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'Feeble' anti-wind farm stories are misleading

UK: A spate of reports in the UK press in March criticising feeble wind farms that operate below par reveals how far the wind industry has to go to counter the common misperception that wind turbines are inefficient.

The story, which originated in The Sunday Times, centres on a study by former power industry engineer Allan Tubb for anti-wind-farm group the Campaign to Limit Onshore Wind Development. Tubb's analysis, is based on 2008 figures for wind farms, released by energy regulator Ofgem. The press reports state that 20 wind farms perform at less than 20% of their maximum capacity, and single out some of the oldest, such as Chelker in North Yorkshire and Blyth Harbour in Northumberland.

Of the 220 onshore projects above 1MW capacity in operation today, these were the fourth and sixth to start operating. Little wonder that they are among the UK's poorest performers; they are 17 years old and both owners had plans to repower the sites with up-to-date technology.

Hainsford Energy has planning consent to replace the nine turbines at Blyth totalling 2.7MW with seven new machines totalling 17.5MW. But Yorkshire Water was refused consent last year to repower Chelker wind farm with more powerful turbines.

Most misleading, however, is the report's condemnation of the low efficiency of wind farms in the UK. Headlines such as "Feeble wind farms fail to hit full power", or "Wind farms operating at less than onefifth full power" are misleading. As a variable source of generation, wind farms only deliver full power when the winds are sufficiently strong.

The Sunday Times also confuses efficiency with load factors, a common error, stating that the best projects achieve 50% "efficiency", while the norm is 25-30%. This debate actually revolves around load factors, a rough measure of the productivity of a wind turbine.

A 30% load factor means that a wind farm delivers 30% of its rated power on average over the year. So a 10MW wind farm with a 30% load factor delivers an average power of 3MW. Indeed, a 30% load factor for the UK is entirely respectable, while a 20% load factor would be regarded as a good result in Germany.

Load factors for wind turbines are generally lower than those of coal, gas and nuclear plants - although higher than solar photovoltaic installations. According to the government's 2008 Digest of UK Energy Statistics, onshore wind achieved an average load factor of 29.4%, with 34.9% for offshore wind. This compares with load factors of 49.4% for nuclear, 35.4% for hydro and 56.7% for coal.

But in assessing the effectiveness of a project, what really matters is the overall cost of energy. This is the bottom line for investors. Currently, the generation costs for wind, nuclear, hydro and coal fall within a similar range - between EUR55 and EUR70/MWh.

The efficiency of electricity generating plants is another matter entirely and is not related to the load factor. Efficiency is a measure of how well a power plant converts the energy in its fuel into electricity. Modern coal-fired power stations have efficiencies in the range of 35-40% (but load factors in the range of 75-85%).

The "fuel", in the case of wind energy, is the energy in the air stream that passes through the rotor. Although peak efficiencies - at one particular wind speed - are sometimes quoted, it is more meaningful to quote values that take into account the energy extraction efficiency over the full range of wind speeds.

An analysis in the latest issue of Windpower Monthly's sister title, Windstats, suggests efficiencies of large machines vary, from around 40% at low wind speeds (4-5 metres/second), falling to 30% at higher wind speeds (7-8 m/s).

Nick Medic, spokesman for trade group RenewableUK, said that it has asked The Sunday Times to correct the story. "If we don't get equal prominence to state our case then we don't rule out going to the Press Complaints Commission."

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