It pains me to say this, but David Rose has a reasonably decent piece in this week’s Mail on Sunday. So, instead of half-baked nonsense about climate science, we have something sensible about bio-energy. I find myself in the strange position of recommending that you read it.
But one thing in it really striked me. This is what a spokesperson for Drax Power Station in Yorkshire had to say in defence of subsidising using biomass (a politically correct term for wood) as a fuel in Drax:
We’re a power company. We’ve been told to take coal out of the equation. What would you have us do – build a dirty great windfarm?
Which gets me thinking. How big would a wind farm need to be to replace Drax?
First, Drax has a total capacity of 3.96 GW and annual output of around 24 TWh. In other words the average output is about 2.7 GW.
Second, David Mackay’s recent paper gives estimates of the power density of British wind farms. Based on his numbers 2.5 watts per square metre is a good upper bound on the power density of English wind farms, that is the average power output per square metre of wind farm. In reality it will be slightly less, but we’ll go with 2.5 for now.
So, to replace Drax we would need a wind farm that covers around 1000 square kilometres of land. Let’s see what that would look like if it was centred on Drax itself:
OK, not so good. Perhaps he has a point about that “dirty, great wind farm.” If you want to know why the UK is building wind farms in the North Sea, instead of building them on land this map tells you all you need to know. We simply have too little land, and too many NIMBYs.
But what is his alternative? Yes, using biomass.
So, basically we cut down a bunch of forest and convert the proceeds to wood chips. This is then shoved into a furnace and we get some electricity using a steam turbine. But how much land does that require?
I said earlier that power density of an English wind farm would be something like 2.5 watts per square metre or less. Biomass however can proudly claim to be able to provide power densities of not much above 0.5 watts per square metre. (see Vaclav Smil’s, or David MacKay’s calculations.) And this is the power density of the energy provided by the biomass. You still need to run the stuff through a 40% or so efficient power plant.
So, instead of a wind farm, how much forest would we need to convert to biomass plantation to provide feedstock for Drax. Well, at least 5,000 square kilometres. Here it is on a map:
This is roughly one fifth of all forested land in the UK. And I should point out that I am being generous with my estimate of power density. You can probably multiple this area by two. It is no surprise that Drax is simply importing almost all of its wood chips instead of converting vast areas of the UK over to biomass plantation.
So, when a bio-energy PR person is complaining about things being dirty and great you probably should not listen to them.