The Falkenhagen plant, set up five years ago in Germany's wind-swept Brandenburg state, already produces green hydrogen by running wind power through water to split it into oxygen and hydrogen. Methane gas would provide a higher quality material with more diversified uses.

"This is an important step for a successful energy transition," Uniper said in a statement after the company opened a new facility at the site for the scheme.

"Green methane, in contrast to green hydrogen, can be used in a wider variety of ways."

Germany is looking at new ways to use and store renewable energy at a number of power-to-gas sites as a boom in wind and solar power in the country has led to excess production.

Billions of euros have also been spent building up a natural gas infrastructure, which the government wants to use to store carbon-free energy rather than retiring it once renewables largely replace coal, nuclear power and gas by the middle of the century under the government's energy policy.

While green hydrogen can be used as a transport fuel or fed into gas grids - which can technically absorb 10 percent hydrogen as a complementary fuel to natural gas - green methane would take renewable energy a step further.

"It can be made available to a variety of markets, such as the manufacturing sector, the electricity and heating markets as well as the mobility sector," Uniper said.

Under the two-year pilot scheme, the company will set out to produce green methane by using carbon dioxide from a bio-ethanol plant and mixing it with the hydrogen, creating a gas-like substance.

If Uniper engineers can show the technology is viable, they hope that Germany can lead the way in providing the renewables sector with an entirely new raw material.

But it is early days.

Uniper says the methanation plant's initial hourly production of gas will be equivalent only to the heat consumed by a 50-square metre (538 square foot) apartment in a month, which is tiny.

The material's potential for industrial use longer term could be huge, however, and there is wide interest in the facility as companies including Uniper, Vattenfall, BASF, and EnBW have all invested individually in power-to-gas research.

Falkenhagen is part of a European energy project called STORE&GO, that is calling for regulatory and policy support, given efficiency rates are still well below marketability.

(Reporting by Vera Eckert, editing by Susan Fenton)

By Vera Eckert