by Cassandra Burgess
The configuration of a tidal energy array partially determines the level of environmental impact. In determining the optimal configuration for a particular area, it is important to consider not only power output, but also environmental impacts. Fallon, et al. discuss the impacts of a tidal energy array located in Ireland in their 2014 paper. They use a two dimensional model and average speeds over the depth of the channel. This simplifies the modeling process, but it also overestimates some of the impacts. They then analyze grid spacing for turbines, with turbines spaced 0.5, 2, and 5 times their diameters apart. The model indicated that the 5 diameter spacing had the least environmental impact. It decreased velocities by 19.9% less than the 0.5 diameter spacing outside the grid, and increased flood velocities by 27.3% less. The 5 diameter spacing also changed the tidal range of heights by only 1% while the 0.5 diameter changed them by 6.4%. Because the 5 diameter spacing has significantly less impact on the hydrodynamic environment around the turbines, the authors conclude that it is desirable to use low density arrays when possible.
The authors discuss the possible environmental impacts of changes in the hydrodynamics of the channel. Increased velocities around the array will likely lead to increased erosion of sediment in the bed and shoreline. This leads to greater amounts of sediment in the water, which increases maintenance costs for the array. The increased erosion will also likely impact those species living in the bed of the channel negatively. Changes in velocities also impact many marine mammals and fish, and it is possible that these species will migrate to areas with more moderate tidal flows in response. While it is unknown what the exact effects of any changes in tidal currents and height ranges will be, they will certainly have some effect. It is important to minimize these effects wherever possible, without sacrificing the efficiency of the tidal array. This can be accomplished by building a low density array with high power output. The low density arrangement significantly reduces the environmental impacts, allowing the turbines to take more energy from the tidal flows without significant damage to the surrounding environment. The authors conclude that a spacing of 5 diameters is sufficient in this particular instance, but urge site specific study for future arrays.
Fallon, David, et al. “The effects of array configuration on the hydro-environmental impacts of tidal turbines.” Renewable Energy 64 (2014): 10-25. [GSSS: Fallon tidal]