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deep in the tunnels of the London Underground, as in many subway systems around the world, it's so hot it feels like hell. And yet in a basement only a few meters away, a boiler is firing to heat water for someone's shower.

rather than stewing in our excess heat. What if we could make it work for us? There is no shortage of waste heat after all, throughout our energy system from electricity generation in a power plant to boiling a kettle, using boilers to warm houses to powering a car, more than 50% of the energy we use leaks into the surroundings as wasted heat recapturing it wouldn't just benefit our wallets. It would reverse some of the damaging effects that waste heat from our towns and cities is having on the climate. The good news is that several cities have found a way to hunt down their waste heat in some unexpected places. The cities are building systems that deliver heat in much the same way that networks handle electricity and water could they point the way to the next energy revolution. waste heat is an enormous problem. A report in 2008 by the US Department of Energy found that the energy lost as heat each year by US industry is equal to the annual energy use of 5 million Americans. Power Generation is a major culprit the heat loss from that sector alone is in considerable excess of the total energy use of Japan. The situation in other industrialized countries isn't much better. The report also estimated that given the right technologies, we could reclaim nearly half of that energy, but that's easier said than done. We often talk about the quantity of waste heat says David McKay, 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 60% is below 230 degrees Celsius. While that may sound pretty hot, it is too cold to turn a turbine to generate electricity the alternative is to just move the heat directly to where it is needed. That is what cogeneration plants do. These are power plants that capture some or all of their waste heat and send it as steam or hot water through a network of pipes to nearby cities. Their buildings tap into the network to warm their water supplies. Or air for central heating. Many countries are encouraging cogeneration a US cogeneration initiative, for example, might save the country $10 billion a year. And cogeneration allows power plants to bump up their efficiencies from 30% to almost 90%. Yet waste heat from power plants is just a drop in the ocean compared with the heat loss from our homes, offices, road vehicles and trains. waste heat from these numerous sources is much more difficult to harnessed than the waste heat from single concentrated sources like power plants, because it leaks out slowly. What's more, it is barely warm enough to earn its theme. reclaiming that is much trickier. As it happens. There is a technology that can siphon energy from slightly warm temperatures and we have long had access to it. Ground source heat pumps have been helping homeowners save on heating bills since the 1940s when US inventor Robert Weber realized he could invert the refrigeration process to extract heat from the ground. The system takes advantage of the fact that the ground is a terrible conductor of heat. In temperate regions regardless of surface temperature. A few meters on the ground, the soil always remains around 10 degrees Celsius. Ground source heat pumps can tap into that stable temperature to heat a house in the winter. The mechanism is simple. A network of pipes makes a circuit between the inside of the home 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 that 10 degrees Celsius soil. While this is not what you might consider hot, it nonetheless causes the refrigerant in the fluid to evaporate into a gas. When this gas circulates back into the house, it is fed through a compressor which vastly intensifies the heat. That heat can then be used by heat exchanger to warm up your hot water or air ducts. This mechanism is powerful enough to efficiently provide heat even in places as cold as Norway and Alaska. It is also cheap. In the UK, the best systems lowered heating bills by 30% Because compressing a gas to heat your home requires far less energy than traditional gas or electric methods of heating.