On a hot summer day, a ride on the metro can be an unpleasant experience with stiflingly hot metro systems and trains. What is the reason for that and why don’t we reuse this heat?

Metro trains generate excess heat when operating. The electric motors create heat due to their electrical and mechanical functioning at acceleration, at constant speed and at deceleration. About half of the heat in an underground system is produced when trains slow down: waste heat is produced by the brakes when a train approaches a station. The ventilation system of the trains also generates excess heat. There are of course ventilation and air conditioning systems in place that try to ensure thermal comfort. Some old metro systems, however, may not have been planned with sufficient ventilation in mind. Also, metro systems are ever expanding and the frequency of the operating trains is increasing.

But the solution to the hot metro systems could be relatively simple: recovering this heat and using it for district heating. The potential is promising: World-wide, there are metro systems are present in 148 cities. They have a total extension of 11,000 km of rails and transport 151 million passengers every day. In the European Union, 50 medium- and large-sized cities have a metro system in place, with a total length of 2,800 km and 31 million passengers every day.

Considering that the average distance between metro stations is only 1 km in Europe, and the fact that all stations are located in densely populated urban areas where heat demand is high, this kind of waste heat can ideally be used for heating. Estimates show that if this potential is exploited well, a total of 6.7-11.2 TWh per year of waste heat from metro systems can be recovered and used in district heating networks at EU level. Reusing this waste heat directly results in higher thermal comfort at the stations for passengers and workers, as well as cheap and low-carbon heat for the end-users.