Power generation by combined fuel cell and gas turbine systems

The oxidation of H, CO, CH₄, and higher hydrocarbons in fuel cells to produce power also produces reject heat. This heat arises from two sources: the entropy decrease, and the loss in work. Heat from these two sources must be rejected from the fuel cell in order to maintain its temperature at a desired level. The heat can be removed and recovered by transferring it across a bounding surface to a heat transfer fluid, but care must be taken to maintain the cell at its desired temperature in this and adjacent regions. Alternatively, heat can be removed in one of the reactant streams passing through the cell-most practically the air, oxidant stream. Also in the operation of a practical fuel cell, some unburned fuel must remain in the combustion products leaving the cell in order to maintain a significant generated voltage throughout the cell. In order to obtain the highest possible efficiency in electrical generation both the thermal energy in the heat and the unburned fuel rejected from the cell must be recovered and converted into additional electrical energy. This can be accomplished by means of a heat engine cycle making use of a gas turbine operating in a regenerative Brayton or combined Brayton-Rankine cycle or a steam turbine operating in a Rankine cycle. The relative merits of these three heat engine cycles depends on their overall efficiencies and on the practical aspects of integration, operation, and cost of the power generation plant as a whole