Energy figures are not comparable

The figures are not comparable.

The government figures assume a national warming scenario based on average emissions from electricity generation, which assumes long-term availability of gas for central gas-fired power stations and local boilers – a surprising assumption when our remaining major source of fossil fuel is coal.

There are two main issues with this scenario. At a global level, to reduce carbon emissions carbon trading is designed to encourage building low-carbon or renewable sources of electricity generation and to shut down high-carbon sources, such as coal-fired power stations. The GLA is thus more correct in using carbon emission from coal-fired plant as the basis for its evaluations, as they reflect carbon trading and address global warming.

The other issue is the inappropriate use of average figures to calculate savings in electricity or heat generation by the addition of new, cleaner generation technologies, or demand side measures. Use of averages does not reflect the actual saving achieved.

The marginal saving is the correct economic measure to use. As an example, you will get two quite different answers to the economics of adding insulation to a building, depending on whether the annual average efficiency or marginal efficiency of a boiler is used.

The average annual efficiency includes radiation losses, pilot lights, electrical consumption for fans and pumps, fuel burn and the effect of the boiler switching on and off.

The marginal efficiency reflects the change in fuel burn for the change in heat demand. The marginal efficiency for an existing gas boiler might be around 82% (gross calorific value), whereas average efficiency might be about 75%.

Electrical transmission and distribution losses on average may well be 7.5-12.5%, depending on the network. However, marginal losses at times of peak demand rise to over 20%, rising as a square function of energy transmitted.

The effect of this is to make generation capacity close to consumers much more valuable than an average analysis would indicate, and means supplying extra electricity at times of peak demand has a high cost.

Heat distribution losses in contrast are on average between 8% and 20%, but they have a marginal loss that tends to zero as the heat loss is solely a function of the temperature of the water flowing in the pipe, not the energy transmitted. This factor is reflected in a change in the mass flow. It means supplying extra heat has a very low cost.

I am indebted to Thonger for raising the anomaly between the government’s and the GLA’s respective analyses. It is a key issue as currently CHP and other renewable electricity technologies are awarded roughly half the contribution they actually make to reducing global warming because of the “national warming and average saving” assumptions used in many government publications, such as Part L and the current white paper.