A Lot of Hot Air? David Mackay Fudges the Figures in Favour of Nuclear Power

2011 October 6
by Chris Vernon

Last week I had the opportunity to attend a lecture at the University of Bristol’s Cabot Institute given by Prof. David MacKay, chief scientific adviser to DECC (UK Government Department for Energy and Climate Change). There were two main focuses of his lecture: firstly, a discussion of various sources of energy and secondly, an introduction to DECC’s 2050 pathways tool.

The pathway tool I like and I would encourage anyone interested in the UK’s future energy system and associated carbon emissions to have a play with it. All I would say is that, like many scenario analyses, it is too narrow. The whole point of carrying out scenario analysis is to explore the possibility space. The DECC tool assumes both population and GDP growth. These parameters may be “out of the scope of consideration”, but I would have liked the energy and emissions tool to allow the exploration of steady-state and also economic contraction scenarios. I got the impression MacKay would also have liked to include this flexibility but he said Westminster wouldn’t allow it.

I did not like his comparison of energy sources though. He promotes the use of a single metric to compare energy sources: power density. This means the amount of power delivered per unit area, expressed in watts per square meter [W/m2]. The lecture focused on wind and nuclear power but the analysis can be done for solar power, energy crops, or fossil fuelled power stations.

The headline results were that wind has a power density of 2.5 W/m2 whereas nuclear delivers 1000 W/m2. Sounds good for nuclear and not so good for wind! But the difficulty is that MacKay is comparing apples and oranges.

In order to compare things quantitatively as MacKay is attempting to, it helps if units are the same. MacKay’s m2 of wind farm are not the same as his m2 of nuclear power station. There are three main problems with this analysis:

  • Layering
  • Time
  • Externalities


Cows under Wind Turbines

Cows in a wind farm

The square km of land underneath a nuclear power station is 100% used up. There is nothing else that land can be used for. However, with many renewables the land isn’t used up in a comparable way. Solar panels can be installed on the top of existing buildings requiring none of the underlying land to be used up. Wind farms use around 5% of the land under the turbines, leaving the remaining 95% available for other uses (such as livestock or crops). This 5% compared to 100% improves the power per unit area of wind turbines by a factor of 20.

MacKay made no allowance for the time dimension. He just divided the power of a wind farm or power station by its area. This fails to consider that the nuclear power station took at least 10 years to build before its ~40 year generating lifespan, followed by a ~100 year decommissioning period. In contrast, the wind turbines are generating within months of build commencing and decommission can be similarly swift. This results in the nuclear power station using up the land for around three times longer than the period of time it is generating for, which effectively reduces its power per unit area by a factor of three.

MacKay also made no allowance for the land requirements outside the perimeter fence of ether the nuclear power station or wind farm. This discounts the land required for the uranium mine, the uranium processing, the water required for cooling and importantly the waste storage. The wind turbines also required an iron ore mine, steel foundry and factory. I am not able to quantify the differences in land requirement but I expect the nuclear power station’s “invisible footprint” to be larger, especially when multiplying up the area used for waste storage by the duration for which the land is required (potentially many thousands of years) as described above. Finally, nuclear power stations have a non-zero probability of catastrophic failure, then requiring exclusion zones of hundreds of km2 for decades (Chernobyl, Fukushima).

A Comparable Analysis?
A comparable analysis would consider the fractional land use (layering) of an energy source, the total duration for which this land was used (time) and the land required beyond the immediate installation (externalities). That MacKay’s analysis doesn’t consider these aspects, and that they impact the final results by many factors suggests to me that this metric of comparison is oversimplified. I do not object to the use of the power density metric but would like to see it done properly; otherwise it is comparing apples and oranges and is not useful information.

I don’t doubt that MacKay has considered the points raised above. I am worried that the seemingly-deliberate omission of these factors is presenting an overly political bias towards one source of energy.

According to the above back-of-the-envelope estimates, I would therefore amend MacKay’s comparison of nuclear (1000 W/m2) and wind (2.5 W/m2) to the more realistic 300 W/m2 (accounting for time) and 50 W/m2 (accounting for layering). These adjustments reduce the difference between nuclear and wind from 400- to 6-fold. A further unquantified adjustment to account for externalities is likely to reduce this still further.

Of course, in the final analysis the total land area that is needed is reflected by the naive energy densities MacKay calculates – to generate most of our power from wind (or solar, or biomass) would indeed require vast proportions of the countryside or sea surface to be utilised, and this is an important consideration. However, given the above considerations, it is clear that the headline numbers MacKay is promoting are unfair to renewables, and overly generous towards nuclear.

8 Responses leave one →
  1. October 6, 2011

    It’s hard to imagine where he gets the 2.5 W/m2 figure from.
    Take the wind turbine down my lane: 800kW from a tower footprint of less than 20m2 – that’s 40000W/m2
    What am I missing?

    Was this a central theme of his lecture?

  2. Chris Vernon permalink*
    October 6, 2011

    He calculates it from:

    power per turbine / land area per turbine

    … assuming 6 m/s wind speed and taking the land area to be five times blade diameter squared (apparently the packing density before wind shadows become an issue).

    See his book here:

    For the turbines near you, you would have multiply the 800 kW by a load factor of around 0.25 taking it down to 200 kW but that still leaves 10,000 W/m2. I guess the footprint may well be larger than 20 m2 and you also have to include access roads through the site.

    But despite the numbers, the key point is layering. Wind farms don’t use the land in the same way as power stations do so it seems odd to compare them in this way.

    Of course MacKay’s calculation only works for wind farms – a single turbine, far from others, has an very high power density, similar if not higher than a nuclear power station.

    Central theme? Before the 2050 pathways section power density, of both generation and various countries’ consumption was the main theme.

  3. October 6, 2011

    I still see this question of nuclear or not comes down to the waste and of course the hazard of failure. How much energy and how convenient or unsightly etc is a bit neither here nor their when we’re already stuck with 250,000 tons of waste we don’t know what to do with and it’s not going to be safe for 100,000 years, unless we discover a way of cleaning it. Which we might. But we might not. Not good stuff to have hanging around on an unstable planet.
    Have you seen Into Eternity (www.intoeternitythemovie.com) – by Danish director Michael Madsen? Poetic, with moments of genius and a few rubbish bits but brings the problem of waste home. Worth a look.
    Thanks for the post.

  4. October 7, 2011

    What about off shore windfarms? Since they provide a de facto no fishing zone and a base of food-web substrate they increase to overall productivity of the area.

  5. Chris Vernon permalink*
    October 7, 2011

    Henrik, yes, I did see Into Eternity. I liked the concept of inter-civilisation communication.

    Regarding waste though, I don’t see it as a major factor when considering new nuclear build in the UK. I rather see the waste problem as binary, we either have a nuclear waste problem (we do) or we don’t. The magnitude, the tonnage or total activity, is less of an issue.

    Whether or not we build new nuclear doesn’t change the fact we have to deal with nuclear waste.

  6. January 16, 2012

    Nicely put Chris. Think the whole nuclear debate should also encompass EROI… nuclear being much closer to the edge of the net energy cliff that wind or solar. Furthermore if we focus too heavily on the supply side of power we miss the fundermental that we consume far too much. To question the demand for energy is to question economic growth which is a step too far for the likes of MacKay.
    Link supply and demand and we might be able to acheive something?…… demandenergyequality.org

  7. Doreen permalink
    March 30, 2012

    You claim that nuclear power cannot layer other activities, whereas wind can. But there’s nothing to stop you layering your nuclear power plant (which in your example would include turbine buildings and staff accommodation/car parking) with a wind farm and PV panels.
    Only the reactor building needs the long occupancy time, and that’s usually a small fraction of a nuclear power station’s footprint.

    The fact that these activities do not usually occur does not prove they’re impossible. McKay’s analysis seems about right to me – “welcome to guerilla [engineering]” (p .31)

    As for the waste ‘lasting’ hundreds of thousands of years: it doesn’t. If you extract the unspent uranium and plutonium from the waste (which you would for continued reactor use), the resultant product is as radioactive as raw uranium ore in 1,000 years; after 300, you could display it safely on your mantelpiece.

  8. March 4, 2013

    1)Wind power is more materials intensive than nuclear power. It requires more mining than nuclear power. (For just concrete and steel see this figure link) If you are interested in checking how much space those uranium mines actually take, check their size. Then divide the size by the number of plants powered with uranium from that mine and compare with similar analysis for all the materials needed for wind turbine construction. (At least Vestas provides necessary figures on their LCA link)

    2) Constructing wind turbines is not faster that constructing nuclear power. You can construct a single wind turbine quickly, but to have the same energy production you need thousands of them. Denmark built about 3GW of wind capacity in 10 years. These deliver about 700MW average power. Finland build 2500MW of nukes in 10 years with a smaller GDP. Germany has built around 30GW of wind capacity in 10 years. These deliver around 5 GW average power. So they rate has been around 500MW/year. France which is a smaller economy built nuclear power at a rate that was closer to 2GW/year.

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