COMBINED HEAT AND POWER 1 ENERGY EFFICIENCIES
of, say, 15%, an 80% CHP total efficiency may be only slightly better than a boiler. Similarly, it is sometimes claimed that
A computer image showing an example of a combined heat and power system with a ground source heat pump, which together supply underfloor heating and hot water. The ‘equivalent heat efficiency’ concept, described in this article, can be used to compare CHP and heat pumps
heat pumps have an efficiency of more than 100% (which is true, provided the CoP x grid efficiency is >100%), and that this must therefore always be better than a CHP, which has a practical maximum total efficiency of 80% (see ‘Sustainable Energy Without the Hot Air’, David MacKay, 2009, page 151. www.
withouthotair.com). Again, this is an erroneous conclusion. As shown above, the CHP- equivalent heat efficiency can be significantly higher than a heat pump, depending on the seasonal CoP of the heat pump, the CHP electrical efficiency and the grid efficiency.
Emissions factors Although the above approach provides some useful insights to help mitigate climate change, it does not take account of the variation in CO2 emissions that result from using different fuel types. An alternative method for comparison is to calculate the CO2 content of a unit of heat
produced by the different technologies. l For a boiler, the CO2 content of heat is given by: fuel emission factor / efficiency
l For a heat pump it is given by: electricity emission factor / CoP
l For a CHP plant it is: (CHP fuel x fuel emission factor – CHP electricity generated x electricity emission factor)/ CHP heat generated This can be expressed in terms of the CHP
thermal efficiency and CHP electrical efficiency as:
Equivalent heat efficiency for CHP and heat pumps
400% 350% 300% 250% 200% 150% 100% 50% 0%
10% 15% 20% 25% 30% 35% 40% 45% 50% CHP Electrical efficiency
grid efficiency 40% grid efficiency 50% heat pump CoP 3 grid effic. 40% gas boiler at 85% effic.
grid efficiency 45% heat pump CoP 3.5 grid effic. 45% heat pump CoP 4 grid effic. 50%
Figure 1: Equivalent heat efficiencies for CHP (CHP total efficiency = 80%) 30 CIBSE Journal December 2011
CO2 content of CHP heat = (Fchp x ef – Echp
x ee)/ Hchp = ef / ηh – ee x (ηe / ηh) = (ef – ee x ηe) / ηh Hchp
where ef = fuel emission factor, ee = electricity emission factor, ηe and ηh are the CHP electrical and thermal efficiencies, Echp, Hchp and Fchp are the CHP energy flows for electricity generated, heat and fuel. Another type of CHP plant is where heat is extracted in the form of low-pressure steam from a power station built primarily for electricity generation, whether fossil fuel, energy from waste, biomass or nuclear. When steam is extracted from the steam turbine at a useful temperature, there will be a reduction in the electricity generated as the steam flow through the low-pressure turbine is reduced. In this case the CO2 content of heat relates to the emissions from power stations on the system that would have to operate to replace this ‘lost’ electricity. The ratio of heat extracted to electricity reduction is termed the z-factor and is typically in the range of 6 to 8. Hence: CO2 content of heat extraction = ee/z
The above equations have been plotted on Figure 2 for comparison, with the main variable being the electricity emission factor for the grid supply. It is assumed that gas is the fuel used for CHP and boiler with an emission factor of 198g/ kWh. Figure 2 demonstrates that the key issue in
the comparison is the emissions factor assumed for the grid electricity. As the electricity supply is composed of a mix of power stations with wide variation in emissions factor (from hydro- electricity and wind energy with near-zero emissions to coal-fired power stations with around 900g/kWhe), it is not obvious which emissions factor to use.
An average emissions factor is the approach taken within Part L of the Building Regulations 2010, where 529g/kWhe is to be used in assessing the benefits from displacing grid electricity. Looking at Figure 2, we can conclude:
l The CO2 content of heat from CHP varies significantly with CHP electrical efficiency and electricity emissions factor.
l The actual CO2 saving arising from CHP can be found by using marginal electricity emission factors (say 690g/kWh), reflecting partly the energy efficiency of CHP and partly fuel switching from coal to gas. This approach shows that gas-engine CHP will result in more CO2 savings than heat pumps.
l However, if the question is ‘how does gas- engine CHP compare with the best alternative use of gas as a fuel?’ then a much lower electricity emission factor should be used (say 394g/kWh), and for this case a mid-range CHP
www.cibsejournal.com
Equivalent heat efficiency
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