Clarifying reversible efficiency misconceptions of high temperature fuel cells in relation to reversible heat engines

High temperature fuel cells are promising energy conversion devices. Optimal design and analysis require a thorough understanding of their second law limitations. Fuel cells do not produce work from thermal energy as do heat engines. This has led to the provocative statement that fuel cells are ‘non-Carnot limited’. This label easily, yet erroneously, connotes that an ideal fuel cell is superior to an externally reversible heat engine. Clarity is achieved by analyzing the corresponding systems of these technologies. Conventional reversible fuel cell efficiency is also addressed, and a modified relation is developed. It accounts for the needed coupling of high temperature fuel cells with reversible heat engines, in order for maximum work to be produced.