A rational choice of energy sources

The Scientific Alliance Newsletter

Despite the intense debates which occur about the merits of different sources of electricity, the fact is that governments have an obligation to keep the lights on for their citizens. Modern societies need a constant supply of energy so that businesses can operate efficiently and individuals can be safe and comfortable. Even short power cuts can be very disruptive: computers stop working, broadband connections are lost, modern phones will not work, central heating pumps and boilers cut out and candles and torches are the only sources of light. Imagine if this was a regular occurrence.

This power, of course, does not always have to be generated in the same way. Many countries have moved from an almost total reliance on coal towards greater use of gas. Nuclear is part of the mix for many and France has chosen to make it its primary source of energy. For countries with the right geography, such as Norway, hydroelectricity is the obvious answer and in a few cases (eg Iceland) geothermal power is viable. But in all cases, a mix has been arrived at which can – barring occasional problems – provide energy security at a reasonable price.

This could change over the next few years. The current drive to reduce the carbon intensity of power generation ahead of whatever changes free market forces might bring risks placing undue reliance on intermittent sources of power, particularly wind generation, which is the most mature and generally the least expensive of the non-nuclear alternative options. Even enthusiasts agree that the intrinsically intermittent nature of the wind means that turbines on average only generate a fraction of their nominal capacity. What is still a very controversial issue is the extent to which this can be accommodated within an overall generating mix.

The John Muir Trust, which describes itself as the ‘leading wild land conservation charity in the UK’ has therefore done a valuable service by commissioning a report on the topic from Stuart Young, called (accurately but somewhat prosaically) Analysis of UK Wind Power Generation; November 2008 to December 2010. Using publicly-available information, this analyses the data to give a rather less positive assessment than we get from organisations such as Renewable UK (until recently the British Wind Energy Association, but now also covering wave and tidal power) or the Renewable Energy Association. It will not have pleased Alex Salmond, First Minister of Scotland, who spoke at a recent conference of ‘putting Scotland at the forefront of the global energy revolution’.

Critics may say that the John Muir Trust has an axe to grind, because it does not want to see wind turbines erected in what is currently wilderness. This is a fair point, but very few reports are published for purely altruistic reasons and it is for the reader to judge for themselves the objectivity of the author. In the case of the JMT report, the facts are allowed to speak for themselves. The National Grid, via its Balancing Mechanism Reporting System, provides very detailed data, publishing information every five minutes since November 2008. This data can be accessed by anyone, including those who may be sceptical of Stuart Young’s conclusions.

The author set out to test a series of statements often made in support of wind power and, in so doing, came to some important conclusions about its ability to make a useful contribution to the mix. For example, based on these data, he concludes that a figure of 30% of rated capacity for the average outputs of on-shore wind farms is a significant over-estimate. For November and December 2008, the figure was 31.7%, but in 2009 it fell to 27.2% and in 2010 was only 21.1%. Over the whole period covered, the average was just 24.1%. It should be said that the NG figures include only metered wind farms, but these are all in Scotland, which we can safely assume to be windier than the UK average. Including on-shore wind turbines in England would only make the picture worse.

This is one aspect; in terms of planning investment, it would be safe to assume that on-shore wind farms would have an average output of only about a quarter of their rated capacity. But even more important is the variability of the output. It has already been argued many times that wind farms can do little to reduce carbon dioxide emissions because conventional generating capacity has to be kept running inefficiently on standby to make up the shortfall when the wind is too weak (or too strong). The ability of the grid to cope with large, rapid changes in supply has also been questioned.

The analysis in the report gives a good illustration of this. To summarise, the metered wind farms operate:

At below 20% of capacity more than half the time

Below 10% of capacity more than one-third of the time

The equivalent of one day in twelve below 2.5% capacity

The equivalent of just under one day a month below 1.25% capacity

To compound this, the total output from the wind farms (about 1600MW metered capacity) was less than 20MW on average more than once a week, and this low output typically lasted for nearly five hours on each occasion. To compound this “Movements in excess of 100MW over a five minute period are relatively common even at the low levels of metered capacity during the study period. As more wind power is connected the magnitude of rapid changes will increase.”

Some people will see this report and its conclusions as an attack on wind power. I prefer to see it as a recognition of the reality and an aid to rational decision making. If the objective is to reduce the carbon intensity of electricity generation, it makes no sense to wilfully ignore the negative aspects of particular technologies. To do so would be to saddle the country with a power grid which is not fit for purpose.

Even enthusiasts for renewables should take proper note of these issues. Professor David MacKay, Cambridge academic and currently Chief Scientific Adviser at the UK Department of Energy and Climate Change is both a committed environmentalist and an excellent and rational scientist. For those who want to look at the broader issues of energy generation and use in more detail, the first point of call should be his extremely useful (and free in pdf form) book Sustainable energy – without the hot air. Rather than make the sort of unwarranted assumptions some are prone to do, he estimates capacities and outputs from first principles. Both he and Stuart Young have made invaluable contributions to the debate and we can only hope that they are the voices which are listened to.

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