NOVELTY - A fuel cell system comprises a coolant reservoir storing coolant for cooling the fuel cell stack and provided with a ventilation device for removing air bubbles generated inside and entraining the coolant flow, thus preventing the coolant circulation from being blocked by the air bubbles.    USE - For transportation vehicles and power generation system, e.g. home power systems and other portable and stationary power generating systems.    ADVANTAGE - The invention provides humidified air having sufficient moisture for performing chemical reaction at high rate even at heavy load condition, thus enables the fuel cell stack to provide large working current. Unreacted and excessive hydrogen is recovered and circulated back to the fuel cell stack. The recycle of hydrogen efficiently enhances the chemical reaction inside the fuel cell stack. The ventilation device effectively expels air bubbles entraining the coolant flow, thus eliminating blockage of the coolant flow.    DETAILED DESCRIPTION - A fuel cell system comprises a fuel cell stack (202), an air supply system supplying an air flow to the fuel cell stack and comprising a blower (206) for driving the air to the fuel cell stack and an air humidifier (216) for humidifying the air supplied to the fuel cell stack, a hydrogen supply system supplying hydrogen to the fuel cell stack, a coolant circulation system supplying a low temperature coolant to fuel cell stack for absorbing heat from the fuel cell stack and becoming high temperature coolant, and a control circuit electrically controlling the flow and pressure regulating device, blower, pump, and fan. The hydrogen supply system comprises a hydrogen storage (218) in which hydrogen is supplied to the fuel cell stack, a pressure regulating device for regulating the hydrogen flow, and a hydrogen recirculator (204) for receiving excessive hydrogen from the fuel cell stack and forcing the hydrogen back into the fuel cell stack to induce a hydrogen flow inside the fuel cell stack. The coolant circulation system comprises a coolant reservoir (208) for storing the coolant, a pump (210) driving the circulation of the coolant, a radiator for removing heat from the high temperature coolant and converting the high temperature coolant into the low temperature coolant. The coolant reservoir comprises a ventilation device for removing air bubbles from the coolant. The coolant recirculation system further comprises a heat exchanger (212) for transferring heat from the high temperature coolant to the hydrogen storage. The radiator comprises a fan (215) for generating an airflow to remove heat from the coolant.    DESCRIPTION OF DRAWING(S) - The figure shows a system block diagram of a fuel cell system.    Fuel cell stack (202)    Hydrogen recirculator (204)    Blower (206)    Coolant reservoir (208)    Pump (210)    Heat exchanger (212)    Radiator (214)    Fan (215)    Air humidifier (216)    Hydrogen storage (218)    Flow and pressure regulating device (220)    Filter (222)    Second filter (224)    Exhaust valve (226)    Valve (228, 230)    First pressure sensor (232)    Second pressure sensor (234)    Control circuit (244)    Solenoid valve (246)