Abstract
Flooded underground mines are attractive for groundwater heat pump systems, as the voids created during mining operations enhance the subsurface permeability and storage capacity, which allows the extraction of significant volumes of groundwater without requiring extensive drilling. Heat exchange at a flooded mine is, however, difficult to predict because of the complex geometry of the underground network of tunnels. A case study is presented here to demonstrate that numerical simulations of groundwater flow and heat transfer can help assess production temperatures required to optimize the design of a heat pump system that uses mine water. A 3D numerical model was developed for the Gaspé Mines located in Murdochville, Canada, where a district heating and cooling system is being studied. The underground mining tunnels and shafts are represented in the model with 1D elements whose flow and heat transfer contributions are superimposed to those of the 3D porous medium. The numerical model is calibrated to simultaneously reproduce the groundwater rebound that occurred when the mine closed and the drawdown measured during a pumping test conducted in a former mining shaft. Predictive simulations over a period of 50 years are subsequently performed to minimize pumping rate and determine maximum heat extraction rate.
Résumé
Les mines souterraines inondées offrent des avantages pour les systèmes de pompe à chaleur d’aquifère puisque les vides créés par les opérations minières contribuent à l’accroissent de la perméabilité du sous-sol et de la capacité d’emmagasinement, ce qui permet l’extraction de volumes significatifs d’eau souterraine sans recourir à de nombreux forages. Les échanges de chaleur dans une mine inondée sont cependant difficiles à prévoir en raison de la géométrie complexe du réseau de galeries souterraines. Une étude de cas est présentée afin de démontrer comment la simulation numérique de l’écoulement d’eau souterraine et des transferts de chaleur peut aider à l’évaluation des températures d’opération pour optimiser la conception d’un système de pompe à chaleur opéré avec l’eau de la mine. Un modèle numérique 3D a été développé pour les Mines de Gaspé, situées à Murdochville, Canada, où l’installation d’un système énergétique de quartier pour le chauffage et la climatisation est à l’étude. Les galeries souterraines et les puits de la mine sont représentés dans le modèle par des éléments 1D dont les contributions à l’écoulement et au transfert de chaleur sont superposées à celles du milieu poreux 3D. Le modèle numérique est calé de manière à reproduire simultanément la remontée d’eau souterraine qui a suivi la fermeture de la mine et le rabattement mesuré lors d’un essai de pompage réalisé dans un ancien puits de ventilation. Des simulations couvrant une période de 50 ans sont ensuite effectuées afin de minimiser le débit de pompage et de fixer le taux d’extraction de chaleur maximal selon les prédictions du modèle.
Resumen
Las minas subterráneas inundadas son atractivas para los sistemas de bombas de calor de agua subterránea, porque el vacío creado durante las operaciones de minería enriquece la permeabilidad del subsuelo y la capacidad de almacenamiento, lo que permite la extracción de volúmenes significativos de aguas sin requerir de extensas de perforaciones. El intercambio de calor en una mina inundada es, sin embargo, difícil para predecir debido a la compleja geometría de las redes de túneles en el subsuelo. Aquí se presenta un caso de estudio para demostrar que las simulaciones numéricas del flujo de agua subterránea y la transferencia de calor pueden ayudar a evaluar la producción de las temperaturas requeridas para optimizar el diseño del sistema de bombeo de calor que usa el agua de la mina. Se desarrolló un modelo numérico 3D para las Minas Gaspé ubicadas en Murdochville, Canadá, donde un sistema de calentamiento y refrigeración distrital está siendo estudiado. Los túneles en el subsuelo y los pozos de mina están representados en el modelo con elementos 1D cuyas contribuciones al flujo y calor se sobreimponen a las del medio poroso 3D. El modelo numérico está calibrado para reproducir simultáneamente la recuperación del agua subterránea que ocurrió cuando la mina se cerró y la depresión medida durante un ensayo de bombeo realizado en un antiguo pozo de la mina. Las simulaciones predictivas sobre un período de 50 años son subsecuentemente realizadas para minimizar los caudales de bombeo y determinar la máxima tasa de extracción de calor.
Resumo
As minas subterrâneas inundadas são locais atrativos para a instalação de sistemas extração de águas subterrâneas com bomba de calor, na medida em que os espaços vazios criados durante as operações de mineração aumentam a permeabilidade do subsolo e a capacidade de armazenamento, o que permite a extração de volumes significativos de águas subterrâneas sem exigir grandes perfurações. As trocas térmicas numa mina inundada são, contudo, difíceis de prever, devido à geometria complexa da rede subterrânea de túneis. Apresenta-se um caso de estudo para demonstrar que as simulações numéricas do escoamento de águas subterrâneas e de transferência de calor podem ajudar a avaliar as temperaturas de produção necessárias para otimizar o projeto de um sistema de bomba de calor que utiliza água de mina. Desenvolveu-se um modelo numérico 3D para as Minas Gaspé, localizadas em Murdochville, no Canadá, onde está a ser estudado um sistema de aquecimento e de refrigeração. Os túneis subterrâneos e os poços de mina são representados no modelo com elementos 1D, cujas contribuições de fluxo e de transferência de calor são impostas às do meio poroso 3D. O modelo numérico é calibrado para reproduzir, simultaneamente, o ressalto de águas subterrâneas que ocorreu quando a mina fechou e o rebaixamento medido durante um ensaio de caudal realizado num poço de extração antigo. Os cenários de previsão para um período de 50 anos são posteriormente executados para minimizar a taxa de extração e determinar o valor máximo de extração de calor.
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Acknowledgements
The town of Murdochville, the Fondation Gaspésie-Les-Îles, the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Fonds québécois de la recherche sur la nature et les technologies (FQRNT) are acknowledged for funding this research. Collaboration from the staff at the town of Murdochville and Xstrata at the Gaspé Mines was greatly appreciated. Comments from Dr. Wolfram Rühaak and a second anonymous reviewer, who both deserve acknowledgement, helped to improve the manuscript.
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Published in the theme issue “Hydrogeology of Shallow Thermal Systems”
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Raymond, J., Therrien, R. Optimizing the design of a geothermal district heating and cooling system located at a flooded mine in Canada. Hydrogeol J 22, 217–231 (2014). https://doi.org/10.1007/s10040-013-1063-3
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DOI: https://doi.org/10.1007/s10040-013-1063-3