Combined Heat and Power (CHP) is a simple enough acronym, but carries with it a lot of meaning. To many policymakers and planners the combination of CHP and District Heating (DH) is a core technology for a low carbon economy, a lynchpin in any strategy that aims to reduce emissions over a wide area and for a long time. To some strategists and policy makers it is a no-brainer. But to others, particularly to building owners who have had these systems installed in recent years, it is a very different story.
The BRE have weighed into the discussion, although in a very quiet and ‘British’ way by publishing an Information Paper on the subject of ‘The Performance of District Heating in New developments’ IP 3/11.
The paper makes a few important points, but disappointingly doesn’t deliver an overall clear message about where CHP/DH is best applied. It highlights the fact that DH is expensive and requires a major capital investment, (not news to anyone) and that to make the best use of DH it should be used in situations where there is a ‘high concentration of heat demand’. So far so good.
The paper rightly points out that the major benefit of CHP is in its longevity and the ability of a heat network to be a carrier for many different low carbon technologies over a long period of time. The expected sixty year life of a network will outlast three generations of plant without needing replacement, and as the heat plant is replaced, say from gas-fired CHP to gas-fuelled fuel cells, to PVT systems there are opportunities to ramp down the greenhouse gas emissions without significant further investment.
But two questions remain about the use of these technologies as a general panacea that has seen their adoption in the London Plan.
1. Is there enough heat demand from new dwellings to justify their inclusion in a design?
2. There are significant heat losses in a Heat Network, the pipes running underground will lose a proportion of their heat no matter how well insulated they are, and the longer the network, the greater the losses. Do these losses reduce the efficiency of District Heating systems to the point where they are no more efficient than competing cheaper systems?
On the first question, the BRE paper points out that even with no heating demand from dwellings there is a significant demand for hot water for washing. But for a low density scheme with little space heating demand then the proportion of losses in the system go above 50%. The same amount of water needs to be pumped around a heat network to keep the temperatures constant, so as the proportion of hot water drops, the proportion of the losses rises. Given that the national Grid is only about 30% efficient, taking generation and distribution losses into account, we are still better by at least 20%, but in a modern system an efficiency of only 50% is pretty poor and one has to ask the question whether District Heating should ever be recommended in such scenarios.
There are many large housing projects being proposed across the UK, including some eco-towns where large scale district heating networks are being proposed for new dwellings, which will be relatively low density and will have a low heating demand. Is it time to ask why?
The BRE IP raises more questions than it answers, but it does provide policymakers and designers of these systems with some questions to ask of each other.
1. If this District Heating system is to work it needs to feed heat to existing dwellings and businesses as well as new ones in order to generate a balanced and substantial load. Existing buildings are less efficient and will require much more space heating for the forseeable future.
2. If there is no reasonable prospect of this ‘District Heating’ system linking to other uses and buildings, then it is likely to be no more efficient than other competing systems but a lot more expensive, so why install it?
For further reading see BRE/IP 3/11 ISBN 978-1-84806-183-5
Also read Max Fordham’s paper on the subject, http://www.maxfordham.com/files/library/District_Heating__CHP_Study.pdf