Design and testing of an off-grid cold room powered by renewable sources for sub-Saharan regions

This study deals with the comprehensive design, installation, and experimental characterization of an integrated system including photovoltaic (PV) system, batteries, and a cold room. To be deployed in a non-grid-connected small-scale farm in the Democratic Republic of Congo, this system addresses food security challenges through the preservation of meat at a temperature of -18°C. The research is intended to provide a scientific foundation for the practical implementation of sustainable energy solutions in off-grid environments within the context of developing regions. The goal is to cool and freeze (in 12 hours) 120 kg of meat from 39°C to -18°C, considering an ambient temperature of 30°C. Hence, a refrigerating system consisting of an air-to-air vapor-compression machine using R290 as refrigerant has been designed. A cooling capacity of 1.3 kW is required to perform the cool-down process whereas 0.4 kW is required to compensate for the heat gains from the external ambient, lighting and openings and maintaining the temperature of the frozen meat constant. The refrigerator is equipped with a scroll compressor, two fin-and-tube heat exchangers, an electronic expansion valve, a liquid receiver, and a suction accumulator integrated with an internal heat exchanger. The whole plant is installed at the SolarTech Laboratory at Politecnico di Milano, specifically the cold room is connected both to 1.5 kW photovoltaic modules, based on Cadmium-Indium-Selenite (CIS) technology, having an active area of approximately 15 m2, and to 5 kWh capacity batteries with a rated power of 3 kW. The system has been tested with two different defrosting methods: (1) electric (2) warm glycol. From the experimental analysis, the warm glycol system resulted in more promising in terms of energy saving whereas, on the other hand, the electric defrosting can be faster.