Wind-power efficiency not quite so simple

HAVING read Daniel Brady’s letter (December 15) I think it is appropriate to put his assertion that ‘modern wind turbines are far from inefficient’ into context.

If we start by considering efficiency as the percentage of energy turbine blades can extract from the wind’s kinetic energy, the theoretical maximum, as defined by Betz’s law, is 59.3 per cent. But, as this based on an ideal rotor having no mass or hub and an infinite number of blades, practical efficiency of the rotor is unlikely to be more than 50 per cent under ideal flow conditions.

But the output of a wind turbine is also dependent on wind-speed – the optimum speed being around 15 metres per second (m/s). The power available from the wind is a function of the cube of the wind-speed so, assuming a linear relationship, if the wind speed drops from 15m/s to 7.5m/s, the turbine output will fall by 75 per cent and, if it drops to 3.75m/s, the output will fall by 88 per cent (some turbines don’t produce electricity at wind-speeds of less than four m/s).

In an optimum location, modern wind turbines may typically generate about 30 per cent of their capacity over the course of a year, but the official figures on the Department of Energy and Climate Change website show that last year the average output of Britain’s offshore turbines was just 26 per cent of their capacity.

By comparison, the conversion efficiency of conventional thermal power stations is typically 50 per cent, ie they are 20 per cent more efficient than a wind turbine in converting their source energy into electricity, and they provide a continuous output all day and every day.

A conventional combined heat and power (CHP) power station can, however, achieve efficiencies of 70 per cent to 80 per cent.

If, however, we consider efficiency in terms of the cost of electricity generated, including the costs of building, and decommissioning, and use the cheapest method, ie pulverised coal, as a baseline, the relative costs, according to a 2010 report of the US Department of Energy, are: gas with carbon capture and storage (CCS) 13 per cent more expensive, nuclear 19 per cent more expensive, coal with CCS 29 per cent more expensive, wind – onshore 49 per cent more expensive, wind – offshore 90 per cent more expensive.

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So again, when compared to current alternative generation technology, even that employing CCS, wind turbines cannot be said to be efficient.

Finally, I have to say that I find Mr Brady’s remark that Mrs Malt’s generation “has had its turn and a fine mess it has made of it, so how about stepping down...” is insulting and very simplistic. To denounce an entire generation as having made a mess is to assume they all colluded in the mess making – if in fact a mess was made.


Bramley Avenue