Effect of an Internal Heat Exchanger on Performance of the Transcritical Carbon Dioxide Refrigeration Cycle with an Expander

Zhenying Zhang,Yanhua Chen,Lili Tian,Lirui Tong

doi:10.3390/e16115919

Abstract

The effect of the internal heat exchanger (IHE) on the performance of the transcritical carbon dioxide refrigeration cycle with an expander is analyzed theoretically on the basis of the first and second laws of thermodynamics. The possible parameters affecting system efficiency such as heat rejection pressure, gas cooler outlet temperature, evaporating temperature, expander isentropic efficiency and IHE effectiveness are investigated. It is found that the IHE addition in the carbon dioxide refrigeration cycle with an expander increases the specific cooling capacity and compression work, and decreases the optimum heat rejection pressure and the expander output power. An IHE addition does not always improve the system performance in the refrigeration cycle with an expander. The throttle valve cycle with IHE provides a 5.6% to 17% increase in maximum COP compared to that of the basic cycle. For the ideal expander cycle with IHE, the maximum COP is approximately 12.3% to 16.1% lower than the maximum COP of the cycle without IHE. Whether the energy efficiency of the cycle by IHE can be improved depends on the isentropic efficiency level of the expander. The use of IHE is only applicable in the cases of lower expander isentropic efficiencies or higher gas cooler exit temperatures for the refrigeration cycle with an expander from the view of energy efficiency.

Highlights

Carbon dioxide (CO2), which is a natural working fluid, has been considered a promising alternative to synthetic refrigerants owing to its virtues such as lack of toxicity, no flammability, low cost, admirable thermal physical properties and absence of hazards to the environment
Whether the energy efficiency of the cycle by internal heat exchanger (IHE) can be improved depends on the isentropic efficiency level of the expander
It is observed that the optimum heat rejection pressure is almost independent of the evaporating temperature, but it shifts to higher values with increase in gas cooler outlet temperature

Summary

Introduction

Carbon dioxide (CO2), which is a natural working fluid, has been considered a promising alternative to synthetic refrigerants owing to its virtues such as lack of toxicity, no flammability, low cost, admirable thermal physical properties and absence of hazards to the environment. The employment of an expander to replace the throttle valve is expected to give the largest improvement of the cycle efficiency This action would increase the cooling effect of the evaporator and can reduce the net compression work. Sarkar [4] showed that the replacement of the throttling valve with an expander of 80% isentropic efficiency resulted in a COP increase of 18% and an optimum heat rejection pressure decrease of 2.5%. Found that the cycle with IHE provided a 15% increase in cycle COP and a 13% decrease in optimum heat rejection pressure compared to those of the basic transcritical cycle at an evaporator temperature of 0 °C and gas cooler outlet temperature of 60 °C. The analysis of the cycle performance has been done based on the first and second laws of thermodynamics

System Descriptions

Thermodynamic Modeling

Energy Analysis

Exergy Analysis

Results and Discussion

Conclusions