During the operation of the subway, the acceleration and braking of the train will generate a large amount of heat energy, which is usually discharged into the air through the ventilation system. However, the practice in Paris shows that this waste heat can be used efficiently, providing new ideas for building heating. German experts believe that this solution also has broad application prospects in Germany and may provide assistance for the energy transition.
Study on waste heat utilization of metro tunnels in Germany
A team of researchers from the University of Stuttgart in Germany conducted such experiments several years ago. Researchers laid temperature sensors and plastic pipes inside the tunnel walls of the Fasanenhof mettro station on the U6 line in Stuttgart, similar to a floor heating system. The water in the pipes absorbs the ambient temperature of the tunnel and is further heated by a heat pump to achieve the heating function.At present, a large amount of industrial waste heat is not effectively used in Germany every year. According to the German Energy Agency (Dena), Germany's industrial and commercial sectors generate about 125 billion kWh of waste heat every year, some of which can be used directly locally, while the other can be heated by the heat network, replacing fossil fuels, which could theoretically meet the heating needs of up to 10 million households.Waste heat heating mode of the Paris metro
The successful practice in Paris has proven the feasibility of this technology. The heat energy from the Paris metro tunnel is transferred to a building with 20 homes via heat pumps, which supplies about a third of the building's heating needs. According to data from the French national railway company (RATP) and Paris Habitat, the Paris housing authority, the project has achieved good results.According to German researchers, similar solutions are also valuable in Germany's large cities, especially in urban areas with high heat demand. Sebastian Blömer, an expert at the Heidelberg Institute for Energy and Environment Research (Ifeu), points out that finding sustainable heating methods is challenging, and metro waste heat is a local heat source to watch.
The waste heat potential of the Berlin Metro is huge
In cooperation with the Institute for Eco-Economics in Berlin, the Bremer team calculated the waste heat of the Berlin subway network. The results of the study show that the Berlin Metro generates about 460 GWh of waste heat per year, equivalent to 4% of the city's district heating needs. This figure is even higher than the waste heat generated by Berlin's industrial sector (340 GWh) and data centers (120 GWh).In addition, the temperature inside the metro tunnel usually does not drop below 10 degrees Celsius, especially in winter, and the frictional heat generated by the acceleration and braking of the train further increases the available heat.
International success stories
A similar project has been launched in London. In an abandoned station, waste heat from the metro tunnel is extracted and fed to the local heating network via heat exchangers, providing heating and hot water to around 1,300 households. Austria, Switzerland and other countries have also implemented related applications.Although the Stuttgart pilot project was terminated as planned in 2015, research data prove that the technology is not only feasible but also costly. According to Bühler, although heat pumps require electricity to operate and are costly in the long term, they are environmentally friendly and do not require additional infrastructure.
Future directions
Experts suggest that the utilization of waste heat in the subway should be included in the future infrastructure construction plan. For example, heat-absorbing pipes can be pre-installed in new subways or road tunnels to reduce the difficulty of retrofitting at a later stage. A similar design has been adopted for the Rosenstein tunnel in Stuttgart and will be used as a habitat for elephants in a temperature-controlled zoo in the future.Christian Moormann, director of the Institute of Geological Engineering at the University of Stuttgart, says the technology is ripe for standardised heating. He stressed that the use of geothermal energy should be fully considered in future tunnel construction, "In the future, no tunnels without heat-absorbing systems will be built." ”
