Exergy, An International Journal最新文献

Partial exergy losses appearing in particular parts of thermal systems have been defined. Balance equations determining these losses have been formulated. The problem of calculation of partial exergy losses in cogeneration processes has been discussed. Sequence method of calculation of partial exergy losses has been presented. Examples of calculation of partial exergy losses have been developed.

This paper outlines a newly emerging body of work that relies on exergy analysis and thermodynamic optimization in the design of energy systems for modern aircraft. Exergy analysis establishes the theoretical performance limit. The minimization of exergy destruction brings the design as closely as permissible to the theoretical limit. The system architecture springs out of this constrained optimization principle. A key problem is the extraction of maximum exergy from a hot gaseous stream that is gradually cooled and eventually discharged into the ambient. The optimal configuration consists of a heat transfer surface with a temperature that decays exponentially in the flow direction. This configuration can be achieved in a counterflow heat exchanger with an optimal imbalance of flow capacity rates. The same optimal configuration emerges when the surface is minimized subject to specified exergy extraction rate. Similar opportunities for optimally matching components and streams exist in considerably more complex systems for power and refrigeration. They deserve to be pursued, and can be approached first at the conceptual level, based on exergy analysis and thermodynamic optimization. The application of such principles in aircraft energy system design also sheds light on the “constructal” design principle that generates all the systems that use powered flight, engineered and natural, cf. constructal theory.

Exergy analysis is applied to a turbojet engine over flight altitudes ranging from sea level to 15 000 m (∼50 000 ft), to examine the effects of using different reference-environment models. The results of this analysis using a variable reference environment (equal to the operating environment at all times) are compared to the results obtained using two constant reference environments (sea level and 15 000 m). The actual rational efficiency of the turbojet decreases with increasing altitude, ranging from a value of 16.9% at sea level to 15.3% at 15 000 m. In the most extreme cases considered, the rational efficiency calculated using a constant reference environment varies by approximately 2% from the variable reference environment value.

An exergy analysis of a solid polymer fuel cell power system for transportation applications is reported. The analysis was completed by implementing the fundamental governing second law equations derived for the system into a fuel cell performance model developed previously. The model analyzes all components of the system including the fuel cell stack and the air compression, hydrogen supply, and cooling subsystems. From the analysis, it was determined that the largest destruction of exergy within the system occurs inside the fuel cell stack. Other important sources of exergy destruction include irreversibilities within the hydrogen ejector and the air compressor, and the exergy associated with the heat rejected from the radiator. The results may aid efforts to optimize fuel cell systems.