Knowledge of how the wind potential will be in a planned wind farm is of large importance for how the economic outcome of the wind farm will be. If the wind is overestimated too large turbines will be built,
hence too costly. Underestimation of the wind potential will give to small wind turbines so that a lot of the energy in the wind will be lost. The wind potential in the farm is also a function of the type of turbines that will be built. Smaller turbines can be located closer to one another; more turbines can be situated in the farm, than what should be the case when localising large wind turbines.
The wind potential in a wind farm is rather complicated to determine. The flow downstream a wind turbine, wake, is determined by a number of parameters, which depends on both the site and the turbine. One parameter, that as been more or less neglected so far is the atmospheric stability. It has been found, however, that it can not be neglected when calculating the wind potential in a wind farm. The structure of a wind turbine wake is also dependent on the atmospheric stability.
For calculations of the wind field downstream a wind turbine the relevant parameter to use is the transport time. Since the development of the wake is a function of how long time the mixing processes have been active. By using the transport time as the crucial parameter, expressions have been developed for calculations of significant parameters for wakes, wind speed, turbulence intensity and length scale. The expressions have been validated against full-scale measurements and wind tunnel measurements.
These expressions are not restricted for neutral conditions but is valid for all atmospheric conditions.
Investigations of results from full-scale measurements and theoretical calculations clearly indicate the importance of stability. The stability do not only effects the wind potential it also modifies the structure of a
wind turbine wake. Roughly it can be said that during unstable situations the turbulence intensity is higher in the wake than during neutral conditions, the wake is also wider for these cases. For stable stratified atmosphere the wake is less turbulent but narrower, creating larger shear than for neutral stratification. The above results lead to the effect that a wake during unstable stratification has lower wind speed at the same distance downstream a wind turbine than during stable stratification (at hub height), if all other parameters are the same. Differences up to 50 percent due to stability effects as been observed. The importance of stability increases when the transport time increases as a result of the increased influence of the differences in mixing time.
The loads on turbines that are situated downstream other turbines will also be depending on the stratification of the atmosphere, due to the different shapes of the wake