The system uses a vapor compression refrigeration system with R134a refrigerant. It has two separate compartments for warming and cooling, with a volume of 50 liters each. The condenser operates at 50 C and lies at the bottom of the heating compartment. The evaporator works at -2 C and is placed at the top of the colling space.
“This arrangement is informed by the physical phenomenon that hot air rises and cool air descends,” the scientists explained.
The warming and refrigerant compartments use the same compressor. The scientists simulated the system’s heat loads using MATLAB, showing a heat load of 433.5 kJ and a cooling load of 3,550.7 kJ. To meet these loads, they selected a 12 V DC compressor with a capacity ranging from 149 W to 186 W. The academics calculated the system’s performance using the modeling system ANSYS, estimating a coefficient of performance (COP) for refrigeration of 3.32 and for heating of 4.32.
They built a prototype of the dual system using two small household fridges as cooling and heating compartments and tested it in Bellville, Cape Town. The system uses two 90 W monocrystalline silicon PV panels from an undisclosed manufacturer measuring 1,200 mm x 544 mm x 25 mm. It also includes a 12 V DC battery and a 10 A charge controller.
The team tested the system between December 2020 and January 2021 – the summer months in South Africa. The results show that the heat pump warmed the hot compartment to a maximum of 55 C during peak sun hours, between noon and 2 pm (SAST).
“However, it was observed that once the door was left open for more than 15 minutes, a drop in temperature was evident at 3 pm … while running the heat pump system,” the investigators said.
The refrigerated compartment reached a low temperature of 2 C. The scientists also compared the performance of the solar modules with zero- and 30-degree tilt for a north orientation. They said that they found that “the 30-degree tilted PV panel received higher irradiance, which caused both systems to reach their design temperatures right before noon; therefore it proved best suited for the entire system’s performance.”
The academics shared their findings in “A solar powered refrigerator and heat pump for urban street vendors,” which was recently published in MATEC Web of Conferences.
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