Is there enough wind energy to be harvested?

To ‘harvest’ the energy in the wind the turbines need winds speeds above a critical amount, typically taken as a mean of 7 m/s (metres per second), so it makes sense to site wind farms in areas that have mean winds above this value. What isn’t so easy is to predict what the winds are likely to be at any proposed wind farm site and at the so-called ‘hub height’.

A first difficulty is that, as anyone will have experienced, the wind is very variable in with rapid changes over very short distances and from time to time. Second, actual measurements from meteorological records are hard to come by. For weather forecasting purposes winds have traditionally been measured at a standard height of 10m above ground level (AGL) and not at the enormous heights at which wind turbines are nowadays installed. Average wind speed increases with height but the rate of increase gets less the higher you go.

Although there are standard formulae for estimating what the winds will be at ‘hub height’ from measured values at 10m AGL, these rely on a series of assumptions that in practice aren’t always justified. This is why it makes sense for potential sites to be screened carefully and, if possible, for winds to be measured at these sites prior to any proposals being submitted. Given the importance of such measurements to the cases for and against developments it would be a service to us all if the developers made these data available to the community when, and if, they decide to submit formal planning applications.

All of that said, it is possible to make some informed guesses. The Department for Business, Enterprise & Regulatory Reform’s (BERR) wind speed database is available from their website

 (http://www.berr.gov.uk/energy/sources/renewables/explained/wind/windspeed-database/page27708.html)

and gives estimates of the annual average wind speed throughout the country. These are derived from a careful model that accounts for the effects on average wind speeds of the shape of the ground and the nature of the ground cover. It is also very much the standard source for the evaluation of wind energy developments. On it, estimates are available for each and every kilometre square of the country.

What does this reveal?

Using these estimates suggests that the site is characterized by relatively low winds, with average values of 5.7m/s at 10m, 6.4 m/s at 25m and 6.9 m/s at 45m. AGL. As you might expect, down the hill by the Brampton Valley Way estimated speeds are lowest (around 6.6-6.7m/s at 45m), then they rise as one goes up the hill to their highest values (6.9m/s at 45m) on the edge of the plateau on which the old airfield sits.

What are the consequences?

This has three consequences that are likely to affect the detail of the proposal:

• In order to access higher wind speeds, the turbines will need to be very high above ground level, well above the 45m height for which the estimates are provided;
• They may also be sited as high as possible on the edge of plateau on which the old airfield stands. Further to the east across the old airstrip the estimated winds drop as the land becomes more and more sheltered;
• If built, their load factor (the proportion of their rated capacity that is actually achieved) is likely to be low, probably less than 25%. At Burton Wold in 2007 the 100m towers with hub height of 64m, at a site with an estimated wind speed at that height of 7.0ms^-1, achieved a very modest load factor of 22.18%.

Meaning?

In turn these mean that:

• The impairment to the landscape will be very high because the turbines will themselves be very high and located in the most visible positions on the proposed site;
• For much of their working lives the turbines may well be harvesting the subsidy inherent in the ‘Renewable Obligations’ scheme rather than the ‘green’ energy they are supposed to.

Given the importance of wind speeds to the entire notion of harvesting wind energy it is perhaps surprising that the guidance given by Government to planning agencies in its ‘key principles’ (see Planning Policy Statement 22: Renewable Energy ) is that when they decide whether or not to oppose wind farm proposals they cannot make assumptions about the ‘technical and commercial feasibility of renewable energy projects’