17 Jun 2011 04:06:02
Which comes first: the wind farm or the turbine?
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%".
In addition to turbine spacing, future operations and maintenance of the site must be kept in mind when deciding on the final layout. For example, a perimeter siting of substations is preferable to embedding substations in the middle of an array, even if it involves additional cabling, notes the report. This is because it reduces vessel access risk and facilitates maintenance throughout the life of the wind farm.
A site's depth and distance from shore also have profound consequences for the engineering of the wind farm, particularly for the foundations, the power transmission and for the engineering and organisation of the construction/ installation and O&M operations.
"In the North Sea and Irish Sea basins, the severity of exposure to wind and waves will tend to increase with distance offshore. Distance to shore also affects other things, such as electrical transmission losses and, beyond a certain distance, the choice of transmission system," explains Tricklebank.
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