NOVELTY - Fuel cell system (100) comprises: fuel cell stack (30) to generate electrical energy by reacting oxidant and mixed fuel, and to discharge non-reacted fuel, oxidant, moisture, and carbon dioxide; mixer (45) to prepare mixed fuel by mixing at least portion of non-reacted fuel, oxidant and moisture with concentrated fuel and to supply mixed fuel to stack; fuel supply unit (10) to supply concentrated fuel to mixer; oxidant supply unit (20) to supply oxidant to stack; first heat exchanger (42) between outlet of stack and mixer; and second heat exchanger (48) between mixer and inlet of stack.    USE - As fuel cell system, and recovery unit for fuel cell system (claimed).    ADVANTAGE - The fuel cell system has improved water recovery efficiency, which is achieved by cooling gas separated by first gas-liquid separator, and separating liquid from cooled materials. In the system, as fluids discharged from first heat exchanger flow into second gas-liquid separator to separate gas and liquid, the recovery efficiency of water is improved in comparison with the related art. When heat exchangers of the same volumes are employed, recovery efficiency of water improves by about 30%. This is because some liquid is first separated in first gas-liquid separator, and fluids generated from cooled gas are then separated into liquid and gas in second gas-liquid separator. As fluids discharged from mixer are cooled by second heat exchanger, temperature of fuel that flows into second inlet through second heat exchanger can be properly controlled. When liquid is separated by first gas-liquid separator and gas is cooled by first heat exchanger, vaporization of fuel due to difference in temperature between non-reacted fuel and non-reacted air is minimized or reduced; whereas in the related art, when non-reacted air and non-reacted fuel are mixed and separated in gas-liquid separator after non-reacted air is condensed, non-reacted air having comparatively lower temperature is heated, such that condensed liquid is vaporized, condensing effect of non-reacted air decreases and condenser having large volume is needed to accommodate vaporization. When only gas is cooled after gas and liquid are separated from each other, size of heat exchanger can be markedly decreased in comparison with related art that cools both liquid and gas. Further, since there are two heat exchangers, size of first heat exchanger can be decreased in comparison with heat exchanger used in related art. Also, as the temperature is decreased by stages by installing two heat exchangers, the temperature can be further decreased and size of heat exchangers can be formed smaller in comparison to related art in which one heat exchanger is used, such that total size of two heat exchangers can be smaller than size of one heat exchanger in related art. Hence, in the fuel cell system the internal temperature of the stack can be more readily controlled, and a more stable output can be more readily achieved.    DETAILED DESCRIPTION - A fuel cell system (100) comprises: fuel cell stack (30) for generating electrical energy by reacting oxidant and mixed fuel, and for discharging non-reacted fuel, oxidant, moisture, and carbon dioxide; mixer (45) for preparing mixed fuel by mixing at least a portion of non-reacted fuel, oxidant and moisture with concentrated fuel and for supplying mixed fuel to fuel cell stack; fuel supply unit (10) for supplying concentrated fuel to mixer; oxidant supply unit (20) for supplying oxidant to fuel cell stack; first heat exchanger (42) between outlet of fuel cell stack and mixer; and second heat exchanger (48) between mixer and inlet of fuel cell stack. The fuel cell system further comprises first gas-liquid separator (41) for receiving non-reacted fuel, oxidant, moisture, and carbon dioxide, separating non-reacted fuel, oxidant and moisture into a first gas and first liquid, and supplying first gas to first heat exchanger and supplying first liquid to mixer; and second gas-liquid separator (43) for receiving output from first heat exchanger, separating output into second gas and second liquid, and supplying the second liquid to the mixer. The first heat exchanger is configured to cool and partially condense first gas from first gas-liquid separator, and second heat exchanger is configured to cool mixed fuel prior to supplying mixed fuel to fuel cell stack. The first gas-liquid separator is configured to receive all discharged non-reacted fuel, oxidant and moisture. The first gas-liquid separator comprises centrifugal pump, electrokinetic pump, or separation membrane. The second gas-liquid separator comprises centrifugal pump, or electrokinetic pump. The fuel cell system further comprises a buffer (47) between mixer and second heat exchanger for temporarily storing mixed fuel to control amount of mixed fuel supplied to second heat exchanger. The buffer comprises a filter (47a) for removing impurities from mixed fuel. An INDEPENDENT CLAIM is included for recovery unit for a fuel cell system comprising: first gas-liquid separator having inlet, first outlet, and second outlet, where the inlet is connected to at least one outlet of fuel cell stack; first heat exchanger having inlet and outlet, where the inlet is connected to first outlet of first gas-liquid separator; second gas-liquid separator having inlet, first outlet, and second outlet, where the inlet is connected to outlet of first heat exchanger; mixer having at least one inlet and outlet, where the inlet(s) is connected to second outlet of first gas-liquid separator, first outlet of second gas-liquid separator, and fuel supply unit; and second heat exchanger having inlet and outlet, where the inlet is connected to outlet of mixer, and the outlet is connected to inlet of fuel cell stack.    DESCRIPTION OF DRAWING(S) - The figure shows schematic diagram illustrating the fuel cell system.    Fuel supply unit (10)    Oxidant supply unit (20)    Fuel cell stack (30)    Electrode assembly (31)    Electricity generating unit (35)    First outlet (38a)    Second outlet (38b)    Recovery unit (40)    First gas-liquid separator (41)    First heat exchanger (42)    Second gas-liquid separator (43)    Mixer (45)    Buffer device (47)    Filter (47a)    Second heat exchanger (48)    Fuel cell system (100)