Illustrating the relationship between the coefficient of performance and the coefficient of system performance by means of an R404 supermarket refrigeration system

M.R Braun,P Walton,S.B.M Beck

doi:10.1016/j.ijrefrig.2015.10.020

Abstract

Because the coefficient of performance (COP) is primarily concerned with the core refrigeration system, using it for optimisation purposes may lead to higher than necessary energy consumption. This hypothesis was studied with a simple equation relating this coefficient to the coefficient of system performance (COSP), and with a software model based on an R404A refrigeration system installed in a supermarket in north east England. In both approaches the condenser fan power usage was excluded from the COP but included in the COSP. The results showed that, especially for part load conditions, optimising the core refrigeration system for minimum power consumption led to an appreciably higher overall energy consumption with the implication that the condenser fan and compressor controls should be developed together. When using this holistic approach it was found that energy savings of 4.5% could have been achieved based on six months' data from the installed system.

Highlights

A common way of referring to the efficiency of a cooling application is to quote its coefficient of performance (COP)
As this does not take any compressor or motor losses into account, HVAC Systems and Equipment (ASHRAE, 2002) substitutes the energy input with the electrical power supplied to the motor terminals or the mechanical power acting on the compressor shaft
Having highlighted the necessity to fully understand a system’s boundary and the interplay between system components, this paper aims to show the relationship between the COP, which is used here to refer to the core refrigeration system only, and the coefficient of system performance (COSP), which includes ancillary equipment

Summary

Introduction

A common way of referring to the efficiency of a cooling application is to quote its coefficient of performance (COP). One way is for the analysis of a theoretical single-stage cycle where ASHRAE (1997) equates the net supplied energy to the mass of the refrigerant multiplied by its enthalpy change. As this does not take any compressor or motor losses into account, HVAC Systems and Equipment (ASHRAE, 2002) substitutes the energy input with the electrical power supplied to the motor terminals (for hermetic or semi-hermetic compressors) or the mechanical power acting on the compressor shaft (for open compressors). Other ways of defining this power or energy input are given in BS EN 13771-1:2003 for compressors and in BS EN 13771-2:2007 for different types of condensing unit

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Conclusion