21 Aug 2007

Carbon Accounting in Electricity

In order to make sensible decisions about how to reduce CO2 emissions, you need good data on what those emissions actually are. In essence, you need to create a robust accounting system, built out of units of CO2, not to mention other greenhouse gases. There will be both a profit and loss account – energy in use – and a balance sheet – embodied energy in buildings and plant. In an ideal world, these carbon units could be linked to money and you could apply a cost-effectiveness filter across everything.

But the basic data isn’t yet in place. Or, to put it another way, there are still wide disparities in attempting to work out just how much CO2 is being emitted.

Take electricity consumption. It’s a complex beast because it is derived from a variety of sources, some carbon rich (oil, gas, coal) some almost carbon-free (nuclear, renewables). Plus it gets distributed over huge areas and undergoes transmission losses and power station inefficiencies. How can you attribute a single figure to the CO2 content of electricity? Well, we do. It is routinely rated in the UK as emitting 0.43kgCO2/kwh, just over twice as much as mains gas, and this is the figure used to calculate the CO2 emissions of electrical appliances, including heat pumps.

A long and involved thread on the AECB forum looked into just how accurate this 0.43 figure actually is. It turns out it was set in 1998 at a time when the CO2 content of electricity was falling because of the dash-for-gas. It never got down to 0.43, but it was thought that this was a good figure to use because, over a decade or so, it was hoped that the CO2 content might fall to somewhere close, and so it was a good basis to plan electrical installations on. The current figure, according to DEFRA, is actually 0.52 and, worryingly, it’s been going up not down in recent years as coal fired power stations have been coming back on line. This figure also takes no account of transmission losses, nor of embodied energy costs in power stations, nor extraction/transportation costs of coal and gas. It could, in reality be much higher than 0.52kgCO2/kWh. Some comments even set it above 1.00kgCO2/kWh. How can we hope to make sensible planning decisions about whether to install kit like heat pumps unless we can be sure that the raw data on electricity use is accurate?

It is yet another puzzle in this complex web we are weaving. It seems to me to be an increasingly important issue that is not being given the attention it deserves. We are being expected to make ball-achingly expensive decisions about future CO2 emissions and energy paybacks, but there are still huge question marks hanging over the accuracy of the data we are using.

1 comment:

  1. Good post Mark. By a quirk of fate I put up a post - http://sustainaballs.typepad.com/my_weblog/2007/08/intensely-compl.html - on carbon intensity on my site on the same day as this one. Spooky. It's one of those issues that is very techie but absolutely basic to any consideration of the efficacy of renewables.