Prof. Eckhard Groll and Ph.D. candidate Leon Brendel prepare to collect data onboard a Zero-G flight. (Stephen Boxall, Zero-G)
27 May 2021. An industry-university team is testing a compact refrigerator that works in weightless conditions and keeps food cold during long space missions. The NASA-funded project brings together researchers from Purdue University in West Lafayette, Indiana with engineers from refrigeration company Air Squared Inc. in Broomfield, Colorado, and appliance maker Whirlpool Corp. in Benton Harbor, Michigan.
Astronauts today eat mainly canned and dry food, which have a maximum shelf life of about three years. NASA is seeking a refrigeration system that allows food to be stored and retrieved for five or six years, for use on missions to the moon or Mars. In addition, says Air-Squared, refrigerators used today on the International Space Station use thermoelectric temperature control, which is inefficient. And earlier attempts with refrigerators on space missions show earth-bound systems are not reliable, particularly in weightless conditions, also known as microgravity.
Air Squared proposed to NASA a more efficient refrigeration system to meet the demanding requirements of space travel. The system uses the same basic vapor-compression process as refrigerators on earth, but the company’s technology employs more advanced scroll components that perform the process more efficiently. The company’s systems use two scrolls, one fixed and one orbiting that creates crescent shaped pockets in the refrigeration fluid between the two scrolls.
The two scrolls, says Air Squared, move refrigeration fluid from the periphery of the scroll to the center, causing compression to increase. As a result, says the company, its systems eliminate the need for valves, cutting the number of moving parts, and reducing vibration and noise. For NASA, the Air Squared refrigerator is about the size of a home microwave oven, and built to fit on the International Space Station’s standard payload racks.
30 separate weightless periods, each 20 seconds long
An engineering research team is now testing an Air Squared prototype refrigerator in weightless conditions. The tests are conducted with Zero Gravity Corp. in Dumfries, Virginia. Zero-G, as it’s called, uses a modified Boeing 727-200 airliner to simulate weightless conditions for scientists and tourists. Researchers at Purdue, led by mechanical engineering professor Eckhard Groll, designed a platform to test the Air Squared refrigerator with sensors to measure effects of microgravity on vapor-compression cycles, and detect flooding of the liquid. Earlier tests on earth measured the system’s performance in various orientations, e.g. sideways or upside-down.
Groll and three Purdue colleagues accompanied their testing platform on the Zero-G test flights, and continue to analyze data they collected. The Zero-G flight provided 30 separate weightless periods, each 20 seconds long, performing parabolic or U-shaped maneuvers that simulate microgravity, as well as reduced gravity on the moon and Mars.
Results so far indicate the system operates as well under weightless conditions and is no more likely to experience liquid flooding as on earth. “We want to have a refrigeration cycle that is resistant to zero gravity and works to normal specifications,” says Groll in a university statement. “Our preliminary analysis clearly shows that our design allows gravity to have less impact on that cycle.”
NASA is funding Air Squared’s work through its Small Business Innovation Research or SBIR program that sets aside a part of the agency’s research funding for U.S.-based and owned companies. SBIR grants fund work by research companies in the U.S., and in most cases are made in two parts: a first phase to determine technical and commercial feasibility, and a second phase to develop and test a working prototype.
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