Underground Energy Storage

The annual consumption of natural gas in France is around 500,000 gigawatt hours (GWh), which is the equivalent of the production of 70 nuclear reactors. You probably think that the gas arrives straight from a pipeline or a gas tanker, but that is not the case. More than half of the gas supplied in winter actually comes from underground storage sites and the same is true for heating oil and petrol.

This decades-old, proven technology provides a safe and low-cost solution for storing very large volumes of fuel with a minimal footprint above ground. As far as natural gas is concerned, this massive storage is indispensable, especially for balancing the gas demand throughout the year. In France, the available storage capacity is spread across 14 sites around the country and characterised by two geological formations: porous rock and salt caverns (see figure opposite).

•  The gas is stored in a porous rock stratum, such as sandstone or limestone, which is capped by a layer of impermeable rock. Such a site may be a former oil or gas deposit, or an aquifer (as is often the case in France). These reservoirs located at a depth of several hundred metres below the ground typically have a surface area of several square kilometres, but are no more than a few dozen metres thick. Today in France, there are 10 natural gas storage sites in aquifers and they represent an approximate capacity of 120,000 GWh.

• Salt caverns are mined in existing salt deposits between several dozen to several hundred metres thick. A well is drilled into the formation and water is pumped down to dissolve the salt, which returns to the surface as brine. This process creates caverns that are structurally sound and, as rock salt is impermeable, they can be used to store both gas and non-aqueous liquids (such as oil). 

In France, there are four natural gas storage sites with around fifty salt caverns between 50,000 and 600,000 cubic metres in size and with a total storage capacity of 12,000 GWh. This type of installation has a lot of potential for storing non-fossil energy.

«ENGIE Campus, the group’s future headquarters in La Garenne-Colombes near Paris, will be equipped with a heating and cooling system based on the underground storage of heat in an aquifer. »

Renewable Energy

• We could notably envisage storing biogas (whose production is on the rise) instead of natural gas. The MéthyCentre project located in Angé (in the Loir-et-Cher department) combines a Power-to-Gas unit and a methanation plant that produces biogas from agricultural waste. A methanation process also recycles the CO2 from the biogas and combines it with hydrogen to produce synthetic methane. Up to 2,200 GWh of gas is injected into the gas grid per year (with a target of 56,000 GWh by 2030); part of it is stored at a nearby site in Céré-la-Ronde.

  
  
• Hydrogen alone or combined with natural gas can also be stored in salt caverns. This has been the case since the 1970s in the United Kingdom and since the 1980s in the USA. In France, the HYPSTER pro-ject (Hydrogen Pilot Storage for Large Ecosystem Replication) launched in 2020 plans to test the storage of up to 44 tonnes of green hydrogen (or 1.8 GWh) in salt caverns. This corresponds to the daily consumption of more than 1,700 hydrogen fuel cell buses. Aquifers are less suited to storing hydrogen, because of the possible presence, depending on the chemical characteristics of the water and the type of rock that comprises the reservoir, of hydrogen-consuming bacteria. As the volumetric energy density of hydrogen is lower than natural gas (it is the opposite in terms of mass), at usual storage pressures, converting all of France’s salt caverns to store hydrogen would correspond to just 3,500 GWh.
  
  

• In flow batteries, two chemical compounds (electrolytes) flow through one or more electrochemical cells, where a chemical reaction on both sides of an ion-exchange membrane pro-duces electricity. In France and Germany, studies are focussing on how to combine flow batteries with the massive storage potential of salt caverns (for the organic electrolytes). There are still many obstacles to overcome, notably the compatibility of these organic com-pounds with brine and the salt cavern walls, nevertheless a first 0.7 GWh battery should be operational in Germany by 2023. Devoting all the salt cavern storage in France to this use would store around 60 GWh.As for compressed air (the term used is Compressed Air Energy Storage, or CAES), the available storage space ranges from 40 to 130 GWh. When released, the compressed air would be used to drive a turbine generator.

Finally, storing electricity in a pumped storage power plant (PSPP) would yield approximately 15 GWh. A PSPP stores electricity using a similar system to that of pumped-storage hydroelectricity: water is pumped up to a reservoir at a higher elevation and produces electricity as it travels back down through turbines to the lower (underground in this case) reservoir.

Storing heat

But would it be possible to store heat instead of gas or liquid? UTES (Underground Thermal Energy Storage) aims to answer this question and such systems could contribute to the heating and cooling of individual homes or several buildings.

A first option is an open-loop system: ATES (the A stands for aquifer). Water is extracted from an aquifer located at a depth of between 40 and 300 metres; in summer, the water is used for cooling and then the heated groundwater is re-injected back into the aquifer. In winter, the previously heated water is extracted and, in combination with a heat pump, used for heating purposes. This type of heat storage system is already widespread in the Netherlands and Sweden, but it is still rare in France. ENGIE will be installing one of the first ones in France at its new headquarters in La Garenne-Colombes, near Paris.

In a BTES (B for Borehole) system, heat exchange takes place within a closed loop in boreholes drilled down into the underlying rock formation.

All things considered, in the short-term salt caverns would appear to be an effective solution for storing renewable energy. They are actually being tested, as part of projects at different stages of maturity, for the storage of synthetic methane, hydrogen and compressed air, and as part of a flow battery system. In the longer term, storage in porous formations will also have a role to play, provided that remaining technical and environ-mental issues can be solved.

To ensure the success of the energy transition and, in particular, to overcome the intermittent nature of renewable energy production, effective storage solutions are surely indispensable - and if they could be underground, i.e. invisible, that would be even better!

To find out more about energy storage: visit Storengy's YouTube channel