After their decommissioning and the accompanying natural flooding, mines represent large water reservoirs. Because of the contact with the rock, the mine water is thermally well coupled to the underground. The large storage mass activated in this way correlates with a high heating or cooling capacity. However, harnessing this energy is associated with high costs due to the drilling and securing of boreholes or shafts. This makes an accurate design of the plant essential in order to optimize energy utilization. Herein lies an obstacle for many potential project initiators. Administrations of mining regions and local energy suppliers most often are not able to conduct detailed thermohydraulic modeling of the mining structures to determine the energetic potential. Subsequently, the scope of this paper is the advancement of a reduced model that will be integrated into a user-oriented tool to allow for preliminary thermodynamical assessments.

As a basis for this development, literature models suitable for mine water geothermal use were reviewed. Simplified analytical models have the advantage of producing fast results compared to extensive mine structure modeling. However, the lack of flexibility or accuracy poses obstacles in the unrestricted integration in a practice-oriented tool. To address these weaknesses, a separate reduced numerical model was developed using a one-dimensional implicit finite volume method.

This developed implicit Finite-Volume-Method (FVM) model was tested against an analytical model as well as against a three-dimensional computational fluid dynamics (CFD) mine water model, the latter being used as a verification benchmark. Reference calculations show that in the case of laminar water flow, the heat extraction power between the self-developed model and the CFD simulation differs by less than 2.5% in the reference scenario. This demonstrates a higher level of accuracy than that achieved by the comparison model. Furthermore, the model can be adapted to accommodate varying heat loads, thereby facilitating the integration of cooling in addition to heating.

The model will be subsequently used to create an easy-to-access tool which is able to yield accurate results for a wide variety of boundary conditions, including properties of the fluid and the underground as well as heat load requirements. This opens up new opportunities for stakeholders in the application of reduced models for the planning of geothermal mine water systems.

Mine water geothermal and thermal energy storage are efficient solutions to re-use subsurface infrastructure and contribute to the decarbonisation of heating. The understanding of groundwater flow and heat transport in flooded mines and the prediction of system behaviour over the long-term operation is key for a successful implementation of the technology and sustainable use of the resource. However, the combined uncertainty about the post-closure mining conditions and the physical processes occurring in the subsurface limits the prediction of the flow and heat preferential pathways and long-term behaviour of the system.

This work describes the results of heat tracer tests performed at the UK Geoenergy Observatory (UKGEOS) in Glasgow (UK), a unique at-scale research facility to study mine water geothermal and thermal energy storage. The observatory includes a geothermal infrastructure, with multi-physical monitoring capabilities and boreholes drilled to different depths in and above a coal mine closed in the 1930s. Six boreholes intersected a variety of mine working types including coal pillars and voids, backfilled, collapsed and fractured zones and roadways, with variable hydraulic and thermal properties. Four of these boreholes are equipped with infrastructure that allow for multiple combinations of abstraction-reinjection and geothermal experimentation.

We present and compare the results of the heat tracer experiments and discuss the influence of the mine geometry and working types on the groundwater flow and heat transfer processes. The analysis of old mine working plans in combination with the data collected after drilling has been used to inform, delineate and parameterise site numerical models used to help in the interpretation of the hydraulic and thermal observations. Data from single-borehole pumping tests and abstraction-reinjection experiments have been used for model calibration and obtain a spatial distribution of hydraulic and thermal properties. Hydro-thermal models were then run to compare outputs with the heat tracer experiments.

Understanding the spatial variability of these elements and their influence on groundwater flow and heat transport processes is fundamental to assess the viability of a mine water geothermal project. Learnings from this work at an extensively monitored site can be applied to other sites. For example, to understand the responses to characterisation pumping tests, to improve the understanding of flow and heat transport processes in various flow regimes, and to predict, and avoid, rapid thermal breakthrough of the reinjected water in the abstraction borehole that would decrease the efficiency of the installation.

As part of the energy and heating transition away from fossil fuels and towards renewable energies, it is important to offer local energy suppliers as many functional and economically competitive options for renewable energies as possible. In former coal mining regions, the geothermal use of mine water can be a substantial building block that can be accessed both as a source and as a storage option when converting the heat supply. The development and implementation of projects for the thermal utilization of mine water present a number of challenges that must be addressed, in order to make such utilization both feasible and economical.

In previous projects that investigated the feasibility of utilizing mine water, a wide range of challenges were identified, necessitating the development of appropriate solutions where possible. Recognizing and compiling these important findings is essential in order to create better conditions for future projects so that obstacles that arise again can be dealt with more effectively using the lessons learned.

The challenges that may arise in mine water utilization projects can be diverse and range from legal, technical, contractual and social to economic challenges. The findings from previous projects clearly show that legal and contractual issues in particular should be clarified as early as possible in the project. Although solving legal problems can be very time-consuming, a solution can usually be found with the right planning and appropriate contracts. Depending on the complexity, technical obstacles can sometimes be very cost-intensive or result in the project being not feasible and alternative heat sources have to be developed. Social challenges are often based on the prejudices and personal impressions of individuals, so they need to be persuaded with sound, scientifically proven arguments to overcome them.

As part of the ongoing heat transition, mine water is intended to play a vital role as a geothermal source in the former mining regions to offer suppliers another sustainable option. It is therefore essential to evaluate and systematically categorize the experience gained, providing an ideal basis for future projects. Leveraging this existing knowledge effectively will enhance both the efficiency, profitability and speed of future project implementation.

The feasibility study for the foreseen development area Richtericher-Dell in Aachen, Germany aims at the supply of heating and cooling via mine water with the focus on the underground development and mine water utilization concept.

The planned heating grid within the Richtericher-Dell development area is around 37 ha with approx. 850-900 new residential units. A maximum grid capacity of approx. 4.2 MW with a simultaneity of 83 % was based on the net energy balance of the foreseen buildings (with an anticipated annual consumption of approx. 9.5 GWh). As an innovation, it should be emphasized that the influence of an additional underground storage system was specifically investigated and evaluated within the feasibility study. This could significantly support the approx. 40 °C network during peak loads by providing an additional hydraulically separated temperature circuit for more than 24 hours at a time. In combination with an underground storage system, the output of the mine water system and the heat pumps could be significantly reduced.

Thanks to an up to 100% sustainable, low-energy and efficient mine water system with regeneration of the heat source through the possible use of surplus heat from cooling in summer and the integration of an underground storage system, there is no need for additional fossil heat sources. Only a power-to-heat system is only planned to be integrated into the system as a backup.

The results of this preliminary study for a so-called 5^th generation heating network (5GDHC) should be seen as a “blueprint” for heating grid expansions in the Aachen city region. This integral concept aims at the reutilization of mine water for the supply of decentralized energy systems, including the integration of additional storage systems. Based on the size and interconnectivity of the abandoned hard coal mines in the Aachen area, an extension of the foreseen heating grid is feasible.

A challenge in the decarbonisation of heating systems is the seasonal mismatch between heat production and heat demand. Mine thermal energy storage (MTES) is an innovative solution that uses mine water as heat carrier to store heat in abandoned, flooded mine workings. In these systems, hot water is injected during summer, when heat demand is low, and extracted during winter to support peak demand, improving efficiency and reducing costs. However, as with the implementation of mine water geothermal (for heating), the commercial uptake of mine thermal energy storage is relatively slow due to multiple technical, economic and regulatory barriers.

The EU-funded project PUSH-IT (Piloting Underground Storage of Heat In geoThermal reservoirs) is piloting the implementation of high-temperature thermal energy storage in aquifers (ATES), boreholes (BTES) and in two mines (MTES) in Bochum (Germany) and Cornwall (UK). In Bochum, an abandoned coal mine will be used to store high temperature waste heat from the university data centre at the Ruhr University Bochum. In Cornwall (UK), the project is evaluating solutions to store the residual heat from deep geothermal power production at United Downs in nearby flooded metal mines. Investigations are being carried out at these sites as part of the project, including water sampling, tracer testing, temperature logging and numerical modelling.

In parallel with the scientific and technical activities at each site, the project aims to understand public knowledge and perception of the technology. As with other new technologies, limited technology awareness and the lack of specific legislation are factors that can prevent faster market upscaling. In this work we investigate how MTES systems are regulated, focusing on regulations relevant to the respective countries and sites. The research involves literature studies as well as semi-structured interviews with operators, permitting authorities and regulators.

By analysing the regulatory frameworks and assessing experiences from the regulatory and operational sides of MTES permitting and construction, we develop an understanding of the benefits and challenges associated with the different approaches for regulating MTES. Through discussing and comparing specific examples, we develop recommendations for good regulatory practices that maintain public participation and environmental protection objectives without creating unnecessary barriers to the development of MTES systems.