A solar-powered vapor-compression refrigeration system developed for Johnson Space Center operates without batteries. The design of this system will make the cost of solar-powered refrigeration systems competitive and enable the use of such systems in long-distance spaceflights, military field operations, and other situations in which electric power for conventional refrigerators and freezers is unavailable.
The elimination of the need for batteries reduces (in comparison with a battery-powered system) the initial cost of the system and the cost of maintenance, as explained below. The system includes control circuitry that is connected directly to a solar photovoltaic (PV) panel and to a compressor. The system features a well-insulated cabinet, generous thermal-storage capability, and a high-efficiency cooling subsystem, that, together with the PV panel, make it possible for the cooled volume to stay cold year-round. The technical feasibility and the potential economic advantage and environmental benefit of the system have been demonstrated in studies and in tests of a prototype of the system — a full-size, solar-powered vapor-compression refrigerator in which the compressor is driven by a variable-speed, direct-current motor.
In a conventional refrigerator, a single-speed, alternating-current motor drives a vapor-compression cooling subsystem housed in a moderately insulated cabinet. Unfortunately, because a conventional refrigerator is designed to rely on an electric-power grid, its utility is restricted in spaceflight, military applications, or wherever electricity is unavailable or expensive. Prior solar-powered refrigerators included large batteries that were recharged by PV panels, or else thermal-cycle heat pumps rated at efficiencies lower than those of vapor-compression systems. Although the use of solar energy is environmentally benign, its widespread application has been slowed by a lack of cost-effective means. Where solar energy systems have been tried, they have lowered overhead, but because of the need for batteries and/or dc-to-ac power conversion, they have not eliminated it altogether. The present system eliminates the weights and costs of batteries and dc-to-ac power conversion subsystems.
The present system concept is flexible and allows variations of the basic design. The size of the cabinet can be chosen, with appropriate matching of the cooler, thermal-store, and PV capacities. In an alternative version of the system, one could reduce or eliminate thermal storage and incorporate electronic controls that would utilize backup power from a power grid or other source; this version could be cost-effective in an urban setting as well as in a remote setting. The scope of potential commercial applications could be widened by extending the concept to solar-powered freezers, ice makers, and air conditioners.
This work was done by Michael K. Ewert of Johnson Space Center and David J. Bergeron III of Space Industries, a Division of GB Tech, Inc.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Johnson Space Center, (281) 483-0837. Refer to MSC-22970.