flow temperature of 95C and a return temperature of 55C. If we assume an average 8 km round trip, this 1C loss per km results in a 20% heat loss, as follows: ((8 km x 1C/km) / (95C – 55C)) x 100 =
20% heat loss to the ground Since fossil fuel-powered CHP only
saves 20% of energy at best, it is clear that these losses become very significant for the carbon emission viability of the CHP/DH system. In addition, the DH system needs to account for its pumping losses, which consume electricity, further eroding the potential energy savings. Even the modest carbon savings claimed
for DH/CHP networks demand further scrutiny to account for heat lost through the transmission network and storage devices; heat lost to heat-dump; and energy used in pumping, controls, inverters and central plant room conditioning.
Making comparisons In order to decrease the carbon intensity of the DH systems, designers often install biomass boilers. Biomass is used because it is assigned a very low carbon factor due to its short carbon cycle. It is clear that the energy savings for the DHS system should be compared against biomass boilers installed within the buildings they serve. However, this is rarely done because the DH notionally benefits from a comparison against a (higher carbon fuel) gas-fired boiler (see Figure 2). DH systems often require top-up heat
to supplement the CHP systems that are correctly sized on a high utilisation factor. Consequently it is usual that gas-fired boilers provide a significant quantity of the overall annual heat demand in the network. For instance, the Olympics DH network currently has a total heat capacity of 92.8 MW, of which 80 MW is supplied by gas- fired boilers (see the Journal article). Again, the central DH boiler efficiencies, together with the pumping losses and heat losses, need to be compared with the modern high-efficiency condensing boilers installed within the buildings in which they serve. Other features of DH systems that
require accurate energy modelling are thermal storage devices and the overall temperature differences within the system. The temperature differences in DH
systems need to be high to minimise the pumping losses. However, to control return temperatures, the heat-emitting devices need to operate with low temperatures, and the water-heating devices need to operate
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Figure 1: The effect of using the average grid carbon factor for CHP
Figure 2: Relative carbon content of biomass heating
Figure 3: Carbon content of a range of heating sources over time to 2060
on a single pass heat exchanger in order to generate low return water temperatures. This means that the DH system must be at a variable volume flow rate to maintain the low return water temperatures.
Zero carbon Following the publication of the government’s Carbon Plan in December 2011 it is clear that any proposed DH system needs to demonstrate how it is to become zero carbon by 2050. Since carbon capture and sequestration (CCS) techniques are currently considered
It is surprising that many local planning policies continue to encourage the installation of fossil fuel- powered combined heat and power installations
March 2012 CIBSE Journal 57
Kg. CO2 per
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Kg. CO2 per
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