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SESSION 2633

2nd Law Analysis of a Rankine Cycle Using the Wicks Cycle as the Ideal Standard Frank Wicks Mechanical Engineering Department Union College Schenectady, New York, 12308

Abstract

For the last century thermodynamics books have said and thermodynamics professors have taught that the Carnot cycle is the ideal or most efficient possible engine that can operate between a hot and a cold reservoir. The corresponding Carnot cycle efficiency is Ecarnot=(Thot-Tcold)/Thot where the temperatures are in absolute.

A decade ago the author recognized that with respect to the Carnot cycle, students were learning a classical equation that they did not understand. Also the Carnot cycle standard was widely misapplied by practicing engineers and also by the National Council of Examiners for Engineering and Surveying which prepares the Professional Engineering licensing exams.

The important fact that is not recognized by the Carnot cycle is that the source of the high temperature heat for virtually all heat engines are the products of combustion of a fuel, which are finite in size and in heat capacity. Thus, heat is not relased at a single temperature , but over the entire temperature range from the maximum combustion product temperature Thmax down to the ambient temperature which is the infinite and constant temperature environment Tcold. Accordingly, the efficiency of the ideal fuel burning engine has been defined and derived as the Wicks cycle efficiency with Ewicks=1- Tcold*ln(Thmax/Tcold)/(Thmax-Tcold).

Prior papers have shown the importance of understanding that the Wicks cycle rather than the Carnot cycle represents the ideal fuel burning engine. These papers have been of practical as well as academic importance. They have explained that while the Stirling engine may be a method to implement the Carnot cycle, a fuel burning Rankine cycle is more efficient than a fuel burning Stirling engine. These papers have also used the Wicks cycle standard to show why a combination of internal and external combustion engines is more efficient than either one separately.

This paper will use the Wicks cycle as the ideal standard for evaluating a fuel burning Rankine cycle. It will use a 1st law technique to calculate efficiency as the product of the boiler efficiency and the cycle efficiency. It will then use a 2nd law method to obtain the same result, but also showing the lost efficiency related to each irreversible process.

Proceedings of the 2001 American Society for Engineering Education Annual Conference & Exposition Copyright2001, American Society for Engineering Education

The irreversibly processes are identified as heat transfer over a temperature difference in the condenser and the water heating, boiling and superheating sections of the boiler along less than ideal turbine and pump and the stack loss. An important idea is that in terms of the first law analysis the boiler with a typical efficiency of 90 % appears to be quite efficient, the 2nd law analysis shows that most of the lost efficiency is related to the boiler because of transferring heat over large temperature differences along with the stack loss.

It is finally shown that the actual efficiency is the Wicks cycle efficiency minus the sum of the lost efficiencies for each process, and that the appropriate 2nd law efficiency of the Rankine cycle should be defined as the ratio of the actual and Wicks cycle efficiencies.

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Wicks, F. (2001, June), 2nd Law Analysis Of A Rankine Cycle Using The Wicks Cycle As The Ideal Standard Paper presented at 2001 Annual Conference, Albuquerque, New Mexico. 10.18260/1-2--8874