Heat storage in abandoned coal mine shafts in Scotland

Following an increase in demand and extent of renewable energy systems, one of the most urgent challenges in the energy transition is the storage of intermittent renewable energy. Storage would enable continuous energy supply and ease current difficulties in balancing the energy grid. For example, it is estimated that the UK spent over £500 million on wind energy curtailment payments in 2023, wasting thousands of GWh during valuable high output times. This curtailed wind energy could be stored as Underground Thermal Energy (UTES) and utilised when demand increases and outstrips production. Hundreds of abandoned legacy mine shafts across the UK contain a large volume of water and concrete lining surrounded by low permeability rocks, suggesting they can be used as valuable pre-existing structures for low-cost heat storage systems. The locality, availability and volume of flooded mine shafts make them an interesting asset of energy storage for space heating and cooling especially in Scotland. The STEaM project (Subsurface Thermal Energy storAge: Engineered structures and legacy Mine shafts) is investigating the challenges and safety of heat storage in abandoned coal mine shafts that have refilled with water via a joint experimental/modelling investigation and an upcoming pilot project at a legacy mineshaft in Scotland. While the overall project includes hydrogeochemical analysis, energy systems modelling, as well as mechanical and material investigation, this presentation focuses on simulating the coupled hydro-thermal-mechanical processes impacting the shaft and near-field environment such as e.g., development of natural convection cells. Specifically, this presentation will showcase a modelling sensitivity analysis of some of the common variabilities in mine shafts (such as shaft dimension, geology and groundwater flow) to evaluate heat storage efficiency for cyclic heating and cooling. Basis for the presented model is a 3D finite element analysis with COMSOL to study spatial and temporal developments of heat gain and loss underground.