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The limits to renewables – what can be done?

The limits to renewables

Prof. David MacKay from Cambridge University has been getting good media coverage for his seminal self-published book ‘Sustainable Energy without the hot air’, in which he attempts to construct and then test a range of possible energy mixes for the UK. It’s a very stimulating- and sobering- exercise. His clearly presented analysis offers a challenging assessment of the renewable resource, and he is obviously worried that enthusiasts for renewables sometimes overstate what they can deliver- he says ‘plans must add up’.

It would be interesting then to see his reactions to a bold new paper in The Electricity Journal (Vol. 22, No.4, May 2009, pp95-111) by Ben Sovacool and Charmaine Watts who ask is ‘Going Completely Renewable’ possible and desirable – and say yes, for electricity in both the USA and New Zeeland, which they select as case studies, and also, ultimately, for the world as a whole: ‘Excluding biomass, and looking at just solar, wind, geothermal, and hydroelectric, the world has roughly 3,439,685 TWh of potential- about 201 times the amount of electricity the world consumed in 2007’.

MacKay’s focus is just on the UK and he is at pains to alert people to the fact that if they want to use renewables to meet their energy needs, the scale of deployment will have to be very large in land use terms (‘country sized’). Even then he doubts if enough can be obtained- we may also need nuclear or CCS, or both.

His approach is based on an assessment of averaged watts /sq. m, and he calculates that, for example, on-land wind delivers 2 watts/sq. m. He says that he is ‘not anti wind, jut pro arithmetic’ However his sums seem to ignore the possibility that the land around wind turbine bases can be used for other activities- e.g. farming or energy crop growing. And going off shore avoids land-use limits altogether, as Mackay recognises- although he points out that very large areas ( ‘the size of Wales’) would have to be involved to get significant amounts of energy.

Looking beyond the UK, Sovacool and Watts mention the potential of concentrating solar arrays in desert areas. As Mackay points out, they certainly use a lot of land, but there are plenty of low value desert areas, for example in North Africa.

Even so, there is major gap between MacKays cautious resource analysis and some of the more speculative data used by Sovacool and Watts. But then again, while we have to avoid over-enthusiastic assessment, there is also a need to challenge overly conservative estimates. One issue is costs. Mackay mostly escapes this by focusing on resources and physical data, By contrast Sovacool and Watts are stronger on the economics – although, once again, there will be disputes about their selection of economic data.

Overall, in looking at these two studies, we have on one hand an attempt at a hard nosed physical assessment, and on the other, a more speculative vision of what we might aim for. That’s not to say MacKay’s book lacks vision- it’s packed with ideas and insights on how we might reduce emissions effectively. However, his overall approach does sometimes feel overly deterministic. While his calculations are clearly valuable in setting order of magnitude boundary conditions, I’m still reminded of Bertrand Russell’s dictum that “Science may set limits to knowledge, but should not set limits to imagination.”

David MacKay’s book can be downloaded for free from

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