# Opportunities for Integrating Underground Railways into Low Carbon Urban Energy Networks: A Review
## 4 Waste Heat Recovery from Railway Tunnels
Based on temperature data from the surrounding soil, the tunnel linings and the tunnel air, the authors calculated that heat could be extracted from the tunnels at a rate of 20 W/m\({}^{2}\)[37]. A different design approach must be taken when dealing with underground rail systems built with mechanized tunneling. A design of railway tunnels with absorber pipes embedded within segments of the tunnel lining, namely the tunnel energy segment (TES) system, was analyzed by the authors of [38], who showed how cooling can be provided to the Crossrail tunnels in London while waste thermal energy is harvested to heat nearby buildings. A finite element thermal-mechanical model was developed using a multi-physics software to simulate the thermal performance of the TES system by analyzing how different heat extraction rates affect the temperature of the heat carrier fluid. The results showed that heat extraction rates are limited by the risk of fluid freezing and can reach up to 30 W/m\({}^{2}\)[38]. A similar system was proposed by [39], who numerically analyzed the feasibility of developing an energy tunnel in Turin. The analysis was based on the development of a finite element thermo-hydro model to simulate the heat transfer between absorber pipes within a tunnel and its surrounding environment. The proposed design also consisted of embedded pipes within precast concrete tunnel linings and could operate in cooling and heating modes. Thermo-active tunnels and ground structures can achieve high heat extraction rates, however, these technologies can only be applied to new underground railway developments and are not suitable to recover heat from existing tunnels. For that reason, geothermal technologies must be incorporated into geotechnical design at an early stage, helping to establish a multidisciplinary cooperation between the teams responsible for designing the bearing ground structure and the heat recovery system, which can reduce delays and installation costs associated with energy active structural elements. A different approach to recover the excess heat that is generated in railway tunnels in London was proposed by the authors of [40; 41], suggesting the use of ground source heat pumps (GHSPs) connected to geothermal heat exchangers (GHEs) to collect the heat that is conducted into the surrounding soil during the operation of trains due to the heat sink effect. A finite element model was developed to analyze how GHEs can have their performance enhanced when placed near railway tunnels and how different configurations of GHEs--with varying numbers of rows and loops, sizes, as well as proximity to tunnels--could affect their heat extraction rate. Results showed that the performance of the system could be improved by up to 43% and the model was validated by comparing simulation results to measured temperature data from the London Underground network. This technology is able to recover heat from existing railway tunnels as it can be deployed independently from the tunnel structural elements. However, the high capital costs related to the excavation of boreholes represent a drawback to the adoption of this heat recovery solution. Another issue related to the development of this technology relates to land ownership, as energy plants would have to be installed along the tunnels, in what could be privately owned areas. An alternative heat extraction technology was analyzed by the authors of [42], who investigated the potential to recover waste heat from ventilation shafts of the London Underground network by deploying an air to water heat exchanger connected to a heat pump. The potential energy, carbon and cost savings of the system were calculated in a comparison against conventional cooling and heating, based respectively on chilled water systems and gas fired boilers. Results showed that the waste heat recovery system could lead to considerable savings, using 79% less energy, emitting 48% less GHG and costing 82% less than the conventional cooling and heating methods analyzed.