Effect of island topography and surface roughness on the estimation of annual energy production of offshore wind farms

Accurate information on the wind flow characteristics within a given site is a prerequisite for making precise prediction of the wind energy production of offshore wind farms. Two typical methods of taking field measurements at an actual site are employed. The first is to install an offshore meteorological mast, and the other is to use onshore meteorological towers on the coast or on adjacent islands. This study explores the feasibility of predicting the annual energy production (AEP) from an offshore wind farm by analysing the full-scale data measured at an adjacent island by using WindPRO and WAsP software. These software programs are used to model the topology of the island and to predict the wind resources and the energy production of wind turbines by using at least one set of measured wind data. The effects of the island topography and roughness on the prediction of the AEP are obtained, and the predicted AEP affected by inaccurate island shape modelling is analysed. When wind velocity data obtained at a height of 60 m are used, the prediction error of AEP decreases to 38% compared with that measured at a height of 30 m owing to inaccurate modelling of the island terrain. Therefore, in order to reduce the prediction error of AEP caused by inaccurate terrain information of the island it is desirable to increase the height of the wind measuring tower. Inaccurate roughness modelling of the island has little influence on the prediction of AEP.