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Heat pumps versus combined heat and power

By Dave Elliott

There has been a long and interesting debate over whether heat pumps or combined heat and power (CHP) plants linked to district heating networks are the best option for efficient low carbon home heating.

In theory, a heat pump, working like a refrigerator in reverse, can deliver heat with around three times the energy value of the electricity fed in to run it, though in practice they may not always achieve these high levels of return, especially in cold damp weather (R Roy, S Caird and S Potter 2010 Getting Warmer: a Field Trial of Heat Pumps Energy Saving Trust). But heat pumps do offer a way of upgrading low-grade heat, from whatever source, including the air, ground, water, direct solar and geothermal, and if they are run using electricity from renewable energy sources, their carbon emissions will be low.

Steam-raising thermal power plants by contrast are much less efficient. However, they can be operated in CHP mode, so that some of the heat that would otherwise be wasted in the conversion process is captured for use in district heating (DH) networks. CHP/co-generation can increase the energy conversion efficiency up to 80% or more, compared with the around 35% typical of conventional stream-raising plants, and so it makes a lot of sense, as long as there is a suitable local heat load, e.g. a city or urban area. It has been claimed that CHP plans linked to district networks are far more efficient than heat pumps, especially small domestic-scale heat pumps.

Heat pumps have an energy output/input coefficient of performance (COP) of maybe 3, but since they are using some of the heat that would otherwise be wasted, CHP plants linked to DH can deliver a COP equivalent of up to 9 or more, depending on the grade of heat that is required (R Lowe 2011 Combined heat and power considered as a virtual steam cycle heat pump Energy Policy 39 5528). And if CHP plants use a renewable source of heat, such as geothermal or biomass, their carbon emissions, already low, should be even lower, and cost less per tonne of CO2 saved than heat pumps (S Kelly and M Pollitt 2009 Making combined heat and power district heating (CHPDH) networks in the UK economically viable: a comparative approach Energy Policy Research Group, Cambridge University, Working Paper 0925). Though there is some debate on this; it may depend on the carbon content of the electricity used by the heat pump and the grade of heat that you want out (P Woods and G Zdaniuk 2011 CHP and district heating – how efficient are these technologies? CIBSE Technical Symposium, De Montfort University, UK; D MacKay 2013 Sustainable Energy Without the Hot Air p147 et passim).

It is true that installing DH mains can be disruptive, more so than installing heat pumps in individual houses. But once installed and linked to the central heating radiators of houses and other buildings, unlike with domestic heat pumps, there is no in-house device to maintain. Moreover, once installed DH pipes can be fed with heat from any source, as they become available, including solar and geothermal energy, and it can become a major infrastructure asset. That helps make large-scale solar heating linked to heat stirs viable in Denmark – it’s claimed to be much more economic than individual house solar heating, if you already have the DH network.

Note also that heat can actually be sent quite a long distance without significant losses: for example, Oslo’s district heating network is fed via a 12.3 km pipe from a waste-burning plant in the city outskirts, and in Denmark there’s a 17 km link from a CHP plant to the city of Aarhus, while heat is delivered by a 200 MW capacity heat main to Prague from a power station 65 km away.

With a CHP/DH system at Odense claimed to have a COP equivalent of near 12, three times that of a heat pump, surely CHP/DH wins hands down? Well no, sadly it’s more complex than that. These systems are not operating in a vacuum. If there is a lot of spare electricity available at night (e.g. from a large nuclear and/or wind programme) then heat pumps can use it, and in off-gas grid areas, which are also to be unlikely to be candidates for DH, domestic heat pumps can be very useful .

It has also been pointed argued that, while CHP may convert 1 unit of combustible fuel into, typically, 0.5 units of heat and 0.3 units of electricity, and so is overall 80% efficient, a heat pump converts 1 unit of electricity into ~2 or more units of heat, so it’s 200% efficient. Ah, but where did the electricity come from? If it’s from a fossil or nuclear plant running at 35% efficiency, you are back down to 70%. And retaliating further, the CHP buffs say that, in theory, since CHP delivers heat that would otherwise be wasted, its COP is infinite, since it gives us heat for no new fuel input!

It certainly can get complicated. For CHP operation, taking heat out from the near final stages of a power turbine reduces its electrical generating efficiency slightly, but to confuse things further, steam is sometimes taken off at several stages at different temperatures and times. So there is no single efficiency figure – CHP plants can vary the heat-to-power ratio depending on market and weather conditions. And of course you can use heat pumps with heat from CHP! And big heat pumps can be good in some locations, taking heat from rivers, lakes or even the sea, as is done in Sweden. The debate goes on…

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