Evaluation of the wind-resource estimation program WAsP for offshore applications

The increasing interest in harvesting offshore wind energy requires reliable tools for the wind resource estimation at these sites. Most commonly used for wind resource predictions on land as well as offshore is the WAsP program. It has been validated extensively for sites on land and at the coast. However, due to the lack of suitable measurements there is still a need for further validation for offshore sites.

Data from ongoing measurements at prospective wind farm sites in the Danish Baltic Sea region are available now. The wind resources estimated from these measurements are compared to WAsP-predictions. They agree well. The only deviation found is for two sites, which are located at about the same distance, but on different sides of the island Lolland.

Here the measurements show a difference in the wind resources, which is not predicted by WAsP. A direction-dependent comparison explains this deviation. Wind speed ratios of several pairs of stations are modeled with WAsP for 12 directional sectors and compared with the measurements. Deviations in the directional wind speed predictions were found to correspond with the length of the sea fetch: For smaller sea fetches WAsP seems to slightly over-predict the wind speed, while for long fetches of more than 30km an under-prediction is found.

An analysis of the vertical wind speed profiles at three sites indicates that a combination of different effects is responsible for the correlation between sea fetch and model behavior. Effects of atmospheric stability as well as a fetch dependent sea surface roughness have to be taken into account.