NOVELTY - A fuel cell system (500) comprises a phosphoric acid polymer electrolyte membrane fuel cell stack (100), to produce an electrochemical reaction using hydrogen and oxygen; a water circulating unit (510) to supply heated cooling water to the stack to increase a temperature of the stack; and an air circulating unit to heat air, and to circulate the heated air to the stack as a source of the oxygen, such that phosphoric acid is not eluted from the membrane due to condensation of water in the stack.    USE - As a fuel cell system for generating electricity from a fuel.    ADVANTAGE - The fuel cell system prevents phosphoric acid from eluting from the phosphoric acid polymer electrolyte membrane, while directly using heat generated from an electrochemical reaction in a stack, during an initial start up operation. The temperature of the stack can be rapidly increased without eluting the phosphoric acid from the phosphoric acid polymer electrolyte membrane. When a rapid temperature increase in a stack is required, such as at an initial start up, the temperature of the stack can be increased by not only using heated cooling water, but also heat generated from an electrochemical reaction, thus greatly reducing time required for the fuel cell system to reach a normal operating temperature. The condensation of water vapor in the stack is prevented. Thus, the elusion of phosphoric acid from the phosphoric acid polymer electrolyte membrane is prevented, thus securing a stable electrochemical performance of the fuel cell system. The heating element to heat entering air can be modified, or the capacity of a blower can be modified. Thus, the modification from a conventional fuel cell system is easy, and can be performed at a low cost.    DETAILED DESCRIPTION - A fuel cell system (500) comprises a phosphoric acid polymer electrolyte membrane fuel cell stack (100), to produce an electrochemical reaction using hydrogen and oxygen; a water circulating unit (510) to supply heated cooling water to the stack to increase a temperature of the stack; and an air circulating unit to heat air, and to circulate the heated air to the stack as a source of the oxygen, such that phosphoric acid is not eluted from the membrane due to condensation of water in the stack. The air circulating unit comprises a heater (153) to heat the air; a heat exchanger (154) to heat the air using exhaust gas from the stack; an injector to inject exhaust gas from the stack into the circulating unit in order to heat the air in the circulating unit; and a moisture remover (156) to remove moisture from the exhaust gas before the exhaust gas is injected into the air circulating unit. An INDEPENDENT CLAIM is included for starting the fuel cell system, involving circulating heated cooling water through the stack to increase the temperature of the stack to a first temperature; and generating an electrochemical reaction in the stack to increase the temperature of the stack to an operating temperature, by supplying hydrogen and air to the stack, such that phosphoric acid is not eluted from the membrane by water vapor condensation in the stack.    DESCRIPTION OF DRAWING(S) - The figure shows a block diagram of a fuel cell system having a phosphoric acid polymer electrolyte membrane.    Fuel cell stack (100)    Heater (153)    Heat exchanger (154)    Moisture remover (156)    Fuel cell system (500)    Water circulating unit (510)