Dual Evaporator, Ejector, Domestic Refrigeration
Increasing air conditioning and domestic refrigeration demands pose a massive burden on grid electricity calling for efficient cooling technologies. In this paper, a novel refrigeration architecture -the Dual Evaporator Dual Ejector Cycle (DEDEC) is proposed. The DEDEC utilizes separate high and low-temperature (HT and LT) evaporators and two ejectors, allowing cooling at two different temperatures levels. Compared to the conventional Dual Evaporator Ejector Cycle, the proposed architecture maintains a higher temperature difference between the two evaporators while reducing the throttling losses. This feature finds application for domestic refrigeration where the temperature difference between the fresh food and freezer section is to be maintained. Additionally, the thermal compression provided by the dual ejectors decreases the compressor work, leading to higher COPs than the dual evaporator vapour compression systems. In this study, a steady-state thermodynamic model for the proposed DEDEC is tested for a domestic refrigerator application. A sensitivity study on various cycle parameters is presented. Results based on R134a show that for fixed condensing temperature and LT evaporator temperatures of -15°C and -20°C, the system is able to maintain the HT evaporator at a temperature of -4.3°C and -7.8°C, respectively. Corresponding minimum load ratios (fresh food : freezer) are found to be 1.67 and 1.75, respectively. Compared to a standard vapour compression cycle (VCC) operating in the same conditions, the DEDEC delivers a COP improvement of 31% and 35% respectively. Lastly, the potential of integrating a low-grade heat source such as solar power with the system is analyzed. The ejector motive fluid is preheated, enabling the system to achieve a higher temperature difference between the HT and LT evaporators. The higher temperature difference reduces the dehumidification of air and build-up of frost typically associated with the conventional VCC refrigeration systems. Results indicate that for an LT evaporator temperature of -20°C, a 10°C preheating delivers an additional 4.6°C temperature rise in the HT evaporator, bringing it to -3.2 °C. Compared to the case without heat integration, a 13.6% decrease in COP is noted with heat integration. The additional heat requirement is 28% of the compressor power to be supplied at a temperature greater than 57°C, which is typical of residential solar heater systems. Therefore, DEDEC refrigerators can be readily deployed in residential buildings with solar heating.