Decommissioned open-pit mines are potential geothermal sources of heating or cooling for nearby population centres

The global shift towards cleaner energy necessitates the early decommissioning of coal-fired power plants and mines. This presents an opportunity to repurpose open-pit coal mines, which have the potential to serve as geothermal heat sources or sinks for space heating and cooling. Currently, post-closure use of these mines for geothermal energy remains largely unexplored. This research addresses this gap by evaluating the techno-economic feasibility of utilizing decommissioned mine pits to provide heating and cooling services to nearby residential areas. The importance of this research stems from the need for sustainable solutions for mine closure and the potential for economic revitalization in coal-dependent communities. Traditional mine decommissioning focused on minimizing environmental risks, but now, it's crucial to incorporate post-mining land use planning that integrates with the economic transition of the affected regions. One promising solution is rapid flooding of the mine void to create artificial pit lakes, which can provide various benefits, including improved water quality and earlier land reuse. However, current post-mine land reuse projects primarily focus on ecological conservation and recreation, leaving significant potential for alternative uses, such as harnessing geothermal energy, largely untapped. Shallow geothermal systems offer an efficient way to extract or reject heat from the ground or water bodies using heat pumps. The integration of these systems into existing mine infrastructure offers an environmentally friendly and economically viable way to leverage the thermal potential of decommissioned mines. The mining sector's limited adoption of this technology, coupled with the often significant distances between open-pit mines and population centers, has hindered the exploration of this option. This study aims to fill this knowledge gap by providing a comprehensive techno-economic analysis of this innovative approach.

Existing literature reveals a limited number of documented cases where shallow geothermal energy has been harnessed from mining environments, especially from open-pit mines. While studies exist on geothermal reuse of abandoned mine infrastructure, there's a lack of research focused on integrating geothermal systems during the mine closure phase. Underground coal mines, often situated beneath population centers and possessing natural geothermal reservoirs, are more frequently considered for geothermal exploitation. In contrast, open-pit mines, typically located further from population centers, have received less attention despite their potential. While the importance of proximity to water bodies for economic feasibility of large-scale heat pump systems is acknowledged, specific cut-off distances and comprehensive analyses of long-distance, low-temperature heat and cooling provisioning are absent from the existing literature. This lack of research underscores the need for in-depth analysis of the techno-economic viability of utilizing decommissioned open-pit mines as geothermal energy sources.

This study employs a techno-economic framework to assess the feasibility of using decommissioned open-pit mines for heating and cooling. The analysis focuses on two case study regions: the Latrobe Valley in Victoria, Australia, and the Rhenish Coal Mining Area in North-Rhine Westphalia, Germany, both regions with several open-pit coal mines slated for decommissioning. The study considers the residential thermal demands of clusters of single-family houses (SFHs) in each region, using freely available typical meteorological year (TMY) datasets and a simplified 5R1C circuit method to calculate hourly thermal demands. Two network configurations are modeled: a decentralized system with individual heat pumps for each consumer and a centralized system with a central heat pump serving multiple consumers. The decommissioned mine serves as a free low-grade thermal source, connected to the consumers through transmission pipelines. A benchmark case with individual thermal sources is also included for comparison. The economic assessment utilizes the equivalent annual cost (EAC), encompassing investment costs, operation and maintenance costs, and commodity costs. The levelized cost of energy (LCOE) is used to compare different scenarios with varying thermal demands and transmission lengths. Sensitivity analyses investigate the influence of source temperature, pipeline insulation (insulated vs. non-insulated PE pipes), and electricity prices. The COMANDO optimization package is utilized for operational optimization using mixed integer quadratically constrained programming (MIQCP). The model considers hourly time steps and employs time series aggregation using typical days to reduce computational burden. The commercial solver GUROBI is used to solve the optimization problem. The economic assessment considers costs for various components (heat pumps, heat exchangers, pipelines, etc.), obtained from literature and adjusted for regional differences and inflation. Commodity costs for electricity and gas are obtained from publicly available sources for each region.

The study's key findings reveal several important aspects of the techno-economic feasibility of using decommissioned open-pit mines as geothermal sources: 1. **LCOE and Q/L Ratio:** The levelized cost of energy (LCOE) decreases significantly as the ratio of total thermal demand (Q) to transmission length (L) increases. A Q/L value of approximately 1 MWh m⁻¹ marks a transition point where the LCOE's dependence on transmission investment costs diminishes. 2. **Centralized vs. Decentralized Systems:** The economic performance of centralized and decentralized systems varies depending on the region and the ratio of industrial to residential electricity prices. In the case of NRW, the centralized system generally shows a lower LCOE due to economies of scale and lower industrial electricity costs, but the opposite is true for VIC, where higher cooling loads and a higher electricity price ratio favor the decentralized approach. 3. **Source Temperature Impact:** Increasing the source temperature consistently reduces the LCOE, particularly for heating-dominated demands. This effect is more pronounced for the decentralized systems. However, at higher temperatures, the benefits of increased source temperature diminish due to heat losses in the transmission lines, especially in centralized systems with long transmission lengths. 4. **Pipeline Insulation:** The effect of pipeline insulation on LCOE is mixed. In the Latrobe Valley (VIC), insulation provides negligible or even negative economic benefits, while in the Rhenish Area (NRW), the effects are more nuanced and depend on the specific combination of source temperature and transmission length. Longer transmission lines with higher temperature sources may benefit from insulation, but this is often offset by higher investment costs for insulated pipes. The optimization model's disregard of system heat losses, along with differences in pipe material properties, influence this observation. 5. **Economic Competitiveness:** The economic competitiveness of the geothermal systems depends significantly on consumer electricity prices. A hinge point around 0.3 EUR kWh⁻¹ was identified, where the optimal design for individual thermal sources switches from RCAC-only to a combination of gas boiler and RCAC. Below this price, the geothermal networks are more competitive. Higher source temperatures generally enhance competitiveness, and this effect is more pronounced in decentralized systems. The study illustrates that changes in commodity prices, such as electricity prices, can substantially affect the relative competitiveness of geothermal networks compared to individual thermal supply sources.

The findings demonstrate that repurposing decommissioned open-pit mines as geothermal sources for heating and cooling is economically viable under specific conditions. The economic competitiveness hinges on several factors, including source temperature, transmission length, the number of connected consumers, electricity prices, and the chosen network configuration (centralized or decentralized). The interplay between heating and cooling loads, as well as the ratio of industrial to residential electricity prices, significantly influences the optimal system design. The limited impact of pipeline insulation suggests that a careful cost-benefit analysis is necessary for each specific application. The study’s insights offer valuable guidance for decision-makers involved in mine closure planning and the development of sustainable energy solutions for coal-dependent communities. The model's simplification of constant source temperature and commodity prices represents a limitation that could be addressed in future studies. The results highlight the need for case-specific assessments that account for unique regional conditions and factors.

This study offers a novel techno-economic assessment of using decommissioned open-pit mines as geothermal energy sources for heating and cooling. The results demonstrate the potential economic viability of this approach under specific circumstances, highlighting the importance of considering factors such as source temperature, transmission distance, consumer density, and electricity prices. The limited impact of insulation suggests that its implementation should be carefully evaluated on a case-by-case basis. Further research could focus on incorporating dynamic source temperature and commodity prices, exploring different network optimization strategies, and investigating the integration of renewable energy sources to further enhance the sustainability and economic competitiveness of this innovative approach.

The study’s limitations include the assumption of a constant source temperature and commodity prices, which may not accurately reflect real-world variability. The analysis also simplifies the hydraulic and thermal behavior of the transmission system by considering steady-state conditions. The model doesn't account for potential environmental impacts beyond those related to the thermal energy systems. Furthermore, the economic analysis doesn't include subsidies or decarbonization incentives that might enhance the competitiveness of geothermal systems. Finally, the costs for the components were assumed to be equal between both cases studies to aid in assessing the competitiveness of the thermal system but these might vary from the true ones.