Comparing COP Optimization with Maximizing the Coefficient of System Performance for Refrigeration Systems in Supermarkets

In recent years the energy usage of supermarkets, in particular that of their refrigeration systems, has been investigated using a variety of approaches, such as floating pressure set points and integrating the heating and refrigeration systems. Something which has not yet attracted much attention is the energy consumption of the dry condenser fans in refrigeration systems. This is surprising as it has been shown for comparable installations that including the energy consumption of these fans when optimizing the system efficiency was beneficial. To address this deficit, COP maximization has been compared to optimizing the Coefficient of System Performance (COSP). The simple refrigeration system used for this investigation was based on a commercially available R404A/CO2 system comprising the basic components, with the condenser having extractor fans. The results show that, when the outdoor temperature is below about 15°C, there is no observable difference between these two approaches. However, when the ambient temperature increases beyond this threshold, the control method which optimizes COSP is significantly better for part load conditions. This indicates that maximizing the COP can lead to a sub-optimal system in terms of energy consumption under part load conditions. When the refrigeration system is at its full load point, however, both approaches produce similar results again.