Renewable Energy 81 (2015) 707e722
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Constraints on extractable power from energetic tidal straits P. Evans b, *, A. Mason-Jones b, C. Wilson b, C. Wooldridge a, T. O'Doherty b, D. O'Doherty b a b
School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, Wales, UK School of Engineering, Cardiff University, The Parade, Cardiff CF24 3AA, Wales, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 April 2014 Accepted 30 March 2015 Available online
National efforts to reduce energy dependency on fossil fuels have prompted examination of macrotidal nearshore sites around the United Kingdom (UK) for potential tidal stream resource development. A number of prospective tidal energy sites have been identified, but the local hydrodynamics of these sites are often poorly understood. Tidal energy developers rely on detailed characterisation of tidal energy sites prior to device installation and field trials. Although first-order appraisals may make macrotidal tidal straits appear attractive for development, detailed, site-specific hydrodynamic and bathymetric surveys are important for determining site suitability for tidal stream turbine (TST) installation. Understanding the ways in which coastal features affect tidal velocities at potential TST development sites will improve identification and analysis of physical constraints on tidal energy development. This paper presents and examines tidal velocity data measured in Ramsey Sound (Pembrokeshire, Wales, UK), an energetic macrotidal strait, which will soon host Wales' first TST demonstration project. While maximum tidal velocities in the strait during peak spring flood exceed 3 m s1, the northern portion of Ramsey Sound exhibits a marked flood-dominated tidal asymmetry. Furthermore, local bathymetric features affect flow fields that are spatially heterogeneous in three dimensions, patterns that depth-averaged velocity data (measured and modelled) tend to mask. Depth-averaging can therefore have a significant effect on power estimations. Analysis of physical and hydrodynamic characteristics in Ramsey Sound, including tidal velocities across the swept area of the pilot TST, variations in the stream flow with depth, estimated power output, water depth and bed slope, suggests that the spatial and temporal variability in the flow field may render much of Ramsey Sound unsuitable for tidal power extraction. Although the resource potential depends on velocity and bathymetric conditions that are fundamentally local, many prospective tidal energy sites are subject to similar physical and hydrodynamic constraints. Results of this study can help inform site selection in these complicated, highly dynamic macrotidal environments. In order to fully characterise the structure of the tidal currents, these data should be supplemented with 3-D modelling, particularly in areas subject to a highly irregular bathymetry and complicated tidal regime. © 2015 Elsevier Ltd. All rights reserved.
Keywords: ADCP Hydrodynamics Marine resources Alternative energy Site assessment Tidal stream turbines
1. Introduction Global climate change is becoming more widely acknowledged and, as such, policy makers worldwide are recognising the importance of greenhouse gas emission reductions. Consequently, there is an international shift towards clean renewable technologies for electricity generation [1]. Furthermore, the finite nature and geographical constraints associated with fossil fuels is motivating this movement towards finding long term clean and renewable alternatives. * Corresponding author. Tel.: þ44 02920 875905. E-mail address:
[email protected] (P. Evans). http://dx.doi.org/10.1016/j.renene.2015.03.085 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
The total estimated tidal-stream (marine-current) resource in the United Kingdom (UK) exceeds 110 TWh yr1, a quantity comparable to the estimate for the rest of Europe, and one that accounts for approximately 10e15% of the total energy from tidal-stream resources estimated for the world [2]. Of that total, approximately 20% is considered extractible [2]. A key document for policy and planning decisions regarding feasibility studies and site leasing is the Atlas of UK Marine Renewable Energy Resources [2], which offers regional-scale descriptions of marine energy resources. However, the resolution of the Atlas is too coarse to capture the tidal dynamics of complicated nearshore zones such as highvelocity straits. Tidal amplitude and current velocities are functions of coastal physical geography and local bathymetry [3e5];
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Abbreviations and nomenclature ADCP CD TST P u, v, w
acoustic Doppler current profiler chart datum tidal-stream turbine power flux (kW m2) instantaneous cross-channel, streamwise and vertical velocity components (m s1) u; v; w spatially-averaged cross-channel, streamwise and vertical velocity components (m s1) ut ; vt ; wt average cross-channel, streamwise and vertical velocity over the vertical diameter of a TST swept area (m s1) vd depth-averaged streamwise velocity (m s1) x, y, z distances along the cross-channel (eastewest), streamwise (northesouth) and vertical axes (m) r water density (kg m3)
therefore this lack of local-scale hydrodynamic data suggests large uncertainties exist in tidal resource estimates [6,7]. Several resource assessments [2,8,9] have identified three primary locations for tidal stream energy development along the coast of Wales: Anglesey [10], Pembrokeshire [11,12], and the Bristol Channel (including the Severn Estuary) [13]. The latter is restricted due to navigational constraints, limited depths, a large tidal range (potential turbine exposure at low spring tides) and relatively low velocities [13]. The Welsh Government has set targets to harness 4 GW of tidal and wave power by 2025 [14]. Meeting this target requires a better understanding of the tidal resource in Wales by constraining estimates of tidal power through site-specific velocity measurements [11e13] and through fully hydrodynamic oceanographic numerical modelling [10,15,16]. In 2011, Tidal Energy Ltd. (TEL), a UK-based commercial energy company, was granted permission to trial its DeltaStream™ tidalstream turbine device (TST) [17] in Ramsey Sound, a strait in Pembrokeshire (Fig. 1) estimated to have an energy potential of approximately 75 GWh yr1 [18]. The original 1.2 MW DeltaStream™ unit supported three 15 m diameter horizontal-axis tidal turbines mounted on a triangular frame with the centre of the hubs set 12 m from the seabed. However, to prove the technology without overcomplicating the design it was decided to install a single 400 kW turbine on one of the smaller foundations, which will still greatly contribute to the energy demands of the communities in St David's [19]. The tip of the DeltaStream™ turbine will be approximately set at 30 m below CD (Chart Datum), so not to restrict boating activity [17]. The constructed prototype device is currently at Pembroke Dock, Wales, awaiting a weather and tidal window for deployment. The device is to be installed as part of a one-year demonstration project to test its integrity and poweroutput capabilities. If successful, the device will be scaled up to full commercial scale and suitable locations identified for a turbine array. Even outside the context of tidal resource development, few studies to date have measured directly the effects of bathymetry on current speed and three-dimensional (3-D) velocity structure of tidal flow through confined passages such as straits [20e29]. Although research into the effects of TSTs on the environment and the effects of the environment on devices is becoming more prevalent, to date, few field studies have investigated the feasibility of installing these devices in areas that are subject to high current speeds [30], which are attractive from an energy generation perspective. Tidal stream turbines are currently being considered
for deployment at sites where local conditions deviate from the ideal with complicated velocity profiles, spatial and temporal variability (over daily and lunar timescales), as well as complexities in the seabed topology. The efficiency and functional longevity of a TST device depends on the interaction of the blades and the tidal flow field [31]. Therefore, quantifying the unsteady loading across the turbine's diameter caused by a non-uniform velocity profile across the face of the turbine is important, as it affects TST performance and cumulative wear on the device. These fine-scale characteristics of tidal velocity fields tend to be overlooked in assessments for potential TST sites. Motivated by the velocity and bathymetric data gap in tidal resource assessments between coarse-scale theoretical potential and fine-scale realities, the viability of tidal energy generation for an area in the complicated flow system of Ramsey Sound has been investigated. It should be noted that a vessel-mounted ADCP study was previously carried out within Ramsey Sound [11] to compare the flux there against that in the two channels to the west of Ramsey Island. This paper builds on this study by utilising additional data and equipment, while covering a larger area of Ramsey Sound to examine the effect of parameters such as tidal asymmetry, variations in the water column and vertical velocities on the natural power flux. The spatial and temporal patterns in flood and ebb tidal-stream velocities near spring peak and use of these measurements to calculate power flux are also quantified. To determine suitable TST locations within Ramsey Sound, the analysis is extended to examine the vertical velocity profile accounting for the water depth relative to turbine diameter and swept area, and the seabed gradient, or bed slope, using the dimensional and geometric specifications of a typical TST. Finally, the effect of depth-averaging tidal velocity data across the entire water column ðvd Þ against that over the TST swept area ðvt Þ in estimates of extractable power, and in assessments of TST site potential is evaluated. Although this paper relates to a single site, the same concerns are present at every site. 2. Methodology 2.1. Study site Connected to the Irish Sea, Ramsey Sound (Fig. 1) is a strait approximately 3 km long and 500e1600 m wide, separating Ramsey Island from the Pembrokeshire coastline near St. David's headland, Wales. Water depth in the strait is typically between 20 and 40 m below CD (where 0 m CD is approximately the level of Lowest Astronomical Tide, LAT), but reaches a maximum depth of 66 m CD within a northesouth trending trench. A submerged pinnacle known as Horse Rock dominates the north-eastern quadrant of the strait. Roughly conical, this natural obstruction to flow has an estimated diameter of 100 m at its base (50 m at half its height) and is approximately 23 m higher than the seabed around it. The crest pierces the water surface and dries (according to the Admiralty Chart) at approximately þ0.9 m CD during spring-tide lows. It should be noted that the data presented here have been reduced to Chart Datum so that all measurements refer to the same reference point, rather than depth below the water surface. The tidal hydrodynamics of the Irish Sea to the west of Ramsey Sound are thought to be as a result of a two Kelvin-type waves, one propagating in a north-easterly direction along St. George's Channel and the other propagating southwards through North Channel [32]. The superposition of these two progressive waves results in two degenerate amphidromes (zones of negligible tidal range, but strong tidal currents), characterised by M2 and S2 co-tidal charts north of St. George's Channel [33]. The Eastern Irish Sea is subject to high tidal ranges and as such, strong tidal currents are generated as
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Fig. 1. Location map of Ramsey Sound, Pembrokeshire (UK). Bathymetric contours show seabed elevation. ADCP survey transects are represented by black lines and red dots represent velocity profile locations. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
flow travels in the vicinity of headlands and islands, creating attractive tidal energy areas [32]. Tidal velocities within Ramsey Sound are dominated by the M2 tide, which has a period of approximately 12.4 h [34]. The area therefore experiences a strong, semi-diurnal tidal regime with a range of approximately 1.6e5 m from mean neap to mean spring, and includes zones of high turbulence [35,36]. Charted tidal streams indicate current speeds of up to 6 knots (~3 m s1). The phase relationship between the M2 and its super-harmonic constituents, which results in a more complex signal comprising overtides, namely the M4 tidal currents, can result in an asymmetry [34,37]. These asymmetries are amplified over shallow continental regions [37]. Although the general tidal dynamics in Ramsey Sound has been known for decades, very few studies have characterised the
hydrodynamics of this area, which is of particular importance given its tidal stream energy potential. One of the aims of this paper is therefore to address this general lack of knowledge and understanding of the local hydrodynamics within energetic macrotidal straits using Ramsey Sound as a field site. Although this study focuses on a single site, many potential tidal energy sites in the UK exhibit similar characteristics to Ramsey Sound, such as the Pentland Firth, Scotland, and Kyle Rhea; a strait of water between the Isle of Skye and the Scottish mainland, for example. 2.2. Survey equipment and design To measure the tidal velocity data, a four-beam 600 kHz broadband Workhorse Sentinel acoustic Doppler current profiler
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Fig. 2. Streamwise velocities ðvÞ downstream of Horse Rock in the xez plane (looking upstream) for transects T1 (a), T2 (b) and T3 (c) at peak flood (positive and negative values denote northward and southward flow respectively; solid black line shows 15 m diameter TST swept area while dotted line represents seabed profile); black circle indicates wake of Horse Rock; and (d) vertically-averaged streamwise velocities over 15 m TST swept area ðvt Þ in the xez plane (plan view) at peak flood.
(ADCP) unit, manufactured by Teledyne RD Instruments, was gunwhale-mounted on Cardiff University's Research Vessel, Guiding Light. The acoustic Doppler current profiler (ADCP) transducers were placed 1.4 m below the water surface; water column measurements presented here begin at a depth of 2.75 m. Streamwise (north-south, v), cross-channel (east-west, u), and vertical (w) velocity components of tidal flow were recorded at 1 Hz (one sample per second) at a ping rate of 10 Hz. Depth to the seabed was measured using the built-in bottom-tracking system, which was also used to calculate the vessel speed. Vessel position and heading data were logged using an external Coda Octopus F180 heading sensor with a horizontal accuracy of 1.5 m, along with the ADCP's self-contained tilt sensor, which has a range of ±15 with accuracy ±0.5 , precision ±0.5 , and resolution ±0.01 [38]. Surveys across the central portion of Ramsey Sound (Fig. 1) were conducted over two consecutive days in June 2012, just prior to a peak spring tidal cycle. Flood-tide velocities were recorded in one day at a set of three transects (T1eT3) downstream of Horse Rock (downstream with respect to flow on the flood tide, and so sited north of the feature); ebb-tide velocities were recorded the next day at a different set of three transects (T4eT6) just south of Horse Rock (but again downstream with respect to flow on the ebb tide, and so sited south of the feature). No upstream transects were made because of the navigational hazard of collecting velocity data upstream of this feature. Downstream distance from Horse Rock varied from 50 m (T3 and T4), 250 m (T2 and T5), and 400 m (T1 and T6). The transects covered a significant area of the northern portion of the Sound encompassing the deeper northesouth
trending trench as well as the shallower outer margins. Each set of transects were surveyed in a continuous, 5-h circuit from 1 h after slack (Slack þ 1) until 1 h before slack (Slack þ 5). Although each three-transect circuit took approximately 30 min to complete, the simplifying assumption made here is that the data recorded during each circuit are representative of one twelfth of a given tidal cycle. Vessel transect time is a well-known limitation of vessel-based surveys relative to bottom-mounted instrumentation. However, the temporal and spatial resolution of the velocity measurements and transects employed herein are consistent with vessel-based methods used in previous studies of this type [11,20]. For the purposes of this paper, only velocity data recorded at the peaks of the flood and ebb tides, when flow through the strait was fastest are presented. Although power availability varies over a spring-neap cycle, no neap tidal surveys were conducted because the aim of this study was to examine the maximum loadings that a TST could be subjected to in these energetic coastal environments, as well as determine the maximum power flux during a typical spring tide. Notwithstanding this, the tidal velocities during a typical neap tide will be lower than a spring tide. Therefore, the power availability will be less. 2.3. Data post-processing Instantaneous velocity measurements (u, v, w) for each transect were spatially averaged with a sliding 5 m window ðu; v; wÞ, equating to an averaging interval of approximately 5e10 s; a filter size significantly smaller than the width of the strait, to reduce uncertainty/standard deviation. This is consistent with the
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Fig. 3. Streamwise velocities ðvÞ downstream of Horse Rock in the xez plane (looking upstream) for transects T4 (a), T5 (b) and T6 (c) at peak ebb (positive and negative values denote northward and southward flow respectively; solid black line shows 15 m diameter TST swept area while dotted line represents seabed profile); black circle indicates wake of Horse Rock; and (d) vertically-averaged streamwise velocities over 15 m TST swept area ðvt Þ in the xey plane (plan view) at peak ebb.
averaging approach adopted by others [11,23]. This post-processing step reduced the standard deviation (s) of the velocity data from ±0.07 m s1 to ± 0.04 m s1. Therefore, a velocity of 2 m s1 represents a random error of ±2%. Many ADCPs automatically average the velocity data over 5e10 s, but it was considered important to capture data at the maximum sampling rate (1 Hz) of the ADCP so that the data could be averaged to a user-defined value. Increasing the averaging period could mask important flow structures, however, there are no pre-determined rules for an appropriate averaging period as it ultimately depends on the application, i.e. longer averaging periods of circa 5e10 min are generally used for moored ADCP data [39] because the instrument is sampling over the same portion of the water column. However, moving platform applications require a much shorter averaging period. The vertical resolution of the data (1 m) remained unchanged to allow the velocity profile and the extractable power to be determined with a meaningful resolution. Dialogue with a Field Service Supervisor at Teledyne RD Instruments (K Grangier, July 2012, pers. comm.) confirmed that these averaging intervals were appropriate for this study.
2.4. Velocity analyses Velocity data were clipped to the 15 m TST swept area (i.e. 4.5 m and 19.5 m from the seabed, equating to a distance of 12 m from the seabed to the centre of the nacelle) using the Eonfusion software (v2.4) developed by Myriax Software Pty Ltd. This configuration was chosen as it represented the same dimensions as the DeltaStream™ TST designed by TEL. The velocity data were subsequently vertically-averaged over the 15 m TST diameter ðvt Þ. Averaging velocity data across the swept width of a turbine is common practice. This is supported by Bryden et al. [40] who noted that ‘it is reasonable to assume that the current speed should be averaged over the swept area of the turbine’. This vertically-averaged depth was subsequently interpolated across the flood (T1eT3) and ebb (T4eT6) survey tracks using an inverse distance weighted (IDW) operator to create a raster of the time-averaged velocities. The streamwise velocity component ðvÞ was used for these analyses because this is the dominant flow direction within the Sound that a TST will be subjected to. Furthermore, examination of the spatiallyaveraged velocity components ðu; v; wÞ revealed that the
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Fig. 4. Vertical velocities ðwÞ downstream of Horse Rock in the xez plane (looking upstream) for transects T1 (a), T2 (b) and T3 (c) at peak flood (positive and negative values denote upwelling and downwelling respectively; solid black line shows 15 m diameter TST swept area while dotted line represents seabed profile); black circle indicates wake of Horse Rock; and (d) vertically-averaged vertical velocities over 15 m TST swept area ðwt Þ in the xey plane (plan view) at peak flood.
streamwise ðvÞ flow component (especially within the central portion of the Sound that experiences the greatest velocities) was approximately 10 times greater than the cross-channel ðuÞ velocity component and vertical 1000 times greater than the vertical ðwÞ velocity component ðv[u[wÞ. Furthermore, scrutiny of the u, v vectors showed a typical angle of approximately ±5 in the central part of the Sound, which is unlikely to significantly affect turbine performance. The effect of misalignment on turbine performance is examined in more detail by Frost et al. [41]. 3. Results 3.1. Tidal asymmetry Fig. 2aec displays the streamwise ðvÞ velocity component interpolated along the cross-sectional transects T1eT3 (shown in Fig. 1) during the flood-tide peak. The solid black lines represent the 15 m diameter turbine swept area. In shallower regions, there is insufficient space to accommodate a TST of this scale (i.e. where the clipped area extends into the air). However, for the purposes of this assessment, data within this region have been vertically-averaged over a smaller distance where the clipped area extends into the air to create the xey plane plots in Fig. 2d. High spatial variability is evident in the tidal velocities, particularly in the cross-channel direction, with the greatest velocity located on the eastern side of the deep channel and in the shallower region to the east of Horse Rock. The wake created by this natural feature is also discernible (particularly at transect T3), albeit to a lesser extent as the distance downstream increases. Flow
reversals are evident near the margins of the Sound as the main current is influenced by the local bathymetry. Fig. 2d shows the streamwise velocities ðvÞ averaged over the vertical diameter of the 15 m swept area ðvt Þ, a similar approach as employed by Mason-Jones et al. [42] during the peak of the flood tide, when water pushes into the Sound from the south. The non-uniform variability in the tidal flow is evident as it separates around Horse Rock. The wake created by this pinnacle is clearly visible and its influence on the currents at transect T1 (circa 400 m downstream) is still apparent. Although spatial-averaging of velocity data could remove eddying effects in the wake of this pinnacle, this paper relates to the general suitability of energetic, macrotidal straits for power generation rather than a detailed examination of the local hydrodynamics. Fig. 3aec shows the streamwise ðvÞ velocity component along cross-sectional transects T4eT6 during the ebb-tide peak. The lower velocities associated with this phase of the tidal cycle result in a reduction in the spatial variability as well as a less noticeable wake of Horse Rock. Fig. 3(d) shows the depth-averaged velocity ðvt Þ over the 15 m TST diameter. During the ebb, when the tide drains through the strait from the north, the highest velocities occur in the corridor of the strait defined by the deep channel. Again, a velocity deficit zone exists in the lee of Horse Rock, and persists for hundreds of metres downstream. Although the velocities associated with this phase of the tide are lower, a flow reversal on the western margin of the Sound occurs as the currents are deflected to the north by ‘The Bitches’ reef. Maximum flood velocity over the 15 m TST swept area (3.8 m s1) is markedly higher than maximum ebb velocity (1.9 m s1), consistent with previous observations of tidal asymmetry around the
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Fig. 5. Vertical velocities ðwÞ downstream of Horse Rock in the xez plane (looking upstream) for transects T4 (a), T5 (b) and T6 (c) at peak ebb (positive and negative values denote upwelling and downwelling respectively; solid black line shows 15 m diameter TST swept area while dotted line represents seabed profile); black circle indicates wake of Horse Rock; black circle indicates wake of Horse Rock; and (d) vertically-averaged vertical velocities over 15 m TST swept area ðwt Þ in the xey plane (plan view) at peak ebb.
Pembrokeshire promontory [11]. This flood-dominated tidal asymmetry is affected by external hydraulic factors (the driving potential for currents) and by the Sound, as noted by Woolf [43]. 3.2. Vertical velocity data The tidal velocity data examined in Section 3.1 relate to the streamwise ðvÞ velocity component. This section, however, scrutinises the vertical ðwt Þ velocity component in order to investigate the magnitude of the positive (upwelling) and negative (downwelling) velocities over the 15 m diameter TST swept area. Very few studies have examined vertical velocities in tidal data. Velocities approaching a TST at an angle to the axial, or streamwise (y), plane are undesirable, affecting turbine performance and structural loading [41]. Placing TSTs in areas of high vertical velocity should therefore be minimised, or ideally avoided, in order to maximise the device design life. The following plots provide an insight into areas that experience greater degrees of vertical velocities. Fig. 4aec shows the
vertical ðwÞ velocity component measured along cross-sectional transects T1eT3 (shown in Fig. 1) during the peak of the flood tide. Away from Horse Rock, the degree of upwelling within the deep channel (particularly at transect T3) is relatively high, peaking at 0.4 m s1. These cross-sections demonstrate the influence of the bathymetry (coupled with the high tidal velocities during this phase of the tide) on tidal velocities, which cause large deviations from the streamwise (y) direction. Fig. 3d shows vertical velocity fields based on the depth-averaged velocity over the 15 m TST swept area ðwt Þ during the peak of the flood tide. Again, over the 15 m TST swept area there are relatively large variations in the vertical velocity component across the Sound with the greatest vertical velocities occurring in the vicinity of Horse Rock and within the deep channel to the west. Fig. 5aec shows the vertical ðwÞ velocity component of flow measured along cross-sectional transects T4eT6 (shown in Fig. 1) during the peak of the ebb tide. The effect of the weaker ebb tidal velocities are clearly evident, resulting in lower vertical velocities both through the water column and across the Sound. Likewise,
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Fig. 6. Streamwise velocity profiles ðvÞ for T3 (flood) and T4 (ebb) e Location A (west of deep channel); Location B (within deep channel); and Location C (in the vicinity of Horse Rock).
the averaged vertical velocities ðwt Þ across the 15 m TST swept area (Fig. 5d) are lower when compared with the same phase of the flood tide, peaking at approximately 0.13 m s1 to the south-west of Horse Rock as the flow is forced into the deep channel. These data demonstrate that the magnitude of the vertical velocity component is largely dictated by the streamwise ðvÞ velocity component, i.e. the greater the velocity in this north-south (y) direction, the greater the vertical velocity component. Although the bathymetry in the vicinity of the Sound, where ebb tide data are available, is still very irregular, the lower velocities associated with this phase of the tide result in reduced vertical velocities at the expense of available power. There is therefore a compromise that needs to be met in these energetic, macrotidal systems between sufficient streamwise ðvÞ velocities for power generation and tolerable vertical velocities. Ideally, the seabed should be wide and flat enough to limit vertical velocities and significant variations in velocities with depth.
3.3. Velocity profiles Velocity profiles of the streamwise ðvÞ velocities at three locations across the Sound are presented in Fig. 6. Negative values represent southerly flow. Again, these data have been spatially averaged with a sliding 5 m window in the horizontal direction. Three locations, displaying differing hydrodynamic conditions, have been selected: to the west of the deep channel (Location ‘A’ in Fig. 1), within the deep channel (Location ‘B’ in Fig. 1) and downstream of Horse Rock (Location ‘C’ in Fig. 1). These locations were examined at the peak of both the flood and ebb tides for the reasons identified earlier. The velocities to the west of the deep channel (Location A) during the peak of the flood (T3) and ebb (T4) tides are relatively low and uniform through the water column, peaking at 0.5 m s1 and 0.2 m s1 respectively. Low flow conditions are experienced at this location during the peak of both the flood and ebb tides given their close proximity to the centre of counter-clockwise (flood)
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Fig. 7. Cross-section of power flux (based on the streamwise, v, velocity component) downstream of Horse Rock in the xez plane (looking upstream) for transects T1 (a), T2 (b) and T3 (c) at peak flood; solid black line shows 15 m diameter TST swept area, dotted line represents seabed profile while black circle indicates wake of Horse Rock; black circle indicates wake of Horse Rock; (d) contour plot of power flux based on vertically-averaged streamwise velocities over 15 m TST swept area ðvt Þ in the xey plane (plan view) at peak flood; and (e) the difference in power flux when depth-averaging tidal velocities over the entire water column ðvd Þ compared to only over the 15 m TST swept area (positive and negative values denote an overestimation and underestimation of power respectively) at peak flood.
and clockwise (ebb) re-circulatory zones at the outer margins of the Sound. The presence of “The Bitches” reef deflects the northward flooding currents to the east, which results in a velocity deficit zone to the north and a zone of accelerated flow as the currents are constrained through passage between this reef and the mainland. During the ebb, the southward currents are able to flow across a wider area. However, as the currents encounter this reef they are deflected northwards resulting in a recirculation zone. The velocities increase towards the deep channel (Location B), peaking at 3.2 m s1 at this location during the flood tide (T3). There is a maximum velocity difference of 1.6 m s1 over the 15 m TST swept area. During the ebb, the profile displays a more uniform shape with velocities peaking at 1.7 m s1. Downstream of Horse
Rock (Location C) the turbulent nature of the flow results in a fluctuation in the streamwise ðvÞ velocities from 3.9 m s1 near the surface to 1.4 m s1 near the seabed. 3.4. Power flux estimation Tidal velocity can be used to estimate the undisturbed power flux (in kW m2) using the expression:
P¼
1 3 rv 1000 2
(1)
where r is the water density (1025 kg m3) and v is the velocity, which here is represented as vertically-averaged over the depth in
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Fig. 8. Cross-section of power flux (based on the streamwise, v, velocity component) downstream of Horse Rock in the xez plane (looking upstream) for transects T4 (a), T5 (b) and T6 (c) at peak ebb; solid black line shows 15 m diameter TST swept area, dotted line represents seabed profile while black circle indicates wake of Horse Rock; (d) contour plot of power flux based on vertically-averaged streamwise velocities over 15 m TST swept area ðvt Þ in the x-y plane (plan view) at peak ebb; and (e) the difference in power flux when depth-averaging tidal velocities over the entire water column ðvd Þ compared to only over the 15 m TST swept area (positive and negative values denote an overestimation and underestimation of power respectively) at peak ebb.
which the TST will occupy. As mentioned previously, the velocity used in this paper is averaged over horizontal distances of 5 m. Physically meaningful estimates of power flux depend on two general conditions: the cross-sectional area of a given TST and the
minimum velocity at which power production is deemed economically viable. Couch and Bryden [44] noted that sites of interest to tidal energy developers tend to have peak spring tidal velocities greater than 3 m s1. Furthermore, according to the UK's
P. Evans et al. / Renewable Energy 81 (2015) 707e722 Table 1 Effects of depth-averaging tidal data.a Depth-averaged velocities Depth-averaged % Difference over vertical diameter velocities over entire of a 15 m TST swept water column ðvd Þ m s1 area ðvt Þ m s1 Flood tide Ebb tide
3.8 1.9
Power (kW/m2) Flood tide 27.4 Ebb tide 3.5 a
3.5 1.8
8.6 5.6
Power (kW/m2) 22.2 3.1
23.4 12.9
Based on area of the Sound that experiences maximum v velocity.
Carbon Trust summary report on tidal stream resources [8], sites with maximum velocity below 2.5 m s1 during mean spring tides may not be capable of generating enough power to warrant development. A recent report by the UK's South West Regional Development Agency [45] suggests a slightly lower threshold of 2 m s1. The minimum velocity for economic viability ultimately depends on the TST design, with the aim of extracting energy more efficiently with reduced start-up torques. With technological advances tending toward more efficient power generation at lower velocities, tidal sites are likely to become more rather than less viable for resource development as velocity becomes a less important limiting factor. However, considering that firstgeneration tidal energy devices require relatively high flow speeds [46,47], the power flux calculations presented here use a minimum velocity threshold of 2 m s1, which is consistent with PMSS [48] and Renewable UK [49]. Fig. 7aec shows the power flux based on Eq. (1) using the streamwise ðvÞ velocities along cross-sectional transects T1eT3 (shown in Fig. 1) during the peak of the flood tide, while Fig. 8aec
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shows the equivalent power flux for the ebb tide (transects T4eT6). The zone of maximum power differs on each tidal peak. During peak flood conditions, maximum power flux is focussed to the east of the deep channel invert and to the east of Horse Rock, while at peak ebb, the power peaks to the western side of the channel invert. Fig. 7d shows the power flux (calculated from the vertically-averaged ðvt Þ velocity over the swept area of the 15 m TST) during the peak of the flood tide, reaching 27.4 kW m2. Figs. 7e and 8e display the effect of depth-averaging streamwise velocities over the entire water column ðvd Þ on power availability during the peak of the flood and ebb tides respectively, by subtracting from the velocity data only averaged over the vertical area of a 15 m TST ðvt Þ. There is a general underestimation of the power flux during the peak of the flood tide; however, power flux in the vicinity of Horse Rock is overestimated by approximately 2 kW m2. As would be expected, there is little difference in the power flux when vertically-averaging the data over the 15 m TST swept area ðvt Þ compared to over the entire water column ðvd Þ in the shallower outer margins of the Sound given the similar flow depths and lower velocities. In the deeper regions, however, there is a greater difference between vertical swept area ðvt Þ and water-column ðvd Þ depth-averaged velocity. During the flood tide (particularly at transects T2 and T3) the velocities accelerate as they pass Horse Rock, resulting in increased velocities to the immediate east and west. Furthermore, within the area of accelerated flow to the west of Horse Rock, the velocities are generally greater within the 15 m swept area compared with higher in the water column. This therefore results in an underestimation in the velocity and power availability when depth-averaging over the entire water column. Table 1 shows the power flux based on both the vertically-averaged velocities over the 15 m TST swept area ðvt Þ and over the entire water column ðvd Þ. This comparison suggests that depth-averaging velocity over the entire water column ðvd Þ tends to underestimate the velocities and power flux in energetic,
Fig. 9. Power flux over a typical spring tidal cycle in Ramsey Sound.
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macrotidal tidal straits. It is therefore recommended to calculate power flux over the swept area of a TST. Temporal variability in the tidal velocity also controls power flux over daily (semi-diurnal), monthly (spring-neap cycle) and yearly timescales. Fig. 9 displays the temporal variability of power during a typical spring flood and ebb tidal cycle, based on the area of the Sound that experiences the greatest streamwise ðvt Þ velocities over the TST swept area. Power across the turbine's 15 m swept area fluctuates over both the flood and ebb tides, peaking at 4840 kW and 620 kW respectively. The flood-dominated tidal asymmetry is clear, with maximum power at the peak of the ebb tide 13% lower than that of the equivalent phase of the flood tide. 3.5. Physical constraints of TST deployment A tool has been developed as part of a separate study using the Eonfusion software [50] to identify suitable TST sites based on the vertically-averaged velocities over a 15 m diameter turbine ðvt Þ, the bed slope and water depth. Fig. 10 from Evans et al. [50] is included for completeness as it is important to this discussion. This plot shows suitable areas for TST deployment based on a vt value of 2 m s1, a maximum bed slope of 5 and a minimum water depth of 30 m. The resultant velocities have subsequently been converted to power flux using Eq. (1). Only 2% of this portion of the Sound meet these requirements during the peak of the flood tide and, given the lower velocities associated with the peak of the ebb tide, no areas are viable. Furthermore, suitable areas are extremely limited in extent and therefore depending on the TST design (i.e. TST arrays) could prove impractical. 4. Discussion This energetic, macrotidal strait exhibits a complicated flow regime, with high spatial variability in tidal velocities and a marked flood-dominated tidal asymmetry to the north of The Bitches reef. Tidal asymmetry, which is the variation in current speed between the flood and ebb phases of the tidal cycle, is an important parameter to consider when designing a TST device. This parameter is not routinely considered when selecting suitable TST sites, but
one that has an important role in quantifying the resource. A 2-D hydrodynamic TELEMAC model of the area [51] suggests that the variability between the flood and ebb tides could be due in part to the narrowing of Ramsey Sound between The Bitches and the mainland, as suggested by Fairley et al. [18], which accelerates the flow as it is laterally constrained through the narrow channel. Furthermore, the configuration of the coastline to the north of Ramsey Sound, particularly the promontory of St David's Head, directs the ebbing flow to the west of Ramsey Island. Tidal asymmetry is not limited to Ramsey Sound with many coastal areas experiencing this phenomenon [52e55]. Neill et al. [52] examined this parameter in Orkney, Scotland using a highresolution 3-D ROMS tidal model. Many turbine designs, including TEL's TST, are two-way generating, i.e. turbines are able to harness both the flood and ebb tidal currents. In Ramsey Sound, maximum power flux during the flood tide is approximately 13% higher than the equivalent phase of the ebb tide. Designing a TST with a yaw system allows the turbine to face into the principal tidal flow. This has its advantages, for example where the flood and ebb tidal velocities are of a similar magnitude. However, many coastal areas (as identified by Neill et al. [52]) are either flood-dominated or ebb-dominated, which raises the question of the need for such a yaw system if, during the peak of the weaker tidal regime, there is insufficient power available for economic viability. This research has shown that the northern portion of Ramsey Sound has a flood-dominated tidal asymmetry, with minimal power available during the peak of the ebb tide. When accounting for the turbine's power coefficient (Cp), the extractable power will be less, which greatly reduces the economic viability. As power flux is proportional to the cube of the velocity, even small changes in the flow lead to substantial fluctuations in poweroutput, which is consistent with previous observations of tidal asymmetry [52]. Although the data presented here reflect the peak of a typical spring flood and ebb tide, the magnitudes of the velocities are lower at other phases of the tidal cycle. This daily fluctuation in energy results in a further reduction in suitable locations for TSTs, which suggests that this portion of the fast-flowing strait is not a viable energy extraction area for large arrays of TSTs, particularly
Fig. 10. Contour plot of power flux for suitable TST areas at peak flood and ebb based on a minimum vertically-averaged streamwise velocity over the 15 m TST swept area ðvt Þ of 2 m s1, a minimum water depth of 30 m and a maximum seabed slope of 5 .
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seabed-mounted, with current technology. As well as the daily semi-diurnal fluctuations, the resource varies over a variety of timescales: seasonal, lunar (spring-neap cycle) and turbulent [52]. Since the data presented in this chapter represent spring tidal conditions, there will be a reduction in the available energy as the tidal range reduces towards neap conditions. Vertical velocities are not often considered by tidal energy developers. However, they can exert undesirable loadings on a turbine, particularly if placed downstream of a significant bathymetric feature, such as Horse Rock. This can compromise the structural integrity of a device. In the northern part of Ramsey Sound, the flood-dominated asymmetry produces greater vertical velocities than the ebb tide. Knowing a site's hydrodynamic and bathymetric characteristic is therefore of vital importance to preserve the design life of a device. Preferably, the flow approaching the turbine is uniform with very little vertical or horizontal variation across the swept area, flowing in the streamwise (y) direction with low levels of turbulence. However, in reality the velocities are likely to deviate from this plane by the turbulent nature of the sites in which these devices will be installed. The bathymetry is a major factor controlling both the magnitude and direction of the flow. The highly changeable bathymetry within Ramsey Sound results in a high spatial variability in the tidal velocities, including the vertical ðwÞ velocity component. Therefore, in order to reduce this variability and ensure the flow is relatively uniform both in the vertical and cross-channel directions, devices should be installed in areas comprising a relatively flat seabed. The slope of the seabed is therefore important, not solely within the footprint of a device, but also upstream of the device as this bathymetry will largely dictate the flow direction. These natural features are undesirable as they introduce crosschannel and vertical velocities across the turbine's swept area. Velocity variations through the water column are another major consideration for tidal energy developers, as large differences in current speeds with depth (especially in the vicinity of the turbines themselves) can create pressure differences across the turbine as it rotates, which can lead to significant stresses and potential failure of a TST. Given the complicated bathymetry of this tidal strait, the velocities are spatially variable across the Sound, with the greatest velocity difference occurring in the vicinity of Horse Rock: an area of increased turbulence. Zones of high-velocity, low-shear flow (optimal conditions for a TST) may neighbour zones of highvelocity flow dominated by high vertical variation in flow (deleterious conditions for a TST). These desirable and undesirable flow conditions may be as little as 20 m apart in the cross-channel direction, a distance barely larger than the swept area of a single turbine. Tidal energy developers should, therefore, have a sound understanding of these velocity differences, particularly across the swept area of a TST since significant variations can exert large, asymmetrical loadings on a submerged structure. It is, therefore, important to fully understand the upstream and downstream (if the turbine is bi-directional) bathymetric configuration of an area proposed for marine energy extraction, to prevent installing a device in close proximity to these unfavourable features. If deploying a device or array of devices downstream of a sharp rise or fall in the seabed is unavoidable, then if cost were no object it would be advisable to install a series of moored ADCPs for a minimum period of 35 days, i.e. two spring-neap cycles (coupled with simultaneous vessel-mounted surveys for validation purposes), to understand the hydrodynamics and expected structural loadings on a TST. Although depth-averaging tidal data over the entire water column is common practice within oceanographic models [10,20,22,23,51,52,57], it can disguise important flow characteristics in these fast-flowing environments, resulting in either an
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underestimation or overestimation, and a subsequent inaccurate estimation of the available power in the system [20]. Waldman et al. [56] noted that where turbines (or indeed naturally-occurring features) are present, the flow is often not uniform with depth and does not conform to a standard log-law velocity profile. Any depthaveraging in regions with a large variation in velocities with depth will therefore prevent the detailed hydrodynamics from being captured, resulting in an inaccurate estimation of the available power. A 2-D, depth-averaged model may therefore be inaccurate in predicting the effects of energy extraction. Three-dimensional models, based on a non-hydrostatic pressure assumption, are therefore required in macrotidal regions in order to resolve the complicated hydrodynamics that exist. Velocities within the Sound vary with depth and the degree of variation fluctuates across the channel. This reinforces the problems associated with depth-averaging tidal velocities over the entire water column (such as the outputs of a depth-averaged numerical model). Clearly, where the ratio of water depth to the turbine diameter is close to unity or a similar order of magnitude as this rotor diameter (i.e. 15 m), there is little difference in the velocities when depth-averaging. However, as the ratio increases, the vertical variability in the velocities (particularly during the flood tide) will not be captured when the data are depth-averaged over the entire water column. Depth-averaging tidal velocities in areas comprising highly variable velocity profiles have the tendency to average out the peaks. The effect of depth-averaging velocities over the entire water column ultimately depends on the velocity profile and where the turbine sits in relation to it. For instance, if the turbine is located high in the water column where the velocities are generally greater, depth-averaging is likely to underestimate the velocities and subsequently the available power, while the opposite is true if the turbine is located closer to the seabed where velocities are generally weaker. Direct measurement allows for a more representative assessment of tidal energy sites, both in terms of regions of flow accessible to a given turbine design, and with regard to characteristics of the flow, such as strong shear zones in the water column, that may adversely affect the turbine apparatus itself. The question, therefore, is whether to use moored or vessel-mounted ADCP instruments. The former offers high temporal resolution but comparatively limited spatial resolution; the latter offers the opposite. A comprehensive survey of the tidal velocity field in northern portion of Ramsey Sound would require a gridded array of approximately 200 moored ADCPs (18 in the cross-channel, x, direction and 12 in the streamwise, y, direction), since each ADCP would have to be deployed with a minimum spacing of 80 m to avoid interference from the 20 beam angles. Hypothetically, the survey would run continuously for a complete lunar cycle at the shortest (i.e. 35 days), but ideally for multiple cycles (and perhaps over multiple years). Although this relatively dense grid would capture the variability in the measured data, there would still be data gaps given the 80 m spacing requirements. Perhaps a compromise would be the deployment of two or more survey vessels to measure the tidal flow over a circuit simultaneously in order to increase the temporal resolution while still maintaining a high spatial resolution. This would more accurately capture the variability in the measured data, which is not possible from these data without smoothing. Furthermore, this would provide the most complete indication of spatial and temporal patterns in the flow field, which depth-averaged numerical models cannot resolve and a single boat survey/moored ADCPs can only partially define. In any case, the inherent nature of measured data means that data gaps will always exist, especially in areas too dangerous to access. These measured data should therefore be supplemented by 3-D models (calibrated with these measured data to ensure confidence in the
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modelled outputs) to provide a broader understanding of the tidal resource. Power flux decreases with downstream distance from Horse Rock, and the maximum power is localised on either side of the pinnacle as the flow accelerates around this feature. This paper has indicated that power flux is minimal near the outer margins of the Sound. Furthermore, although there appear to be power hotspots in the high velocity region that separates around Horse Rock, the steep bed slope presents a new complicating factor for TST placement, especially for gravity-based systems [18]. Seabed topology is rarely flat and conditions vary significantly around the coast. An irregular or undulating seabed is more suited to piled foundations because bed preparation is very difficult for gravity structures [47]. Arrays that mount multiple turbines on a single structure (as opposed to the one tower, typical of wind turbines, for example) require reasonably low gradient beds. A number of TSTs are gravitymounted and can only tolerate relatively low bed slopes. TEL's DeltaStream™ device can tolerate a maximum bed slope of 5 (P Bromley, December 2013, pers. comm.), which results in a base width of 20 m having a vertical drop of approximately 1.7 m across the structure. Lower bed slopes are desirable since the base has to remain stable. The maximum tolerable bed slope is dependent on the device mounting/anchoring arrangement but could be increased for a piled device. For the purposes of this study, a gravity-based device or small array of turbines sharing a common structure was used. Assuming no blasting or excavation of the Ramsey Sound channel bottom, local bed slope in several locations within the channel render high-energy zones functionally inaccessible. Water depth is another limiting factor. The majority of TSTs are deployed in water depths exceeding 30 m [24] since many devices extend approximately 20 m from the seabed to the tip of the turbine. A minimum 5 m clearance is normally recommended for recreational activities (small boats, etc.), as well as to minimise turbulence and wave loading effects on the TSTs and damage from floating materials on the assumption that an exclusion zone be created restricting vessels with a draught greater than 2 m [58]. This generally results in a minimum water depth of 25e30 m with the inclusion of a 5e10 m freeboard. The bathymetry data (Fig. 1) shows that there are large areas that meet this criterion. However, these are generally confined to the deep, steep trench. Bryden et al. [40] noted that where there is no exclusion of shipping, the top tip of the turbine has to be at the lowest astronomical tide (LAT) with additional safety factors to account for the lowest negative storm surge (1.5 m), the trough of a 5 m wave (2.5 m) and shipping and waves (5 m). Therefore, based on this guidance and using TEL's DeltaStream™ device configuration, a 15 m diameter rotor with the hub set 12 m from the seabed requires a minimum water depth of 33.5 m. However, vessel activity within Ramsey Sound is restricted to local fishing and coastal vessels, which have a draught rarely extending 5 m below the water surface. Bryden et al. [40] also noted that the bottom tip of the turbine must not be within 25% of the water depth from the seabed. This portion of the water column is subject to large vertical velocity shears due to bed friction. To take maximum advantage of the tidal stream resource both in the UK and on a global scale, it will be necessary to design devices that can operate in water depths less than 30 m, subject to navigational and other physical constraints. This was realised by Pacheco et al. [24] who noted that deploying devices in shallower water has the added benefit of being in closer proximity to the electrical grid and associated infrastructure. Given the importance of bed slope in determining suitable TST locations, a high resolution (2 m in the horizontal plane) bathymetric grid has been used. This detailed bathymetry accounts for small-scale irregularities in the seabed, which are masked by either coarser bathymetric grids and/or low mesh resolution; an inherent
limitation of far-field oceanographic models. These models generally employ an unstructured mesh with a relatively coarse grid away from the area of interest with increasing resolution as distance to the site decreases. However, grid sizes are often still too large at the area of interest to capture the local bathymetric irregularities, which, as shown previously, can have a significant influence on the flow. Haverson et al. [59] developed a 2-D depthaveraged TELEMAC model of the Pembrokeshire coast, refined at Ramsey Sound with a mesh resolution of approximately 35 m. Aside from the fact that the model is depth-averaged, which has been shown generally to underestimate the velocities in these macrotidal straits thereby masking the detailed flow structures, a bathymetric resolution of 30 m has been mapped onto the 35 m mesh. This relatively coarse grid is likely to ignore the small-scale bathymetric features, such as Horse Rock, which are highly influential on the local flow field. Much higher resolution bathymetric data of the area exist (~2 m resolution) and have been used to inform this paper. Embedding this grid into this TELEMAC model (coupled with a finer mesh) would improve the accuracy and confidence in the modelled outputs (particularly if further validation is undertaken). Sensitivity tests using smaller mesh sizes and ideally higher resolution bathymetric grids should be a prerequisite for these types of models to determine their accuracy. These models are therefore appropriate for larger-scale far-field modelling of sediment dynamics, for example, but become problematic when attempting to resolve medium- to near-field (i.e. CFD) modelling issues, such as TST array impacts on the local flow field using an extra sink in the momentum equations. Furthermore, use of these far-field models as a tool to identify viable TST sites based on bed slope tolerances may not be practicable, as the coarser grids can smooth and even ignore important bathymetric features that may otherwise (i.e. if a higher resolution bathymetric grid was used) prevent a site from being developed. Until computer technology advances such that finer grids can be utilised to resolve these small-scale features, it is prudent to use these finer grids outside a numerical model, such as within GISbased software, to ensure seabed gradients can be accurately defined at proposed TST sites. 5. Conclusions This paper has examined the viability of narrow, fast-flowing tidal straits for TST development, using Ramsey Sound as a field site. The bathymetric configuration of this area largely dictates the magnitude and direction of the tidal currents as they pass through the Sound, which gives rise to a high spatial variability and a flooddominated tidal asymmetry within the northern portion of the Sound. This raises questions as to whether a yaw system in this location is required. From a structural integrity perspective, it may be more practical to have a yaw system despite there being a strong tidal asymmetry in order to direct the turbine into the oncoming current, thereby reducing the loading. Site-specific resource assessments of any potential tidal energy area should therefore be undertaken prior to the TST design stage. Depth-averaging streamwise velocity data over the entire water column in these energetic, macrotidal environments generally underestimates the velocities and therefore the power flux in the system when compared to vertically-averaging over the TST swept area. This is likely to result in an inaccurate estimation of the available energy in the system. Therefore, reliance on 2-D depthaveraged numerical models in dynamic tidal regions should be avoided and substituted with a 3-D model. Based on a survey that encompasses a significant portion of Ramsay Sound, very few areas exist (at least with the specifications of the device intended for Ramsey Sound) where streamwise
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velocity ðvt Þ, bed slope and water depth are accounted for. These parameters are typically overlooked but essential to extractable resource estimates and for insight into realistic TST performance. Locating a flat area of seabed to install a device with a large footprint is difficult in areas with an irregular bathymetry. Designing a device with a smaller footprint would allow a greater area of the channel to the exploited. The TST design is therefore an important consideration since this ultimately dictates viable locations for deployment. This paper has demonstrated that physical conditions at energetic tidal straits reduce the extractable areas quite considerably, particularly as TST devices will have to be installed in arrays if they are to achieve viable costs. This viability ultimately depends on the TST design, which should be considered after a site's tidal velocity field and bathymetry have been characterised. Although the data used to inform this paper are related to Ramsey Sound, many other tidal regions worldwide exhibit similar physical and hydrodynamic characteristics. The present findings are therefore transferrable and can be used to help understand the hydrodynamics at other energetic areas, which will ultimately dictate the feasibility of and most suitable location for TSTs. The interest of the work presented here therefore goes beyond this macrotidal strait, and ultimately the same methodology for a range of TST designs can be applied elsewhere. Acknowledgements This work was undertaken as part of the Low Carbon Research Institute Marine Consortium (www.lcrimarine.org) (WEFO: 80366). The authors wish to acknowledge the financial support of Welsh Government, the Higher Education Funding Council for Wales, the Welsh European Funding Office and the European Regional Development Fund Convergence Programme. The authors also acknowledge the collaboration of LCRI Marine partners, RNLI St David's and St Justinian's Boat Owners Association. The authors also wish to thank Tidal Energy Ltd. for their valuable advice and support. Warwick Gillespie is also acknowledged for his ongoing technical support with the Eonfusion software. Finally, Dr Eli Lazarus of Cardiff University's School of Earth and Ocean Sciences is acknowledged for his invaluable input into this paper. References [1] Denny E. The economics of tidal energy. Energy Policy 2009;37(5):1914e24. [2] Black and Veatch. Tidal stream energy resource and technology summary report. Carbon Trust; 2005. [3] Easton MC, Woolf DK, Bowyer A. The dynamics of an energetic tidal channel, the Pentland Firth, Scotland. Cont Shelf Res 2012;48:50e60. [4] Bryden IG, Couch SJ, Owen A, Melville G. Tidal current resource assessment. Proc IMechE Part A J Power Energy 2007;221:125e35. [5] Dewey R, Richmond D, Garrett C. Stratified tidal flow over a bump. J Phys Oceanogr 2005;35:1911e27. [6] O'Rourke F, Boyle F, Reynolds A. Tidal energy update 2009. Appl Energy 2010;87:398e409. [7] Cooper B. Enhanced tidal resource assessment within the Western Isles. In: Wave and tidal 2011; 2011. [8] Black and Veatch Ltd. Phase II UK tidal stream energy resource assessment. Carbon Trust; 2005. [9] ABPmer. Atlas of UK marine renewable energy resources: technical report. Department for Business, Enterprise & Regulatory Reform; 2008. [10] Serhadlioglu S, Adcock T, Houlsby G, Draper S, Borthwick A. Tidal stream energy resource assessment of the Anglesey Skerries. Int J Mar Energy 2013;3e4:98e111. [11] Fairley I, Evans PS, Wooldridge CF, Willis M, Masters I. Evaluation of tidal stream resource in a potential array area via direct measurements. Renew Energy 2013;57:70e8. [12] Evans PS, Armstrong S, Wilson C, Fairley I, Wooldridge CF, Masters I. Characterisation of a highly energetic tidal energy site with specific reference to hydrodynamics and bathymetry. In: Proc. 10th European wave and tidal energy conference (EWTEC), Aalborg, Denmark; 2013. [13] Willis M, Masters I, Thomas S, Gallie R, Loman J, Cook A, et al. Tidal turbine deployment in the Bristol Channel: a case study. Proc ICE Energy 2010;163(3): 93e105.
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