Energy harvesting controls for solar direct-drive medical cold chain equipment

Solar direct-drive (SDD) appliances including refrigerators and freezers are widely used for storing vaccines and other medical products in locations without reliable mains electricity. The capacity of the solar array powering an SDD appliance must be large enough to provide sufficient compressor runtime during periods of reduced solar irradiance. SDD appliance solar arrays therefore create useful quantities of excess electricity that are currently not utilized. An energy harvest control (EHC) that prioritizes the power requirements of the appliance and diverts excess electricity to other purposes has the potential to power a wide range of electrical devices, including health facility lights, appliance data loggers, medical devices, and mobile phone chargers. This paper discusses results of laboratory testing of two EHC prototypes employing different control logics using simulated and actual solar power, as well as results from field testing. In laboratory testing, the prototype EHCs demonstrated diversion of useful amounts of electricity to secondary loads without adversely affecting refrigerator performance in most cases. Some problematic interactions between the refrigerator compressor controller and EHCs were observed. Field tests of the EHC prototypes in Colombia demonstrated the usefulness and acceptance of EHCs in providing power to community- and district-level health facilities. Development of more low-powered medical devices that can take advantage of the limited power provided by EHCs would be of great benefit to health facilities in areas without reliable mains electricity. Prioritized control of loads on solar arrays may have applications beyond medical refrigeration appliances.