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Wind power can substitute for base-load coal

By Mark Diesendorf - posted Wednesday, 6 February 2008


This article is a response to one entitled "Australia: where too much wind will never be enough" by Tom Quirk and published in On Line Opinion. Quirk’s article is a critique of an article of mine called "The Base-Load Fallacy" and published in www.energyscience.org.au as Briefing Paper No. 16.

Since most of Quirk’s points have already been refuted in "The Base-Load Fallacy" and in my book, Greenhouse Solutions with Sustainable Energy (UNSW Press, 2007), I suggest that interested readers start with my original article. Here I’ll offer some clarifications of Quirk’s misunderstandings.

The underlying issue is whether a mix of renewable energy sources can substitute for a conventional electricity supply system based on fossil fuels. While Quirk raises the underlying issue, he then evades it and focuses on a sub-issue: namely whether wind power can substitute for coal-fired power stations. Based on my scientific research experience in this field, I say “yes” to both questions, while Quirk says “no” to the second question.

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Quirk’s arguments are very similar to those of the anti-wind power organisation, Country Guardian, which appears to have a close relationship with the nuclear industry in the UK. Quirk, a member of the Institute of Public Affairs, has links to the uranium and other minerals industries through CRA Ltd.

Quirk commences by obfuscating a basic problem of electricity supply: namely that all power stations, whether they are powered by coal, uranium, wind or some other source, are unreliable, to the extent that they fail to generate unexpectedly from time to time. A coal-fired power station fails infrequently, but then can be out of action for weeks or even months. A single wind turbine “fails” more frequently (from lulls in the wind), but is “down” for much shorter periods of time: usually hours or days. As a consequence, both coal-fired power stations and large quantities of wind power in an electricity grid need partial back-up.

Quirk’s response to this basic problem is to try to draw a distinction between what he calls “naturally intermittent” supply and “supply which goes off-line unexpectedly”. However, wind power for the next hour and the next 24 hours can be predicted quite well, while forced outages of coal-fired power stations cannot. Quirk’s argument merely obscures the essential unreliability of all power stations and the fact that an electricity supply system cannot be constructed out of coal-fired power stations alone.

To handle the variations in supply and demand, a mix of different types of power station is needed. In a conventional system, these types are known as base-load (24-hour power), intermediate-load (generating from dawn to midnight) and peak-load (for power variations over a few hours at most). This mix of conventional power stations can also handle the variations introduced by a small amount of wind power in the grid and a large fraction of the variations from a large penetration of wind power into the grid.

Applying Ockham’s Razor, the simplest scientific description of intermittency is to say that the availability of coal power and wind power are both random variables, that is, they are governed by probability distributions, which can be derived from empirical data.

Further confusion is introduced by Quirk’s treatment of the capacity factor of a power station, which is its annual average power output divided by its rated power. Quirk claims, incorrectly, that capacity factor is a measure of performance. This is obviously untrue for Snowy Hydro and gas turbines, which have typical capacity factors of 5-10 per cent, and is also untrue for wind turbines, which have typical capacity factors of up to about 40 per cent.  Incidentally, some coal-fired power stations in New South Wales have capacity factors less than 60 per cent, so they are not much “better” than wind farms in this regard.

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The different capacity factors of wind and coal power are, of course, taken into account in the calculation of the cost of electricity in cents per kilowatt-hour.

Since Quirk is clearly not confident in the strength of his argument about capacity factors, he then shifts ground to claim that wind and coal should be compared by a so-called “reliability figure”. His “reliability figure” is arbitrary, except that it is obviously designed to disadvantage wind. However, there is an objective way of assessing the reliability of wind versus coal, one which my former CSIRO and ANU colleagues and I used when we constructed generation planning models of electricity supply systems with and without wind power.

This is to calculate the Loss-of-Load-Probability (LOLP) or similar indicator of reliability of the whole generation system before and after adding wind power. Even in the absence of wind power, LOLP cannot be zero, because that would require an infinite amount of back-up and hence an infinite cost. If LOLP in the presence of wind power is greater than LOLP in the presence of wind power, then some additional peak-load power plant (gas turbine or hydro) is added to bring the reliability of the generating system back to its pre-wind power level.

According to our computer modelling, subsequently confirmed and extended by overseas researchers, a 20 per cent wind energy contribution to electricity generation needs a relatively small amount of peak-load back-up, which is operated infrequently, in order to restore the reliability of the supply system to that of the pre-wind system. (References are given in The Base-Load Fallacy.) In general, the amount of back-up declines with increasing geographic dispersion of the wind farms.

The South Australian data is a peculiar case, because the four wind farms are approximately lined up perpendicular to one of the prevailing wind directions. Hence they tend to experience some lulls almost simultaneously. A much more detailed study by Graham Sinden from Oxford University, published in Energy Policy, vol. 35, pp.112–127, draws on 30 years of wind data from 66 sites in the UK and shows that a high degree of reliability results from genuine geographic dispersion.

To conclude, wind power, with a small amount of peak-load back-up, which is operated infrequently, could substitute for several of Australia’s coal-fired power stations. Several additional base-load coal-fired power stations could be retired by implementing efficient energy use and solar hot water, while banning electric resistance hot water systems. A little further down the time track, bioelectricity, generated from combusting the residues of existing crops, and hot rock geothermal power could replace the remaining coal-fired power stations. The barriers to this transition are not primarily technological or even, with a significant carbon price, economic. They are the political power of the big greenhouse gas emitting industries.

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About the Author

Dr Mark Diesendorf is Deputy Director of the Institute of Environmental Studies, UNSW. Previously, at various times, he was a Principal Research Scientist in CSIRO, Professor of Environmental Science at UTS and Director of Sustainability Centre Pty Ltd. He is author of about 80 scholarly papers and the book Greenhouse Solutions with Sustainable Energy. His latest book is Climate Action: A campaign manual for greenhouse solutions (UNSW Press, 2009).

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Australia: where too much wind will never be enough - On Line Opinion

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