Climate Change Committee: “We can have cake and eat it”

Pretending that competing demands don't exist is no way to plan policy

According to the Committee on Climate Change's report on getting the UK to net zero carbon dioxide emissions, it should be possible to generate the majority of our future electricity using a fleet of 7500 offshore wind turbines, each of 10 MW capacity. This 75-GW fleet will occupy 9,000 square kilometres and will deliver 369 TWh of juice to the grid, or so they claim.

Colour me unconvinced.

For example, the committee is assuming that they will get load factors of 58%. While some of the newest windfarms have briefly achieved such levels in windy months, it is highly questionable whether this can be sustained over whole years or over turbine lifetimes - performance necessarily declines in the harsh marine environment. And it's only some of the newest windfarms that have managed such super-performance. Others have been very poor.

The other thing that jumped out at me was the area of sea required. Crunching the numbers again, the CCC is implying that they can get 4.8 watts per square metre. This is extraordinarily ambitious. Take a look at what the late Professor David Mackay, the chief scientist at the Department of Energy and Climate Change, said on the subject more than ten years ago.

The Kentish Flats wind farm in the Thames Estuary, about 8.5 km offshore from Whitstable and Herne Bay, which started operation at the end of 2005, was predicted to have an average power per unit area of 3.2W/m². In 2006, its average power per unit area was 2.6W/m².

The numbers have barely improved since. Kentish Flats is currently running at 2.8 W/m².* More recent windfarms seem no better. Gwynt Y Mor, commissioned in 2013, has just had quite a successful year, with a load factor of 36%, well above its average since it started operations. But this still yielded just 2.6 W/m² over its 80 square kilometres. Meanwhile, Westermost Rough, the poster-child for the offshore wind industry, with its headline grabbing load factor of 48.5% last year (a notably windy year), only achieved 2.9 W/m².

David Mackay saw 3 watts per square kilometre as a generous maximum, and the data over the last 10 years suggests that he was broadly correct. Wind farm designers have to balance competing demands. If you want large turbines, you need to space them out. The more you space them out, the higher your load factor, but the fewer you can get on your allocated area of sea. Thus the high load factor at Westermost Rough seems to have been achieved by installing the 6-MW turbines at very low density, just one per square kilometre. Compare this to the 3.6-MW machines at Sheringham Shoal. These are installed at 2.5 per square kilometre, but achieve a lower load factor of around 40.

Meanwhile, the Committee on Climate Change is saying that they can get 10-MW turbines installed at a density of 0.8 per square kilometre, only slightly lower than at Westermost Rough. This is a case of having your cake and eating it. They are saying that with only a marginal increase in density, they can use turbines almost double the size and still achieve extraordinary improvements in load factor.

As I said: colour me unconvinced.

*Per Wikipedia, Kentish Flats has an area of 10km² and 30 3-MW turbines. Its rolling load factor per the Renewable Energy Foundation database is 31.1%. (3.3*15*0.311)/10 = 2.8.