Deep in the tunnels of London’s underground railway, as in many around the world, it’s so hot it can feel very uncomfortable. And yet in the basement of a building only a few metres away from the station a boiler is firing to heat water for someone’s shower.
Recapturing it wouldn’t just benefit our wallets. It would reverse some of the damaging effects on the climate. The good news is that several cities have found a way to hunt down their surplus heat in some unexpected places. These cities are building systems that deliver heat in much the same way that suppliers handle electricity and water. Could they point the way to the next energy revolution?
It was also estimated that given the right technologies, we could reclaim nearly half of that energy, although that’s easier said than done. ‘We often talk about the quantity of waste heat’, says David MacKay, chief scientific adviser to the UK Department of Energy and Climate Change, ‘but not the quality’. Most of what we think of as ‘waste heat’ isn’t actually all that hot; about sixty percent is below 230°C. While that may sound pretty hot, it is too cold to turn a turbine to generate electricity.
There, buildings tap into the system to warm their water supplies or air for central heating. Many countries are encouraging such cogeneration, as it is called. A US initiative, for example, might save the country $10 billion per year. And cogeneration allows power plants to bump up their efficiencies from thirty percent to almost ninety percent.
As it happens, there is an existing technology that can siphon energy from such temperatures, although applying it on a large scale to capture waste heat is as yet unachievable. Ground source heat pumps have been helping homeowners save on heating bills since the 1940s, when US inventor Robert Webber realised he could invert the refrigeration process to extract heat from the ground.
The mechanism for this is simple. A network of pipes makes a circuit between the inside of the dwelling and a coil buried underground. These pipes contain a mix of water and fluid refrigerant. As the fluid mixture travels through the pipes buried underground, it absorbs the heat from the 10°C soil.
This system is powerful enough to efficiently provide heat even in places as cold as Norway and Alaska. It is also cheap. Scientists around the world are now working on the idea that the way ahead is to develop city-wide grids using source-heat pumps to recycle waste on a grander scale, from sources such as subways and sewers.