Performance investigation and optimisationof CO2 refrigeration systems in retail food stores

Abstract

Natural refrigerants are recognized as the most promising working fluids to replace conventional HFCs in food refrigeration systems. Due to its negligible GWP, zero ODP and attractive thermophysical properties, the CO2 working fluid has grown in popularity over the last decade, especially in supermarket refrigeration systems. Accelerated tax relief schemes for new investments in environmental friendly and energy efficient technologies such as CO2 refrigerant solution in supermarkets are available across the Europe and rest of the world. The first part of this work presents an experimental investigation into the performance of CO2 finned-tube gas coolers/condensers with different designs in a CO2 booster system. The heat exchangers were mounted in a specially designed test facility that allowed the control of different test conditions and parameters, including air-on temperatures and flow rates, approach temperatures and CO2 operation pressures. The integrated refrigeration system can provide specified CO2 fluid parameters at the heat exchanger inlet, through which the system efficiency can be calculated. Subsequently, extensive measurements were recorded from this test rig, with insightful indications into system performance and the most influential parameters for system optimisation. These include heat exchanger designs, air on temperatures and flow rates, supercritical and subcritical pressure controls and cooling capacity controls, which are analysed in this work. The second part of this work includes simulation model of a transcritical booster CO2 refrigeration system which has been developed to investigate and evaluate the system performance. The model includes a detailed gas cooler model which can predict accurately the temperature, pressure, air and refrigerant heat transfer coefficient profiles across the heat exchanger. The component and system models were verified using test results from the experimental CO2 test rig built at Brunel University London. Mathematical models were also developed to investigate different refrigeration system applications in supermarkets. The control parameters for the systems with CO2 at the high pressure side were derived from the experimental work of this study. The models compared the system performance, annual consumption and TEWI at ambient conditions of London and Athens. The proposed natural refrigerant systems show good improvements compared to the HFC counterparts in terms of power consumption and annual electricity cost. In particular, the CO2 refrigeration systems show 20% to 50% reduction in terms of TEWI in case of London and 9% to 35% in case of Athens comparing to cascade R134A and R404A refrigeration system respectively