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          Site testing and feasibility studies

          The wind is a fickle energy resource.

          The wind is a fickle energy resource: it comes and goes, changes direction, and blows more or less strongly in different places and at different times. Measuring the wind regime on a potential site is important, both for choosing a suitable wind device, and for finding exactly where on the site to install it.

          A useful first step is to find out the average wind speed for a site, which will give a rough idea of how windy it is.  However, the average wind speed is not enough on its own to predict the typical annual output of a wind turbine with any degree of accuracy. For that, it is necessary to work out the wind profile of the site, showing how frequently the wind blows at different speeds and from different directions.

          Testing a site for a wind turbine (or a whole wind farm) can be expensive and time-consuming. Typically, data is collected for at least one year and is often correlated with decades of readings from the nearest meteorological station, in order to gain a realistic impression of the wind regime at the test site. There are also computer software packages that can simulate the wind regime of a site, but these are expensive too.

          [pictures of anemometer and wind vane]

          It is also possible to model how the wind behaves using Computational Fluid Dynamics (CFD).  This is particularly useful where buildings predominate, as the funnelling effect of the buildings can lead to a large range of wind speeds in quite close proximity. Setting up an anemometer in such conditions would only capture data relevant to the one spot: CFD gives a clearer overview. Computer modelling is clearly the only option if the building has not yet been built.

          In practice, the thoroughness of site testing or computer modelling will depend on the cost of the proposed project. For a multi-million pound wind farm or a major property development, it may make sense to invest thousands of pounds on testing. For a small stand-alone turbine, it makes more sense to use a simpler wind speed logger costing hundreds of pounds, rather than thousands. The cheapest option for a small turbine is to set it up and monitor its performance in use. If the energy output seems low, the turbine can be moved and monitored again until a better site is found.