Heat pumps 1 CHP Gas power station with heat pump and with combined heat and power Best gas power station with heat pump

62% Additional energy transferred by heat pump from another heat source

62%

100%

Prime mover

~19 Transmission Generator 53% 47% 47% heat Losses

8% transmission loss (~4% system loss)

~49%

Heat pump (C0P 4.2) Consuming ~19% of electricity

~30% electricity

System totals: 100% Fuel in 80% Heat

~30% Electricity

80% Heat

Gas power station CHP

Assumed 4% reduction in electrical efficiency from heat reclamation

100%

Prime mover

Generator 51% 34%

34% heat losses (An additional heat resource?)

Figure 3: Tw0 Sankey diagrams showing, top, a ‘best case’ CCGT; and, bottom, assuming a 4% reduction in electrical efficiency from reclaiming the heat Source: Huw Blackwell/ Hoare Lea

>

Potentially >8% transmission loss (~4% system loss)

49% ~15 Transmission ~45%

48% Additional energy transferred by heat pump from another heat source

48%

Heat pump (C0P 4.2) Consuming -15% of electricity

~30% electricity 17% Heat

System totals: 100% Fuel In 80% Heat

~30% Electricity

63% Heat

heat. The same transmission loss is assumed for comparison’s sake, and 30% of the energy output is still supplied as electricity, with any electrical output higher than this used in a heat pump. Only 17% of the original system energy (about a third of the heat) needs to be reclaimed to equal the outputs given in the first example. In this simple thought exercise, the CHP system has

been able to match the performance of the ‘best gas power station and heat pump’ scenario. What is more, if more than 17% of the system heat is reclaimed, we can improve on the ‘System totals’ in these examples and perform better than the ‘best gas power station and heat pump’ scenario. To summarise another of the concerns that Professor

MacKay raises in his book, if everyone had a ground- source heat pump in a dense suburban environment, potentially the quantity of heat required to heat all the homes in winter could not be drawn through the ground. He states: ‘I therefore suggest air-source heat pumps are the best heating choice for most people.’ (Page 305.) In this case it can be argued that district heating with CHP is a better solution. This is one of the ideal scenarios for CHP because of the diversified heating

38 CIBSE Journal August 2010

and hot water load. A CHP can then operate efficiently at full load constantly, and supply high-quality thermal energy (80C to 120C is usual) to all properties as required.

Conclusion This article has attempted to show that there are a number of practical reasons why a heat pump, particularly an air-source one, may not operate at a COP of 3 or 4. The main cause is where the temperature of the receiver is substantially different from the source. This is a particular problem wherever there is a requirement to produce large quantities of hot water at high temperatures; heat pumps should always be used carefully where this is the case. It should also be possible for CHP to compete with

the best gas-fired power stations, generating electricity, where a proportion of that electricity is then used in a heat pump for heating (in place of gas boilers). CHP systems, therefore, should not necessarily be discounted in a dense urban environment. l

Huw Blackwell is a senior sustainability consultant at Hoare Lea. David MacKay’s book can be downloaded at: www.withouthotair.com

www.cibsejournal.com

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