A team of Danish researchers have developed a method to assess the efficiency of different sized onshore and offshore wind farms. The team investigated whether the farm power density - the power per unit area - of very large wind farms was limited, and related this to their efficiency and annual energy production. The study used regional atmospheric model simulations to assess the power production and wind speed through the farm area.
Lead author Dr Patrick Volker, from the Technical University of Denmark, commented: “Wind farms covering large areas need to remain sufficiently productive. Inside very large wind farms, winds can decrease considerably, to a point where an equilibrium wind speed is reached. If the equilibrium wind speed is too low, this can have a negative effect on the wind farm’s efficiency, so it’s important to find out whether future wind farms that cover large areas will be productive enough to be of benefit.”
To assess the velocity field and power production of hypothetical wind farms, the researchers used the Weather Research and Forecast (WRF) model to simulate the wind speed field inside different wind farms, ranging from small (25 square kilometres) to very large (100,000 square kilometres), under different wind and surface conditions (land and water).
They considered three regions in which to place the wind farms - onshore with moderate winds; and offshore with strong and very strong winds, respectively. The simulations were designed to understand the effect of free-stream wind and surface roughness on the time-averaged velocity reduction in a very large wind farm.
From the velocity reduction in very large wind farms, they determined the degree to which the power density – in the limit of very large wind farms – depended on regional conditions. Their results also showed which wind farm type, in terms of wind farm size and turbine density, would be most efficient in a given region.
Dr Volker said: “Our results show that the power density even of very large wind farms depends on the local free-stream wind speed and surface characteristics. In onshore regions with moderate winds, the power density of very large wind farms reaches 1Wm−2, whereas in offshore regions with very strong winds it exceeds 3Wm−2.”
“Despite a relatively low power density, onshore regions with moderate winds offer potential locations for very large wind farms with wide turbine spacing. In offshore regions with strong winds, meanwhile, clusters of smaller wind farms with narrower turbine spacing are generally preferable. However, under very strong winds, small wind farms become extremely efficient power generators and very large offshore wind farms also become efficient, as the farm power density is significantly larger than previously thought.”