Picture: Karin Lohberger / RAG

Stor­ing the sun in the soil

In the win­ter months, 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, 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 andCO2 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 andCO2 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 asCO2-neu­tral nat­ur­al gas.

The search for suit­able locations

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 suit­ableCO2 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.

On the way to decar­bon­is­ing our ener­gy system

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.