Title: Heat engine efficiency enhancement through chemical recovery of waste heat
Abstract: The thermodynamic efficiency of a chemically fueled heat engine (work out divided by heat of combustion of fuel input) is traditionally compared to the efficiency of a Carnot cycle having the same temperature limits. This paper presents a distinctly different viewpoint, based upon the Gibbs free energy equation. It shows that the theoretical efficiency of such an engine is limited by the chemical potential of the fuel with a secondary dependency on the temperature limits. The maximum theoretical efficiency of chemically fueled heat engines is shown to be much higher than the corresponding Carnot efficiency. For example, a Carnot engine fueled by coal (carbon) with chemical recovery of waste heat has a maximum thermodynamic efficiency of up to 143% of the Carnot efficiency. An example system based upon thermo-electric devices is discussed. Engineering constraints upon the application of these principles are identified, along with other potential application areas. Declining fossil fuel reserves and increasing concern for our environment have resulted in the reevaluation of many of the accepted practices throughout the engineering and scientific communities. One of these is found in the design and analysis of chemically fueled heat engines: the acceptance of an equivalent closed cycle system as the basis for evaluation. We have tended to overlook the fact that Carnot efficiency is not defined for an open cycle. The thermodynamic analysis of chemically fueled engines is, more logically, based upon the work of J. Willard Gibbs (1)*. The Gibbs function decrease expresses the maximum work capability of the fuel oxidation energy conversion system. This paper presents a concept for the chemical recovery of waste heat. It shows that the maximum theoretical efficiency of converting chemical energy to work is not limited to the Carnot efficiency of the heat engine subsystem. A partial dependency upon the Carnot efficiency is established for the maximum efficiency of a real chemically fueled heat engine. The engineering constraints on the chemical recovery of heat is discussed with an example system. The physical embodiment of this concept consists of a chemical reactor, through which the fuel and a portion of the exhaust products are passed. The reaction of the fuel with the exhaust is endothermic. Thus, heat, which would have been rejected, can be absorbed by conversion to chemical energy.
Publication Year: 1972
Publication Date: 1972-01-01
Language: en
Type: article
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Cited By Count: 14
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