COMBINED HEAT AND POWER 1 ENERGY EFFICIENCIES
One of the energy centres serving the Olympic Park and Stratford, as part of a district heating scheme that is near completion (see CIBSE Journal, August 2011, page 16)
T
he growing need to improve energy security and reduce greenhouse gas emissions is often focused on cutting energy demand and using renewable
energy sources. Other important approaches include
using combined heat and power (CHP) systems or heat pumps to provide heating or cooling – but both of these approaches need to be rigorously assessed to determine their ‘equivalent heat efficiency’ – that is, equivalent to boiler efficiency. This analysis also needs to take account of the carbon intensity – or ‘carbon factor’ – of different fuels used by CHP and heat pump systems. The carbon factor of electricity supplied by the grid is crucial here. When CHP or heat pumps are used with
district heating (DH) systems, there are three other factors that also need to be considered: the heat losses from the DH network; the pumping energy required; and the additional use of boilers to meet peak demand. These factors can dilute the benefits obtained from the higher efficiency of larger-scale CHP.
Comparing efficiencies The ‘equivalent heat efficiency’ concept can be used to compare CHP and heat pumps. The calculations are shown in the panel on page 32.
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All of the CHP-equivalent heat efficiencies
are higher than for boilers, which are typically 80% to 90% efficient. Figure 1 can also be used to compare CHP
with heat pumps. For a grid efficiency of 40%, a heat pump with a coefficient of performance (CoP) of 3 has an equivalent heat efficiency of 120% (3 x 40%); and a CHP with an electrical efficiency of 20% would have the same equivalent heat efficiency.
When CHP or heat pumps are used with district heating systems, there are factors that need to be considered
However, for a grid efficiency of 50%, a heat
pump with a CoP of 4 would have an equivalent heat efficiency of 200% (4 x 50%), and a CHP would need to have an electrical efficiency of 40% to be as efficient. The current grid efficiency is about 39%, as calculated from DUKES (Digest of UK Energy Statistics, Chapter 5, Table 5.6, 2010.
www.decc.
gov.uk). This excludes transmission and distribution
electrical losses, so the average efficiency for delivered energy is around 36%.
Typical efficiencies calculated from the equations shown left are: Current technology and a grid efficiency of 40%: Individual Gas Boiler
85% Air source heat pump CoP 2.5
Gas-engine CHP 35% elec/ 45% thermal
100% 360%
Future technology and 50% grid efficiency: Individual Gas Boiler
Air source heat pump CoP 2.5 CHP 35% elec/45% thermal
Heat pump CoP 4, 50% grid efficiency
CHP 40% elec/40% thermal, 50% grid efficiency
85%
125% 150%
200% 200%
The above shows that, as the grid efficiency improves, CHP efficiency will also need to improve if the efficiency benefits are to be maintained, especially when compared with heat pumps that benefit from the improved grid efficiency. CHP efficiencies are quoted as being at
80%, referring to the total efficiency, and then compared with the grid efficiency of 40%, thus indicating a doubling of energy efficiency. As the analysis above shows, this is a false comparison: for low CHP electrical efficiency
December 2011 CIBSE Journal 29
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