Above: typical geothermal mine water system. Image based on U.S. Department of Energy, National Renewable Energy Laboratory, March 1998, www.eere.energy.gov
Over two centuries of mining in Canada have left thousands of underground mines that are no longer used but are still largely intact. With increasing energy prices, price instability, and the focus on energy independence, it may make sense to extract geothermal energy from some of these underground workings and use it to heat nearby buildings. The idea might sound far-fetched, but it has been around for decades and has many benefits for the community and the environment. Upon closure of a mine, turning it into a renewable geothermal energy source provides a welcome parting gift to the community and can improve a mining company’s public image.
There are a number of geothermal underground mine water systems in operation or under active consideration around the world, including in Canada, the U.S., U.K., France, Germany, the Netherlands, Poland, Spain, Slovakia and Ukraine. Site-specific conditions vary widely, with a range of mine types (e. g. coal, lead, gold, copper), volumes and flow rates. Consequently, energy savings and payback periods also vary widely.
One system in Canada that has been successfully operating for many years is at Springhill in Nova Scotia.
Potential in Canada -a Survey
In 2006, Marbek and FVB Energy completed a study for the Mining Association of Canada and Natural Resources Canada. We examined the potential for using geothermally heated mine water from inactive underground mines to heat nearby surface facilities.
Based on the best available data in regional mine databases, the study estimated that 2,500 to 7,500 inactive underground mines exist in Canada.
The following five criteria were used to assess the preliminary technical suitability of these mines for geothermal mine water systems:
• Depth of at least 30 metres as a proxy for sufficient mine water temperature.
• Extensive underground workings as a proxy for size and renewability of the geothermal reservoir.
• Closed after 1950 as a proxy for mine workings in good condition and availability of accurate mine data.
• Located near potential end-uses to minimize the cost of insulated pipe.
• Low possibility of re-opening to ensure long-term project financial viability.
Based on the limited data on these thousands of mines and their nearby communities, the study found that around 25 sites adequately met the screening criteria above. Most of the 25 sites are now owned by provincial and territorial governments, with only a few still owned by mining companies. The remaining thousands of sites were deemed unsuitable because they appear to be too shallow, small, old, or remote, although more accurate site data may improve their suitability for geothermal.
According to the 2006 study, even 25 such geothermal mine systems could help reduce approximately 10 kilotonnes of carbon dioxide equivalent per year (ktCO2e/yr) of greenhouse gas emissions.
Some of the more promising sites in Canada are:
• Noranda copper mine near Murdochville, Quebec, with extensive workings from 40-50 years of mining (the town has recently applied for funding through the Federation of Canadian Municipalities to implement a geothermal mine water system).
• Con gold mine under Yellowknife, Northwest Territories, with 130 kilometres of workings to a depth of 1,900 metres.
• Wellington coal mine under Nanaimo, B.C., with workings to a depth of 125 metres.
• Frood nickel-copper mine in Sudbury, Ontario with workings to a depth of at least 900 metres.
• Falconbridge nickel mine near Sudbury, Ontario, with workings to a depth of 2,000 metres.
• Various coal mines (e. g. Dominion and Old Emery) near Sydney and Reserve Mines, Glace Bay, Nova Scotia. They have underground workings spread over 10-15 square kilometres and reaching a depth of 1,000 metres.
Geothermal mine water system design is very site specific and requires detailed, locally-drawn information to more accurately assess the technical suitability of a site. Consequently, the 25 potential sites noted above provide only a first approximation of the potential in Canada.
What makes a mine suitable?
The most important considerations for determining the suitability of an underground reservoir as a geothermal resource are its overall shape, condition, thermal capacity, and thermal sustainability (i. e. whether the surrounding earth can replace the heat that is continually removed). Also important is the quality of water that would be circulated through the reservoir. Water quality issues such as dissolved solids and acidity may require specialized filtration and materials selection, which would directly affect project costs.
The illustration shows the system configuration and main components in a typical geothermal mine water system. Other key technical considerations are:
• Mine condition and connectivity
• Water quality (filtration and treatment)
• Temperature change by depth
• Water flow rate to avoid exhaustion
• Distance from extraction point(s) to thermal loads
• Heat pump configuration
• Temperature lift from mine water to desired temperature
• Electricity price relative to fossil fuel prices
• Electricity generation source(s) and associated emissions.
Heat extraction and injection system. The heat contained in the geothermal reservoir water must be extracted and transferred to thermal loads, where it can be used. If the water temperature in the mine is close to that required by the end-user, then a simple heat exchanger can be used to transfer heat. Most suitable inactive underground mines in Canada, however, would require a heat pump to lift the temperature adequately.
Heat transfer and distribution system. Most systems require a significant amount of medium-or low-temperature water/fluid to be moved from the geothermal extraction point to the thermal loads. This transfer is usually achieved through an insulated pipe network. The insulated pipes are very costly to purchase and install, which adds to the overall project costs.
End-use heating applications. Typical end-uses for the extracted heat include medium or low-temperature applications such as space heating, domestic hot water, and processes in industrial, commercial, residential and municipal buildings. Since heat pump performance is particularly sensitive to the required application temperature, it is important for the system design to minimize the temperature difference between the mine water and the intended heat use.
Geothermal mine water systems can benefit both the mining sector and the local community in several ways.
Renewed life for inactive infrastructure. Legacy mine workings can continue to serve the community with renewable energy. Industry spends a great deal of money creating huge cavities in the earth over decades of mining. Depending on the mine characteristics, it may make good sense to repurpose the inactive workings as ready-made geothermal energy collection fields.
Improved economic development. The lower heating costs from the geothermal systems can help attract businesses and jobs to communities where facilities can be built above or near underground mines, potentially revitalizing old mining towns. The use of the systems can also strengthen local interest in renewable energy and green technology, especially when the geothermal energy is used to heat buildings with a high level of public access.
Lower energy costs from renewable energy. While the current energy-savings-based payback can range from 5 to 20 years for suitable sites, this will improve as fossil fuel prices continue to rise in the long term. With a renewable energy source at hand, businesses and institutio
ns can operate more competitively, with lower and more stable energy costs and increased energy independence.
Fewer air emissions. Using low-impact renewable energy offsets fossil fuel use in the connected facilities. This reduces the greenhouse gases and air contaminants produced from fossil fuel combustion.
Improved mining legacy. Transitioning suitable mines into renewable geothermal energy sources provides a welcome parting gift to the community.
Brad Kynoch, B. Sc., is an associate at Marbek, an energy and environmental consulting firm in Ottawa that serves public, private and non-profit sector clients.