Operational Performance Envelope for Irreversible Fuel Cell Heat Engine Hybrids

Abstract

The performance envelope for the fuel cell heat engine hybrid (FCH) with irreversibilities is developed and explored. 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. While the perfect fuel cell and heat engine would have no irreversibilities and undergo no degradation, practical fuel cells and heat engines exhibit irreversibilities and will degrade. Various working reactions (WR) can be used in a FCH, for example, hydrogen oxidation, carbon monoxide oxidation, etc. For the FCH each working reaction has a characteristic transition temperature and a characteristic optimal operating temperature for maximum power. The characteristic optimal operating temperature of 1070 K for the FCH with an ideal heat engine and hydrogen oxidation WR is not a function of ASR for ASR not a function of temperature. The path of maximum power can be pursued as a FCH operational strategy. In the case of degradation, or changing ASR, the path can be followed by varying the fuel flow rate (fuel cell current) at constant fuel utilization. This is a continuation of work previously published by the authors in the ECS Transactions.