Sustainable Heat Extraction from Abandoned Mine Tunnels: A numerical model
Because abandoned mines are often associated with enduring liabilities, often involving significant costs long after the mine has been decommissioned, capturing usable heat from mine water can help to improve the sustainability of the mine. This extracted heat can then be used as an energy resource for communities living in areas close to the inactive mine.
Underground mines are commonly flooded after closure and are considered potentially viable geothermal resources, in particular open loop resources, because of the presence of essential components: heat, water, and permeability. Water can reach temperatures of 25-30°C 1000 meters below the mine, and is infiltrated into the mine network by precipitation, underground aquifers, or artificial flooding. Infiltration of water into the mine cavities is heavily dependent on permeability, which can be inherent or natural permeability of the rock, or a function of man-made factors (such as man-made openings). Furthermore, because the flooded mine has a great water storage capacity, it can be used as a geothermal reservoir to manage heat-loads during periods of peak demand. Therefore, short-term energy storability in underground mine cavities, which is not easily available in conventional open loop systems, provides a great opportunity for peak shaving.
Open looped geothermal systems require the presence of an underground aquifer; in this type of system, underground water is chilled using a heat pump in which heat is extracted from the water, and then it is returned to the aquifer. A closed loop system, by contrast, involves geothermal energy being extracted from water that is circulating in a closed network of heat exchanged tubes (embedded in the ground). In terms of the feasibility of open loops systems, much depends on the cost of drilling and well maintenance (which can be quite significant). Hence, the utilization of underground mines for the purpose of geothermal resources can provide the opportunity of decreasing these costs and improving feasibility.
The application of underground mines as a geothermal resource began in Canada in the 1980s, and the Springhill, Nova Scotia project is one of the pioneering studies in the field. In this case, a coal mine is used to heat a plastic factory that has an approximate surface area of 14 000m2. The production well is located 140 m deep and a flow rate of 2401/min of mine water is fed to the heat pump system, consisting of 11 heat pumps; these are used for heating in winter and cooling during the summer. The water has a temperature of 18°C upon leaving the system, and returns at about 13°C in winter and 25°C in the summer. The water is then re-injected into the mine at 30m below the surface. The company’s saving, using the geothermal system, is estimated to be $160 000 per year in energy costs over a conventional oil furnace system.
While the Springhill project is of small-scale and does not raise many sustainability issues, larger scale projects that provide heat to larger communities require more of a focus on resource assessment and sustainability. Larger scale mines, such as Wismut in Germany, can provide upwards of 690 kW of heat capacity.
With the successful application of low temperature open loop geothermal systems in some flooded mines, more attention is drawn towards more systematic works to find criteria for the viability of different mines for large-scale heat extraction. Studies were carried out on mines such as the Con mine in Yellowknife, in which water temperatures are around 35°C in the deeper levels of the mine. Because this Nordic Canadian city has a high dependency on fossil fuels for heating, extraction of geothermal energy is expected to significantly reduce the city’s energy costs and carbon footprint. It is estimated that Con mine can contribute up to 10MW of usable heat to the city.
Using abandoned mines as open loop geothermal resources is controlled by implementation costs, and high risks associated with thermal behaviour of these resources. The risk is mostly related to the fact that much of the available information about the underground mine workings cannot be incorporated as firsthand information into the geothermal models, due to the complexity (or even in some cases the uncertainty) of the basic geometry information of the underground openings and in-mine water movement.
The risk issue is also associated with the resource sustainability. Since the heat capacity of a mine is limited and should be matched with the demand, the total rate of sustainable heat extraction from the mine should be carefully assessed. Otherwise, if over-exploited, the mine will not be able to provide sustainable heat in the long-term. One way to assess the resource sustainability is to employ a numerical simulation model which takes into account the most important and better known factors and deals confidently with the uncertainty or absence of data on some less important parameters.
This paper focuses on fully flooded mines where the heat flow from the rock mass to the mine cavities is dominantly controlled by conduction in the rock mass. The sustainable heat flux into the mine workings is assessed using a transient two dimensional axisymmetric heat transfer model. The Finite volume method is applied to solve the model and simulate the transient temperature fields in the rock mass and within the water (flowing through cavities). The model is capable of controlling the rate of heat extraction through continuous adjustment of the rate of water flow through the mine. The sustainable rate of heat extraction is calculated for seasonally varied heat loads and for different project life cycles. It is shown that, with proper resource management, each kilometre of a typical deep underground mine tunnel can produce about 150kW of usable heat in a sustainable manner. The study also shows that underground mines can be a significant source of green heat for Canadian mining communities, where decommissioned mines have so far been liabilities rather than assets.
Link to full article:
S. A. Ghoreishi Madiseh; M.M. Ghomshei; F. P. Hassani; and F. Abbasy. May, 2012.