Winter is here, together with ice, snow, and cold weather. Many people have already started using furnaces fueled by oil, natural gas, coal or other forms of energy to keep their homes, offices and schools warm. These types of energy usually work well. However, they also pollute the environment and no one really knows how long supplies will last. In recent years, many scientists and inventors have been working on alternative energy sources, such as solar or wind power. In Paris, heat from sewers is part of the government’s plan for keeping the city’s buildings warm. This new kind of geothermal heat might eventually help other cities use their resources more efficiently.

Sometimes people need to be very creative to find a solution for the world’s energy needs. An alternative fuel source that people have been developing is geothermal energy, which uses natural heat from inside the earth to provide power for running machines or keeping buildings warm. The center (core) of the earth is very hot, and the layers above the core are also warm.

Usually, using the earth’s warmth involves digging holes deep into the ground to find the heat. People have found that the earth’s temperature increases by about 30˚C for each kilometre. Using this kind of heat helped keep the athletes at the 2010 Vancouver Olympics warm, and Iceland has heated its buildings with the energy from its many hot springs and geysers for years. The idea of using sewer water, however, is new.

The city of Paris is famous for its sewers. Almost 2400 kilometers of tunnels run under the city, carrying waste water from sinks, dishwashers, bathtubs or showers. Much of this water is still warm, with an average temperature of about 20˚C. The sewer heating system uses steel plates containing a heat-transferring liquid, which sits in the water and warms up. The plate is connected to a pump which sends the heat throughout the building. This new heating system can even use the pipes that are already in the building. A school was the first building to receive the new heating system, with the president’s palace, a swimming pool and a town hall next in line. About 10% of the city’s heat is already generated through this system.

Geothermal heat is much cleaner than traditional fuels like oil and gas, but it also creates some problems. Digging into the ground can release gases like hydrogen sulfide, which pollute the air, and it can also produce deadly liquids in the process of producing heat. Using sewer water is different. With the Paris project, the main problem is in keeping the smell from the sewers from coming into the buildings, but the inventors believe that they’ve solved that problem by creating a tightly sealed system that lets heat through, but nothing else.

If heat generated from sewers continues to work as well as it has so far, it might help the people of Paris and other cities around the world to create cleaner, more sustainable energy for the future.