NOVELTY - A fuel cell system comprises fuel feeding unit (116) for feeding fuel to fuel cell system and for providing information on the fuel; unit (118) for generating information about fuel utilization in fuel cells and oxygen to carbon ratio (O/C ratio) in the fuel cell system process; active unit (120) for controlling loading of the fuel cells; a first closed loop controller (122) for controlling the fuel feeding unit; and a second controller (124) for controlling active fuel cell loading unit.    USE - As a fuel cell system for producing electricity with fuel cells containing anode side, cathode side, and electrolyte between anode side and cathode side (claimed).    ADVANTAGE - The fuel cell system is more environmentally friendly; has better efficiency; and is capable of adapting well and fast enough to internal and/or external changes in operating conditions of the fuel cell system.    DETAILED DESCRIPTION - A fuel cell system comprises: fuel feeding unit (116) for feeding fuel to the fuel cell system and for providing information on the fuel; unit (118) for generating information about fuel utilization in the fuel cells and oxygen to carbon ratio (O/C ratio) in the fuel cell system process; active unit (120) for controlling loading of the fuel cells; a first closed loop controller (122) for controlling the fuel feeding unit by taking into account the fuel utilization information as process feedback information, and by implementing a constraint function taking control of O/C ratio by means of restricting output of the first controller when the O/C ratio deviates from an allowed range; and a second controller (124) for controlling the active fuel cell loading unit by implementing a constraint function taking control of fuel utilization by means of restricting output of the second controller when the fuel utilization deviates from an allowed range. An INDEPENDENT CLAIM is included for producing electricity in a fuel cell system, involving feeding fuel to the fuel cell system by providing information on the fuel; generating information on fuel utilization in the fuel cells and oxygen to carbon ratio (O/C ratio) in the fuel cell system process; actively controlling loading of the fuel cells; controlling the feeding of fuel by taking into account the fuel utilization information as process feedback information, and by implementing a constraint function taking control of O/C ratio by means of restricting a first control output when the O/C ratio deviates from an allowed range; and actively controlling loading of the fuel cells by implementing a constraint function taking control of fuel utilization by means of restricting a second control output when the fuel utilization deviates from an allowed range.    DESCRIPTION OF DRAWING(S) - The figure shows a diagram of a fuel cell system.    Feeding unit (116)    Unit (118)    Active unit (120)    First closed loop controller (122)    Second controller (124)