The temperature in London Underground cars can reach around 40°. To ensure the well-being of passengers, extra heat generated by the trains needs to be vented outside. This has led to the idea of using this source of heat to provide heating not for the Underground itself of course, but for nearby homes and businesses during the winter months. As of the end of 2019, a Tube line is due to test a system which involves piping water heated by air from the Underground. The UK is starting to use waste heat captured from various sources including factories and abandoned mines, and from the difference in temperature between a river and the air, for example. Other countries already have substantial experience in heat recovery: Iceland uses geothermal energy, Copenhagen relies on a giant incinerator, and Boston heats its hospitals by means of a power plant.


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