Unless an offshore wind farm is optimally designed, even the most powerful offshore wind turbine will under-perform. While there will always be a trade-off between turbine performance losses and wind farm lay-out cost savings, the trick is knowing how to strike the right balance, says the latest Wind Energy Update Offshore Wind Installation and Construction Report.
A host of factors need to be considered when fine-tuning an offshore wind farm’s final layout – geology and seabed conditions, variable water depths, construction, operations and maintenance operations, to mention a few.
But only one factor determines the wind farm size for a given capacity: the spacing between the turbines. If turbines are spaced too closely together, the wake effect, (turbulence and reduced wind velocity resulting from wind flow past the static tower and moving blades), reduces overall wind farm performance. On the flip side, however, there are costs attached to the occupied sea area, as well as to the transmission required to link the wind arrays.
“The prevalence of wake effects defines a shadow area in which the energy capture of a downwind turbine would be reduced and its fatigue loading increased,” explains the report’s author, Alan Tricklebank.
“The full restoration of the wind from these effects may take 20 rotor diameters (D) or so. Given that there are costs attached to the area occupied and the transmission distances, it is economic to trade off performance losses against savings in other costs arising from the reduction of the spacing from the aerodynamic ideal,” he advises.
According to the report, which provides rigourous analysis of offshore wind farm installation and construction practices and costs, wake effects have been found to be more severe in large arrays than small arrays. The report advises that nominal spacings “might reasonably be expected to keep array losses some way below 10%”.
