NOVELTY - The fuel cell system uses a dry compressor to provide compressed air used as the oxidant. Controlled amounts of water are sprayed directly onto the rotor(s) of the dry compressor to reduce the energy required by the compressor to compress a given amount of air.    USE - Fuel cell system for continuously producing electrical energy using oxygen and hydrogen.    ADVANTAGE - The energy efficiency of the dry compressor is optimized, particularly at low speeds, in order to reduce the parasitic load the compressor places on the fuel cell system.    DETAILED DESCRIPTION - The fuel cell system comprises    (a) a fuel cell (2) having a cathode inlet admitting compressed air into the cell and a cathode outlet for exhausting effluent containing water generated in the cell;    (b) a collector (18) downstream of the cathode outlet for collecting water from the effluent;    (c) a pump for removing the water from the collector;    (d) a dry compressor (22) for compressing input air and supplying it as compressed air to the cathode inlet via a first conduit over a range of operating pressures suitable to the fuel cell, the compressor having at least one rotor;    (e) an injector (30) communicating with the compressor to spray a mist of the water from the collector directly onto the rotor;    (f) a second conduit communicating the pump with the injector to supply the injector with water;    (g) a mass flow meter for measuring the mass flow rate of the input air into the compressor; and    (h) a controller (34) communicating with the mass flow meter and the injector to modulate the flow of water through the injector as a function of the mass flow rate to the input air into the compressor to optimize the energy efficiency of the compressor at a particular mass flow rate.    An INDEPENDENT CLAIM is included for a method of operating the fuel cell system.    DESCRIPTION OF DRAWING(S) - the drawing shows a schematic view of the fuel cell system.    Fuel cell (2)    Anode side (4)    Cathode side (6)    Cathode outlet (10)    Anode outlet (12)    Combustor (14)    Water collector (18)    Dry compressor (22)    Rotor (24)    Liquid injector (30)    Controller (34)