TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Method: Storing energy in the form of methane further comprises: step (d), where at least the oxygen of step (b) is used for producing at least partially the carbon dioxide used in step (c) by oxygen enriched combustion or gasification. The electric energy of step (a) is generated using a renewable energy source. The renewable energy source comprises solar energy, wind power, wave power, tidal power, water or hydro power, geothermal energy, biomass and biofuel combustion, or combinations. The methanogenic microorganisms are methanogenic archaea, where the methanogenic archaea is selected from Methanosarcinia barkeri, Methanobacterium thermoautotrophicus, Methanothermobacter thermoautotrophicus, Methanococcus maripaludis, Methanothermobacter marburgensis, Methanocaldococcus jannaschii, and mixtures. Step (c) comprises at least one methane production phase, where in the at least one methane production phase the ratio of the partial pressure of hydrogen to the partial pressure of carbon dioxide inside the reaction vessel is maintained at 5:1 or higher (parts hydrogen : parts carbon dioxide), and where step (c) further comprises at least one cell growth phase, where in the at least one cell growth phase the ratio of the partial pressure of hydrogen to the partial pressure of carbon dioxide inside the reaction vessel is maintained at a ratio different from the ratio in the at least one methane production phase. Preferred System: In the system, the bioreactor further comprises at least one device for measuring the head pressure and the off-gas concentration and at least one device for adjusting or maintaining the partial pressure ratio of hydrogen to carbon dioxide inside the reaction vessel. The system further comprises at least one device for oxygen enriched combustion or gasification for producing carbon dioxide, preferably the device for oxygen enriched combustion or gasification uses the oxygen produced by the electrolysis of water, the carbon dioxide being transferred to the bioreactor; and at least device for recovery, purification, enriching, storing, recycling and/or further processing of off-gas, hydrogen, water, oxygen, chlorine, carbon dioxide, hydrogen sulfide (H2S), sodium sulfite, methanogenic microorganisms, used medium and medium components, methane and other substances of the process. The renewable energy source comprises solar energy, wind power, wave power, tidal power, water/hydro power, geothermal energy, biomass and biofuel combustion. The methanogenic microorganisms are methanogenic archaea. EXAMPLE - No suitable example given.