Wind

Wind energy has been used for many thousands of years for propulsion, for powering grain mills and for water irrigation.

Wind energy is to all intents and purposes Solar energy. Wind is driven by the effect of the Sun’s energy both at a Global Level with differences in heating of the Earth at the Equator and towards the Poles, and at a Local Level by differences in the heating and cooling of land and sea - on the coast for example.

(The Danish Wind Turbine Manufacturers web site www.windpower.org provides a very good description of this and many other topics associated with wind energy).

Both Global and Local mechanisms generate winds sufficient to extract energy although they tend to operate on different time frames and in different ways. Coastal thermal winds for example such as those in California involve winds driven by the temperature differential between the Pacific ocean successive heating of the desert during the day and cooling at night i.e. a pronounced diurnal effect.

Storm front winds off the Atlantic such as those largely experienced by Ireland and the UK are driven by more Global effects and although they may have a diurnal component this tends to be overshadowed by the significant variations over a period of 3 – 4 days as the front moves through.

Wind energy and the design of a wind farm to extract that energy is thus very dependent on the type of wind, the location of the site, and topography of the site itself.

Two heights are given in the left hand table, one corresponding to a typical first exploration measurement height (10m) and the second closer to the turbine hub height (50m). As is clear from this table the average wind speed, and thus the energy in it increases (the Wind Power Density) with height above the ground. This is because friction due to vegetation - trees, hedgerows and even grass slows the wind down and this effect obviously reduces as the height above ground is increased. This speed up effect is known as the wind shear.

Wind Measurements
The effects of the wind are often illustrated by its effect on natural vegetation. Long living vegetation such as that of mature trees in particular gives a strong indicator of the local wind regime. This effect well illustrated in the image below is known as flagging and is often more pronounced in some species than others.

Methodologies do exist for using the degree of flagging as an indicator of wind resource, however, these must be treated with some caution since the effects may be enhanced by local topographic wind effects which might be less evident at the turbine hub height. Apparent flagging may also be induced by salt burn from ocean spray in certain directions if the site is close to the coast or any number of other growth deformation effects.

Predictive methods including sophisticated computer modelling exist for determining wind speeds from local meteorological stations and knowledge of the intervening terrain but the accepted and most reliable method of determining a sites resource is to measure wind speed and direction on an hourly basis and correlate those measurements against the nearest appropriate meteorological station.

Preliminary investigations might take place with a 10m wind monitor of the type (left), and an assumption made for wind shear before continuing the measurement campaign with a taller mast of up to 80m in height with multiple measurements at different heights (right).
The advantage of starting with a 10m tower is that it can be hand carried to site if necessary and erected by 1 man in less than half a day whilst the larger towers need more equipment and more manpower.

Typically a monitoring programme would continue for a minimum of 12 months in order to cover all four seasons, and would in the event of the site being deemed suitable for a wind farm continue until the wind farm itself was constructed.

Wind Result
The result of the speed and direction measurements is a site wind rose, which describes both the annual average wind speed from any given direction, and the amount of time that wind comes from that direction. The wind rose is site specific and it is this, which is used to design the wind farm layout and to predict the energy yield.

Although there are differences from site to site most of the wind roses for open exposed sites in the British Isles would illustrate similar characteristics namely a heavy bias for winds from the southwest and westerly directions, with a much lower frequency of occurrence from the northeast. The wind rose on the left from a site in the West of Ireland is based on 2 years measured data and illustrates a fairly typical distribution.