Tidal Stream Energy Research of the Sustainable Energy Research Group

Importance in Design of Tidal Stream Farms

Resource assessment forms part of the design process for an array of tidal stream generators - as indicated in the flow diagram in Figure 1 - and consists of the following tasks:

Selection of sites suitable for placing of tidal stream generator arrays. This is primarily constrained by a minimum value of mean cube flow speed (For a fixed generation efficiency, this value will be proportional to the average power output for a single turbine.); and a suitable range of depths for a particular type of generator. Site selection will also be constrained by integration with the power distribution network and environmental impact.
Initial sizing and rating of the generating device to maximize energy extracted over the life of the device taking into account factors such as the long term variations in flow speed; deviation of the flow from rectilinear movement; vertical profile of flow velocity.
Investigation of different arrangements of generators within the selected area given the device parameters above, in order to maximize combined power output. Revision of generator parameters if necessary.
Investigation of the geographical extent of significant effect of the proposed tidal stream generator array on tidal parameters (extracting tidal energy in one location may lead to a reduction in available energy elsewhere). If necessary, corrections made to power output estimates due to resulting changes in boundary conditions.

Table 1 lists methods for generating tidal flow data for use in resource assessments. 1, 2 and 3-D models can also be used to assess the effect of the generator arrays on the tidal regime, whereas previous assessments have used empirical corrections to flow values in the natural state to take account of the effect of multiple generators.

Fig. 1 - Processes involved in assessing the optimal power output of a tidal stream generator array.

Table 1 - Modelling of tidal stream generator arrays in order of increasing complexity: advantages / disadvantages according to approach.

Tidal flow data	Advantages	Disadvantages
Nearest representative values from navigational charts	Simple, data easily obtainable.	Inaccurate over large areas and with sparse data. Corrections made for effects of generators can only be included empirically.
Interpolate between data points	Simple, more accurate than above.	Doesn’t account for changes in flow due to depth and other effects.
1D model	Simple, suitable for ‘fences’ of generators across well-defined channels. May be solved analytically.	Not suitable for complex topography. Can not simulate flow acceleration between generators.
2D numerical model	Well established for coastal tidal modelling. Energy extracted through added roughness.	Increasing computational expense. 3-D Wake structure not simulated. Requires tuning and validation.
3D numerical model	Can simulate wakes of turbines and include vertical velocity profile.	Complex. Currently suitable only for highly localized models. Data may be lacking on turbulence quantities.

Table 2 - Tidal stream energy resource for five sites in the Channel Islands according to three different reports.

.	.		Black and Veatch, 2004		Joule 2, 1996		ETSU, 1993
Tidal Race	Area	Max. Speed	Rated Power	Load Factor	Rated Power	Load Factor	Rated Power	Load Factor
.	km²	m/s	MW	%	MW	%	MW	%
Race of Alderney	102	4.4	394	49	1973	44	2407	23
Casquets	190	2.6	538	35	370	50	2943	13
NW Guernsey	222	2.1	170	33	422	54	2186	22
Big Russel	59	2.6	101	43	219	47	1001	22
NE Jersey	58	3.1	57	33	196	45	1179	13

Previous Estimates

Assessments of the tidal stream energy resource have taken the form of desktop studies, produced using navigational data, for the purpose of providing government and industry with broad estimates of the economic potential for the development of tidal stream power.

Recent assessments of tidal stream energy resources around the UK have estimated the exploitable resource, when averaged over a year, in the range 2 - 7 GW, which may be compared to an average electrical power consumption in the UK for 2005 of 46 GW (Digest of UK energy statistics 2006). There is considerable uncertainty attached to these resource estimates, however; all of the assessments to date have either ignored the change in flow conditions due to the effect of the generating devices, or have been based on more or less arbitrary proportions of kinetic energy flux through a site.

The results from three reports for the five key sites in the Channel Islands (shown in maps.live.com) are shown in Table 2 above. This clearly highlights a large discrepancy between reports and a need to make more accurate estimates of the resource.

Modelling of Tidal Currents

Portland Bill on the South coast of the UK (view in maps.live.com) is a promising site for tidal stream energy, with high tidal stream velocities of up to seven knots (3.6 m/s) around the headland. Although the area with high tidal streams is smaller than other proposed sites in Scotland or the Channel Islands, the location is closer to population centres than these sites.

In order to produce high resolution data on tidal streams, independent of navigational charts, a 2D tidal hydrodynamic model of this promising site has been produced using the TELEMAC system. The model was forced by tidal elevations around the outer boundary and the results validated using tidal elevation records at Weymouth (within the model domain) and tidal diamonds from the relevant Admiralty charts. One of the finite element meshes used for the model can be seen in Figure 2, along with the bathymetry of the area. Some of the results of the model can be seen in Figure 3 and the animation of tidal flows around the headland below.

view Portland Bill animation

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Work has been published in:

Blunden L.S. and Bahaj A.S. (2006) Initial evaluation of tidal stream energy resources at Portland Bill, UK. Renewable Energy, Volume 31, Issue 2, February 2006, pp 121-132. view paper

Forecasting

For example tidal data, simulation results around Portland Bill have been used, this site has a significant swing from the 180º flow reversal, allowing comparisons between fixed orientation and yawing devices. The variation of depth-averaged speed over 28 days from the simulation is shown in Figure 4.

The T_TIDE package for MATLAB was used to determine the constituent ellipse properties by harmonic analysis, in which nodal corrections were applied, based on the central time of the input time series. Based on the solved constituents, predictions can be made from any start date with any time step. For the predictions in this paper, the tidal stream speeds and directions from the start of 2006 for 18 years have been generated at one minute intervals. A hodograph showing a forecast for 18 years is presented in Figure 5 and first 5 days are shown to demonstrate a typical cycle. The ellipse is offset south due to the constant flow component. This and the constituents with inclination close to 0° or 90° cause the tidal stream to swing away from rectilinearity.

Some marine current turbine concepts do not turn to face the tide and instead rely on the tidal stream at the site to be bi-directional. Thus the rotor orientation is fixed but the blades are able to twist through 180 degrees capturing the energy by running the generator backwards. For this case direction is also important. The east and west directions is presented in Figure 6. This averaged data set provides a basis for comparing designs of turbines based on know device characteristics and for use in optimisation studies.

Further Topics on Tidal Stream Energy

Fig. 2 - Part of a finite element mesh used for modelling tidal flows around Portland Bill, with sea bed elevations in metres above chart datum (scale on right). Co-ordinates are OSGB National Grid.

Fig. 3 - Tidal flows around Portland Bill headland. Colour scale is in m/s and vectors show direction and relative magnitude of the velocity field. Rectangle shows the approximate area with highest average flow speed (actually highest mean cube speed).

Fig. 4 - 28 day prediction for speed and direction from TELEMAC simulation.

Fig. 5 - Hodograph showing direction and strength of the tidal flow. The first 5 days are the black lines and the shaded gray area represents the full 18 year forecast.

Fig. 6 - Binned data set showing a histogram of the times in the east and west. (Red denotes higher number of hours). The bins are defined by 1° intervals and in cubed speed steps of 1 m3/s3 from 0 to 35 m3/s3 and averaged data over 18 years.