HEAT FROM UNDERGROUND ENERGY LONDON (HEAT FUEL)

Recovering waste heat from urban infrastructures is becoming increasingly important as governments around the world strive to decarbonise heat supply, which remains one of the main challenges in the transition towards net zero. The Bunhill Waste Heat Recovery (WHR) System represents a first-of-its-kind scheme that recovers waste heat from a ventilation shaft of the London Underground (LU) transport network. The system is based upon the installation of a heat recovery heat exchanger that consists of cooling coils and a reversible fan; the coils are connected to a heat pump (HP) that supplies low-carbon thermal energy to a heat network in the London Borough of Islington. One advantage of district-scale HP systems is the possibility of coupling them with thermal energy storage (TES) in order to reduce operating costs while delivering significant carbon savings. Furthermore, depending on the operation of the reversible fan, the WHR system enables the supply of cooled air to the Underground tunnels whilst simultaneously providing heating to the local heat network.
This thesis investigates the potential benefits that could be claimed by recovering waste heat from underground railways (URs), based upon the development of a mathematical model of the WHR system. This WHR model, which was validated with operational data, is able to calculate system performance under different heat source conditions, which vary throughout the year and depend on the operation of the reversible fan. The analysis focused on the influence of condensation and air temperatures on the performance of the WHR system, evaluating how these parameters may affect its efficiency and capacity. In order to fully realise the cooling potential when operating in a bivalent heating/cooling mode (Supply Mode), an investigation was carried out using a numerical model of the local LU environment to assess the impacts of cooling provision in terms of alleviating peak temperatures at nearby stations, with reductions of up to 7.2 K being calculated for adjacent stations in 2030.
The WHR model was also coupled with a techno-economic model of a heat network, which was applied to assess how different volumes of TES could improve the levelised cost of heat (LCH) and carbon abatement costs (CAC) when compared to meeting the same heat demand with communal air-source heat pumps (ASHPs). Results indicate that, if the WHR system operates in Supply Mode for half the year, savings of approximately 9% and 18% could be
obtained for the LCH and CAC, respectively, in comparison with ASHPs. The potential for replicating this technology across the UK was also investigated, focusing on the LU and Tyne and Wear Metro networks, with 30 MW being estimated as the recoverable waste heat, which could be reclaimed to provide 351 GWh of thermal energy annually. The different analyses that were carried out indicate the opportunity for waste heat from railway tunnels to become a key resource for decarbonising heat supply in cities with underground transport systems.