NOVELTY - Regenerable fuel cell system comprises (i) a combination of fuel cell and water electrolyzer comprising a polymer membrane and electrodes, which are respectively arranged on either side of the polymer membrane, (ii) an oxygen container (1) for storing oxygen gas, (iii) a hydrogen container for storing hydrogen gas, (iv) a first storage for storing water and oxygen gas, and a second storage for storing water and hydrogen gas, or a storage (2) for storing water and oxygen gas, and (vi) a vapor pressure control means for controlling vapor pressure of the water in the first and/or second storage.    USE - Used as a regenerable fuel cell system.    ADVANTAGE - The regenerable fuel cell system: exhibits a design, which adjusts the amount of water present in the polymer membrane and maintains the moistening of the polymer membrane without external water supply; and allows the re-use of oxygen gas, hydrogen gas, and water without depositing them to the outside.    DETAILED DESCRIPTION - Regenerable fuel cell system comprises (i) a combination of fuel cell and water electrolyzer comprising a polymer membrane and electrodes, which are respectively arranged on either side of the polymer membrane, (ii) an oxygen container (1) for storing oxygen gas, (iii) a hydrogen container for storing hydrogen gas, (iv) a first storage for storing water and oxygen gas, which is not used for energy generation from oxygen gas supplied during the energy generation period, and a second storage for storing water and hydrogen gas, which is not used for energy generation from the hydrogen gas supplied during the energy generation period or a storage (2) for storing water and oxygen gas, and (vi) a vapor pressure control means for controlling vapor pressure of the water in the first and/or second storage or a vapor pressure control means for controlling vapor pressure of the water in the storage. The regenerative fuel cell system is operated for generating electrical energy during energy generation period. The oxygen gas of the electrode arranged on one side of the polymer membrane and hydrogen gas of the electrode arranged on the other side of the polymer membrane are respectively supplied during energy generation period, and operated for the production of oxygen gas and hydrogen gas during charging time. The water is electrolyzed in the combination of the fuel cell and water electrolyzer. The combination of fuel cell and water electrolyzer, the oxygen container, the water container, the first storage, the second storage and a connecting liquid/gas path are provided as closed path. The oxygen container and the first storage are connected with each other such that the oxygen gas moves back and forth between the oxygen container and the first storage. The water supply container and the second storage are connected with one another such that the hydrogen gas moves back and forth between the water supply container and the second storage. The water, which is required during energy generation period, is supplied to the polymer membrane, and the vapor pressure of water is controlled by the vapor pressure control means.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic representation of the regenerable fuel cell system.    Oxygen container (1)    Storage (2)    Pump (7)    Stack (10)    Differential pressure measuring device (11)