Control strategies for solar direct drive refrigerators to achieve off-grid cold storage

There are about 760 million people worldwide without access to electricity. Achieving off-grid refrigeration helps maintain nutritious food while reducing food loss and waste in remote areas. This paper presents the performance of four control strategies for solar direct drive refrigerators with integrated ice thermal energy storage. The strategies, namely, on/off, perturb and observe, proportional-integral, and voltage rate of change were developed and compared based on the criteria of maximizing the utilization of the available power generated by the photovoltaic (PV) panels. Experimental tests were carried out using a prototype comprising of a PV emulator, a cabinet with 200 Liters storage capacity, an integrated energy storage with phase change material, an AC compressor, and a variable frequency drive for compressor speed adjustments. The experiments were designed according to Taguchi method considering three factors representing the control strategy, irradiance profile, and PV cell technology type. Results indicate that the proportional-integral controller provides the most rapid and smooth response for a wide range of irradiance changes. It was also shown that the controller can maintain stable operation at low irradiance levels down to 128 W/m2 with 130 W of supplied power, thus opening market prospects in areas with varying levels of access to solar energy.