The Canadian mining industry has created thousands of underground mines and due to the nature of mining they eventually exhaust the resource and shut down, leaving a system of underground openings that could be profited for their geothermal energy. Low temperature geothermal energy can be extracted from these mines using current heat pump technology. This paper will describe recent developments that make heat extraction from mines feasible, as well as analyze current systems in place. A case study of a Canadian company that has been extracting geothermal heat from an abandoned mine will also be presented. Major conclusions include the feasibility of this technology in regions located in close proximity to abandoned mines. Geothermal heat pumps using mine water boast similar COP to classic geothermal systems, without such large initial drilling costs due to the mines presence.
Geothermal heating is known to be more efficient than air source heat pumps and electrical resistance heating, reporting a coefficient of performance () of 3-4 (Watzlaf and Ackman, 2006). This is from systems that are specifically designed for heating and cooling; the objective of this paper is to consider the efficiency and feasibility of geothermal heat pump systems using underground mines as a heat source.
2.0 Geothermal Heating Principle
2.1 Heat Pumps
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To appreciate the ability of utilizing the earths near constant temperature, the system involved must be explained. According to Banks et al. (2003), the cycle pumps a collector fluid from the warm source (eg mine) to the evaporator. This warm collector fluid interacts with the refrigerant fluid in the evaporator, resulting in the refrigerant being warmed and evaporated. The refrigerant is then heated further by compression to a temperature that is ideal for use. During the condenser stage, the air or fluid that is to be heated strips the refrigerant of its heat. To complete the cycle, the refrigerant goes through an expander, which cools the refrigerant. The cycle repeats itself with the collector fluid returning to the heat sink where it is warmed by the ambient earth temperature. Figure 1 summarizes the system and cycle, focusing on the heat pump.
Source: Banks et al., 2003
Figure : Heat Pump System (Closed Loop)
An important part of this system is the refrigerant's characteristics, for the heat pump system to function correctly the temperature of the collector fluid must be high enough to cause the liquid refrigerant to boil. Furthermore, once the refrigerant expands and cools, it must be at a lower temperature than the ground source liquid, defined here as the collector fluid. (Watzlaf and Ackman, 2006)
2.2 Geothermal Systems
There are two main geothermal heat pump systems that can be used in the application to underground mines, the closed-loop system shown in figure 2 and the open-loop system shown in figure 3. The most important difference between the two systems is that the open-loop system extracts water from the source and uses it to heat the refrigerant fluid. The mine water returns to the source cooled and usually downstream of the intake location.
Source: Watzlaf and Ackman, 2006
Figure : Closed Loop Geothermal Heat Pump System
Source: Watzlaf and Ackman, 2006
Figure : Open Loop Geothermal Heat Pump System
The closed loop system may be used if the hydraulic conductivity of the soil is low, this would be the case with some clays and intact rock. Also, if hazardous liquids are present, they could cause corrosion to occur in the heat pump system therefore a closed loop system would be ideal (Watzlaf and Ackman, 2006).
A more common situation is to have large bodies of water (pools) stored in abandoned mines and so an open loop system is more convenient.
3.0 Current Situation
3.1 Groundwater Heat Pumps
Bodies of trapped underground water, known as aquifers, are common in abandoned mines since pumping out ground water would have ceased, resulting in the tunnels and shafts flooding. According to Raymond et al. (2009), there are 165 inactive mines and 94 exploration sites in Quebec that have geothermal potential. Communities in the vicinity can use the geothermal energy extracted from these inactive sites.
Another use for groundwater heat pumps (GWHP) is in active mines where water is already being pumped out to avoid flooding during normal operation. Installing GWHP's in these situations can be highly beneficial for mine operations, as the infrastructure needs to be heated or cooled. (Raymond et al., 2009)
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GWHP systems would use an open loop design since water permeability is high and large reservoirs are present. Figure 4 below shows relevant mines in Quebec that could be exploited for geothermal heating.
Source: Raymond et al., 2009
Figure : Active And Closed Mines In Relation To Significant Towns
3.2 Surface Water Heat Pumps
Flooded surface mines, depending on the depth, will have an adequate temperature gradient to generate energy savings using a SWHP system. Retention ponds at active and inactive ponds would also benefit from this system. Once again open loop designs would be ideal. (Raymond et al., 2009)
3.3 Ground Coupled Heat Pumps
Mines usually have waste piles and tailings on the surface that could be easily trenched to allow for closed loop GCHP's to be installed at cheaper costs. Increased ground temperatures can be present in these waste dumps due to oxidization reactions underway. This is significant since increased temperatures dramatically shorten the exchanger length. (Raymond et al., 2009)
4.0 Energy Resource Potential
According to Ghomshei (2007), geothermal resources consist of three main constituents.
Heat reserves or the total heat of a resource can be calculated using the equation shown below (where H can be expressed in BTU, kcal, or kWh).
The parameters are:
Vg = Volume of subsurface ground reservoir
Gg = Specific density of soil/rock
Cg = Specific heat of the soil/rock
Vw = Volume of subsurface water reservoir
Gw = specific density of the pore water
Cw = specific heat of the water
Tg = average ground temperature
Tm = minimum ground temperature after heat extraction
Most of these parameters are difficult to accurately quantify therefore Ghomshei (2007) warns that this approach should only be used to obtain preliminary order of magnitude values. The most accurate values are the temperatures, which can be controlled.
Water is the main mean of recovering heat from mine shafts and tunnels. To benefit from an open looped system the mine (abandoned) must be at least partially flooded. Water at deeper levels is warmer due to increased rock temperatures, therefore it is beneficial to extract water from the deepest mine levels to achieve maximum efficiency. To be a truly renewable source of heat and to remain environmentally friendly, the water pumped out must be returned to the mine. The location of the input well and the relevant rates are a function of the hydraulic conductivity. (Ghomshei, 2007)
4.3 Hydraulic Conductivity
The rate at which water flows through the mine rock mass is a deciding factor in designing a geothermal system. During mining operation, large pumps are constantly dewatering most mines. Although this can be a nuisance for active mines, the fact that the hydraulic conductivity is high and once mine operations shut down it will not take much time for the mine to flood to suitable levels for initiating open looped systems. (Ghomshei, 2007)
Another important design characteristic that is dependent on the hydraulic conductivity is the rate of extraction and reinjection. Rates should coincide with the hydraulic conductivity to avoid heat exhaustion. If the conductivity is high, than the reinjection point should be further away from the extraction well to allow for adequate heat recovery. (Ghomshei, 2007)
The idea of using geothermal heating and cooling methods in active mines is rarely mentioned but there are instances when this could be beneficial. As mentioned, when the hydraulic conductivity of the rock mass is high, and rates of dewatering are substantial, the possibility of using this water as part of a heat pump system is possible.
The pumped water could go through a similar open looped system as discussed in the flooded mine scenarios, and the energy could then be used onsite for mine activities. The most cost intensive part of drilling and well installation is already present thus mining companies would have very little to invest and the returns are very beneficial, as shown in the case study. The mining industry would also be shaping a new face in terms of environmental friendliness.
A preliminary study for a small scale demonstration project in the city of Yellowknife was completed. A 300 kW heat demand is equivalent to the needs of a residential apartment building. Table 1 shows the capital costs of such a project, it is worth noting that the most expensive component of the system is the insulated pipes, at 2 km's in length it could vary widely based on the location of implementation.
Table : Development Costs For A Conceptual 300 kW Open Loop System
Source: Ghomshei, 2007
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This is a large initial investment but the energy savings will show the obvious benefits. Assumptions on local energy prices for the Yellowknife area and an efficiency of 75 percent, the heat pump's electricity costs are $105,000 per year but the heating costs saved are $200,000 per year. Resulting in a net savings of $95,000 per year. The payback period is therefore under 8 years; a short time considering the buildings life expectancy. (Ghomshei, 2007)
6.0 Case Study: Springhill, Nova Scotia
6.1 Mine History
The coal mine at Springhill was in operation from 1872 until 1958. This was the town's principle industry therefore economic stability was difficult to achieve afterwards. By 1985, with increasing oil prices, the town began experimenting with heat pumps that would use the water from the flooded mine. The system provided significant energy savings and by 1994 eight users of either industry or commercial size were using geothermal heating from mine water. (Jessop, 1995)
6.2 Geological Background
Mines in the Springhill area are coal seams that dip at approximately 30 degrees, there are 7 mined seams and all are interconnected. The mine used room and pillar, and some longwall methods for extracting the coal. (Jessop, 1995)
The volume of water now present in the mine is obtained by calculating the volume of the mined seams and the total coal production. Then assuming that 25% of this space remains, this is a conservative value, which gives a mean value of 4x106 m3. (Jessop, 1995)
Source: Ghomshei, 2007
Figure : Section Of Mine Water System
6.3 Geothermal Conditions
The geothermal gradient is the rate that the Earth's temperature changes with depth. Temperature gradients in the surrounding areas average at 15 mK/m. The mine rooms, shafts and interconnecting tunnels allow deep ground water to circulate, although it is unsure how many of these are still intact it is assumed that water is able to flow through these passageways. (Jessop, 1995)
Springhill boasts surface water temperatures that peak at 20 degrees Celsius. This is assumed to be from convective overturn of mine water, flow from deeper areas due to water entering the mine, and finally oxidization reactions. (Jessop, 1995)
Taking an average water temperature of 17, subtracting the temperature drop due to cool water being returned, and the volume of the mine, the potential is 70 GWh. The heat exchange between the water and walls has very little effect on the wall temperatures due to the slow heat transfer, meaning that this amount of energy could be exploited for possibly centuries. (Jessop, 1995)
6.4 Efficiency Results
Ropak Can-Am Ltd is a plastics manufacturer with an 8300 m2 building that pumps mine water from the 140 m level at a rate of 4 L/s using an open looped system. The recorded water temperature at this level is 18 degrees. During the heating mode, water is returned to the 30 m level of the mine, at a minimum temperature of 11 degrees. With some degree of interconnection still existing between mine rooms, the output water is recirculated and reheated resulting in a continuous cycle. (CADDET, 1992)
During the first year of operation, thorough data on energy usage was gathered. The results are a net heat input of 660GJ to the mine (draw of 890 GJ during heating and return of 1550 GJ while air conditioning). The electrical power consumption of the heat pumps was averaged at 128 MWh (460GJ) and the energy output of the heat pumps was approximately 1660 GJ. (Jessop, 1995)
Computing the coefficient of performance as . This value coincides with that of similar heat pump systems, making it a more efficient heating method when compared to electrical and oil heating. Furthermore the source is renewable, leading to many other environmental benefits, unlike fossil fuels.
6.5 Economic Benefits
The capital costs involved with the heat pump system, including the two wells was 20 percent higher than the conventional oil furnace alternative, resulting in an $110,000 initial investment. Thanks to the integrated humidity control in the heat pumps there was no need for dehumidifiers in the factory, this cost alone was estimated at $180,000 ($15,000 per unit). Therefore in this situation, this saving would have compensated the capital cost; but this will not always be the case so this factor will be disregarded. The relevant savings come from the total cost of electricity, which were $18,000 per year. When compared to an oil furnace system, Ropak Can-Am claims that they save approximately $160,000 per year. The result is a payback period of less than one year for the higher initial capital costs. (Jessop, 1995)
Major conclusions include the feasibility of this technology in regions located in close proximity to abandoned mines. Geothermal heat pumps using mine water boast similar COP to classic geothermal systems, without such large initial drilling costs due to the mines presence. Active mines can also benefit from such heat pump systems giving them a more sustainable image. Quebec and Canada as a whole is in an opportune position to benefit from this renewable energy source thanks to the hundreds of suitable mines and our countries climate. Resulting systems could greatly decrease our dependency on fossil fuels for space heating and cooling.