Assessment of seasonal thermal energy storage in flooded mines through numerical modelling

Abstract

A complex challenge for the energy transition in the UK is the decarbonisation of the heat sector, whose significant seasonal fluctuation evidences the need for large-scale technologies to store (intermittent) renewable energy or industrial waste heat. A prospective method involves making use of the exceptional hydraulic conditions in abandoned coal mines for underground heat storage schemes. Through the numerical modelling of subsurface thermal processes, this research aims to investigate the efficiency of seasonal heat storage/extraction cycles in 3D models of representative pillar-and-stall mine geometries; in particular, the influence of certain subsurface characteristics on the system performance, the optimisation of water extraction cycles and the comparison of the energy seasonally recovered from the subsurface with the ‘mined’ coal in the 3D model. The simulations show that up to 45% of the energy transferred to the host-rock in the first six-month heat storage phase could be retrieved by cyclical water extraction during the following six months in favourable geological scenarios. In the following years of scheme operation, the heat recovery percentage can quickly exceed the 50% of the energy seasonally stored. Pillar-and-stall panels with large coal extractions, small pillars and surrounded by dense and thermally conductive lithologies increase the heat transferred to the host-rock and result in higher heat retrieval percentages at water extraction rates > 20 m3 ·h-1 . A 50% lower heat retrieval was evidenced for extraction panels around thick coals, which are more suitable for longer cycles. The energy analysis shows that it is possible to reach the coal energy (modelled panel) in less than 100 years of (optimised) scheme operation in favourable geological contexts, with a feasible range of 100-300 years for different scenarios. In financial terms, the seasonally extracted heat could be worth the value of the ‘mined’ coal in a period from 6 to 30 years of operation, which shows a promising outlook for this heat storage technique.