Limits of Performance for Alternate Fuel Energy to Mechanical Work Conversion Systems

The major alternatives for producing work from fuel energy include combustion systems and fuel cells. Combustion systems are subject to several performance-limiting constraints. Key amongst these is the fact that combustion is an uncontrolled chemical reaction and is typically highly irreversible. The requirement to operate below the metallurgical limit adds to the irreversibility of practical combustion systems. Furthermore, the use of heat exchangers, which must have finite temperature differences between fluid streams, compounds the exergy consumption. The fuel cell conversion system is a major alternative to combustion systems. It operates as a direct conversion device and is often cited as having a potential for 100% second-law efficiency. Realistically, however, the chemical reactions involved are not reversible. More importantly, the available fuel resources must be reformed to make the chemical energy of the fuel convertible to work. The significant exergy input required must be factored into the determination of the overall exergy conversion efficiency attainable. This paper gives a simplified first- and second-law analysis for the limits of efficiency of these alternate systems for the conversion of fuel exergy to mechanical work, thus providing a more realistic comparison of the potential of both systems.