Prospects for offshore wind energy: The state of the art and future opportunities. Offshore wind climate and wind energy applications

Exploitation of offshore areas for wind power has two main advantages-higher wind speeds than are found on land (except in areas of significant topography) and fewer environmental constraints. The disadvantages lie in the additional costs of offshore installation and maintenance although policy aspects of exploiting the offshore resource in the immediate future also have to be considered. Siting of offshore turbines is much more complex than may initially appear to be the case. In addition to physical constraints such as sea bed properties and water depth, the offshore region, particularly in the near-coastal zone, is already used for a wide range of activities. The lack of suitable meteorological data from offshore areas is a major problem. Whilst spatial variability is expected to be low, the sparsity of suitable data makes the description of the wind climate a challenging task. Wind speed observations are available from many different sources, including oil and gas platforms, meteorological stations, light vessels, the voluntary observing fleet and ocean weather ships. The method of observation varies from source to source, as does the length of record. Much of the data is of questionable quality and the coverage remains limited with the North Sea having the most observations. For wind energy purposes it is therefore necessary to use a predictive method. Most techniques use upper air or geostrophic data, although some use measurements from onshore coastal stations. The predicted mean wind speeds are then compared with observations. Other aspects of the offshore wind climate such as the seasonal variability and the possibility of a diurnal cycle in wind speed are also examined. Since turbine hub-heights range from 30 to 100 m stability corrections to the logarithmic wind speed profile may be necessary if conditions deviate significantly from near-neutral. Finally, the particular problems which wind energy production presents in the coastal zone are considered. It is apparent that siting relatively close to land has many advantages in terms of cost but in this area the wind climate is particularly complex due to the changing nature of the surface. A further problem is that the modification of the wind speed moving from land to sea is far from well-known, mainly due to lack of suitable data with which models can be developed and tested. The monitoring project at Vindeby in Denmark should provide useful data for modelling the wind climate of the near-coastal region.