Defra figures dent CHP case

James Thonger’s article (BSj 05/07) stirred up debate with the GLA on the carbon content of grid-supplied electricity and questioned the carbon savings that CHP and CCHP can really achieve.

Defra has further fuelled the debate with the publication of its latest greenhouse gas conversion factors for company reporting. The aim of these is to help businesses convert data such as utility bills into a carbon dioxide equivalent, allowing an environmental cost to be allocated to items such as electricity use. Using this data calls into question the viability of gas-fired CCHP as a low carbon technology and reduces the carbon saving possible with gas-fired CHP.

One of the key figures is the long-term marginal factor for electricity. This is the figure Defra recommends for companies that want to calculate the emissions reductions that can be achieved by investing in on-site renewables and low carbon technology such as CHP and CCHP.

The long-term marginal factor assumes that in the immediate future, over a period of, say, a decade, avoided electricity use will displace generation at a new combined cycle gas turbine (CCGT) electricity generating plant. The reasoning is that, unlike a coal-fired power station, a CCGT plant can be shut down virtually at the flick of a switch, making it the easiest way to moderate supply to the grid and more economical than shutting off a coal-fired plant.

Defra’s long-term marginal factor is 0.43kg CO2/kWh. The target grid-supplied carbon content used in the Building Regulations is 0.422kg CO2/kWh.

However, the regulations also provide a figure for grid-displaced electricity that must be used in calculating carbon savings when CHP or CCHP is installed. This figure is substantially higher than the target grid-supplied content and is given as 0.568kg CO2/kWh.

Brian Mark, director of Fulcrum Consulting, says it is very hard to understand where this figure comes from. “Presumably it comes from the power generators turning off more carbon-intensive sources such as coal-fired power stations first, and I honestly can't see they’re going to do that."

Mark argues that a more realistic figure for the grid-displaced electricity should be used, such as Defra’s long-term marginal factor. Fulcrum Consulting used these figures (and the Defra figure for natural gas) to determine what the carbon reduction would be on a scheme if CHP and CCHP was installed.

It gave some startling results. Rather than cutting CO2 emissions by 36%, the case for CHP is almost halved, to 20%. The case for CCHP is reversed: it would produce more CO2 than a conventional chiller powered from the grid with gas-fired heat production.

However, Mark adds that it is impossible for a scheme with CHP to achieve a 20% reduction in CO2 emissions. This is because to provide systems defined as “good quality CHP” by the requirements of climate change levy exemption, there must be a use for the waste heat otherwise all you have is a generator acting at a lower carbon efficiency than the mix of grid-connected energy sources.

For most residential developments the only heat load in summer is hot water, and this is what generally determines the economic size of CHP plant. So in reality the plant produces only a fraction of the site’s electricity needs, the rest coming from a grid connection. A 10% to 12% saving in total carbon emissions for gas-fired CHP is normal for a typical mixed-use mainly residential development.

In July the government published Forward Looking, a paper indicating changes to Part L of the Building Regulations before the formal consultation document, expected in 2008.

A DCLG spokesperson said: “The Forward Thinking paper reflects current projections that the carbon burden of grid electricity is likely to come down slightly over coming years. The final decision on assumed carbon burden of different types of fuels will be made in the full consultation on changes to Part L.”

Mark, like most engineers, fully supports the likes of the GLA in the bid to cut CO2 emissions.

What he’s concerned about is the prescriptive way they set about this. “If it is a requirement of planning to have, for example, CCHP, you have to invest in a rather expensive piece of kit and you won’t then have the money to invest in a cooling system utilising renewable energy that would save more CO2,” he says.

The GLA’s cooling and heating hierarchy is for solar water heating, CHP and CCHP for heating and cooling, then renewable technologies. “Renewable cooling systems such as interseasonal thermal storage appear on the hierarchy below the requirement for CHP and CCHP,” says Mark. “That’s the nub of what we are concerned about.”