During an electricity overproduction phase, hydrogen is produced by electrolysis, compressed and stored for later use in a fuel cell for electricity generation. The disadvantage of hydrogen is the tow energy efficiency of the storage system, at slightly above 50 per cent, when electrolysis is applied for hydrogen production, in comparison to 75-85 per cent for PSH, batteries or compressed air (Hammerschlag and Mazza, 2005). The biggest advantage is the longer time horizon over which energy can be stored.