If you are planning to install a wind turbine it is sensible to carry out a full site assessment and measure the wind speed first, to find out how windy your potential turbine site really is. We can't emphasise this enough, because if the wind speed isn't good enough you'll be wasting your money - and with the best intentions in the world - a turbine that doesn't turn is hardly going to help your energy bills (or climate change for that matter).   

Measurement

Winds are influenced by the ground surface of heights up to 100 metres. As most wind turbines are likely be positioned below 20 metres this is a vital factor to consider in choosing your perfect site. So not only do you need to assess the wind speed of your area but you need to think about local factors to your site such as the roughness of the surrounding terrain and obstacles such as trees and buildings might affect the output.To fully take such factors into account you really need to use an anemometer which measures the wind speed and direction at your exact location.

Roughness

Roughness relates to the friction played out by uneven ground on the wind. In general a greater level of roughness will slow down the wind speeds more. Smooth surfaces such as water, concrete or tarmac will have little impact, whereas long grass, buildings, hedges and trees are all important factors to consider when siting your turbine as they can greatly reduce the speed of the wind. These factors also increases turbulence which again reduces wind speeds.

Roughness is defined by class; smooth sufaces have a low roughnesss class such as concrete runways are 0.5, while landscapes with many trees and buildings have a high roughness class of 3 to 4. To calculate the projected wind speeds at different heights above the ground, roughness length, or the height above ground level where the wind speed is theoretically zero, is used.

Wind shear

Wind shear shows the relationship between wind speed and height above ground. As the graph below shows the speed is slower nearer the ground and then, as the surface friction influence lowers, the speed increases logarithmically.

The natural logarithnic function this graph is based on is:

V = V_ref ln (Z / Z _o ) / ln (Z _ref / Z _o).

Where V is the wind speed at a certain height Z, V _ref, is the known speed at height Z _ref, and Z _o is the roughness length.

Obstacles

It is very important to look for potential obstacles to your wind turbine such as buildings and trees - the speed is reduced as the wind swirls around the obstacle, creating turbulence.

The porosity of the obstacle must also be considered, for instance trees in winter are fairly open and will still let some wind through, however buildings are solid and so will completely block the wind creating wind shade behind the obstacle. Porosity is defined as the open area divided by the total area of the object facing the wind

Wind shade causes a loss of energy due to a slow down effect. This is more pronounced closer to the object and near the bottom, and increases with height and size of the object. Any obstacles should be taken into account if they are within 1km of the potential turbine site. E.g. a building 20 metres tall will cast a shadow that encompasses a full size turbine with a hub height of 50 metres, 300 metres away. So the further away you place your turbine from any obstacle the lower the impact on the wind speed.

Prevailing wind direction

Winds for a specific site will generally from come from one direction for the majority of the time. This is called the prevailing wind. General rule for prevailing wind speed directions are:

Other local factors

Sea breezes are caused by the land mass heating more quickly than the sun, as the warm air rises it flows out to sea, creating an area of low pressure on land which draws in the cooler sea air. This will potentially alter the prevailing wind direction. During the night as land temperatures decrease the effect is reversed, although these land breezes are much weaker.

Factors that can increase the wind speed

The tunnel effect is the increase in wind speed between two buildings or hills due to the increase in air pressure as the air is compressed between the obstacles to the wind. So if you can site your turbine in one of these natural or man-made tunnels it is likely you will see a significant increase in the wind speed and so wind energy produced. This is closely linked to the Venturi effect where the wind speed increases when the air becomes even more compressed inbetween these obstacles as they get closer together.

The hill effect also increases wind speeds. If you're lucky enough to have access to the top of a hill this will most likely be the best place to site your turbine, other factors considered. The wind becomes compressed on the windy side of the hill (facing the prevailing wind direction) as it reaches the top it is able to expand and flow into the low pressure area on the lee of the hill. But! if the hill is too steep or has an uneven surface it will experience turbulence negating the benefits of siting the turbine there.