Transforming disused mines into pumped storage sites is a technological solution to compensate for the intermittency of wind and photovoltaic energy. However, it is unprofitable and potentially polluting.
Storage needs
The sixth synthesis report on climate change published on March 20, 2023 by the IPCC recalling that CO2 emissions from the combustion of fossil fuels place us on a warming trajectory above 1.5°C is no longer a scoop. However, the generations who will still be alive in 2050 should start to worry about their future. Limiting environmental and demographic damage that will result from global warming requires a shift away from fossil fuels, especially for electricity production, which is expected to triple by mid-century (see IEA, page 293). But relying mainly on wind turbines and photovoltaic panels compels adaptation.
On the demand side, we need to adjust our consumption to the day-night cycle and to the mood swings of the weather. On the supply side, we have to invest in the electricity transmission network and energy storage infrastructure. All of these solutions are encouraged by the European Commission in its proposed regulation to improve the electricity market and in its recommendation on storage. They will be very expensive. Particularly, the storage of electrical energy involves its temporary transformation into kinetic, chemical, thermal or gravity energy, which requires expensive equipment and losses of the order of 25-30% before its restitution in electrons. To date, the most widespread technique remains Pumped Storage Hydropower (see IEA).
Pumped storage hydropower (PSH)
The principle of PSH stations is very simple. Imagine two reservoirs located at different altitudes. When electricity is abundant, it powers pumps to bring water to the upper reservoir. When electricity is needed, water is released to produce electricity with turbines, before being recovered in the lower reservoir.
These installations are very valuable for balancing the grid since they are quite flexible. They both consume electricity (when there is water downstream and the upstream reservoir is not full) and produce electricity (when there is water in the upstream reservoir and free space downstream). But they share, with conventional hydroelectric dams (those without pumping), the disadvantage of using a large space, especially in their preferred areas which are the mountains, since the power of the installations depends a lot on the difference in level of the two reservoirs. To reduce the environmental impact of the used space, why not use industrial wastelands and bury at least one of the reservoirs, if not both?
PSH, mines and quarries
For a partially or totally underground PSH, the main obstacle is the cost of excavating the reservoir(s). Most projects seek to overcome this obstacle by using excavations left by abandoned mines and quarries. In Alberta, Canada, the operator of the Tent Mountain site, which was the country’s first supplier of coal for power generation, wants to turn it into a PSH. The characteristics of the project are an installed capacity of 320 MW and a storage of 4,800 MWh of energy, thus a continuous production for 15 hours, for an expected duration of 80 years.
In France, on a much smaller scale, a micro-PSH project (1.5 MW in turbines) was launched in 2014 in Berrien (Finistère) to convert a kaolin (clay rock) quarry, with the pumping energy provided by wind and photovoltaic farms. But it was abandoned in late 2017, apparently due to administrative hurdles.
Technical and environmental issues
The main difficulty encountered by underground PSH projects is that the mines were not dug and exploited to have large masses of water circulate, nor the air pushed out by water, and that, several times a day. It threats the solidity of the galleries. It also might contaminate the water table. In addition to the costs of pumps and turbines and of the connection to the grid, groundwater contamination is a hindrance to the development of these projects. They certainly allow savings in digging costs, but they do not exempt the need to solidify and seal the galleries and shafts.
Burying PSH is not a bad idea, but there is still a lot of research to be done for them to make a substantial contribution to green energy storage (for more details, see Ineris).
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The energy transition requires major investments in energy storage. For this activity to be profitable, electricity prices must vary on a wide range. They must be very low, even negative, when the storage space is empty, and very high when the storage space is full. This is the only way to cover construction, maintenance, and operating costs.
But the 2022 energy crisis has shown that such price variations are unpopular. To have both low fluctuating electricity prices and sufficient storage capacity, storage operators will have to be remunerated differently, according to their capacity (per MW and not per MWh) or their flexibility (European Commission). We see that, for PSH in general, and underground PSH in particular, the obstacles are not only technical and environmental. Their future is highly dependent on national and European regulations.
