Exergy analysis of the cross vane expander-compressor unit

Abstract

This project conducts an exergy analysis on the Cross Vane Expander Compressor (CVEC) unit. Exergy, which is also known as available energy or availability, is a property to determine the useful work potential of a given amount of energy at some specified state. At a specified state in a system, the useful work potential is defined as the maximum useful work that can be obtained from the system. Exergy analysis is very useful in this study of the CVEC unit as the exergy analysis consists of the first and second law of thermodynamics analysis. Thus, an exergy analysis can bring out a more in-depth and comprehensive efficiency analysis of the CVEC unit. The objective of this project is to numerically investigate the second law efficiency of the expander-compressor unit for the refrigeration system using exergy analysis. The total rate of exergy destruction can be divided and grouped into four categories namely: friction, heat transfer, adiabatic throttling and fluid mixing. The causes of high inefficiencies of the CVEC unit can be then determined and quantified more precisely, so that improvements and enhancements can be made to the CVEC to improve the second law efficiency. A mathematical model is used to model the working principles of the CVEC unit. A computer simulation program, MATLAB, is used to simulate the working principles of the CVEC unit. The working fluid used in this computer simulation is R-134a. Through the exergy analysis of the CVEC, under reference operating conditions, the second law efficiency is found out to be 34.1%. A parametric study is being conducted to find out how different parameters affect the rate of exergy destruction for each area and the overall second law efficiency of the CVEC. The parameters that are being varied are: inner cylinder radius, while keeping the shaft radius constant, and operating speed. A study of using various refrigerants as the working fluid is also being conducted to find out how the various refrigerants can affect the overall second law efficiency of the CVEC. From the study, it is found that the second law efficiency of the CVEC unit is highest when the working fluid is carbon dioxide, when it is rotating at a high operating speed and when it has a low slender factor. The slender factor is defined as the ratio of the expander-compressor unit’s length to the outer cylinder’s radius. Thus, carbon dioxide is being considered to be used as a potential refrigerant in the CVEC unit, in the near future.