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

Abstract

The major alternatives for producing work from the chemical energy of fuels 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 or exergy consumption in 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; such processes require significant exergy input that must be factored into the determination of the overall exergy conversion efficiency attainable. This paper gives a first- and second-law analysis of the alternate systems for conversion of fuel exergy to mechanical work thus providing a more realistic comparison of the potential of both systems.

Key findings

• ▸Combustion systems are subject to performance limiting constraints due to uncontrolled chemical reactions and irreversibility.
• ▸Fuel cells have potential for high efficiency but are limited by the need for fuel reformation, which requires significant exergy input.
• ▸Both combustion and fuel cell systems have exergy consumption due to heat exchanger inefficiencies.

Keywords

Identifiers

DOI

10.1115/imece2003-41285

Journal

Advanced Energy Systems

Year

2002