Thursday, October 30, 2008

Efficiency, capacity factor, and value

To the Editor, Geelong Advertiser:

As a science editor, I share Heinz Dahl's frustration with the inaccurate use of terms in characterizing wind energy on the grid ("Winds of change", Opinion, October 30th).

But while pretending to clarify terms, Mr. Dahl only further confuses them as he evades their unique application to wind energy.

Efficiency is not the issue. It is well understood that burning coal for electricity is only around 30 per cent efficient.

It is also well known that wind turbines generate power at an average rate of around 30 per cent of their full capacity. And although it is technically incorrect to call that their "efficiency", the word nonetheless conveys the problematic nature of wind turbines.

In common use, we don't consider our car to be only 5 per cent efficient because we drive it only an hour or so each day. If we're lucky, we consider it to be 100 per cent efficient because whenever we need it, it runs.

Mr. Dahl says that wind turbines are nearly always available, which is true. Except that if the wind isn't blowing, they aren't. And if the wind is blowing, but not within an ideal range of speed (roughly 30-60 mph), the power generated is much less than the turbines' capacity. In that sense, they are much less efficient than conventional plants which when you turn them on run reliably at full throttle.

That's the difference. Unlike conventional generators, wind turbines respond only to the wind, not to actual demand on the grid.

To pretend that there is some value in that, Mr. Dahl invents a new term, "availability capacity factor", but seems only to apply the attributes of a dispatchable conventional plant to the intermittent and variable nondispatchable output of a wind plant.

He says that when wind energy facilities have a capacity factor of 30 per cent, that means that "30 per cent of the time they are generating at full capacity".

He is completely wrong. That is what a 30 per cent capacity factor means for a conventional power plant, i.e., that it is used 30 per cent of the time.

But because the output from a wind turbine varies with the wind speed, 30 per cent capacity factor for a wind plant means that its output averages 30 per cent of its capacity. In fact, it very rarely reaches full capacity and generates at or above its average rate (i.e., 30 per cent) only about 40 per cent of the time.

Which brings us to the measure that Mr. Dahl ignored: capacity value. When power is needed on the grid, can wind plants provide it? Only by chance. Their capacity value is effectively zero. The rest of the grid still has to be kept up and running.