On a hot summer day, the temperature inside a subway car in the London Underground can sometimes climb above 100 degrees Fahrenheit, and even in the winter, the city’s Tube lines can be so toasty from the operation of the trains that extra heat needs to be vented outside. But by the end of the year, one subway line will begin making use of that heat—sending it to hundreds of nearby homes and businesses to help keep them warm in cold weather.
It’s one example of a growing number of projects in the U.K. that use waste heat from various sources, from factories to abandoned mines, to replace standard gas furnaces. “You’re taking someone that needs heat and taking a source of waste heat that we’d otherwise be throwing into the atmosphere and making it into something valuable,” says Tim Rotheray, director of the nonprofit Association for Decentralised Energy. “Because it’s waste, it’s relatively low cost. And because if you didn’t capture that waste heat you’d have to make it somewhere else, it’s more energy efficient and lower carbon.”
In London, hot water heated by the Underground will be piped into an existing “district heating” system that already harvests waste heat from a power plant to warm 700 homes. The system heats water so it can be used for hot showers or laundry and also connects to radiators, replacing a typical furnace in a home with a heat unit that transfers heat from the larger community system. Similar district heating systems are common in some countries—in Iceland, for example, more than 90% of homes are connected to a district heating system that taps into the country’s volcanic power or underground reservoirs. In Copenhagen, almost every building is connected to a district heating system. It’s less ubiquitous in the U.K., but that’s changing. Heating accounts for nearly half of the energy used in the country and about a third of carbon emissions; by 2025, new homes will no longer be able to use gas furnaces. Expanding the use of waste heat is one way to fairly easily reduce emissions.
The source of heat varies depending on the location. In the English city of Stoke-on-Trent, where coal mines have closed, heat from water in the mine shafts will be captured in a new project that will be completed next winter, helping the city cut an estimated 12,000 metric tons of greenhouse gas emissions each year. In Glasgow, Scotland, the city will capture heat from the difference in temperature between a local river and the air, sending the heat into a network that includes 1,400 homes, businesses, and city buildings. In a city in Norway, a massive new data center will send waste heat to nearby buildings. Copenhagen gets some of its heat from a giant incinerator. In Boston, steam piped from a power plant helps heat buildings like a local hospital.
The system makes the most sense in densely populated areas, since heat is lost as it travels longer distances between homes. “They work really well in cities and dense urban areas, and they’re not so well suited to suburban or, particularly, rural areas,” says Rotheray. “That said, we’re seeing many, many more people moving to cities, and more than half of the [U.K.] population lives in a city. And so it’s an opportunity to have a really major impact in meeting Britain’s heating needs and taking a very significant amount of fossil fuel out of our heating system.”