Making use of metro heat 08 Aug 2019

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.

Researchers investigated different types of underground construction to determine their impact on heat transfer. “We compared three typical cross-sections: a circular TBM cross-section, a drill+blast horse-shoe excavation, and rectangular cut-and-cover work,” explained Peltier. Although the differing cross-sections had some impact, overall the effect was negligible. This means “constant values can be used to describe the overall heat transfer phenomenon in underground structures, regardless of their cross-sections.”

In contrast, the roughness of the walls was found to have a great impact with rougher walls “leading to higher values and increased heat transfer rate,” said Peltier.

Lausanne M3 Metro modelling

The scientists applied their research to the new 4km M3 metro line in Lausanne, which will carry passengers between the main train station and the Blecherette district in the north. According to the research, fitting a heat recovery system along 50-60% of the planned route would cover the heating needs of 1,500 x 80m² apartments. In addition to providing heat during the winter, the system could also be reversed in the summer with heat taken from the surface and stored underground.

“To provide low-temperature district heating or direct heating to surrounding buildings, heat is extracted from the air, as well as the surrounding ground, via heat exchangers and water-based heat pumps,” said Peltier. “To provide cooling, heat from the surface is injected back into the tunnel, where it dissipates or, in some circumstances, can be partially stored.”

“This research shows the technology is mature and could be deployed at district-wide scale,” said Lyesse Laloui, Head of the Laboratory of Soil Mechanics at EPFL. “Despite this, we have only seen systems like this used on test sections.” One of the first pilot installations was in Vienna and more recently, an underground station was equipped with a heat-recovery system in Geneva. The application has also been tested in the Crossrail project in London.

The results of the research have been presented to the main contractor working on the Lausanne M3 metro line, as well as the local utility agency and local public transport operators in Lausanne, and the regional government. “It remains to be seen whether Swiss companies are now prepared to take the lead,” concluded Laloui.