Work Package 1: Modelling Mine Water Geothermal Systems

Why?

Natural gas usage globally has been increasing, leading to a significant rise in greenhouse gas emissions. In the UK, natural gas is heavily relied upon for residential heating, contributing to a large portion of energy consumption and carbon emissions. Establishing a secure and sustainable energy supply is crucial. As part of the effort to reduce carbon emissions and achieve net zero targets by 2050, the UK government plans to phase out natural gas boilers in new buildings. Mine water geothermal heat (MWGH) extraction is a proposed low-carbon heat resource that utilises the temperature of water in disused and flooded mines. By employing heat pump technology, MWGH can be transferred to a closed-loop system for heating. However, MWGH extraction faces challenges such as environmental concerns and logistical complexities. An open-loop system, where mine water is pumped up, heat extracted, and cooled water re-injected, is a common method. Our WP1 research focuses on developing a modelling tools called GEMS Toolbox which can assess the feasibility of MWGH extraction, optimize flow rates, and identify suitable site locations. This tool aims to provide a faster and scalable approach for early-stage feasibility studies of mine water projects.

Concept

The conceptual model used in this approach focuses on an open-loop system for heat extraction from flooded mine systems. Due to the uncertainties associated with the mine system, a simplified model is used, treating the mine roadways as a network of connected pipes. The injection and extraction of water at specific rates and locations create a hydraulic pressure gradient, facilitating the flow of water through the mine system. This approach assumes that the water injected is equal to the water abstracted from the mine workings. Heat exchange occurs between the flowing water and the surrounding rock formations, leading to a temperature gradient within the rock over time. Thermal interference between neighbouring galleries is considered, particularly over longer time scales, to assess the lifetime feasibility of utilizing these mines for geothermal purposes. See video here for more information.

Method

The hydraulic model used in this approach is based on the conservation of energy and the conservation of mass principles. The model utilizes Bernoulli’s principle to describe the conservation of energy, considering the hydraulic heads and friction induced head loss in the network of pipes. The conservation of mass requires that there is no net in- or outflow of water at each node in the network. The model uses the Newton-Raphson Global Algorithm to compute the hydraulic heads and flow rates in the network as presented in Todini and Pilati 1988.

The heat exchange model describes the flow of water through galleries in a mine and calculates the temperature of the water at each node based on the average temperature of the water from all the galleries feeding into that node. The model also considers the heat transfer between the rock mass and the water in the galleries, using equations to determine the heat flow, heat transfer coefficient, and temperature change. Two analytical models are presented: one for cylindrical heat transfer (modified from Rodriguez and Diaz 2009) and another for planar heat transfer in a fracture plane (Loredo et al. 2017, Lauwerier 1955). These models provide equations to calculate the temperature change of the water in relation to the rock mass.

Output

The outputs show the temperature distribution within the mine workings and helps the user understand which pathways control the outflow temperature of the system.

These models provide equations to calculate the temperature change of the water in relation to the rock mass.