The performance envelope for fuel cell heat engine hybrid cycles is developed and explored. The benefit of fuel cell heat engine hybrids in conversion of chemical energy was thermodynamically established. Fuel cell hybrids are combination of energy conversion sub-systems - fuel cells and heat engines. Fuel cell hybrids are important for the future for they are the most efficient devices when converting chemical energy of methane from renewable fuels to electricity. The power produced from an ideal fuel cell hybrid is a function of the current and the standard Gibbs free energy at unit activities. On the other side the operation of heat engines heated by a chemical reaction is based on the same principles, however the technical potential of direct electron transport is principally not used. The fuel cell allows an almost reversible operation at ambient temperature while the Carnot type engine operates principally reversible at a reversible Carnot temperature where the free enthalpy of the reaction is zero. Hybrid systems unify the benefits of both technologies and expand the possible range of operation to all temperatures between ambient and the above mentioned reversible Carnot temperature. The fuel cell hybrid performance is always higher than or equal to the heat engine combustor performance. It is higher until the Carnot temperature of 4,300K in the case of hydrogen combustion. By the same token, the fuel cell hybrid performance occurs over a limited temperature range due to the narrow operating temperatures of fuel cells. The practical operating temperature of a fuel cell, and hence the fuel cell heat engine hybrid, is less than 1,400K. At temperatures from 298K to 1,400K, the fuel cell hybrid performance exceeds all other performances, of course. The cooled fuel cell power case exceeds heat combustion performance until around 1,200K. This paper builds from and is an extension of several papers recently published in the Journal of The Electrochemical Society (ECS), ECS Transactions, and the Journal of Fuel Cell Science and Technology, and the International Journal of Hydrogen Energy.
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