Energy storage from renewable sources is maybe the main issues for the energy sector. Hydrogen seems to be the more compact, flexible and clean solution. However a complete replacement of fossil fuels with hydrogen will probably require some decades. Therefore a bridge solution which concentrates investments on renewable hydrogen production is desirable.
A first solution could be the direct injection of hydrogen into the existing natural gas pipeline. A further way to increase the need for renewable hydrogen bypassing the problem of its distribution is the production of a substitute of natural gas.
If the power excess is due to lack of demand, biomass could be gasified with electrolytic hydrogen to generate directly a gas rich in methane. After water condensation such a gas could be fed to a methanation process to convert almost completely carbon in methane. Hydrogasification and methanation are exothermic processes: the heat recoverable could be used for thermal application or to supply extra power to the electrolyser.
If the power excess is due to problems of grid stability, biomass could be gasified with electrolytic oxygen and the syngas could be fed, together with other electrolytic oxygen, to a power unit. The power units considered are steam and gas turbines, internal combustion engines and high temperature fuel cells. The exhaust gas is composed almost exclusively by CO2 and H2O. After water condensation, CO2 could be fed together electrolytic hydrogen to a methanation process to obtain the substitute of natural gas. Heat from exhaust gas cooling and from methanation process could be recovered to generate power suitable for imput into the grid.
In all cases the input is low value energy (biomass and electric power which cannot be absorbed by the grid) and the output is high value energy (substitute of natural gas and eventually stable electric power).