Gas & Electricity, a marriage of convenience

In the context of this inexorable march forward, gas and electricity networks can no longer be considered in isolation. They must, on the contrary, be considered as two inseparable elements of a much larger ensemble - the energy system. 

Gas storage provides the connection between the gas and electricity networks by reason of its role as the main tool for cross-sector flexibility. The connections between the two have become notably stronger in recent years. More flexibility is required to compensate for the intermittent nature of the renewable energies that are increasingly being integrated into the electricity system. In addition, grid balancing is becoming more difficult because of the abandon of energy sources capable of maintaining the equilibrium between supply and demand (nuclear power, coal, oil, etc.).

Moreover, the electrification of part of the final energy demand means that the electricity network will have to cope with much larger peaks and fluctuations in demand. The power grid has been spared this need for flexibility up until now because demand has been relatively stable. Henceforth, sufficient storage capacity will be required to provide support to the increasingly unpredictable and fluctuating production of electricity.

The question of flexibility is becoming a key issue for the stability of the system. Every type of storage can play a role, but most are limited in terms of capacity and withdrawal time (see figure below).

This is not the case for gas storage, which is set to become the main link between gas and electricity. Flexibility today is mainly based on the storage of natural gas; tomorrow, it will have to turn to renewables, a principle that informs the “Power-to-Gas” concept. The idea is to transform renewable electricity into hydrogen by electrolysis. In this process, electricity is used to split water into hydrogen (H2) and oxygen (O2). The hydrogen can either be used directly or, in a second stage, be transformed into methane (CH4) by combining it with CO2 by means of a catalytic process.

Underground Hydrogen

Renewable gas will therefore be complementary to renewable electricity. ENGIE’s subsidiary Storengy is investing in technologies that will enable it to adapt its existing storage assets to a zero carbon future. This will make it possible to store the massive amounts of excess renewable electricity that are currently lost or offloaded, avoid further constraints on installations and help to maintain grid balance. These principles are at the heart of the “STOPIL H2” project, a pilot scheme for the salt cavern storage of hydrogen in Étrez (Ain, France), 20 kilometres north of Bourg-en-Bresse.

Twenty-five operational salt caverns are located below the ground around the town, artificial cavities that have been made by drilling down into a 650-metre thick underground salt formation at a depth of between 1,250 and 1,900 metres. The large quantities of gas that the cavities can contain are easily available and will make it possible to meet both basic and peak consumption needs.

This proven and highly effective technology has more than 50 years’ experience with natural gas behind it and provides intrinsic environmental and safety benefits. Salt caverns have low space requirements above ground, are gas tight and are inaccessible due to their depth. In addition, there is no risk of interaction with oxygen.

The STOPIL H2 project, funded by France’s Agence Nationale de la Recherche (ANR), brings together leading French stakeholders (Storengy, Geostock, Air Liquide, BRGM, INERIS, Brouard Consulting and Armines, which represents the École des Mines de Paris and Polytechnique). It aims to rise to the specific technical challenges of hydrogen storage in salt caverns. The first phase of the project consists in answering two questions: is a salt cavern storage facility impervious to hydrogen and can frequent, high frequency cycles of hydrogen injection and withdrawal and fast flow rates damage the mechanical stability of the cavity?

As far as the first question is concerned, the imperviousness of the cavity and well tightness is indeed a key concern from the point of view of storage safety and efficiency. The tightness of hydrocarbon storage caverns, thousands of which exist worldwide, is tested with nitrogen. Within the framework of the STOPIL H2 project in Étrez, a hydrogen tightness test will be carried out in a cavity at a depth of approximately 1,000 metres located (see figure opposite). Its goal is to obtain information with a view to devising industrial scale tightness tests for hydrogen storage caverns in the future.

Salt Cavern Operation

As for the second question, a test is planned in which the pressure of the hydrogen in the cavity will be increased to the maximum (in the range of 150 bar). The pressure will then be rapidly decreased and increased several times in succession, before finally being returned to its initial level. This test will provide the information to judge whether short and intense pressure cycles, as are currently used successfully in natural gas storage, will affect the cavern and possibly cause a significant decrease in performance. Once the concept has been validated, the cavity could eventually store up to 40 tons of hydrogen, i.e. around 1.5 gigawatt hours, intended in the short term for local consumption.

Once the principle has been established and supported by sound scientific studies, stakeholders will have to ensure the social acceptability of underground storage facilities, in particular by further reducing the environmental impacts on this technology. In the context of the energy transition, the underground storage of gases other than natural gas will be of prime importance and essential to the development of new sources of electricity production and new forms of consumption. The performance of storage facilities will have to adapt to the intermittent nature of renewable sources of electricity.

Programmes of technological innovation must be launched now if we are to meet future challenges, in particular to reduce the investments required and make the regulatory framework more conducive to the integration of renewable gases and their storage as part of a 100% renewable electricity mix. Only then will we be able to succeed in meeting the deadlines set by the European Union. 

Written by: Carole Le Henaff, Grégoire Hevin, Christian Hue & Delphine Patriarche, Storengy