Geothermal system optimization in mining environments

Tableau d’honneur de la Faculté des études supérieures et postdoctorales, 2010-2011Les ressources particulières à l’environnement minier, tel que l’eau inondant des galeries et des stériles exothermiques, permettent de diminuer les coûts d’installation des systèmes de pompes à chaleur géothermique. Les particularités de l’environnement minier posent toutefois un défi de taille lors de la conception d’un système puisque l’approche utilisée doit considérer, par exemple, la conductivité hydraulique accrue par les excavations ou la génération de chaleur provenant de l’oxydation de minéraux. L’objectif de ce projet de recherche est de simuler l’opération de systèmes géothermiques issus de sites miniers dans le but de démontrer les économies d’énergie potentielles et faciliter l’installation des systèmes. Des approches numériques sont développées avec le programme HydroGeoSphere pour application à des études de cas réalisées aux Mines Gaspé à Murdochville et à la Halde Sud de la Mine Doyon en Abitibi. Un système de pompes à chaleur d’aquifère aux Mines Gaspé est optimisé avec un modèle numérique où sont superposés des éléments 1D et 3D pour représenter les excavations. Les simulations démontrent que le système peut être opéré à partir d’anciens puits de ventilation afin d’éviter d’effectuer des forages. Les stériles de la Halde Sud sont d’abord caractérisés avec un test de réponse thermique conventionnel, analysé avec l’équation de la ligne-source et le principe de superposition considérant les variations du taux d’injection de chaleur. Une méthode numérique est aussi développée pour analyser l’essai influencé par l’hétérogénéité des matériaux et le gradient géothermique prononcé. Le mort terrain et le roc sous la halde sont caractérisés à l’aide d’un nouveau test de réponse thermique effectué avec des câbles chauffants. Des simulations numériques reproduisent ensuite la distribution de température lors d’essais types, laquelle devient rapidement homogène durant la période de restitution thermique ce qui permet d’analyser la température dans le forage sans connaître la position du capteur. L’opération d’un système de pompes à chaleur couplées au sol aménagé sous la Halde Sud est finalement simulée. L’optimisation des charges de chauffage indique que l’échangeur de chaleur situé sous les stériles peut fournir plus d’énergie thermique qu’un échangeur situé dans un environnent conventionnel, réduisant la longueur de forage à l’installation.Resources associated to mining environments, such as mine water and exothermic waste rock, allow a reduction of installation costs of ground source heat pump systems. Compared to other environments, caution is required when designing systems in mining environments because of enhanced hydraulic conductivity created by mine voids or heat generation due to oxidation of minerals. The objective of this study is to simulate the operation of geothermal systems on mine sites to demonstrate energy savings and promote installation. Numerical modeling approaches are developed with the program HydroGeoSphere applied for case studies conducted at the Gaspé Mines in Murdochville and at the South Dump of the Doyon Mine in Abitibi. A groundwater heat pump system at the Gaspé Mines is optimized with a numerical model, where 1D and 3D elements are superposed to adequately represent the mine voids. The simulations show that the system can be operated using former mining shafts to avoid drilling boreholes. Waste rock of the South Dump is initially characterized with a conventional thermal response test analyzed with the lines-source equation and the superposition principle accounting for variations of heat injection rates. A numerical method is also developed to analyze the test that was affected by the heterogeneity of materials and the strong geothermal gradient. The overburden and the host rock below the dump are characterized with a novel thermal response test using heating cables. Numerical simulations then reproduce the temperature distribution during typical tests, which homogenizes rapidly during the recovery period allowing the analysis of temperature inside the borehole without knowing the position of the sensor. The operation of a ground-coupled heat pump system installed under the South Dump is finally simulated. The optimization of heating loads indicates that the heat exchanger located beneath the waste pile can provide more thermal energy than an exchanger located in a conventional environment, reducing bore length required for a given system

Low-Temperature Geothermal Potential of the Gaspé Mines, Murdochville

Évaluer le potentiel géothermique d’une mine inondée est une tâche complexe qui nécessite la réalisation d’un test hydraulique et la modélisation de l’écoulement de l’eau souterraine à travers un réservoir non conventionnel. Cette tâche peut être efficacement complétée en réutilisant les anciennes infrastructures minières. Lors de cette étude, un puits de ventilation débouchant dans les galeries souterraines a été converti en ouvrage de captage d’eau profond. L’ancien puits de ventilation 1100 des Mines Gaspé à Murdochville au Canada a été utilisé pour effectuer un essai de pompage durant 3 semaines à un débit moyen de 0.062 m3/s. L’objectif principal des travaux encourus était d’évaluer le potentiel géothermique du site minier. Lors de l’essai, moins de 3,65 m de rabattement ont été observés et la température moyenne de l’eau pompée a été de 6,7 °C. Le comportement de la nappe souterraine durant l’essai a été reproduit à l’aide d’un modèle d’éléments finis tridimensionnel simulant l’écoulement de l’eau à travers les galeries. Les prédictions du modèle ainsi qu’un bilan énergétique simplifié suggèrent que le taux d’extraction d’énergie durable est atteint à un débit de pompage de 0.049 m3/s, ce qui indique un potentiel géothermique de 765 kW. Cette énergie pourrait être extraite à l’aide de pompes à chaleur géothermiques afin de chauffer les bâtiments du parc industriel de Murdochville.Assessing the low-temperature geothermal potential of a flooded mine site is a complex task involving hydraulic testing and modelling of an unusual man-made reservoir. It can be achieved efficiently taking advantages of the former mine infrastructures such as mining shaft that provided here a deep well directly connected to the mine workings. The former mining shaft P1100 of the flooded Gaspé Mines near Murdochville, Canada, was used to perform a pumping test during a study with the objective of assessing the geothermal potential of the mine site. Water was pumped during 3 weeks at a rate averaging 0.062 m3/s with a mean recovery temperature equal to 6.7 °C and less than 3.65 m drawdown was observed. The hydraulic response of the pumping test was reproduced with a threedimensional finite element model that simulates groundwater flow through the mine workings. Model predictions and a simplified energy balance calculation suggested that the sustainable energy extraction rate is attained at a pumping rate of 0.049 m3/s which yield a geothermal potential of 765 kW. This energy could be extracted with geothermal heat pumps used for space heating at Murdochville industrial park

New concept of combined hydro-thermal response tests (H/ TRTs) for ground heat exchangers

Current thermal response tests, used to estimate the subsurface thermal conductivity in the geothermal domain, are not designed to take into account groundwater flows. To measure the flow parameters, a new concept has been developed. Heating cables are installed within a borehole in contact to the formation, with three temperature probes strategically located at the edge of the borehole. Study of the evolution of temperature for each probe during both a heat injection phase and a recovery period allows determining ground thermal conductivity, groundwater flow velocity and orientation. Numerical simulations have been used to validate the proposed concept and establish its limit

Measurement of internal and effective borehole resistances during thermal response tests

In a conventional thermal response test (TRT), the main parameter evaluated is the bulk subsurface thermal conductivity surrounding the borehole. It is also possible to evaluate the borehole thermal resistance. Several approaches were proposed in the literature to evaluate the possible combination of these two parameters. For example, it is often suggested to measure the temperature during the injection and the recovery periods, where the thermal conductivity is found with the recovery response whereas the borehole resistance is calculated with injection measurements. For this calculation, some authors suggested to use different means for the borehole temperature considering the asymmetric temperature distribution along the pipe legs that affects the borehole resistance. Some confusion about the borehole resistance that should be obtained may come from the difference between the 2D borehole resistance and the effective (3D) borehole resistance taking into account the internal heat transfer between the pipe legs inside the borehole. In practice, the latter one should be used in a design algorithm since it provides a more representative approach. In many cases, the difference between the two is rather small. However, since the borehole length is becoming an important variable to optimize, this difference in borehole resistance may represent a factor to better assess in the design of future systems. This effective resistance depends on the 2D borehole resistance, the water flow rate, the length of the borehole and the so called "internal resistance." To our knowledge, the in-situ assessment of this internal resistance has never been achieved. In this paper, we present our first investigation of a method that can be used to evaluate both the 2D borehole resistance (Rb) and the 2D internal resistance (Ra). The method uses the temperature at the bottom of the borehole at the same time as the inlet and outlet temperatures that are measured in a conventional TRT. Interesting results were found by comparison with theoretical resistances calculated with the multipole metho

Ground heat exchangers with large diameter pipes: What are the benefits?

The geothermal industry has recently been opting for large high-density polyethylene pipes to design vertical ground heat exchangers of ground-coupled heat pump system. Thus, we hypothesized that large diameter pipes can help improve heat exchange rate with the subsurface and made this study with the objective of quantifying the benefits gained with U-loop configurations. The finite line source equation and the multipole model were used to evaluate the maximum heat exchange rate that can be achieved with increasing pipe diameter. Sizing calculations were then performed for a school building in Boston. Ground heat exchangers with a single U-pipe having a nominal diameter of 1.25, 1.5 and 2 inches, as well as a double U-pipe, having a nominal diameter of 1.5 inches, were considered. Results highlighted that the double loop is by far the most efficient configuration, followed by the single loop with a 2 inches pipe diameter, respectively providing heat exchange rates that were 16% and 6% greater and total borehole length that were 22 and 9% smaller when compared to single loop with a 1.25 inches diameter. The use of large diameter pipes was shown without a doubt to benefit ground heat exchanger performances

Identification of Thermal Refuges and Water Temperature Patterns in Salmonid-Bearing Subarctic Rivers of Northern Quebec

In summer, salmonids can experience thermal stress during extreme weather conditions. This may affect their growth and even threaten their survival. Cool water zones in rivers constitute thermal refuges, allowing fish to be more comfortable to grow and survive in extreme events. Therefore, identifying and understanding the spatiotemporal variability of discrete thermal refuges and larger scale cooling zones in rivers is of fundamental interest. This study analyzes thermal refuges as well as cooling zones in two salmonid rivers in a subarctic climate by use of thermal infrared (TIR) imagery. The two studied rivers are the Koroc and Berard Rivers, in Nunavik, Quebec, Canada. On the 17 km studied section of the Berard River, four thermal refuges and five cooling zones were detected, covering 46% of the surveyed section of the river. On the 41 km section studied for the Koroc River, 67 thermal refuges and five cooling zones were identified which represent 32% of the studied section of the river. 89% of identified thermal refuges and about 60% of cooling zones are groundwater-controlled. Continuity of permafrost and shape of the river valley were found to be the main parameters controlling the distribution of refuges and cooling zones. These data provide important insights into planning and conservation measures for the salmonid population of subarctic Nunavik rivers

Underground thermal energy storage in subarctic climates: A feasibility study conducted in Kuujjuaq (QC, Canada)

Underground thermal energy storage can provide space and water heating and has been used in temperate climates so far. A step forward is to evaluate the efficiency and viability in arctic to subarctic environments, where rather low ground and air temperatures can make the design of such systems difficult. The present contribution describes the design of an underground storage system in Kuujjuaq (Québec, Canada) to heat the drinking water distributed in the town. The system was designed and modeled with TRNSYS and a parametric study was carried out to improve its efficiency based on 5-year simulations. The 20% of the 425 MWh annual demand can be satisfied by a solar collector area of 500 m2 coupled to a 10,000 m3 underground storage through two short term tanks. Further improvements could be adopted to reach the target of 50% energy from the underground store

New methods to spatially extend thermal response test assessments

Thermal response tests (TRTs), used to evaluate the subsurface thermal conductivity when designing ground source heat pump systems, are spatially limited to the vicinity of the borehole where a test is carried out. The subsurface is heterogeneous and the thermal conductivity assessment provided by a TRT is likely to vary beyond the tested borehole. New methods have, therefore, been developed to extend subsurface assessments at the building site and the urban district scales. The first method relies on temperature profiles measured at equilibrium in ground heat exchangers that are reproduced with inverse numerical simulations to infer the terrestrial heat flow and the subsurface thermal conductivity beyond a first TRT. Inversion of temperature profiles was verified at a pilot site in the Appalachians where TRTs had been performed and showed a thermal conductivity estimate within less than 10 % for both approaches. The second method is based on geostatistical simulations to map the distribution of the subsurface thermal conductivity in areas where several ground source heat pump installations are anticipated. A first mapping exercise was achieved to the north of Montreal in the St. Lawrence Lowlands with fours TRTs and ten laboratory measurements interpolated with sequential Gaussian simulations

Nanocomposite materials used for ground heat exchanger pipes

This study compares the performance of single U-pipe, double U-pipe, and coaxial ground heat exchangers (GHE) equipped with standard HDPE and thermally enhanced (TE) pipes. Sizing calculations and 10-year hourly simulations were carried out with the GLHEPro software using as input a synthetic thermal load profile of a reference, heating-dominated, medium office building located in the U.S. climate zone 5B enclosing Colorado. Energy consumption by the ground heat and ground loop pumps were then calculated from the simulated outputs. Finally, a life-cycle cost analysis was performed to compare the total costs (construction and operation) net present value of the GHEs equipped with TE pipes with those equipped with standard HDPE pipes. Results showed that the double U-pipe with thermally enhanced pipes was the best option for the conditions considered in the study. Depending on the configuration, the use of TE pipes instead of standard HDPE pipes allowed a reduction of the GHE length between 9 and 14.8% and a reduction of the construction cost between 3.3 and 8.6 %. For each configuration tested, the operation costs were similar between the GHEs equipped with HDPE and TE pipes. This study demonstrates that GHEs equipped with TE pipes can be a financially viable and environmentally beneficial solution, especially if secondary benefits are factored in such as saved footprints on available real estate