Despite the lack of high mountains or obstacles predicting offshore wind resources is complicated by a number of factors. Low roughness gives low turbulence and wind shear but thermal effects are important. Not only can wind speed profiles deviate from logarithmic on average but strong temperature gradients can produce thermal flows such as sea breezes and low level jets which are not well accounted for by current models. Useful references include: (Coelingh et al., 1998), (Barthelmie, 1999b), (Smedman et al., 1996), (Smedman et al., 1997). Additional uncertainty is introduced by the prospects of very large wind farms offshore. Wake effects within large wind farms are not well-known and offshore wakes are not well studied. Some useful references are: (Frandsen S. (editor) et al., 1996), (Crespo et al., 1999), (Magnusson et al., 1996). Interactions between the wind and the sea surface is also complex, particularly for extreme wind/wave studies e.g. (Donelan et al., 1993), (Johnson et al., 1997). Successful planning for operation and maintenance is crucial (DEA/CADDET, 2000) to maximise availability when large offshore wind farms up to 40 km from the coast will have access problems.
Some sources of uncertainty relating to resource assessment are given below:
Some sites without onsite data or nearby long-term records - very high uncertainty
Mean wind speed (measurement error, year-to-year variability)
Wind speed distribution (length of record, methodology)
Contribution of thermal flow (sea breeze, low level jets)
Vertical profile extrapolation beyond measurements (IBL, stability)
Power curve (measured, offshore)
Offshore wakes (lack of data)
Large wind farms (lack of wake data/lack of offshore data, models need further development)
Interaction between large offshore wind farms and coastal effects (lack of data, models need further development & evaluation)
Availability of wind turbines (lack of experience with larger wind turbines, problems with planning for maintenance, access problems).