Energy Extraction by Large Marine Current Turbine Arrays at Sites around the Channel Islands

Abstract

Tidal flows contain a predictable source of renewable energy that can be extracted using large tidal turbine arrays to generate clean, secure electrical power to meet rising demand. Studies of specific coastal sites around the UK generally conclude that electricity generation using large tidal turbine arrays can supply up to 20% of UK electricity demand. However these estimates vary depending on the method used, with many reliant on low temporal and spatial resolution flow data and crude methods for modelling energy extraction.

To investigate these potential sources of error, a new 2D hydrodynamic model was built with significantly improved spatial and temporal resolution to simulate tidal flows around the Channel Islands, located off the West of the Cotenin Peninsula in Normandy, France. The most energetic flows at Alderney Race, Casquets and Big Russel were selected for detailed study.

Energy extraction by large arrays was simulated using the distributed drag method, where an area averaged array drag is applied uniformly over the array plot area. Laboratory experiments were conducted using porous fences to simulate flow through large, regular, multi-row arrays to quantify the accuracy of a commonly used array drag parameterisation. Results show agreement between experimental load cell measurements of fence drag and the numerical formulation of array drag within 10%. This was in part due to close agreement between the depth averaged velocity and the level of wake recovery between each row, which was robust over a wide range of fence spacings. Results from a simple force balance indicate that for rough beds, the presence of the porous fence arrays increased the contribution of drag from the bed by up to 95% due to an increase in pressure drag on roughness strips secured to the flume bed. These findings have implications for regional scale tidal turbine array modelling, where array layout along with site specific characteristics such as turbulence intensity and bed profile determine the validity of the distributed drag approach.

The distributed drag method was implemented in the hydrodynamic model to quantify an upper bound for energy extraction at each site. Based on the distribution of mean ambient kinetic power and suitable depths, ambient flow simulations estimate the total area suitable for tidal energy development is likely to be up to 70% smaller than previously predicted. Energy extraction results confirm that Alderney Race contains the majority of the Channel Islands resource, where an upper bound of 5.1 GW exceeds that of the Pentland Firth, the best known site for tidal energy development in the UK by 35%. This was followed by Casquets (0.47 GW) and then Big Russel (0.25 GW). Increased drag in Alderney Race caused flow to be diverted through Casquets, resulting in an increase in volume flux of up to 25% and an increase in extracted power of up to 75%. This interdependency highlights the need for array layouts at Alderney Race and Casquets to be designed in tandem, otherwise the total energy extraction is likely to be under estimated.

Within Alderney Race simulations were run with arrays overlaid on regions of high mean kinetic power distribution. To assess array feasibility, results were compared with the output of the London Array, the world’s largest offshore windfarm, using mean generated power per swept area as a suitable metric for comparison. The analysis indicates that an array in the most energetic region of Alderney Race with the same array density but covering an area a tenth of the size can achieve a mean power output per total swept area of 2.2 kW/m2, ten times that of the London Array.

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Identifiers

ORCID for Abubakr Bahaj: orcid.org/0000-0002-0043-6045

ORCID for Luke Blunden: orcid.org/0000-0002-0046-5508

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