 SIR - The Duke of Edinburgh often displays a fine range of medals. He deserves another for speaking out so forthrightly against useless, expensive wind turbines (report, November 20). Ralph Bradley Harrogate, North Yorkshire SIR - Investment in wind turbines would have been better spent on large plants for the anaerobic digestion of sewage to generate methane, which can be extracted and fed into the communal gas system.  Small plants have been successfully run in a number of countries. The feedstock is unlikely ever to fail and the residue from the process is claimed to be a valuable agricultural compost.  In my view, this would be an excellent example of natural renewable energy.  alan S Lindsey Great Bookham, Surrey SIR - In Fair Isle, in northern Scotland, where there are no trees, the wind is constant and the installation of a turbine has revolutionised the lives of the islanders. But this is a specific exception. In most cases, wind turbines are expensive disasters. Michael Plumbe Hastings, East Sussex SIR - From the standpoint of saving CO2 or of simple engineering efficiency, wind farms achieve very little. at best, their actual output is around 28 per cent of installed capacity.  This is poor compared to other electricity generation methods, and it does not seem to coincide with when it is needed in cold weather, as there is often little wind in periods of high pressure.  In addition, wind power needs at least 85 per cent back-up from other generation methods for when the wind stops blowing, which has to be kept on continuous "spinning" stand-by.  Denmark has not managed to switch off any conventional power stations 30 years after introducing wind power as an alternative, and Germany is disappointed with the results of its wind programme and is having now to build more conventional power stations.  The cost of our wind power programme is simply enor 