Heat pumps 1 CHP


Sustainability without the hot water?


Combined heat and power solutions or heat pumps – which are the key to delivering low carbon services? A Cambridge professor and author of a book on ‘sustainability without the hot air’ has cast doubt on the efficacy of CHP. But, says Huw Blackwell, is this the whole story?


D


avid MacKay’s book, Sustainable Energy – Without the Hot Air, is an internet phenomenon. This is partly on account of it being free to download in a particularly


altruistic move by the author, a professor in the Department of Physics at Cambridge University. MacKay uses simple maths and physics to


demonstrate some fallacies often quoted by the press, and explores some of the challenges we face as fossil fuels become scarce and we seek to move to a low carbon economy. One of MacKay’s assertions that may be of particular interest to building services engineers is that:


www.cibsejournal.com


‘Heat pumps are superior in efficiency to condensing boilers, even if the heat pumps are powered by electricity from a power station burning natural gas … I thus conclude that combined heat and power even though it sounds like a good idea, is probably not the best way to heat buildings and make electricity using natural gas, assuming that air-source or ground-source heat pumps can be installed in the buildings.’ (See page 151 of the online edition.) This article explores this point and presents a


counter-argument as to why heat pumps might not always be a suitable low or zero carbon system. It also looks at how to improve heat pump efficiency when


> August 2010 CIBSE Journal 33


Simon Weir www.simonweir.com


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