The reaction products of the electrochemical reaction in a fuel cell are water, electricity, and heat. The heat energy released in a fuel cell stack is approximately equal to the electrical energy generated and must be managed properly to maintain the fuel cell stack temperature at the optimal level. If this thermal energy (waste heat) is properly utilized, it will considerably increase the efficiency of a fuel cell system. In low- temperature (<200oC) fuel cells (PEMFC, AFC, and PAFC), the stack is cooled by supplying excess air in low power (<200-W) systems, whereas a liquid coolant (deionized water) is used for large-size systems. The waste heat carried out by the coolant is utilized for cogeneration (space heat­ing, water heating, etc.). In high-temperature (<6000C) fuel cell (MCFC and SOFC) systems, all the heat of reaction is transferred to the reac­tants to maintain the stack temperature at the optimal level. The ther­mal energy of the high-temperature exhaust may be utilized to preheat the incoming air stream, or in internal or external fuel reformer. The high-temperature exhaust may also be used for cogeneration or elec­tricity generation in a downstream gas turbine system.