Wind power not intermittent

The Magazine of Engineers Australia, Page: 8
Monday, 17 July 2006

Graham Shepherd's letter (Engineers Australia June) exaggerates the problems of intermittency of wind power. For a start the term intermittency is misleading, because it implies incorrectly that wind power commonly switches on and off sharply. In general, the power output from a number of dispersed wind farms varies smoothly and very rarely drops to zero, as shown by the forthcoming paper in Energy Policy by Graham Sinden, based on over 30 years of data from multiple sites in the UK. Wind power is more appropriately described as variable and the particular kind of variability associated with forced outages of thermal power stations is better described as intermittent.

There is no such thing as totally firm or reliable power. Even in the absence of wind power, the electricity system is a balancing act between two stochastic variables - demand and supply. Integration of wind power merely adds a third stochastic variable, which can be predicted quite well on an hour-to-hour and day-to-day basis. In the modelling of the integration of wind power into grid, there are studies of diverse quality.

Some of the better studies find that the additional costs incurred by quite large penetrations of wind power (eg in balancing supply and demand) are small. And they still over estimate the total costs, because they do not take into account the economic benefits of optimising the mix of conventional base, intermediate and peak load plant in the presence of wind power. In doing this, my own studies, in co-operation with Martin, showed that wind power substitutes for the capacity and cost of baseload power stations with the same annual energy generation. To maintain reliability of the generating system at the pre-wind level, some additional peak-load plant (eg. gas turbine) has to be installed.

But, since this is rarely used (for wind energy penetrations below 20%), it is simply a form of reliability insurance with low premium. There is no sharp technical limit to the penetration of wind power into a grid. In at least three isolated small grids in Australia (Denham and Hopetoun in WA and the Australian Antarctic base at Mawson) wind power supplies more than 40% of annual electricity generation, while low-load diesels supply the rest. Technically, it is easier to integrate wind power into a large grid, which offers both dispersed sites for wind farms and a mix of conventional base, intermediate and peak load plant for balancing.

The real issue is economics: as the wind penetration increases beyond about 10%, the additional costs gradually increase. Nevertheless, Denmark already generates 20% of its electricity from wind and is planning to increase this to around 30% in the near future.