Expansion device, control strategy, small cooling capacity, energy efficiency
The result of an analytical and experimental study in developing a control strategy for expansion devices in small commercial refrigeration systems has been presented. The objective of the study was to improve system efficiency by optimization of the control of the expansion device. For this product type, typically capillary tubes are applied as the expansion device and compressor cycling is used to control product temperature. It is shown that for products with short operating periods (i.e. more than 4 cycles per hour), large energy saving potential exists by improvement of the evaporator filling during the first minutes after compressor activation and avoiding refrigerant migration during the compressor off cycle. A validated control strategy, based on the use of an electronic expansion device with closing valve functionality, is presented and the impact on cooling system design is discussed. Experimental validations using a dedicated test set-up, showed a reduction in cooling system energy consumption of 12 and 20% for respectively, a R-744 and a R-404A based cooling system of a vending machine in comparison to the benchmark capillary based system. Analyses showed that approximately 50-70% of this gain can be contributed to improving the cyclic average evaporator filling and that the remaining gain is resulting from avoiding refrigerant migration using closing valve functionality.