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Where there’s muck, there’s heat!

This might seem odd – or perhaps downright disgusting – but we need to talk about our waste, and whether we can get a grip on it.

I love the idea of using ‘low grade’ heat and upgrading it with heat pump technology to more usable ‘high grade’ heat. Reusing heat makes sense, far more so than making new heat all the time and literally flushing it down the drain.

The fact is there is a steady flow of the warm stuff passing under our streets right now. It may not be warm enough to heat your house at around 16 degrees, but with a little boost from a heat pump there is a real opportunity to save carbon and recycle the heat that passes through our buildings (and us) every day.

I think often these sorts of projects are seen as a bit left-field, and I suspect I’m not the only one. However there are some technological and social trends that make utilising waste heat such as this a likely growth area.

Firstly, urbanisation. Our cities are growing, and by 2050 a further 2.5 billion people are expected to live in cities globally. The one thing cities do well is concentrate sewage. The Tideway tunnel project in London shows how London is concentrating its sewage in a £5 billion super sewer. This urbanisation creates huge challenges but also huge opportunity; the concentration of people enables us to think of new ways to serve them – using the close proximity and population density as an advantage not a problem.

Technological change is also pushing us towards more innovative solutions for heating. Yes, we can use waste to create more renewable biogas for use in CHP schemes – and we are already using that technology at the pioneering Elephant Park low carbon redevelopment project in south London. But ever more ambitious carbon goals mean we can’t just keep burning gas forever and if we pursue an electrification pathway for heat then heat pumps are likely to become commonplace.

Air source heat pumps have the potential to fill our roofs and open spaces – all increasingly valuable and desirable real estate – with large plant machinery, whereas water source heat pumps linked to sewage wouldn’t have this down side and could potentially be smaller and more efficient.

E.ON’s district heating networks in Malmo, Sweden, for example use four heat pumps to capture heat from a sewage treatment works to boost heat to the existing network. The heat pumps use up to 40MW of electrical power to deliver heat, getting 3.5kw of heat for each kilowatt of electricity used.

So just how much of our heat could be provided from sewage? Thames Water, which serves 15 million people around London, treats 4.4 billion litres of sewage each day. That is an incredible amount. The energy in moving that much water by just 1 degree Celsius is equivalent to 5000MWhrs– that’s 5GWhrs a day of energy, enough to heat 147,000 houses. That’s not an insubstantial amount of energy flowing right under our feet!

The challenge is getting to this untapped resource and recycling the heat through heat networks. Concepts such as E.ON’s Ectogrid do this by creating a breathing network where sources of lower temperature heat (i.e. sewage or cooling) are able to push their waste heat back in to the system for other users. It’s an exciting time in heating and I wouldn’t be surprised if in a few years heating our homes from our own waste wasn’t a fair bit more common!

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