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.

Over half of UK's energy demand is from heat, and most of it is provided by fossil fuels. While coal mining has stopped, the water within flooded abandoned mines provide a huge source (2.2 million GWh) of low-carbon, geothermal heat for the future, enough to heat all UK houses for >100 years! The mine water is only lukewarm (12-20 degC), but with heat pumps, temperatures are increased to a more comfortable 40-50 degC. Heat pumps produce 3-4x the energy than they use, making mine water geothermal heating (MWGH) an efficient energy source. But research is required to make MWGH competitive, technically and logistically feasible, and desirable: which collieries are suitable for sustainable heat extraction? MWGH requires district heating networks between premises, so how can we overcome the associated hurdles for setting those up? Can MWGH handle seasonal heat demands reliably? Can MWGH financially compete with the established gas boiler? Do local communities want such change to greener heat? This project will examine these components of MWGH, from the initial geothermal heat extraction, to the logistics of heat storage and delivery, the political and financial landscape for MWGH, and involving local communities in all this. Detailed knowledge of mine water circulation and thermal interaction with the rocks is essential for the success of MWGH. Prior to expensive drilling, numerical models help predict how suitable a mine system is. WP1 will address this using innovative, detailed mine thermal flow models that are fast, so can easily run thousands of flow scenarios to find optimal settings, and are easily tailored towards individual mine plans to investigate case studies. Simulations will be calibrated against flow experiments at GGERFS, the UKGEOS Geothermal Research centre, while project partners provide mine plans, pumping and geological data from several sites. Valuable, unrecorded mine information available within former mining communities will be collected to supplement the mine knowledge and accuracy of the simulations. Heat pumps will increase the temperature of the extracted mine water for local heating purposes. But to meet seasonally fluctuating heat demands, heat storage is essential. WP2 will address this through novel solar-geothermal heat collection that utilizes both underground and overground storage. Solar heat drives sorption reactions, and access heat is released to mine water and stored underground, thereby supporting the long-term heat capacity of the mines. The experimental design of such storage system will be tested and optimized at GGERFS. The success of introducing MWGH depends on many political, financial and social aspects too. Without a favourable regulatory and financial landscape, the major undertaking of installing a MWGH system may be too risky. And without closely working with local councils, the Coal Authority, the Environmental Agency, and local communities, these schemes often fail. WP3 addresses these aspects, by critically analysing the regulations and procedures to start new mine geothermal heating schemes, map out and analyse the financial landscape, and investigate how local communities, scientists, and government agencies can work together to create financially successful and socially just interventions. Present and historic case studies from NE England and Wales will serve to test all aspects of the proposal. So MWGH projects require an interdisciplinary approach as we are proposing here. WP4 will oversee the project and ensure, 1) that learning within and across WP's is shared and integrated to enrich the whole and, 2) that the communities, various research groups, local industries and project partners have opportunities to fully integrate and collaborate across the entire project. In summary, this project provides technical, logistical, political, financial, and social solutions for MWGH projects to decarbonize heating in the UK.