5.2.2.2 Average wind speeds

Wind speed is normally specified as the hourly wind speed, which is the average speed measured during one hour, during which time a large range of speed oscillations may occur. Although the hourly wind speed itself varies very considerably, this period is chosen as that giving the most consistent measure available, in the sense that there is always a good chance that two consecutive hours will yield similar values.

 

Fig 5.7 Typical Speed Duration curve

These hourly wind speeds can then be measured at a given site over at least a year, from which data the variation can be analysed and an annual average wind speed obtained. A typical variation is shown in figure 5.7, which gives the number of hours in a year for which any given wind speed is exceeded. This also shows that the wind blows harder than the average for slightly less than half the 8760 hours in a year (the median wind speed is slightly lower).

5.2.2.3 Choice of site

Hills and mountain passes offer higher wind speeds than normal, and were initially proposed as major sites for large turbines or wind farms. However the limited number of accessible sites of this kind and local planning objections have resulted in relatively few such developments, notable examples being a few large wind farms in windy passes in California. Assessment of the wind regime at less dramatic sites is therefore all the more important, as well as careful investigation and information of local public opinion. On a smaller scale, whereas urban sites (for either community or household use) are generally difficult to develop for reasons of planning and safety, there is a large number of isolated rural dwellings or communities where wind power is competitive by reason of the cost of cable connection to the nearest electricity grid substation.

Source: ref 5.3 Figure 5.8 Annual average winds (EU 1989, for sheltered terrain)

 

In all cases, a good enough estimate of the annual wind speed can usually be made from national data bases or isopleth maps (e.g. fig 5.8), combined with suitable factors to account for local topography such as hills, cliffs, forests etc. A particularly favourable position is near the top of a long slope leading upwards away from the direction of the prevailing wind. Where a large investment is proposed, it may be necessary to measure wind speeeds at the site over a substantial period before the final turbine specifications are fixed.