I agree, no machine is 100% reliable. The reasons are mechanical failure, lack of fuel etc. etc.

In the case of a steam turbine powered by steam generated from a nuclear reaction, or the burning of coal, oil or gas, the fuel is under the control of the operatives. In the case of a wind turbine the fuel is controlled by nature.

Coal, oil and gas fuel starvation is rare.

Wind starvation is quite common. There are many days during the year when over the whole of the UK and the seas surrounding the UK the wind does not blow at a sufficiently high speed to enable any or hardly any electrical output from a large array of wind turbines. In winter, these periods usually coincide with cold spells and high electricity demand.

Rather than talk about statistical reliability perhaps it would help if the problem was thought of in the following manner:

In the UK the generating capacity of conventional plant, coal, gas and nuclear, is approximately 75GW some 14GW, 23%, more than maximum demand. Historically this margin of capacity over demand has proven to be sufficient to provide 100% reliability of electricity supply.

When 40GW of wind capacity is added the excess capacity over demand rises to 54GW.

But, if we look closely, we find that the average output of this amount of wind capacity is about 11GW which equates to a wind speed of 8m/sec. If we study the shape of the power output curve of a wind turbine we see that the area under the curve between zero wind speed and the average wind speed is small compared to the area under the curve between the average wind speed and the rated wind speed.

This leads to the conclusion that a wind turbine operates, for perhaps 75% of the time, in the zone of the curve between zero and the average wind speed and for a large part of this time its output is zero or near zero.

Consequently, the additional wind capacity does not always add to the margin over demand and therefore often does not provide any capacity credit.

The corollary of this is that a large array of wind turbines requires 100% backup whilst a large number of conventional power generating plants operating together require about 20% to 25% backup.

Denis Stephens