# Opportunities for Integrating Underground Railways into Low Carbon Urban Energy Networks: A Review
## 5 A Case Study for London
### The Bunhill Waste Heat Recovery System  
This paper was produced as part of a PhD project that aims to investigate the real life performance of the Bunhill Waste Heat Recovery System, a novel scheme based on utilizing the London Underground as a heat source. The waste heat recovery scheme, which is also referred to as Bunhill 2, is the extension of an existing heat network, Bunhill 1, located in the London Borough of Islington. Bunhill 1 deployed a combined heat and power (CHP) unit to produce both electricity and heat, which is distributed via a district heating network in order to supply the local community. The Energy Centre for Bunhill 1 comprises a 1.9 MW\({}_{\text{e}}\)/2.3 MW\({}_{\text{th}}\) gas-fired CHP and a 115 m\({}^{3}\) thermal store. The network currently operates with flow and return temperatures of 95 \({}^{\circ}\)C and 75 \({}^{\circ}\)C, respectively. A total of 788 homeswithin three housing estates and one residential building are being supplied with heat from the network, which also feeds another four commercial buildings and two leisure facilities in Islington. The Bunhill Waste Heat Recovery System followed an effort by Islington Council to further expand the provision of low cost and low carbon heat to its residents. The project is the first of its kind in Europe and is part of the EU CELS IUS Project, which aims to promote the use of sustainable heating and cooling in European cities. The decision to use waste heat sources is also related to the decreasing carbon savings of CHP systems due to the growing decarbonization of electricity in the UK. The scheme is expected to provide heating to 455 dwellings in 5 building blocks of local housing estate and a local primary school. A new mixed use building complex that is currently being developed will also be connected to Bunhill 2, adding 720 residential units, a 160-bed student accommodation and a 125-bed hotel, as well as areas for offices, retail and restaurants as new end users of the network. Bunhill 2 will collect heat from one of the shafts that constitute the ventilation system of the Underground network. The heat will then be upgraded using a heat pump before being transferred into the DHN, which will be extended to connect the buildings to the new Energy Centre. The first stage of the system involves capturing the heat using an air-to-water heat exchanger in the ventilation shaft. The heat exchanger used for this waste heat recovery system is a fan coil unit. A head house, where the coils were installed, was built on top of the ventilation shaft, which was upgraded to accommodate the variable speed reversible fan that enables the system to work in both Extract and Supply modes. The benefits of operation in these two different modes have been previously investigated by the authors of [42] and consist of reversing the flow direction of the fan, allowing the system to operate by either extracting hot air from the Underground, which can then be used to warm the water in the coils, or using ambient air, which would be cooled by the coils and then supplied to the Underground, with the heat extracted being recovered by the heat exchanger. Extract mode would be used during the colder months of the year, when the heat demand is at its peak and the underground air is warmer, at temperatures from 18 to 25 \({}^{\circ}\)C. Supply mode would be used to supply cooling to the Underground during the summer, when ambient temperatures are higher, with a daily average temperature of 17 \({}^{\circ}\)C [46]. Throughout the year, heat can be collected at these varying temperatures, which explain the different flow and return operating temperatures of the first water loop, which distributes the recovered heat to the heat pump. A schematic of the ventilation shaft and the head house is illustrated in Figure 2, showing the coil heat exchanger and the reversible fan.