Modelling heat and fluid flow during heating of a water-filled mine shaft

Decarbonizing heat in combination with energy storage can be a key component on the way to achieving net zero. Hundreds of thousands of flooded, post-closure coal mine shafts are estimated to exist based on relevant mining authority databases across industrialized countries, and we are proposing they should be considered an unused asset for large-scale thermal energy storage. These numerous, pre-existing structures are often situated close to high population density areas and surrounded by insulating bedrock. There have been no completed mine shaft thermal energy storage (MSTES) pilot studies to date; as a result, its feasibility and performance as a heat storage system for district heating networks remains untested. The objective of this study was to conduct a computational fluid dynamics (CFD) simulation of heat storage in a flooded Scottish mine shaft with a vertical closed-loop heating system suspended in the top 50 m of the mine shaft to gain a better understanding of the temperature distribution in the shaft and surrounding environment, as well as any buoyancy-induced mixing with deeper mine shaft water below the heating system. The presented simulations focus on the heating phase of an MSTES operation over 10 days. In this scenario an unstratified shaft, selected for a MSTES pilot study, was heated from a small closed-loop pipe with varying temperatures and heat transfer coefficients from the pipe to the water. The results provide assurance that a pilot test at the site can be delivered safely, and opens the possibility of future full commercial-scale projects.