Research by the Swiss Federal Institute of Technology (EPFL) is currently working on systems whereby heat generated in metro systems is captured and utilised, effectively turning the tunnel into a large geothermal heat-recovery system to supply municipal heat systems. EPFL researchers have for the first time estimated precisely the coefficient that allows them to establish the amount of heat that the air contains. This coefficient, called the convection heat transfer coefficient, defines the rate of heat transfer between the airflow within a tunnel and the surrounding ground. A higher convection heat transfer coefficient could therefore imply a higher potential for heat recovery.
Turning tunnels into heat-recovery systems
“A good estimate of the convection heat transfer coefficient is essential, as it can affect the final thermal potential,” said Margaux Peltier, a researcher at EPFL. “Nevertheless, the convection heat transfer coefficient is limited by the tunnel environment. Other parameters also affect the heat transfer rate, which is why the whole tunnel environment should be taken into consideration to evaluate the heat recovery potential,” the engineer told TunnelTalk.
In addition to the convection heat transfer coefficient, the cross-section, the roughness of the walls and the air temperature were also found to impact the heat transfer rate and therefore the heat recovery potential.