One of the beneficial features of SDD refrigerators is its relative simplicity. Electrically two wires from the solar array are directly connected to the compressor. This simplicity minimizes problems in the field and simplifies repairs. Energy harvesting technology introduces a great deal of electronic complexity and the complexity increases as a larger percent of the available energy is harvested. In the US the primary mode of failure of new refrigerators is the electronic circuitry. In the field there will be different types of energy harvesting devices coupled to different brands of refrigerators and the loads connected to the device will be of varying types, ie batteries, resistive loads, pumps etc. these factors will complicate the determination of a cause of failure and will also complicate training of the repair person. If the energy harvesting device does fail there may not be the power necessary to call a repair person.
If a USB charging port is created it is desirable to provide power throughout daylight hours so devices such as smart phones and I pads can get fully charged. The simpler harvesting technologies only provide auxiliary power when the refrigerator is not running and the more complex technologies could provide a USB port throughout the day if there is sufficient excess power when the refrigerator is running.
A simpler strategy to supply energy for auxiliary loads may be to add a small stand alone power system. A 100 watt $100 solar module could typically provide 500 watt hrs of continuously available reliable power. A USB charging station can be powered by a panel between 6 and 20 watts, the cost of the system would be between $15 and $50, and these systems do not require a battery.
Although the idea of energy harvesting is intriguing it may not be the most reliable and cost effective way to supply power for auxiliary loads.