From Rick Jefferys
T. Robertson rightly highlights our unreasonable focus on renewable power and neglect of renewable heat (1 November, p 20). But his proposed solution, running wind turbines for heat at very low wind speeds, will not help. Turbines may have a tough time synchronising to the grid at low speed, but this is not a problem, since there is hardly any power in these winds.
A typical turbine will output its rated power at a wind speed of around 12 metres per second, and power increases with the cube of the wind speed. So at 3 metres per second only 1/64 of the rated power is available; a 1-megawatt turbine would be producing about 16 kilowatts.
Air source heat pumps running on renewable power with thermal storage (such as a hot water tank) are a far better solution.
Your statement that a “large 10-MW wind turbine would create at least 10,000 MWh/year” is also wildly off-beam? No one would build a turbine which ran on a load factor of 11 per cent, only generating full power for 1000 hours per year. Offshore turbines typically run at over 40 per cent load factor and thus generate 35,000 MWh per year.
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Anthony Higham refers to a 10-megawatt wind turbine for every 1000 homes. Even at a very modest output of 10,000 MWh per year, which implies a load factor of only 11.4 per cent when over 30 per cent is the expectation offshore, that implies annual consumption of 10 MWh per home or 10,000 kWh. In Germany the rule of thumb is that a home consumes about 4500 kWh per year. Perhaps Higham’s home is among those already heavily festooned with seasonal lights?
• We were making a conservative worst-case estimate of the power output of a such a turbine, to show that even in these cases they perform impressively. We agree that it is likely the load factor could be a lot better.
Berkhamsted, Hertfordshire, UK
