The fact remains – nine wind turbines are not enough

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As was very politely pointed out by your reader J Bennett in last Thursday’s Guardian, I had made an error over wind turbine output – although not quite to the power of ten in my calculations. For which I apologise.

However the actual result is somewhat more complicated. It is, as noted, important not to confuse efficiency with effectiveness and capacity factor.  Also note that power ratings are measured in units of MW, whereas electrical energy/output is measured in MWh (or kWh – a unit of electricity).

If the 1,720MW of total capacity (of the enlarged Spalding gas-powered station) will supply two million homes, then it is claimed that it will operate at a capacity or load factor of: CF = (2,000,000 x 4.7)÷(1720 x 8760) = 0.624 = 62.4 per cent. Where 4.7MWh (4700kWh or units) is the average home consumption of electricity in a year, 8,760 is the number of hours in a year.

The capacity factor is the average amount of electricity produced compared to the total amount of electricity it could produce if it operated at full capacity of 1,720MW. 

The 62.4 per cent figure is a modest claim; it could operate at a much higher value, so I suspect it will operate in load-follow for much of the time; that is the demand of the National Grid will regulate the power station output not other external factors.

For wind turbines 25 per cent is a supportable capacity factor in England.  So to produce sufficient electricity for two million homes you would need an installed capacity Y = (2,000,000 x 4.7)÷(0.25 x 8760) = 4290MW, which would require 4290÷2 = 2,145 turbines (2MW rated turbines).

Wind turbine capacity factors are generally relatively low because of the variability of the wind: no wind, no electricity.

For those of you that use electricity as the main source of household energy (ie no gas or oil heating) you may be thinking that 4,700 units a year is a little low. You would be correct; the total energy consumption of an average home is between three and four times this figure (gas supplying the majority).

I guess 2,145 turbines is still quite a lot given that there are 3,825 across the whole of the UK already (and this includes offshore as well). The fact still remains, nine 2MW turbines are insignificant when compared to Spalding Power Station’s energy output and its contribution to reducing the emissions of greenhouse gases.

JANE DAVIS

Little London

Spalding