LOW CARBON ENERGY DISTRICT HEATING SYSTEMS
It is argued that the use of gas in CHP will offset coal burning in power stations, but the more likely outcome is less gas being available to combust in power stations
CHP will offset coal burning in power stations. However, the more likely effect is that there is less gas available to combust in power stations, given that natural gas (along with all other fossil fuels and biofuels) has a limited supply. That is why the directive makes the logical assumption that all fuels should be consumed at their highest efficiency possible. This way we strive for maximum efficiency, minimum fuel consumption and the best possible carbon reduction. The use of an average grid carbon factor
Energy savings Extract from EU directive* (b) Calculation of primary energy savings
The amount of primary energy savings provided by cogeneration production defined in accordance with Annex II shall be calculated on the basis of the following formula:
1- PES =
CHP H Ref H
Where: PES
is primary energy savings.
CHP H is the heat efficiency of the cogeneration production defined as annual useful heat output divided by the fuel input used to produce the sum of useful heat output and electricity from cogeneration.
Ref H is the efficiency reference value for seperate heat production. CHP E is the electrical efficiency of the cogeneration production defined as annual electricity from cogeneration divided by the fuel input used to produce the sum of useful heat output and electricity from cogeneration. Where a cogeneration unit generates mechanical energy, the annual electricity from cogeneration may be increaced by an additional element representing the amount of electricy which is equivalent to that of mechanical energy. This additional element will not create a right issue guarantees of origin in accordance with Article 5.
Ref E is the efficiency reference value for separate electricity production.
*Source: EU Directive on Co-Generation:
http://europa.eu/legislation_summaries/energy/ energy_efficiency/
l27021_en.htm
56 CIBSE Journal March 2012 1 +
CHP E Ref E
x 100%
in the engineer’s computations has the disadvantage that the carbon content of the supplied heat will increase over time as the electricity grid carbon content decreases. Moreover, the carbon content of heat from a typical gas-fired CHP in 2050 will be considerably higher than conventional gas- fired boilers (see Figure 1).
Distribution losses It is evident from the published data that many calculations for DH systems fail to correctly account for the energy used in the distribution of the heat energy. Many engineers still make the assumption that the comparison for a DH system should be made against other DH systems; in so doing they fail to account for distribution or pumping losses or for the differing characteristics of energy requirement timings of different types of building connected to the DH/CHP network. Clearly this is incorrect because pumping
and distribution losses do not occur when the fuel source is consumed within the building itself (using, for example, a gas boiler). And, of course, the heating may simply be turned off when it is not needed or when the building is unoccupied. It is regrettable that the CHP quality assurance system (CHPQA) fails to take DH pumping and distribution losses into account, since it is clear that the losses can be very significant. In an article in the CIBSE Journal last year (August 2011, page 16), heat losses from one of the country’s newest DH networks, for the London Olympics, were stated to be just 1C per km – which sounds quite reasonable until it is realised that this is at maximum flow on a 16 km distribution network with a
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