Zero-Gravity Fridge Could be used on Mars missions

Written by popularmechanics.com on June 21, 2021. Posted in Current News

Astronauts sustain their appetites in space with weird, and often gross, freeze-dried food. These vacuum-sealed packets of everything from applesauce to cream of chicken soup have been around since the Project Gemini days in the mid-1960s, yet one major flaw remains: even these foods will eventually spoil.

Researchers at Purdue University say that typical astronaut food lasts just three years; that’s a problem if we want humans to make it to Mars—a journey that would probably take at least 21 months round-trip, at minimum. So, researchers there have developed a zero-gravity refrigerator that could make space-borne food last for up to five or six years.

This design is somewhat similar to the prototypical vapor-compression refrigerator in your kitchen, but the key difference lies in its oil-free compressors. For that, the team from Purdue’s School of Mechanical Engineering turned to Air Squared, a Broomfield, Colorado-based research and development firm that specializes in them.

In a terrestrial refrigerator, a refrigerant is used to remove heat from everything inside the storage area, including the food itself, as well as the surrounding air.

Refrigerant, like tetrafluoroethane, undergoes phase changes as it works through the components in the refrigerator. The compressor circulates the refrigerant throughout the system and constricts the refrigerant into a vapor, pushing it through coils on the outside of the fridge. When that hot gas meets up with cooler kitchen air, the refrigerant becomes a liquid, which then cools down as it travels into the evaporator coils inside the freezer and fridge.

There, it absorbs heat. Finally, the refrigerant evaporates into a gas and flows back to the compressor to start its journey again.

So why doesn’t a regular refrigerator work in space?

“In a typical fridge, gravity helps to keep liquid and vapor where they are supposed to be,” Leon Brendel, a Ph.D. student involved in the zero-g refrigerator project, said in a Purdue statement. “Similarly, the oil lubrication system inside of a fridge’s compressor is gravity-based. When bringing new technology into space, making the entire system reliable in zero gravity is key.”

This means that common refrigerators will not work when tilted at a 90-degree angle, let alone in the vacuum or microgravity of low-Earth orbit—the whole system goes topsy turvy. So, the Purdue team needed to engineer a refrigerator that could bypass these phase change issues.

The crucial difference between our terrestrial fridges and Purdue’s zero-gravity prototype is the compressor that Air Squared designed for the project.

Normally, a refrigerator’s compressor pulls oil from a sump, or a low space where liquids pool due to gravity. But without gravity, that oil would never make it to the compressor. So, Air Squared designed and manufactured a proprietary oil-free compressor that makes use of greased bearings to keep the mechanical components lubricated.

“No gravity means that oil isn’t flowing where it should. Our design provides a higher reliability by not requiring oil in the compressor so that the fridge can run for a long period of time and not be challenged by a microgravity environment, where oil might leave the compressor, become trapped in the system and render the compressor inoperable,” Stephen Caskey, a project engineer at Air Squared, said in the Purdue statement.

Household appliances kingpin Whirlpool also provided components for the fridge and offered advice on how to best integrate the various parts to ensure that the prototype would be reliable in space. “If you have a problem with a fridge in space, you can’t just call a service team to come fix your fridge like you can on Earth,” Alberto Gomes, senior principal engineer at Whirlpool Corporation, said in the statement.

Last month, the Purdue team loaded up their refrigerator prototype in a microgravity lab where they could test it, according to a report in IEEE Spectrum. The lab is inside a special plane, outfitted for parabolic flights that can approximate the zero-gravity environment of space.

Picture a bell curve—the graph that illustrates a normal distribution, with most of the data congregating at the center, with long tails at the high and low ends. The plane takes a course that looks very similar, flying upward at a sharp angle and remaining suspended in the air for about 20 seconds before nose-diving back toward the Earth (side note: people onboard often get really sick on these flights).

Data analysis from these flights is still pending, but the researchers told IEEE Spectrum that the prototype seemed to work in microgravity just as well as on the ground. The next step is continuing to fine-tune the liquid containment controls to make sure that even when the fridge is powered down, it’s still gravity safe.

See more here: popularmechanics.com

Header image: Purdue University

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