“Underground Sun Conversion”: The technology with the exciting name, patented by the Austrian energy company RAG Austria AG, offers a way to store renewable energy seasonally and on a large scale and make it available all year round. In summer, surplus renewable energy – for example solar power – is converted into hydrogen (H2). This is then stored together with carbon dioxide (_CO2) in natural underground storage facilities – for example former natural gas deposits – at depths of over 1000 metres.

This is where the little helpers come into play: microorganisms from prehistoric times, so-called archaea, convert hydrogen and_CO2 into renewable methane (CH₄) via their metabolism. Archaea are widespread throughout the world, predominantly in anaerobic, or low-oxygen, environments, and they were responsible for converting biomass into natural gas millions of years ago. By feeding hydrogen and_CO2 into suitable porous sandstone deposits, this process is virtually started anew. The methane “produced” down there can then be withdrawn from the storage facilities in winter and used in a variety of ways as_CO2-neutral natural gas.

Carbon cycle: The greenhouse gas CO2 is converted into methane in the soil. This is how “climate-neutral” natural gas is produced. Illustration: RAG

Austrian and Swiss energy companies and research institutions have now joined forces to further develop the technology. In a project funded by the European research framework programme ERA-Net and in Switzerland by the Swiss Federal Office of Energy (SFOE), the technical and economic potential in Switzerland and Austria will be explored over the next two years. In Switzerland, the energy company Energie 360°, Empa, the University of Bern and the University of Applied Sciences of Eastern Switzerland OST are involved. Empa is developing a perspective on the entire energy system: “We are looking at when and where surplus electricity occurs, where suitable_{CO2 sources} would be, and ultimately where there is demand for renewable natural gas,” explains Martin Rüdisüli from Empa’s Urban Energy Systems department. Together with the geological conditions, which are being investigated by the University of Bern, and the economic boundary conditions, which are being worked out by OST, a map of possible locations for the application of the “Underground Sun Conversion” technology is to be created.

Martin Rüdisüli considers the technology to be promising. In particular, because in addition to biological methanation, it also provides an answer to the seasonal storage problem: “Even with a large increase in methane gas production, there would be no need to expand the above-ground storage infrastructure thanks to the natural storage facilities in the earth’s interior,” he says.

The volatility of renewable energy sources is one of the great challenges of the energy transition. Basically, we have too little renewable electricity in the winter and too much in the summer. In an earlier study on the potential of “power-to-gas” technology – i.e. the conversion of renewable electricity into chemical energy carriers such as hydrogen or methane – in Switzerland, Empa researcher Rüdisüli predicted a surplus of a good 10 TWh of solar electricity in Switzerland over the next few decades – provided that a large proportion of suitable roof surfaces were developed with photovoltaics, which in turn is necessary if it is to replace the nuclear electricity that is being phased out. If the surplus electricity is converted into methane in the summer, this would allow around one million gas-powered vehicles to be operated on a renewable basis all year round. “Converting renewable electricity into seasonally storable energy sources is an important pillar of a decarbonized energy system,” Rüdisüli said. The results of this earlier “power-to-gas” study also serve as the basis for the current project and are now being refined there in accordance with the techno-economic boundary conditions of the “underground sun conversion” technology.