NOVELTY - Alleviating deficiencies associated with those of prior art, such as response of the system in real time to the fuel needs of the fuel cell and achieving the potential hydrogen density of the system.    USE - For operating fuel cell stack and chemical hydride hydrogen generation system with a catalyst including a reactor for generating hydrogen for the fuel cell stack and controlling the temperature of the fuel cell stack and the chemical hydride generation system.    ADVANTAGE - Hydrogen recycled through a filtration system allows for higher system efficiency as well as increased chemical hydride energy density. The circulation of heat transfer fluid between the fuel cell and the chemical hydrogen generation system provides a mutual warming of the chemical hydride reactor and the fuel cell stack, thus shortening the time needed for the system to achieve optimum operation condition, and further improving the system efficiency.    DETAILED DESCRIPTION - Operation of fuel cell stack, chemical hydride hydrogen generation system and reactor, comprises supplying chemical hydride solution to the reactor, and permitting catalyst to catalyze reaction of solution to generate hydrogen; supplying hydrogen and oxidant to cell stack for generation of electricity; and circulating a heat transfer fluid between cell stack and chemical hydride generation system.    An INDEPENDENT CLAIM is also included for an energy system comprising:    (1) a fuel cell stack (3) for generating electricity from hydrogen and oxidant to form water;    (2) a chemical hydride hydrogen generation system comprising a storage tank (5) for storing a chemical hydride solution having a solution of chemical hydride solute in a solvent;    (3) a reactor (20) containing a catalyst, for catalyzing reaction of the chemical hydride to generate hydrogen;    (4) a first pump (26) connected between the storage tank and the reactor in a first circuit, for circulating the solution through the storage tank and the reactor, so that the chemical hydride reacts to generate hydrogen in the presence of the catalyst;    (5) a first connection between the chemical hydride generation system and the fuel cell stack for supplying hydrogen to the cell stack; and    (6) a heat transfer circuit including second connections between the chemical hydride generation system and the cell stack, for circulation of a heat transfer fluid through the cell stack to effect heat transfer between the cell stack and the solution.    DESCRIPTION OF DRAWING(S) - The figure is a schematic view of an energy system.    Fuel cell stack (3)    Storage tank (5)    Heat exchanger (14)    Gas-liquid separator (16)    Reactor (20)    Switch (24)    Pump (26)