Store the sun in the soil
In the winter half-year, there is too little renewable energy in our latitudes to bridge the cold season. Research into seasonal storage and conversion technologies is therefore in full swing. Empa is involved in an international research project that envisages an unconventional solution: Renewable hydrogen and carbon dioxide are pumped together into the ground, where naturally occurring microorganisms convert the two substances into methane, the main component of natural gas.
"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, this involves converting surplus renewable energy - solar power, for example - 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 meters.
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 found all over the world, mainly in anaerobic, i.e. low-oxygen environments, and they were already 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 all over again. The methane "produced" down there can then be withdrawn from the reservoirs again in winter and used in a variety of ways as CO2-neutral natural gas.
The search for suitable locations
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 program ERA-Net and in this country 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 OST in eastern Switzerland 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. Particularly 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 aboveground storage infrastructure thanks to the natural storage facilities in the earth's interior," he says.
On the way to decarbonizing our energy system
The volatility of renewable energy sources is one of the great challenges of the energy transition. In winter, we basically have too little renewable electricity; in summer, too much. In an analysis of 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 - assuming that around half of the suitable roof surfaces were developed with photovoltaics, which in turn is necessary if it is to replace the nuclear electricity that is being eliminated. If the excess electricity is converted to methane in the summer, it could be used to power around one million gas-powered vehicles on a year-round renewable basis. "Converting renewable electricity into seasonally storable energy sources is an important pillar of a decarbonized energy system," says Rüdisüli.
