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Johnson Space Center’s solar-powered refrigeration system employs a PV panel, vapor compressor, thermal storage and reservoir, and electronic controls. The process that makes the refrigeration possible is the conversion of sunlight into DC electrical power, achieved by the PV panel. The DC electrical power drives the compressor to circulate refrigerant through a vapor compression refrigeration loop that extracts heat from an insulated enclosure. This enclosure includes the thermal reservoir and a phase change material. This material freezes as heat is extracted from the enclosure. This process effectively creates an “ice pack,” enabling temperature maintenance inside the enclosure in the absence of sunlight.

Proper sizing of the highly insulated cabinet, phase change thermal storage, variable speed compressor, and solar PV panel allow the refrigerator to stay cold all year long. To optimize the conversion of solar power into stored thermal energy, a compressor control method fully exploits the available energy. Other power optimization measures include:

• Smoothing the power voltage via a capacitor, providing additional current during compressor start-up
• Monitoring the rate of change of the smoothed power voltage using a controller to determine if the compressor is operating below or above the available power maximum, enabling adjustment of the compressor speed if necessary
• Replacing the capillary tube in the refrigerator system with an expansion valve, improving energy efficiency in certain operating conditions

These adjustments to the compressor operation contribute to the conversion of the majority of the available solar power into stored thermal energy. Applications may include a cold side water loop or incorporation of the evaporator into the thermal storage. Electronic controls also can be added to provide backup power from an alternative power source such as an electric grid.

Why It Is Better

The standard use of AC electricity supplied by the electric utility to power a single-speed vapor compression cooling system in a moderately insulated cabinet ties refrigerators to an electric grid and limits where they can be used. This prohibits their use in off-grid applications and maintains a dependence on fossil fuels for power. For these reasons, the demand for solar appliances of all kinds is increasing. However, other existing solar refrigerators use batteries, presenting a number of disadvantages. Batteries add expense, and their use and disposal cause undesirable maintenance and environmental consequences. Heat-driven cooling systems, such as absorption cycle, can also be solar powered, but their thermodynamic efficiency is not as good as vapor compression, they require more complex solar collectors, and they do not scale down in size as well.

In contrast, the solar-powered refrigeration system developed at Johnson Space Center is environmentally friendly because it eliminates the need for an electric grid or batteries and provides enough reserve thermal storage for cooling in the absence of continual sunlight.