Throughout the world, electricity generation is based more and more on environmentally friendly wind and solar energy. The problem with such energy sources is their unreliability. Depending on the weather or time of day (or more specifically, night) the amount of electricity generated may be deficient or surplus to current requirements. Storing surplus energy in batteries for later use is one solution, but now researchers are developing a way to store surplus renewable electricity as natural gas.

Until now, electricity has been generated from gas. But a German-Austrian cooperation says it has found a way to go the other way. Their process involves transforming surplus electricity as climate-neutral methane, and storing it in existing gas storage facilities and the natural gas network.

The process was developed by the Center for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW), in cooperation with the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) and combines the technology of hydrogen-electrolysis with methanization. A demonstration system already operating successfully in Stuttgart separates water with surplus renewable energy using electrolysis to produce hydrogen and oxygen. A chemical reaction of hydrogen and carbon dioxide generates methane – which is nothing other than synthetically produced natural gas.

The researchers say that the rapid expansion of renewable energies results in the need for new storage technologies and they believe their process should be of special interest to energy utilities and power companies. The new technology aims at facilitating the integration of high shares of fluctuating power generation from renewable energies into the energy system. One goal is to structure the delivery of power from wind parks on a scheduled and regular basis.

"So far, we converted gas into electricity. Now we also think in the opposite direction, and convert electricity into ’real natural’ gas," explains Dr. Michael Sterner of Fraunhofer IWES, who is investigating engineering aspects and energy system analysis of the process. "Surplus wind and solar energy can be stored in this manner. During times of high wind speeds, wind turbines generate more power than is currently needed. This surplus energy is being more frequently reflected at the power exchange market through negative electricity prices." In such cases, the new technology could soon keep green electricity in stock as natural gas or renewable methane.

"Within the development of this technology, ZSW has been guided by two core issues," said Dr. Michael Specht of ZSW. "Which storage systems offer sufficient capacity for fluctuating renewable energies that depend on the wind and weather? And which storage systems can be integrated into the existing infrastructure the easiest?" The storage reservoir of the natural gas network extending through Germany is vast, equaling more than 200 terawatt hours – enough to satisfy consumption for several months. In comparison the power network has only a capacity of 0.04 terawatt hours by itself. The new process would allow natural gas substitute to be stored like conventional natural gas in the supply network, pipelines and storage systems, in order to drive natural gas cars or fire natural gas heating systems.

While the efficiency of converting power to gas isn’t perfect, equaling more than 60 percent, the team says it is definitely better than a total loss that may result if, for instance, wind power has to be curtailed.

In order to push the new energy conversion technology forward, the two German research institutes have joined together with the company Solar Fuel Technology of Salzburg. Starting in 2012, they intend to launch a system with a capacity of approximately 10 megawatts.