On reading the letter ‘Energy league table’ by J. Halstead in the August 2012 issue of this periodical, I am concerned about the definition of ‘availability efficiency’ of a wind turbine, and the statement that it is the fraction of the time for which the wind ‘blows’.
The following is taken from a pdf document published by the British Wind Energy Association which an interested reader can easily find:
“Availability is the percentage of time a wind farm is technically able to produce electricity. This does not depend on the wind speed, because even at low wind speeds the turbines are producing electricity. The theoretical maximum availability is 100% meaning a wind turbine produces electricity 365 days a year, 24 hours a day. However, as with any other equipment, wind turbines need to be maintained and may also have to be repaired...”
To this I will add the following. The above concept of availability is applied to conventional, thermal generation of electricity on the basis that boiler furnaces have to be closed down sometimes for maintenance and (especially if high-ash coal is being used) for de-fouling. Secondly, the word ‘available’ or its noun form sometimes slip into discussions of conversions of heat to work according to the principles of the Second Law, as anyone can confirm by a brief web scan. I personally dislike such usage, but that it takes place is a fact. So in discussing availability efficiencies or similar terms (e.g., availability factor, which I have also seen) we are in a semantic minefield.
Dr John Jones, School of Engineering, University of Aberdeen