| Summary: | The following report, commissioned by the States of Guernsey Commerce and Employment’s Renewable Energy Team (RET), assesses the potential for the extraction of tidal stream energy from two key locations in the Big Russel, located between the islands of Guernsey and Sark, and one location to the north east of Sark. The three locations were chosen by Members of Parliament, for the Isle of Sark, to provide information on the potential tidal resource found at each location and to begin a record of baseline flow through these areas before the placement of any technology. Site 1 was positioned in the centre of the Big Russel, Site 2 in the Big Russel closer to Sark and Site 3 on the north east coast of Sark. The data were collected using an Acoustic Doppler Current Profiler (ADCP) positioned on the seabed looking up through the water column. All three sites had a full month of data on which analysis could be carried out. Due to the limitations of the data provided analysis is based on a single turbine device. The data has been collected from one specific location for each site therefore analysis can only be conducted for these three specific locations. Analysis demonstrates that Site 1, located in the centre of the Big Russel, contains the highest resource with a maximum flow velocity of 2.6 m/s and a potential annual power output of 8,378,640 kWh from a single Hammerfest Strom 1 MW turbine. This is equivalent to 2.09% of Guernsey’s annual energy requirements which is in the region of 400 GWh per year. Analysis shows there is vertical shear in the horizontal flow and in order to extract the maximum amount of tidal stream energy the turbine needs to be placed as high in the water column as possible, whilst still complying with Royal Yachting Association (RYA) guidelines regarding safe limits for turbine blades. The maximum height of the turbine blades in accordance with RYA guidelines is 32.5 meters for Site 1. The Hammerfest Strom turbine can be positioned with the turbine blades reaching a maximum height of 30 meters in the water column, allowing the recreational shipping industry and commercial shipping industry to traverse the area safely at all states of tide. Site 2 contains the second highest energy resource with a maximum velocity of 2.45 m/s. The potential annual power output was significantly smaller than Site 1 with an annual power 2 | P a g e output of 3,547,320 kWh. This decrease in power potential is due to the larger flow velocity fluctuations at Site 2. The annual power output is equivalent to 0.89% of Guernsey’s annual energy requirements. The proposed technology used at Site 2 is a single OpenHydro 2.2 MW turbine. The maximum height of the turbine blades in accordance with RYA guidelines is 28.19 meters for Site 2. This height prevents the use of the Hammerfest Strom turbine. The OpenHydro turbine complies with RYA regulations reaching a height of 21 meters from the seabed. Site 3 contains the lowest amount of energy. The maximum velocity reached 2.15 m/s and the potential annual power output was 2,781,600 kWh with an OpenHydro turbine. This is equivalent to 0.70% of Guernsey’s annual energy requirements. However Site 3 was chosen as a site that could provide energy specifically to Sark’s energy grid. The exact amount of energy Sark consumes is unknown but it is known to consume less than 1MW of energy at peak times, and less than 1 GWh in a year. Based on these estimates, one OpenHydro turbine could produce 278.16% of Sark’s annual energy requirements. This site has definite potential for micro-generation. Further research is required in order to fully understand the tidal stream energy potential in the surrounding area and to identify the optimum location for a device. A schematic has been created in this report for a potential array of turbines in the Big Russel. The array has, of necessity, been designed based on assumptions due to insufficient available data. The array has been constructed based on an extrapolation of the data provided for Site 1. Site 1 was chosen as it contained the highest tidal stream energy potential. Based on the assumption that the potential energy output as found at Site 1 is roughly the same throughout the length of the Big Russel, then any array would have to be narrow and positioned down the centre of the channel. Using this assumption and the findings from the Roosevelt Island Tidal Energy (RITE) project, which gives an indication for the spacing for tidal turbines, approximately 40 turbines would fit down the centre of the Big Russel and produce a combined power output of 335.15 GWh per annum, equivalent to 83.79% of Guernsey’s annual power consumption. Much of the economic data surrounding the tidal energy industry is unknown and unpublished. The industry is still in its infancy and only a few devices are commercially ready. There are no commercial scale arrays in place and therefore trying to cost a project such as this is difficult. These results are only indicative at present and based on published industry estimates. As a mere indication the Guernsey Renewable Energy Commission (GREC) have stated that they don’t expect a commercial tidal array to be operational anywhere before 2017 3 | P a g e and therefore Guernsey’s tidal stream energy projects would not be developed until after this date. Overall the cost for one turbine, along with the required infrastructure such as cabling substations, together with the cost of installing the device on the seabed, can be estimated to cost in the region of £8.4 million based on industry estimates of £5 million per MW of installed capacity. Using economies of scale, as suggested by the Electric Power Research Institute (EPRI), the total cost of a 40 turbine array would be £107.1 million. Allowing time for the tidal energy industry to mature past the installation of the first commercial array would be advisable due to the wealth of knowledge that such an array will provide, including invaluable data on how an array interacts with the surrounding environment, the interaction of the turbines with each other and their resulting efficiency. The first array would also allow greater understanding of the costs involved. Ultimately the economic viability of an array will come down to the cost-benefit analysis of installing such devices. Considering the high economic investment needed for such a scheme Guernsey would be well advised to utilise knowledge gained from other arrays in order to capitalise on cost reducing strategies. Overall Guernsey has the tidal resources around the island to make tidal energy a reality for the island. However further data needs to be collected from multiple locations along the length and breadth of the Big Russel to highlight exactly how the current velocity varies throughout the channel. A detailed understanding of the current in the channel is necessary before considering the design of any array.
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