The last deep coal mine in the UK closed in 2015. The Coal Authority has a record of 177,000 known mine entries. This proposal examines the potential to use abandoned mine shafts for interseasonal storage of curtailed wind energy in the form of thermal energy. In 2020, wind curtailment payments in the UK were £282M: enough to power 1.25 million homes and equivalent to £4 per MWh of energy generated. There is 120GW of 'spare' electricity in East Ayrshire alone. Thermal stores have been studied previously but are limited by size and the need to insulate. Flooded mine shafts are ubiquitous across much of the UK, yet the thermal storage opportunity within shafts has never been explored. The rock mass around the shafts are insulators and pilot work by our consortium has shown that as the rocks heat up the efficiency of the heat extraction rises considerably in as little as three years. We will investigate the feasibility of using the spare electricity on windy days to heat up water in abandoned mine shafts, to be extracted on cold days by heat pumps into homes and businesses. The UK is peppered with mine shafts from the days of coal mining - we want to turn these holes in the ground into thermal stores to help balance the electrical grid and to decarbonise homes and businesses.

Mine shafts were lined with concrete or brick (sometimes unlined). To safely and efficiently utilise this legacy subsurface infrastructure we need to understand the effect of heating up the water in the mine shafts on: the water body in the shaft, which may be naturally stratified and will contain minerals that could cause contamination or scaling; on the lining material, which is likely to have degraded in the decades since mine closure; on the surrounding rocks and the water they contain (in pores and fractures). We will develop sophisticated coupled thermal-hydraulic-chemical-mechanical (THCM) modelling informed by case studies we develop from an assay of the UK's shafts, as well as data collected from a test site. We will also take a whole-systems approach to looking at how such an energy store could sit within the wider energy system, taking into account the economics of such a project, and any carbon emissions generated through construction and operation of a site. We are planning a test at a site where we drill into a shaft to retrieve samples of water and capping materials for analysis, and then monitor the injection of heat to validate our models. The example shaft that we are proposing to work on is the Barony colliery, once the deepest in Scotland. Our project partners, East Ayrshire Council have funding for an observation hole close to the site that will provide a baseline of data for the modelling and for observing the progress of our experiment.

The outputs of this work will be applicable for assessing the mine shaft thermal store resource at mine shaft sites across UK coalfields, any risks associated with utilising that resource, and the optimal way to use that resource within the local energy system. We will also provide useful new data for the more well-understood concept of extracting natural geothermally recharged heat from mine workings; for consideration of the best way to abandon active mines so that they are thermal storage-ready; produce a fully coupled THCM model of mine shafts and the surrounding rock mass; and develop the first integrated energy system model to include subsurface infrastructure and geology.