20MW-1GW provide the required flexibility whilst ensuring the economical part load operation.

Next, we move on to a case study of a particular configuration of a gas/PV hybrid plant, looking at power profiles resulting a simulation based on hourly irradiation values. Considering the effect of cloud shading and short term fluctuation in PV production, it is shown that energy storage, e.g. based on batteries, is required to balance these fluctuations. The relative power rating and required energy storage depend mainly on two factors: 1) the start-up time and power ramp rate of the generators 2) the size of the solar field: the larger the field, the longer it will take for a cloud to shadow the entire plant. The case study conclude with a novel configuration in which the generator can be disconnected from the engine by means of a clutch. This enables continuous supply of inertia, reactive power and short circuit current capacity to the grid, as well as significantly reduced start-up time for the engine when active power is needed.

Finally, the concept of microgrids is discussed, and how they may pose multiple answers to the technical and commercial challenges of including ever more variable renewable energy into the power mix. Also in this setting, reciprocating engine generators are ideal to provide the required flexibility, fast start-up time and dynamic load response. References

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