You need to understand the algorithm

I am indebted to Professor Bob Lowe for his concept of thinking of city-wide CHP as a heat pump with a CoP of 10 or 11. This may help engineers considering the merits of heat pumps and cooling. In large pass-out steam turbines a small amount of electricity is sacrificed to produce a great deal of heat.

The suggestion CHP should be shut down in summer ignores domestic hot water loads. Many engineers do not understand how large pass-out city-wide CHP units operate. They adjust their operation by passing out steam from only producing electricity at peak demand times during the day to producing varying amounts of electricity and heat when electricity demand is lower.

CO2 footprints in the paper are all presented on a “same fuel comparison basis”, not against 25-year-old coal-fired power plant.

James is correct that absorption cooling is not least CO2 in some cases, but for city cooling, such as for Vesteras in Sweden, it is really good. Starting with heat with a CoP of 11 and a CoP of 0.8 for the absorption plant, mechanical plant needs a CoP of 8 to equal the heat option.

Global warming is caused by CO2. Surely we want to use more wind energy and solar energy and stop waste heat from power generation. This low CO2 heat must be classified as renewable source in Part L, if electric heat pumps are to continue to receive this status.

I am baffled by his claim that Dukes table 6D shows CCGT CHP made a primary energy saving of minus 1%. Dukes reads “the carbon emission savings from CHP in 2006 as compared to the fossil fuel basket was 4.2 MtC”. Dukes, however, uses a flawed algorithm; unless you understand the algorithm, it is difficult to derive useful information about the impact of CHP on the electricity and heat sectors.

Savings from CCGT CHP calculated in Dukes may be smaller than they should be. The explanation will lie with CHPQA and its incentive on electricity, which ought to be on heat (see chart, right). The incentive to use the heat stops at an overall efficiency of about 60%, when 86% or more can be achieved with condensing CHP. A further flaw is that there is no incentive to supply heat to the domestic sector as the domestic sector do not pay climate change levy.

In Lithuania, different CHPs fed into a common piped heat network. Working with economists we realised Dukes, CHPQA and the EU methods of analysing CHP cannot reflect the economic “perfect market” condition of one CHP competing with another and are unsuited for analysing the options for heat supply and demand side management of the heat sector.

The most common analysis assumes heat displaced in the heat sector comes from an arbitrary boiler, not another CHP. In fact, when CHP is installed across a city it displaces all forms of heating including electric with its high CO2 footprint. The actual savings are thus much greater than analysed in many government papers.

William Orchard, Orchard Partners, London