Wind power is an exciting technology for anyone interested in generating their own clean and renewable energy, but unfortunately it is not accessible to everyone. A question that we’re often asked is “how much energy can this wind turbine produce?”, but the answer to this is completely dependent on the site where it is installed and the amount of wind available to it. There are several things that you can do to assess the suitability of a potential site for wind turbine.
The theoretical power available in the wind is proportional to the cube of the wind speed, which means that if you double the wind speed you can extract up to eight times as much energy. Because of this, its important to site your turbine in the windiest site available to you. To get an idea of the wind resource in your area you can look at the NOABL database, which gives a fairly rough prediction of the annual mean wind speed at your grid reference. To check this have a look at the BWEA page here – http://www.bwea.com/noabl/index.html
This will give you a figure for the average predicted windspeed at your site, but is not particularly useful for a few reasons:
This is an average figure. Because the relationship between power and windspeed is cubic, not linear, a wide spread of windspeeds around this average would mean that more energy is available than in a relatively narrow spread. There is no way of telling from the NOABL database what sort of distribution of windspeeds to expect.
The values in the database do take into account neither local features such as trees and buildings nor variations in topography inside a grid reference square.
The windspeeds are not actual readings, rather they are calculated by interpolating data from fixed sites such as airports nearby. This further reduces the accuracy especially if you’re a long way from the nearest piece of true data.
In practice NOABL is useful only as a rough guide – if it tells you that there is an average windspeed of 3m/s at your site then it probably isn’t worth investigating wind turbines any further. If, on the other hand, NOABL tells you you’ve got 5 or 6m/s average then a turbine could be worthwhile.
The next step is to look at your local topography. This means landscape but it also means buildings and vegetation around your potential site. Wind turbines need a clean flow of wind free from turbulence to function effectively and efficiently, and every obstacle near to the turbine will affect this flow of wind. Even if an obstacle is below the proposed height of the turbine it may disrupt the airflow above it. Buildings are particularly bad in this respect, because they are so solid and because they generally consist of hard flat surfaces perpendicular to the flow of the wind. Behind and above a building the airflow will be disrupted for a surprisingly long distance, and you should take this into account when planning to erect a turbine near buildings. As a general rule, V3 do not recommend putting wind turbines in built up areas because of this, but like all rules it can and should be broken sometimes in exceptional circumstances.
In general wind speed increases with height above the ground, and also the higher you go the less turbulence there is from obstacles on the ground. Because of this you should always put a turbine on the tallest tower possible. Tower height is limited by up front cost, but in most cases a taller tower will pay for itself over its lifetime by enabling the turbine to capture more energy. Of course, the major limiting factor on tower height is generally planning permission, which varies from place to place. Our advice there is to talk to your neighbours early on and get them on board with your plans.
If you’re looking at a large expensive turbine then you should invest the time and money in a proper wind survey of your site. This involves putting up a mast and logging equipment to measure wind speeds over the course of a year or two years and will give you a very accurate prediction of the energy available at a site. Combining this data with the wind turbine manufacturers performance data (the power curve) will give you a very good idea of how much electricity you can expect to generate annually, and these predictions are essential if you’re looking for investment.
If you’re looking at a smaller turbine, the wind survey may actually cost more than the machine itself, so it is probably a better idea to assess your site qualitatively yourself, or get someone with more experience to do this for you.
As well as grants for buying turbines there are now grant schemes to help pay for the cost of feasibility studies. Check the low carbon buildings trust website for details.
Tom
