In the absence of wind data from the Airfield Farm site, it is necessary to rely on data from the Met Office Station at Thurleigh which is 5 miles from Airfield Farm. The Thurleigh site is at 85 metres above sea level and the sites for the turbines at Airfield Farm are on average 95 metres above sea level. This would give an advantage to the Airfield Farm site of an increased wind speed of about 1.1%.

Met Office measurements are taken at 10m above ground level so it is necessary to calculate a shear factor to adjust the wind speeds to those that can be anticipated at a turbine hub height of 80 metres. The easiest method of calculation is to use the Danish Wind Industry Association’s (DWIA) Shear Calculator to be found on their web site. This gives a shear factor of 1.25 at Thurleigh, if a terrain roughness level of “.5” is used, and equates to an open airfield site without neighbouring hedges and trees. The roughness level at Airfield Farm should realistically be assessed at “1”, or more, because there are many adjacent hedges and woods. Using 1 for roughness gives a shear factor of 1.16 when relating Airfield Farm to Thurleigh wind data. The DWIA website explains ‘roughness’ and other factors involved in calculating a Shear Factor.

In order to reflect the advantage of higher wind speed at an 80m hub height at Airfield Farm, but with increased terrain roughness, a shear factor of 1.16 has been used to increase the wind speeds in the spreadsheets showing Load Factors (LFs).

To evaluate the true level of ‘intermittency” or ‘variability’ and to determine the usefulness of the electricity produced in the grid context, average hourly figures are crucial.

UK (Mainland) Wind Resource

To be able to evaluate the wind resource across mainland UK, it has been assumed that all 64 mainland Met Office stations have a Nordex N901 erected on the site. The LFs have then been calculated daily at 7.00 pm (1900 hours) using the average for that one hour since 1st March 2005. Wind speeds from the 64 Met Office sites plus 25% to allow for shear have been used to calculate the LFs on a daily basis.

The average annual load factor obtained from the data, without any deductions for down time or for any other reason, is about half that claimed by the wind industry and significantly below the UK onshore 5 - Year average (2000-2004) provided by DTI in the “Digest of UK Energy Statistics 2005” (Table 7.4, page 188). The charts also show unpredictable peaks and troughs of the supply indicating the severe problems of integrating wind energy in a grid system. On one occasion the average LF across the whole country dropped from 83% to 9% in 24 hours. This important point identifies why wind energy can only ever be considered as an additional energy conversion technology and is unlikely to replace firm sources of generation. It is also the basis of work done by Michael Laughton, Lewis Dale and Hugh Sharman.

The power supply is intermittent as demonstrated by the figures, and is much more volatile than is generally admitted. Arguments to the effect that clustering and national diversity will reduce variability to an acceptable level are not sound.

Supporting Document

Further detailed explanation is given in UK Wind Energy Resource : Variability, Intermittency and Dispersal

Monthly Data Charts

The monthly data may also be downloaded as pdf files.

2007

Bedford Hourly Load Factor : 2007

UK Averages from 64 Met Office Weather Stations : 2007

2006

Bedford Hourly Load Factor : 2006

UK Averages from 64 Met Office Weather Stations : 2006

2005

Bedford Hourly Load Factor : 2005

UK Averages from 64 Met Office Weather Stations : 2005