# Opportunities for Integrating Underground Railways into Low Carbon Urban Energy Networks: A Review
## 4 Waste Heat Recovery from Railway Tunnels
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Technology** & **Location** & \begin{tabular}{c} **Heat Source** \\ **Medium** \\ \end{tabular} & \begin{tabular}{c} **Heat Source** \\ **Temperature** \\ \end{tabular} & \begin{tabular}{c} **Heat Extraction** \\ **Rate** \\ \end{tabular} & \begin{tabular}{c} **Service** \\ **Disruption** \\ \end{tabular} & **Reference** \\ \hline \hline Ground-water used in platform air handling units & London, UK & Air & 22 to 32 °C & 50 kW/unit & Medium & [34; 35] \\ \hline Energy pile walls and diaphragm walls & Vienna, Austria & Air/ground & \(>\)20 °C & \begin{tabular}{c} 30 W/m\({}^{2}\) of earth-contact area \\ \end{tabular} & High & [36] \\ \hline Energy foundation slabs & Vienna, Austria & Ground & \(>\)20 °C & \begin{tabular}{c} 10 to 30 W/m\({}^{2}\) of earth-contact area \\ \end{tabular} & High & [36] \\ \hline \hline \begin{tabular}{c} Absorber pipes attached to geotextile between tunnel linings \\ \end{tabular} & \begin{tabular}{c} Stuttgart, Germany \\ \end{tabular} & Air & 6.5 °C & \begin{tabular}{c} 20 W/m\({}^{2}\) of tunnel surface area \\ \end{tabular} & High & [37] \\ \hline \hline Embedded absorber pipes in tunnel segments & London, UK & Air & 17 to 36 °C & \
\begin{tabular}{c} 7 W/m\({}^{2}\) to \\ 30 W/m\({}^{2}\) of tunnel surface area \\ \end{tabular} & High & [38] \\ \hline Ground source heat pumps next to tunnels & London, UK & Air & 14 °C &
\begin{tabular}{c} 53 W/m\({}^{2}\) of tunnel surface area \\ \end{tabular} & High & [39] \\ \hline Heat exchangers within ventilation shafts & London, UK & Ground & 20 to 30 °C & 20 to 29 W/m of borehole length & Low & [40; 41] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Potential technologies for waste heat recovery from underground railway tunnels. potential source for waste heat recovery, this technology might be limited to use at station platforms only, as the deep tunnels of the London Underground are too narrow to fit fan coil units. In addition, the installation of platform AHU is likely to cause train service disruptions. Geothermal energy technologies, which are based on the capacity of the soil to store and provide heat, have also been investigated as a means of recovering waste heat from underground rail networks, resulting in innovative solutions that are both environmentally friendly and cost-effective. The authors of [36; 43] investigated how foundations and underground structures, including tunnels, can be designed as thermo-active elements, enabling them to capture heat from their surrounding environment. Based on a case study in Austria, the authors of [36] researched how absorber pipes could be applied to bearing structures, such as bored piles, diaphragm walls and foundation slabs, in order to capture waste heat from railway tunnels and stations in Vienna. The case study results showed that this system was able to provide 150 kW of heating to a school building adjacent to the tunnel, reaching up to 214 MWh in one heating period. This technology is suitable for tunnels built in the cut and cover method, which consists of bored pile walls, as well as reinforced concrete base slabs and roofs. The authors of [36; 43] also highlighted the potential of heat extraction based on absorber pipes attached to an energy geocomposite placed between the outer and inner linings of a tunnel, which is applicable to tunnels built in the New Austrian Tunneling Method (NATM), which consists of a primary support of reinforced shotcrete, rockbolts and anchors, and a secondary lining of reinforced concrete. The absorber elements can also be connected to the anchors, an important structure that supports the tunnel excavation and could be utilized as a new energy active element. In Stuttgart, Germany, a similar heat recovery trial was conducted on the city's metro system, where absorber pipes were attached to a geotextile and placed between the tunnel linings.