Development of a Solar Heat Pump for Space

As humans explore space, we find even greater temperature extremes than on Earth. Apollo astronauts, for example, did not stay on the moon past noon when surface temperatures soar to over 100°C. As we have on Earth, future space explorers will likely turn to mechanical heating and cooling systems to maintain comfortable living spaces. Most locations in Earth’s neighborhood have an abundant natural energy source from the sun, thus the development effort has taken a systems engineering approach and tried to optimize the heat pump and solar power source together. Earlier studies have shown that vapor compression cycle heat pumps are the most mass and power efficient for space applications. Photovoltaic (PV) cells are the most mature solar power conversion technology to provide electricity to the vapor compressor in space. Hence, these two technologies have been integrated into a solar PV vapor compression heat pump. A “ground-test unit” Solar Heat Pump designed to provide 15 kilowatts of cooling was completed in 2002 and underwent preliminary testing at SunDanzer’s previous location in Nevada. Following buildup of a heat pump testbed that will allow simulation of radiator loop temperatures on the moon or Mars, a complete system evaluation will be conducted in 2005 at the Johnson Space Center. Following that, the gravity dependent components of the system, most notably the compressor, will be analyzed and upgraded for the required gravity environment(s). A gravity-independent approach would allow NASA mission designers the most flexibility to use heat pump technology in future mission scenarios, but technical challenges may lead to machines optimized for a particular application. Design data and analysis, as well as the preliminary test results, indicate that the Solar Heat Pump will achieve very good efficiency, near 45% of Carnot efficiency, at a temperature lift of 45°C. A novel motor controller and computer control software allow the heat pump to use solar power directly from the PV array at about 400 volts direct current. These features, referred to as “PV-direct”, along with a good thermodynamic design, form a solid basis for the design of a future space-qualified Solar Heat Pump.