Seasonal energy storage in forms of renewable gases will play an important role in future energy systems. As the expansion of renewable electricity production means that production and demand do not coincide sufficiently in terms of time, amplitude and magnitude, the need for different energy storage solutions is given. Besides short-term energy storage, seasonal storage of energy in forms of gas will play an important role. In the context of Power-to-Gas, Underground Sun Conversion - Flexible Storage combines renewable methane production with large-scale underground energy storage. The used process of geo-methanation describes the conversion of H2 and CO2 to CH4 in underground reservoirs. In order to enable implementation on a commercial scale, some topics need to be further addressed. These concern operating parameters at the technical, microbiological, and systemic level as well as the embedding in the higher-level energy system, the underlying geology, and the associated infrastructure. In the course of this project the potential and possibilities for Underground Sun Conversion were elaborated and tested. A special focus is set on the flexible operation of an Underground Sun Conversion system, including conceptional considerations, the search for optimal locations in terms of geology and infrastructure as well as the actual operation of geo-methanation in field, accompanied by laboratory-scale research.
In the field tests, the safe storage and conversion of H2 and CO2 in an underground reservoir was demonstrated. In addition, an increase in conversion rates was achieved through the use of certain operation modes. This is supported by results from laboratory experiments. Here, the reactions taking place, the microorganisms involved, and the boundary conditions of geo-methanation could be further described.
Variations in the operating parameters result in different operation concepts of Underground Sun Conversion. The three most promising ones were developed and considered in detail. Particular emphasis was placed on their flexible interaction with a future energy system. Regarding the embedding of Underground Sun Conversion into this environment, different system scenarios show the potential for Switzerland as well as for Austria. For Switzerland, numerous promising locations for the implementation of an Underground Sun Conversion facility were identified, which are accessible to the necessary infrastructure (renewable electricity production, gas network, CO2 source). Whether these sites fulfil all geological criteria needs to be proven by commercial exploration. Additionally, three use cases for Underground Sun Conversion – Flexible Storage were developed, serving as product and sector coupling service.
The results obtained will form the basis for scaling the technology. With the combination of storage and conversion in the context of Power-to-Gas, Underground Sun Conversion – Flexible Storage can make an important contribution to seasonal energy storage and thus to the energy transition.