Say No to the Proposed Wind Farm at Brent Knoll Home Page

ecotricity, the company proposing the Brent Knoll wind farm, has published an Information Pack in which it is claimed that the wind turbines would produce enough electricity each year to power just less than 8,000 homes and stop over 23,300 tonnes of chemicals being pumped into our atmosphere.

In this document we consider these claims, offer some criticisms, and produce additional information to put the claims into context.

1. Generation Claims
The wind park applicants estimate (1) that the proposed five 2 MW turbines "will together generate around 25 GWh of electricity per year". This is "based on known wind speeds for the locality and the performance of the turbines".
We are taking steps to examine the validity of these claims, by focusing on the actual wind speeds we can expect at the site itself, and the claimed electrical energy generated in a year.

Figure 1 depicts the proposed installed capacity,(2) which implies a capacity factor (3) of just over 28%. Capacity Factor, sometimes called Load Factor, is the energy generated during a given period divided by the energy that would have been generated had the wind farm been running continuously at maximum installed output, i.e.:

The peak instantaneous load of a domestic house is between 5 and 10 kW. Thus even at maximum output the turbines could support the peak loads of, roughly, between 2,000 and 1,000 houses. But in fact the turbines would only infrequently be operating at maximum output. However, as is well known, the number of hours during which the wind-farm would produce its maximum output is small. Without access to the wind profile at the site a precise statement is not possible, but data from Denmark suggests that wind turbines will only produce above 50% of their rated capacity for ca. 16% of the time, and that for 70% of the time they will be generating less than 25% of their rated capacity (and for some time will generate no energy at all). It should further be noted that the level of generation is randomly intermittent in relation to grid demand, and, as is clear from the E.ON Netz Wind Report 2005, even with the best wind forecasting the output cannot be predicted with great accuracy.

In conclusion it is fair to say that a wind turbine power station cannot make, as conventional plant can, a confident promise of power to any houses. That is to say they cannot guarantee, with a high degree of confidence, to meet the power demands of a single house, let alone several thousand. To avoid confusion ecotricity should have made it quite clear that they are talking about aggregate energy, delivered in dribs, drabs, and sudden spurts over a year, rather than reliable power.

The claims are liable to mislead in a second respect. Domestic electricity is typically only 1/3 of total electricity consumption in a region, so expressing electrical energy consumption in terms of homes is a misleading sleight of hand, and tends to give an exaggerated sense of significance. Furthermore, electrical energy is typically only 1/3 of total energy consumption in a region.
In fact, to put the energy claim into perspective we need to consider it in national perspective. 25 GWh is a very small fraction of the UK's total generation (400 million MWh), in fact it's only 0.00006 of the UK's total generation.

In addition, ecotricity's assumed domestic consumption is a surprisingly low We have consulted the Energy Efficiency Advice Centre for Bristol & Somerset. The Centre advises that the electricity consumption for lighting and appliances in an average 3 bed semi household is currently 4,000kWh. This is 20% higher than the developers' claim. This implies that, assuming the developers' claimed capacity factor, the five turbines would supply 6,250 homes. Even the British Wind Energy Association recommends the use of an assumed 4,700 kWh per year. On that assumption the energy supplied would be enough for just over 5,000 homes.

However, neither of these figures includes heating, which can of course come from other sources than electricity. Nevertheless, if a household is using electricity for space heating it may us as much as 13,500kWh/year, and for hot water around 2,500kWh/year. Bearing all the above points in mind it seems that the applicants' claim of number of households supplied is a significant overestimate.

If we now look at the claimed capacity factor. As stated in Ref. 2, capacity factors vary, primarily due to changes in local wind speed as well as the design of the turbine. The proposed turbines, designed and manufactured by ENERCON, are called the E-70 class. Enercon technical data (Reference 4) gives the electrical power generated by the E70 turbine at each metre/second of wind speed, above a stated cut-in (low) wind speed and up to a stated cut-out (high) wind speed. Low wind speed conditions are especially significant, being likely to occur at individual sites in around 15-20% of all hours.5 We have assumed that the quoted electrical power generated figures takes no account of the power consumption (perhaps 2% of installed capacity) by the turbine's own control systems.

Combining the turbine manufacturer's data with historic wind data already obtained from the Met Office for a similar nearby location, we estimate that the turbines would generate only about 17.5 GWh, which is some 30% lower than claimed by the applicants for the wind park. We have requested detailed wind speed historic data from the Met Office (Reference 6) for the turbine hub height at the actual site grid reference, with which we hope to obtain an even more accurate view of what generated electrical energy could be expected from this development. Note that electrical energy generated in a year will be reduced by turbine unavailability for maintenance, claimed by the manufacturer4 to be no more than 1.5% each year. (It is at present unclear whether this unavailability is for routine maintenance, or whether it includes an element of breakdown maintenance.)

The effect of these factors suggests that the applicants claimed production levels are in all probability a significant over-estimate, which would also have a bearing on its CO2 displacement benefit. In fact ecotricity's claims in regard to CO2 saving are also questionable, and we consider these in the next section.

2. CO2 Emissions Savings
ecotricity claims that the Inner Farm Wind Park will save 23,000 tonnes of CO2 per year. It is self-evident that this claim must be based on the assumption that the wind-farm will displace coal generation when it supplies energy to the grid. However, analysis reveals that this claim is not sound, and is not in keeping with government practice.

ecotricity claims that the wind-farm would generate some 25,000 MWh per year. Thus, they are claiming that approximately 900 kg of CO2 will be saved by every MWh generated. This can only mean that the wind energy is expected to displace coal generation, which emits roughly 0.9 tonnes per MWh. However, it is far from certain that wind energy will actually displace coal (because of very high gas prices at present it is almost certainly displacing mch less polluting gas turbines).

Because of this uncertainty, the DTI recommends use of a grid average emissions "factor" when calculating wind turbine savings. This factor is 0.43 tonnes per MWh, so the Inner Farm wind turbine saving according to DTI methodology should be less than half of what ecotricity claim.

In fact, as is well known, looking into the future it is very likely that wind would work in tandem with very high efficiency new gas generation, which is much less polluting than even modern gas plant. The emissions from such plants could be as low as 0.3 tonnes per MWh, about 1/3 of the savings claimed by ecotricity.

Even ecotricity's number, 23,000 tonnes of CO2 is just 0.00004 of UK emissions (550,000,000 million tonnes per annum), and just 0.0000001 of world emissions (24,000 million tonnes).

3. Income from the Inner Farm Site
In return for this very small and unreliable electricity generation, and tiny emissions savings, the Inner Farm station would earn little short of £2 million a year: