Unravelling the complexities of waste heat…

Henrietta Cooke

25/09/2013 Written by: Henrietta Cooke

Making use of waste heat is a low carbon holy grail. On the surface it looks like a no brainer; waste implies it is both free (has no market value) and zero carbon (after a bit of mathematical acrobatics regarding the allocation of emissions). But dig a little deeper and you begin to see the challenges and complexities it represents.

BuroHappold’s sustainability team has recently completed an in-depth study for the Greater London Authority (GLA) to help illuminate the issue. The GLA is particularly concerned with capturing waste heat for use in district heating networks to supply homes and offices in the capital. Their concern in the context of future carbon reduction targets and the need for greater resource efficiency is that the availability and viability of primary fuels such as gas, used to provide heat through heat networks, is likely to reduce. Consequently heat networks can and must begin to make use of alternative sources to facilitate the low carbon transition. Our study sought to understand not just where and how much waste heat might be available but also the extent to which this heat could be effectively used in the city’s wide range of buildings.

Heat is by no means a uniform commodity and its usefulness depends on a multitude of factors; for example boiling an egg requires a different temperature to warming a room; warming a drafty room requires a different temperature to warming a well-insulated one; and making an elderly person feel cosy requires a different  temperature to making an athlete feel cosy (see my earlier blog on the importance of people in the mix).

That may seem self-evident, but they all have implications regarding what is done upstream and ultimately on the cost of delivery. Once you’ve established what a useful temperature is, there’s the challenge of matching the temperature of supply with that of use. Many of the waste heat sources available are warm rather than hot, and so they need to be upgraded to a higher temperature to become useful.  Waste heat from sewers, for example, is available at temperatures of around 14-22°C and it’s similar for environmental heat sources: air, water and ground. Upgrading these temperatures is achieved by using electricity via so-called heat pumps; these in turn have varying efficiencies, also dependent on temperature, and operate at varying scales. An additional constraint with waste heat sources is that they are (generally) geographically fixed so they are only useful if they are sufficiently close to a centre of demand.

Therefore we need to address a number of factors when looking at waste heat: people, the buildings in which they live and work, how we extract the heat, the technology and electricity required to upgrade it, and the design of the networks that deliver it. This complexity alone seems somewhat mind boggling. But it becomes even more so when you overlay the commercial and institutional challenges of interfacing with a range of industries: power station operators, industrial processors, supermarket chains, infrastructure and utility operators, government departments and ultimately building owners, operators and designers. Each has its own business cycle and objectives which might not align totally with the requirements of a reliable heat supply.

The approach to tackling this complexity is perhaps similar to that of eating an elephant; one bite at a time! This is where our study really begins to come in to its own; it establishes a methodology that starts with clever GIS mapping of sources and sinks, combines these with building type and energy use data, and thereby helps a planning authority to pin point areas which technically and geo-physically could be the focus for a low carbon, low temperature heat network. One such site is Bunhill in Islington, where the GLA is already helping the local council to explore the connection of waste heat from such diverse sources as a local sub-station and a tube ventilation shaft into an existing heating network. It will be fascinating to see how this experiment develops and how the lessons learnt can begin to make the capture of waste heat the no brainer that at first sight it appears to be…

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