This edition of the blog discusses the potential for applying geothermal systems to mines so that useful heat energy can be extracted, for use in district heating systems or other applications.
LOW ENTHALPY GEOTHERMAL SYSTEMS
In many geological settings where there are reservoirs of groundwater at relatively low temperatures (typically 10 to 35 Degrees Celsius), low enthalpy geothermal systems can potentially be used to extract useful heat energy. Favourable conditions for such geothermal systems commonly involve three interconnected elements:
There are two principal configurations of geothermal systems – open loop and closed loop – differentiated by the type of infrastructure (termed the “ground collector”) used to extract heat from the ground and pass it to the heat transfer/energy conversion system.
The benefit of obtaining heat energy from these types of geothermal systems is that the heat energy is produced using less energy and with lower carbon emissions than fossil fuel sources. Appropriately designed and maintained systems can produce heat energy at significantly lower unit costs than conventional fossil fuel sources.
POTENTIAL APPLICATIONS IN THE MINING INDUSTRY
The use of geothermal technologies is well established outside of the mining industry, and is increasing used to heat and/or cool residential and commercial properties. However, to date it has seen relatively few applications to extract heat from active or abandoned mines.
Outside of the mining industry, geothermal heat energy in soils and rock (and associated groundwater) is typically exploited via either:
Unfortunately, while the unit cost per generated kilowatt hour (kWh) can be significantly less than that of comparable traditional energy sources, a geothermal system typically requires significant up-front capital cost to construct the necessary geothermal wells or ground collectors. This capital expenditure is required before a single kWh of energy can be generated, and is probably a key factor why geothermal systems have not been implemented in a wider range of settings and by a wider range or organisations than at present.
However, in the case of the mining industry, much of the existing mining infrastructure has the potential to be adapted to allow extraction of useful heat energy. The mining industry routinely: constructs major structures that penetrate the subsurface (open pits, underground roadways); pumps groundwater as part of dewatering operations; and may leave behind artificial water features (pit lakes or flooded workings). An obvious opportunity is to use these engineered features associated with mine sites, where the capital and operational costs are already committed, to exploit heat energy from geothermal sources.
POSSIBLE CONFIGURATIONS FOR MINING GEOTHERMAL SYSTEMS
Several configurations of geothermal systems are possible on mine sites, tapping into different types of heat reservoir.
Natural ground: It is possible to install open loop or closed loop ground collectors in natural ground. Such systems are little different to those applied widely outside the mining industry. Natural ground may have particular potential to provide heating and cooling to exploration camps (before the start of mining), by making use of the exploration rigs to drill closed loop ground collectors.
Mine dewatering systems: Where dewatering pumping is carried out for underground or open pit mines either during operation or as part of a managed closure regime, it is a fairly straightforward modification to pass the water through a heat transfer system configured to form an open loop geothermal system. This will allow heat energy to be exchanged with the pumped dewatering water.
Flooded mine workings: Where abandoned mines (both underground and open pit mines) are flooded with water this can represent a significant potential heat reservoir which can be exploited by low enthalpy geothermal systems.
A particular issue with geothermal systems that exploit mines is that the water pumped from the mine may have a complex chemical composition, resulting in aggressive corrosion conditions, risk of chemical precipitation, or release of hazardous mine gases. This can be mitigated by appropriate engineering design, such as the use of a secondary water loop to reduce the amount of equipment exposed to the aggressive groundwater and can allow the pumped groundwater to remain pressurised, thereby reducing the risk of degassing, chemical precipitation and clogging. The secondary loop will circulate ‘clean water’ and equipment connected to the loop can be designed for less onerous chemical conditions, saving capital costs.
HEAT ENERGY FROM ABANDONED AND LEGACY MINES
Preene and Younger (2014) reviewed published case histories on geothermal systems in mines. They identified that, to date, there were less than 20 documented examples of operational geothermal systems on mine sites worldwide. The review also indicated that by far the most common mining application of geothermal systems was the extraction of heat from flooded underground mine workings, including those mines that have effectively been abandoned and passed into the legacy phase.
In some situations, regional management of minewater problems requires a net abstraction of water from interconnected workings, for example to control rebound of regional groundwater levels following mine closure. In this configuration, the water can be pumped out, passed through a heat transfer system and then disposed of to the surface water environment. Where there is no requirement for net pumping of water, and an interconnected set of mine workings exists, it may be possible to recirculate water through the workings, effectively using the mine workings as a huge closed loop ground collector. One of the simplest arrangements for pumping from flooded underground mines is where pumps are installed directly in existing shafts.
One operational risk is that older shafts may be unstable and it may be necessary to install a perforated ‘well liner’ within the shaft to provide a clear and protected vertical void into which the pump can be installed. If the shafts are inaccessible, have collapsed or have been backfilled, the experience of the operational geothermal system at Heerlen in The Netherlands in the early 2000s shows that it is possible to successfully drill boreholes from the surface into flooded roadways to extract/reinject water.
Pit lakes formed post-closure in open pit mines can also form a large heat reservoir, which can be exploited by either open loop or closed loop systems.
Geothermal systems exploiting abandoned mines will almost certainly be supplying energy to demands not associated with the mine itself (because the mine is closed or no longer exists). Such geothermal systems could offer a potential new revenue stream to the bodies responsible for management of mine legacy issues. They can also offer a chance to contribute to Corporate Social Responsibility targets by supplying nearby communities with low carbon energy.
There is significant potential to use geothermal technology to exploit the heat energy in mines at all stages from exploration through to closure and beyond, by providing low carbon heat energy to displace some use of fossil fuels. There are particular opportunities in exploiting the potential heat energy in flooded or abandoned mine workings, particularly where there is a requirement to pump minewater as part of the management of regional groundwater levels.