Resource analysis of the Pentland

A. Owen and I.G. Bryden

Abstract

Understanding the complexities of tidal current flows is fundamental to successfully harnessing their energy. The usual picture given by proponents of devices (if it is even mentioned) is of a well behaved, unsteady, uniform flow that reverses every 6.2 hours. This assumes that the flow is constrained by a smooth channel with no significant surface defects to propagate vortices and eddies. This is clearly not the case for real flows where the channel is not straight, usually varies in depth, and has a number of bumps and holes that will disturb the flow[1]. In addition, the flow will usually draw in water from all sides at the channel entry, but then jet into the bay at the channel exit. The tidal current resource is often perceived as being potentially large [2] but the magnitude, and the potentially exploitable proportion, of the energy embodied within a tidal current, is only now beginning to be properly understood. The characteristics of tidal currents may vary considerably over the 12.4 hour flood/ebb cycle, and a turbine installation will need to take this variability into account. It is not possible to develop an indicative value of the exploitable energy simply from knowledge of the harmonic constants or local tide tables, and over- optimistic resource forecasting has resulted from attempts to do so. Tidal currents are bounded, finite systems with no capacity for energy replenishment from other sources, whereas the atmospheric energy source is, from a wind turbine standpoint, almost infinite, being easily replenished from the upper atmosphere or neighbouring weather systems. Cross-application of the wind farm resource model led to tidal turbines offering to extract more energy than existed within the stream. The nature of the flow at the proposed site must be appropriate for extraction, and many sites (particularly the ), exhibit sizeable areas of organised and random vortex activity in ebb and/or flood directions. These large areas of coherent vortices will not provide viable power, at least not with current technology, and must therefore be excluded from resource models. Earlier attempts to quantify the exploitable resource used the wind farm model and forecast a 2025 extractable resource figure of 25TWh/yr [3] for the tidal stream

1 in the Pentland Firth, which is now thought to be capable of providing about 1/5 th of that figure.

Data sources

Tidal data and predictions are readily available for most of the world, particularly areas of regular shipping movements. The UK Hydrographic Office (UKHO) publishes navigational charts and tidal atlases, as does the US equivalent, the National Ocean Survey (NOS). All major UK harbour authorities publish tide tables appropriate to their locality, and many hundreds of smaller ports usually administered (in the UK) by the local authority, will extrapolate their local time- shifted data from these major port sources. The importance of local characteristics in the analysis of tidal currents cannot be overstressed, for example the tide times at Burra Sound in vary by as much as 40 minutes from the published tables for Kirkwall, only 5 miles away. It may be the case that such specific knowledge is not widely published and that local community knowledge must be consulted. Similarly, for particular features of tidal currents, the knowledge and experience of fishermen, ferry operators and pleasure craft pilots will be invaluable in building an accurate picture of vortices, jetting and debris transport within the stream.

The UKHO predicted tidal heights and current velocities are based on mean values with any extreme events removed, and therefore the predictions will not be appropriate for severe weather conditions or anomalistic astrological events. Tidal heights are referenced to LAT (Lowest Astronomical Tide) in the and MLW (Mean Low Water) in the United States. Most tidal current velocities are based on data sets that would be considered inadequate for tidal height prediction [4] and inspection of nautical charts reveals much of the information included thereon can be over 50 years out of date. The availability of data is therefore, often much greater than its continuity or accuracy, and this fact must be borne in mind when modelling the resource. Some readily available on-line sources of tidal data are given in Table 1, below

2 Institution URL Data type

National Tidal and Sea UK tidal predictions, British http://www.pol.ac.uk/ntslf/ Level Facility dependencies tidal data British Oceanographic Tide gauge, bathymetry, http://www.bodc.ac.uk/ Data Centre bottom pressure Permanent service for Sea level data and tidal http://www.pol.ac.uk/psmsl/ mean sea level constants Harbour Authorities http://www.portoftyne.co.uk Tide times, levels, predictions (examples) http://www.aberdeen-harbour.co.uk National Oceanic and Historical data and predictions Atmospheric http://tidesandcurrents.noaa.gov/ for a range of locations. Administration Principally US. University of South Tide predictions for many http://tbone.biol.sc.edu/tide/ Carolina global sites UK Hydrographic Office http://easytide.ukho.gov.uk/EasyTide/ Tides predictor Channel Coastal Tidal and meteorological data http://www.channelcoast.org/ Observatory Canadian Hydrographic http://www.waterlevels- Tides, currents and water Service niveauxdeau.gc.ca levels for hundreds of Canadian sites.

Table 1: Some readily accessible world sources of tidal data.

The essential point to be made here is that any desk study using data extrapolated from harmonic constituents, or aged historical data, is at best a general approximation. The only accurate method of predicting the energy available for exploitation at a given point in a tidal stream is to measure the tidal current behaviour at that point, and to account for external factors such as prevailing meteorological conditions.

Assessment methodologies

The acceptability of pre 21 st century tidal stream resource assessments has diminished over recent years. A report for the Carbon Trust [5] concentrates on post 1990 work on the grounds that earlier works had been superseded or rewritten. It is evident however, that the two principal works [6,7] used as the basis for most work prior to 2003-4, utilised the farm methodology previously employed for wind turbine applications. The farm methodology assumes that an infinite supply of kinetic energy is available and that the power output of an installation is governed by the number of devices deployed and their spatial distribution. This model is acceptable for wind energy since its volume of operation is essentially boundless with the exception of the earth‘s surface on

3 which it stands. For tidal currents, improvements in the understanding of the effects of energy extraction on the flow dynamics of a tidal currents [8], and inclusion of the limitations of a hydraulic channel, indicate that the farm methodology can result in predictions of hyper-extraction, i.e. forecasting a greater energy output than is physically possible from the flow available. The earlier works also placed a minimum flow velocity constraint of 2m/s [6] and 1.5m/s [7] before considering a site to be appropriate, whereas velocities of 1m/s may now be more representative of the lower limit. More recent work has sought to quantify the environmentally acceptable level of extractable energy flux, which was then placed tentatively at 10% [9], on the basis that in many cases, the flow speed reductions would be less than the turbulent fluctuations. Further work [10], developed the flux methodology and examined the feasibility of defining a variable Significant Impact Factor, (SIF) that could be applied to a site and applied the mothod to the resource in the Channel Islands. This work was interesting in that it was applied to an open water environment, which did not lend itself to a more analytic approach, without substantial numerical analysis. The SIF can be viewed as a limiting factor, almost certainly site specific, that takes into account the characteristics of, and constraints applicable to, a tidal stream, and can thereby indicate the maximum acceptable level of energy exploitation. Recent work has been directed towards the derivation of a general formula for any channel [11] or fundamental numerical analysis of the nature of the sensitivity of generic tidal sites [14]. This latter approach offers the enticing prospect of generic parameterisation of any candidate site allowing subsequent detailed assessment without costly numerical analysis. It is interesting to note that this recent work supports the hypothesis that knowledge of the undisturbed flow velocities is a necessary but not sufficient criterion for resource assessment and that further understanding of the sensitivity of a site requires knowledge of the bathymetry, topography and boundary conditions. Under some conditions, it can be demonstrated that it might even be possible to extract more energy than appears to be present in the kinetic flux [15][16], which would indicate that the kinetic flux alone does not represent the total available energy and that it might be more appropriate to define a —total flux“, which includes consideration of potential energy and energy dissipated by boundary friction. In this work it has been assumed that 22% of the apparent kinetic flux can be extracted. This is compatible with published work, referred to previously, or work presently in press.

4 The EPRI approach

The Electric Power Research Institute methodology was applied to the North American east coast sites in Massachusetts, Maine, New Brunswick and Nova Scotia[17] and it references much of the early work for the flux methodology, outlined above. The site survey begins with an initial screening process to identify sites that may be worthy of closer examination and the criteria for this are, • Peak flow speeds (ebb and flood) of the tidal stream, EPRI places a minimum value of 1.5m/s on this factor. • Acceptable cross sectional area (CSA) of the channel, ie narrow and deep is more attractive than wide and shallow. • Suitable seabed geology for anchorage of tidal stream devices • An existing grid interconnection point capable of accepting a commercially viable input of power. • Accessibility for installation and local infrastructure to provide facilities for inspection and repair. The EPRI methodology also notes that hydrographic charts may not be the last word in accuracy and that sites can be included if there is anecdotal or empirical evidence of high tidal stream velocities. In many potential North American locations the tidal stream data are sparse by comparison with the detailed knowledge of UK waters, and for site comparison purposes generic flow data is extrapolated sinusoidally from available measured data. Velocity variation with depth uses the 1/10 th power law in the EPRI model, and this offers a rather more optimistic flow velocity than the Prandtl 1/7 th power law that has so far been employed to model depth values from surface velocities. The difference (up to 10% higher flow velocity) is particularly noticeable in the 5% - 40% (of total depth) elevation above the seabed, where the majority of commercial devices are expected to operate. The EPRI model does not accommodate cross channel velocity variations, and by neglecting the reduction caused by shoreline proximity, effectively overestimates the resource.

Graphical flux method: The Pentland Firth

The mechanics of the graphical flux method are discussed in detail in [10], and the method will be applied here to the Pentland Firth as an illustrative example. The output of the model will be compared with the ETSU report [18] which examined five sites in the area straddling the very strong tidal race between mainland and the Orkney archipelago.

5 Source Data

Pictorial data can be found from a variety of publications including bathymetry from British Geological Survey maps, tidal stream vectors from the Admiralty Tidal Stream Atlas and a variety of relevant data from navigational charts. For the Pentland Firth study, bathymetry data was used from BGS Sheet 58N 04W () [19], bathymetry and tidal diamond data from Admiralty Chart 2162 [20] (the same chart was used for the ETSU report), and tidal stream data was taken from Admiralty Tidal Stream Atlas NP209 (Orkney and Islands) [21]

Bathymetry

The bathymetry is defined using individual colours for each of the bathymetric contours and for the landmasses, leaving the spaces in between as unknowns. The programme then scans the picture and generates an array of numerical contour values from the colour found at each vertex, using a linear interpolation algorithm to produce values for the vertices where no colour is identified. Tidal current data

The vector field is input by overlaying the relevant tidal stream vector image over the bathymetric contour image and using the mouse click event to indicate the start and end points of each vector. Any land mass is given a zero vector value and boundary conditions for the graphics‘ edge are found by using an average value of the nearest available vectors. Substantial downstream vortices due to surface piercing land masses and promontories, and sub-surface detail, will alter the resource magnitude by rendering areas in their shadow unsuitable for extraction at either flood or ebb tide, and the vector input of tidal stream data reflects these unexploitable zones.

Runtime

The programme first identifies what information it has available to it by scanning the image, recognising any landmasses present, and imports the appropriate flow vectors. The programme then scans the picture, attaching known vector X and Y component values at each vertex, interpolating for any missing values and passing the results to an array, the coordinates of which coincides with the bathymetric coordinate system. Zero value vector components attached to landmasses are reasserted at this point to prevent the algorithmic erosion of the coastlines. The vectors are assembled and their magnitude and direction (in degrees) are written to a final array. The image is then redrawn using the vector

6 magnitude to govern the colour used in the image i.e. white (RGB (255,255,255)) indicates <0.05m/s flow and dark grey (RGB (50, 50, 50)) indicates a flow speed in excess of 6m/s. (Figure 1)

Figure 1: Overview of tidal stream intensity around the Orkney archipelago. Time series interpolation

For the Pentland Firth, the data files for each one-hour interval are read into the program and assembled into a three-dimensional array. By extracting the data at any chosen section, a 13-point, approximately 1-hour interval, time series is found for the tidal stream velocities at that section, (in actual fact the flow/ebb cycle is generally taken as being 12.4 hours). Application of a second order Lagrange interpolating polynomial generates intermediate values at quarter hour intervals. Similarly, for the 14-day Spring/Neap cycle, tide tables provide twice daily high water and low water values for a nearby port that can be used to model the cyclical variation of the tidal stream velocities at the point. Taking the difference between the HW and LW heights and normalising for the Spring peak, gives a factor which, when applied to the Spring values used by the program, models the Spring/Neap cycle from Spring values only.

Between each vertex in the cross section, which on the scale used, represents a distance of 51m, the power is calculated as follows:-

The program has generated X and Y vector components at each vertex ( Xvect,

Yvect ), from which, the velocity vector ( Vvel ) may be defined.

7 2 2 Vvel = (X vect +Yvect )

Error! No text of specified style in document. 1

The length of the section can be found from the start and finish X,Y co-ordinates,

2 2 2 2 Lsec tion = ( Xstart −X end ) + (Y start −Yend )

2

The CSA ( A) is defined by the scale width (51), the length of the section in terms of the graphics X,Y co-ordinates and the section depth ( D) at the vertex,

A = 51 * D * Lsec tion

3

To obtain hourly power (Whr) figures through the section from ³ hour intervals, (4) is used for each 15 minute interval and the sum taken of four consecutive intervals.

3 P = 5.0 * ρ * A*Vvel

4

The resulting hourly figures are summed for the 12.4 hr flood/ebb cycle giving a total power flux through the section in Whr per flood/ebb cycle.

13 P = P FE ƒ1

5

These power totals are then transferred to a spreadsheet where the equivalent velocity that would be required to generate that power in that period is calculated from the total power flux,

3 Veq = (PFE /( 5.0 * ρ * A)

6

8

The ratio of high water to low water for a nearby port, in this case Wick, provides a reasonable model for the Spring/Neap cycle. Normalising the ratio to the Spring maximum gives a factor ( γ), which may be applied to the 14 day cycle

HW LW Range Normalised Day Normalised Normalised (m) (m) (m) () 

1 2.85 0.913462 0.71 0.514493 2.14 0.862903 2.7 0.865385 1.25 0.905797 1.45 0.584677 2 2.9 0.929487 0.67 0.485507 2.23 0.899194 2.78 0.891026 1.12 0.811594 1.66 0.669355 3 2.96 0.948718 0.65 0.471014 2.31 0.931452 2.87 0.919872 0.98 0.710145 1.89 0.762097 4 3.01 0.964744 0.66 0.478261 2.35 0.947581 2.96 0.948718 0.85 0.615942 2.11 0.850806 5 3.02 0.967949 0.69 0.5 2.33 0.939516 3.04 0.974359 0.74 0.536232 2.3 0.927419 6 3 0.961538 0.75 0.543478 2.25 0.907258 3.09 0.990385 0.67 0.485507 2.42 0.975806 7 2.95 0.945513 0.81 0.586957 2.14 0.862903 3.12 1 0.64 0.463768 2.48 1 8 2.88 0.923077 0.88 0.637681 2 0.806452 3.1 0.99359 0.65 0.471014 2.45 0.987903 9 2.79 0.894231 0.96 0.695652 1.83 0.737903 3.06 0.980769 0.7 0.507246 2.36 0.951613 10 2.71 0.86859 1.04 0.753623 1.67 0.673387 2.98 0.955128 0.78 0.565217 2.2 0.887097 11 2.62 0.839744 1.13 0.818841 1.49 0.600806 2.89 0.926282 0.87 0.630435 2.02 0.814516 12 2.54 0.814103 1.23 0.891304 1.31 0.528226 2.79 0.894231 0.96 0.695652 1.83 0.737903 13 2.48 0.794872 1.32 0.956522 1.16 0.467742 2.7 0.865385 1.03 0.746377 1.67 0.673387 14 2.44 0.782051 1.38 1 1.06 0.427419 2.64 0.846154 1.05 0.76087 1.59 0.641129

Table 2: HW/LW difference for Wick, normalised to spring peak. [22]

Since the equivalent velocity Veq represents the velocity required to generate the calculated power through any given section at Spring peak over a period of 12.4 hrs, variation of this velocity in proportion to the tidal range (γ in Table 2 above), will permit a reasonable approximation of the velocity variation with the

Spring/Neap cycle (7). The resulting total, (P cycle ) multiplied by 26 will give an

9 annual power output, (P annual ), at the section, based on the Spring peak Veq for that section. (8).

28 3 Pcycle = ƒ 0.5* ρ * A (* γ * Veq ) 1

7

Pannual = Pcycle * 26 (GWh)

8

Whilst the method is clearly an approximation, it does accommodate the variations both within the flood/ebb cycle and the Spring/Neap cycle based on 15 minute intervals.

Define area & sections of interest

For the purposes of this study, the general area to be examined is outlined by the lat/long co-ordinates, 58.380oN, 2.450 oW to 58.520o N, 3.250o W (Figure 2 overleaf) and the six Pentland Firth sites examined in the ETSU report will be revisited here.

1. : 58.758 oN 3.250 oW 2. : 58.733 oN 3.016 oW 3. Stroma / : 58.716 oW 3.133 oW 4. South Ronaldsay / : 58.700 oN 2.916 oW 5. Pentland Skerries: 58.675 oN 2.983 oW 6. : 58.650 oN 2.983 oW

The Pentland Firth separates the north coast of mainland Scotland from the southern edges of the Orkney Isles and has sustained a justifiably fearsome reputation in mariners‘ minds for over two thousand years. The flow is highly complex and notoriously difficult to predict, being very sensitive to meteorological influences in addition to the strong tidal forces. A contemporaneous account of a storm in December 1862 has the eastgoing flow clearing the vertical cliffs on the west of Stroma and depositing seaweed and shipwrecks on the top [23]. Rora Head, on the west coast of Hoy and 27km north of Holborn Head, serves as the dividing point of the eastgoing tidal stream, and, therefore the hydraulic entrance to the Pentland Firth. The geographical entry and exit points of the firth

10 proper are considered to be a line from Tor Ness on Hoy to on the mainland (12km), and a line from Old Head on South Ronaldsay to Duncansby Head on the mainland (11.2km), which gives the firth a length of about 20km. The islands of Stroma (5.4km NW of Duncansby Head) and Swona (11km N of Duncansby Head) impose a further constriction of about 5km on the flow, which has already been forced to narrow from its original hydraulic entrance width of 27km. This funnel effect accelerates the flow to charted values of 6m/s in the Outer Sound and generates substantial vortices, some of which are charted, but many are not, due to lack of information or inherent instability in the driving hydraulics. The eastgoing flow, issues from the eastern end of the firth, and divides around the Pentland Skerries, a small group of surface piercing rocks, some of which are tidal. Anecdotal records have reported flow speeds up to 8m/s in this area. As the eastgoing flow develops, it draws water in from north and east of Old Head resulting in a very large organised vortex off the southern tip of South Ronaldsay. The returning westgoing flow initiates at the northern and southern edges of the main channel and, once established across the full channel, creates another organised vortex, this time off the south west of South Ronaldsay. Incoherent turbulence is generated in the waters to the west of Stroma and Swona and, as the flow matures and begins to recede, the organised vortex off South Ronaldsay dissipates. During both westgoing and eastgoing flows, a variety of minor flows are initiated and dissipated around the smaller Orkney islands of Fara, and Flotta, feeding into and out of the waters of Scapa Flow. A variety of continuous eddies, contra flows and random vortices are regular features of the Pentland Firth tidal streams, which as will be shown, may significantly reduce its tidal energy generating potential.

11

Figure 2: Pentland Firth and surrounding area showing position and orientation of sections.

The Lat/Long co-ordinates are converted to X,Y co-ordinates in relation to the graphic.

Start End section X Y Long Lat X Y Long Lat 1 154 203 3.273 58.767 153 243 3.274 58.747 2 446 260 3.002 58.739 403 274 3.042 58.732 3 351 282 3.090 58.728 326 323 3.113 58.708 4 509 354 2.943 58.692 508 309 2.944 58.714 5 505 365 2.947 58.687 451 408 2.997 58.666 6 431 447 3.016 58.646 457 417 2.991 58.661

Table 3: Section co-ordinates for the six Pentland Firth sites.

The site graphic as used by the program measures 718(W) x 472(H), producing data at 338896 vertices with depths varying from 0m to 100m, in increments of 1m. The model is run for each image combination representing 13 x 1hour (approx) intervals of the tidal cycle. The resulting greyscale image is then checked for correlation with the known values as given in the Tidal Stream Atlas. By clicking on the image, a text box shows the X, Y co-ordinates at the point and

12 displays the vector speed and direction at that point. The methodology examines the flux crossing the section, regardless of direction, and assumes that any energy extraction method would be capable of aligning itself with the prevailing flow.

Results and Discussion

The output from the software is collated into tables, which provides numerical values for the Pentland Firth tidal resource and appropriate comparisons with the 1993 ETSU report. Other factors which may impinge on the viability of a site are also discussed.

Site1, Hoy: ETSU 1993 Flux 2006

58.758oN 3.250oW

Power Output (GWh/year) 1948 @20% 1223 Maximum tidal stream velocities (m/s) 4.4 3.9 Max = 88m Average =62m Bathymetry Min = 64m Site width 2000m 2041

Table 4: Site 1 data.

Site 1 is due south of Hoy between Tor Ness and Brims Ness and is situated in the northern end of the Men of Mey, a formidable race in the westgoing stream. The chart suggests a reasonably flat seabed with no holes or projections. Being relatively distant from the high speed, turbulent flows around the main firth channel, but still benefiting from strong bi-directional flows, this area is possibly the initial starting point for exploiting the tidal stream energy in the Pentland Firth, from the Orcadian perspective. An indicated resource of approximately 1.2 TWh/yr is potentially commercially attractive, as is the relative proximity of Hoy as landfall for power take-off and/or processing. Grid connection is not necessarily the best use of , and a number of alternative added value products are conceivable though most do require land access.

13 Site2, South Ronaldsay / Swona: ETSU 1993 Flux 2006

58.733oN 3.016oW

Power Output (GWh/year) 1811 @20% 395 Maximum tidal stream velocities (m/s) 4.9 4.5 Max = 65m Average =56m Bathymetry Min = 51m Site width 2300m 2306

Table 5: Site 2 data.

Site 2 fits between the south west point of South Ronaldsay and the eastern side of the island of Swona. The significant difference between the ETSU 1993 value and the Flux 2006 value produced here, is due to the fact that the area, though highly energetic, is very turbulent, and large sections of this site are subject to a sizeable coherent clockwise vortex off the south west point of South Ronaldsay during the westgoing flow. Much of the area is then shaded from the strong, eastgoing flow by Swona and the area of good quality flow is relatively small. This site is not thought to be commercially attractive at present.

Site 3, Stroma/Swona: ETSU 1993 Flux 2006

58.716oW 3.133oW

Power Output (GWh/year) 4823 @20% 2384 Maximum tidal stream velocities (m/s) 5.1 3.9 Max = 89m Average =55m Bathymetry Min = 53m Site width 2500m 2450

Table 6: Site 3 data.

On opening the tidal stream atlas, site 3 is probably the most instantly attractive in terms of its apparent energy capability and bi-directional flow quality. The navigational chart suggests a reasonably flat and level seabed for the site itself, though there is a sharp feature rising from 71m depth to 47m depth, approximately 1800m due east of Stroma. The feature is approximately 900m long and 200m in width, and is aligned well with the dominant flow. Moving 1500m due north, there is a narrow trench, about 1600m in length, that varies in depth from 30m œ 40m below the surrounding seabed, which is 70m below the free surface, and its principal axis is horizontally perpendicular to the flow. It is centrally positioned on the line of flow between Stroma and Swona and could potentially be a source of submerged, coherent, rolled up vortices up to 1500m in

14 length, with their axis of rotation horizontally across the dominant flow (Figure 3 below).

Figure 3: Scale sectional representation of the trench situated east of Stroma/Swona showing possible effect on westgoing flow.

It is suggested that seabed features such as these, are of equal importance as the surface velocities when assessing a tidal stream sites‘ energy exploitability. Offering a potential of about 2.3 TWh/yr, this site is commercially attractive from an energy extraction perspective and has a useable landfall in the form of either Swona or Stroma.

Site4, South Ronaldsay / Pentland Skerries: ETSU 1993 Flux 2006 58.700oN 2.916oW

Power Output (GWh/year) 1948 @20% 740 Maximum tidal stream velocities (m/s) 4.4 4.8 Max = 73m Average =58m Bathymetry Min = 53m Site width 2300 2296

Table 7: Site 4 data.

The fourth site, off the southernmost tip of South Ronaldsay, is in the region of a strong anti-clockwise vortex, marked as the Liddel Eddy, which at times during the eastgoing flow occupies the width of the site from Banks Head on South Ronaldsay, to . At various stages of the tidal cycle, there are also stretches of contra-flow up to 5000m in length and trench features similar to those discussed for site three, which are up to 70m deeper than the surrounding

15 seabed. Therefore, though the site demonstrates a potential of approximately 700 GWh/yr, it is considered unsuitable for development at this point in time.

Site5, Pentland Skerries: ETSU 1993 Flux 2006

58.675oN 2.983oW

Power Output (GWh/year) 11005 @20% 3128 Maximum tidal stream velocities (m/s) 6.2 6.2 Max = 75m Average =69m Bathymetry Min = 42m Site width 3500 3520

Table 8: Site 5 data.

Site 5 has a strong, mainly bi-directional stream and a flat, even seabed with bathymetrical variations of no more than a few metres in any direction. If the horizontal vortices suggested for a westgoing flow through site 3 prove to be applicable, then the same feature may generate similar flow perturbations for site 5. It is however, an apparently straightforward task to relocate sites 3 and 5 to maximise the distance from the trench and thus minimise the level of disturbed flow, due to this feature, impacting on installed turbines. It must be borne in mind that, due to the governing topography and its influence on the nature of the flow in the area encompassed by Stroma, Swona, Muckle Skerry and Duncansby Head, the stream is capable of generating and carrying substantial coherent vortices that defy prediction.

Site 6, Duncansby Head: ETSU 1993 Flux 2006

58.650oN 2.983oW

Power Output (GWh/year) 3221 @20% 1088 Maximum tidal stream velocities (m/s) 5.1 4.1 Max = 71m Average =65m Bathymetry Min = 60m Site width 2000m 2058

Table 9: Site 6 data.

Site 6 is essentially an extension of site 5, and the two could be considered as one unit, offering a total of about 4.2TWh/yr with the added benefit of the easiest mainland landfall.

16 An hourly graphical representation of the surface flow velocity of the tidal stream, as it passes through the Pentland Firth is shown below. The thin pale areas, indicating areas of contra-flow, off the southern tip of South Ronaldsay are clearly visible in the 3 rd , 2 nd , and 1 st hour before high water at Dover. Darker areas indicate areas of higher flow velocity, and the shadowing effect of Stroma, Swona and the Pentland Skerries on both flood and ebb tides is well defined as regions of white or pale colouration.

6 hours before HW at Dover, 5 hours before HW at Dover Eastgoing flow initiated

4 hours before HW at Dover 3 hours before HW at Dover

2 hours before HW at Dover 1 hour before HW at Dover

17

HW at Dover 1 hour after HW at Dover, Westgoing flow initiated

2 hours after HW at Dover 3 hours after HW at Dover

4 hours after HW at Dover 5 hours after HW at Dover

6 hours after HW at Dover 6 hours before HW at Dover

18

The flux method of analysis, applied to the Pentland Firth, suggests that the area is not as easily exploited nor as productive as the earlier reports have claimed, and that the exploitable area is limited to the shaded region in Figure 4, below.

Figure 4.: Exploitable region of the Pentland Firth tidal stream.

Conclusion

The sum total of the exploitable power forecast by the 1993 ETSU report is 24.7TWh/yr which is in agreement with the 25 TWh/yr forecast as exploitable in 2025 by the Scottish Executive 2001 report. This flux model suggests that sites 2 and 4 are unlikely to be commercially viable, and that, taking account of the fact the tidal stream energy is finite in any one cycle, the energy can only be extracted once. Therefore, the energy extracted at sites 5 and 6 will not be available at sites 1 and 3 or vice versa, and, since sites 5 and 6 together offer the largest single exploitable energy flux in the sequence, it is therefore apparent that the exploitable resource of the Pentland Firth is between 4 TWh/yr and 5 TWh/yr.

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1 Dewey R, Richmond D, Garrett C, Stratified tidal flow over a bump, 2005, J. Phys. Oceanogr., 10, 1911-1927 2 Bahaj AJ, Myers L, Analytical estimates of the energy yield potential from the Alderney Race (Channel Islands) using marine current energy converters. Renewable Energy 2004 29: 1931-45 3 Scotlands Renewable Resource 2001 œ Volume 1: The Analysis, Scottish Executive 2850/GR/02, December 2001 4 Hagerman G, Polagye B, Bedard R, Previsic M, Methodology for estimating tidal current energy resources. EPRI, EPRI-TP-001 NA Rev 2 June 14th 2006: p18 5 Black & Veatch Consulting Ltd, UK Europe and Global tidal stream energy resource assessment, peer review issue, The Carbon Trust, September 2004: p13 6 ETSU T/05/00155/REP Tidal stream energy review, 1993 7 Commission of European Communities, DGXIII, The exploitation of tidal and marine currents, Program JOULE II. Report: EUR16683 EN. No. JOU2-CT93- 0355 8 Bryden IG, Couch SJ, ME1 Marine energy extraction: tidal resource analysis; Renewable Energy: 31, 2006, p133-139 9 Bryden IG, Grinsted T, Melville GT, Assessing the potential of a simple channel to deliver useful energy, Applied Ocean Research 26 2004, 198-204 10 Owen A, Bryden IG, A novel graphical approach for assessing tidal stream energy flux in the Channel Isles, Journal of Marine Science and the Environment 2006; C4 49-55 11 Garrett C, Cummins P, The power potential of tidal currents in channels, Proc. R. Soc. A 2005 461, 2563-2572 14 Couch SJ and Bryden IG , —Tidal Current Energy Extraction: Hydrodynamic Resource Characteristics“, Proc IMechE —M“ Engineering for the Maritime Environment, Proc IMechE Vol 220 Part M: J Engineering for the Maritime Environment- DOE:10.1243/14750902JEME50 15 Bryden, S.J. Couch, G.P. Harrison; "Overview of the Issues Associated with Energy Extraction from Tidal Currents", Proceedings 9th World Renewable Energy Congress (WREC IX), 19-26 August 2006 16 Bryden IG and Couch SJ, —How much energy can be extracted from moving water with a free surface: a question of importance in the field of tidal current energy?“, Journal of Renewable Energy, Accepted and awaiting publication. 17 http://www.epri.com/oceanenergy/streamenergy.html 18 ETSU T/05/00155/REP Tidal stream energy review, 1993 19 British Geological Survey, Caithness Sheet 58N 04W Solid Geology 1:250000, Director General of the Ordnance Survey; 1976 Crown Copyright. 20 Pentland Firth and Approaches, sheet 2162, Hydrographic Dept Admiralty Charts and Publications, Taunton 1979 21 Orkney and Shetland Islands, NP209, edition 4 ,Hydrographic Dept Admiralty Charts and Publications , 1986 22 http://www.pol.ac.uk/ntslf/tides/?port=0035 23 http://www.geo.ed.ac.uk/scotgaz/features/featurehistory6716.html

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