LOW CARBON ENERGY DISTRICT HEATING SYSTEMS
Combined heat and power and district heating systems are being assessed against the carbon content of electricity supplied by coal-fi red power stations
POWER
Many building services engineers are using wrong calculations to evaluate heating systems. As a result, argues James Thonger, they are overstating the benefi ts of district energy networks
BROKERS I
n recent years there has been a lot of enthusiasm in the industry for the adoption of district heating (DH) systems as a way of reducing carbon
emissions – and helping to meet the government’s target of an 80% emissions cut by 2050. Moreover, many local planning policies continue to encourage the installation of fossil fuel-powered combined heat and power (CHP) installations, often attached to DH networks. This is surprising because it is well known
to engineers that CHP systems cannot deliver the optimum fuel (and therefore carbon) savings unless truly low-grade waste heat is extracted from the generators. It is worth considering why these
DH and CHP schemes appear to have such an enthusiastic following amongst planning authorities despite such modest carbon savings.
Calculating carbon It is clear that, despite the EU Directive on Co-Generation, many proponents of CHP and DH are still using incorrect carbon
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reference values for the determination of their potential carbon savings. The directive says that CHP should be assessed against central same-fuel, grid-supplied electricity and a modern high-effi ciency same-fuel boiler (because it is the alternative uses of the same fuel that are being compared for effi ciency of use). Energy (and carbon) savings can be determined by following the formula in the directive (see the box on the next page). However, many engineers still seem to
use calculations where average grid carbon content – or, worse, coal-fi red grid carbon content – is used as the electrical effi ciency reference value when justifying fossil fuel gas-fi red CHP or DH. There are a number of reasons why this is
a wrong calculation methodology, not only because it goes against the directive but, more importantly, because it makes no sense to justify the ineffi cient combustion of a low carbon fossil fuel (gas) against the carbon emissions of a high carbon fossil fuel (coal). The justifi cation is made on the
assumption that the burning of gas in March 2012 CIBSE Journal 55
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