Ben Krasnow 3D-Printed a See-Through Einstein Refrigerator That Gets Cold From an Alcohol Flame

Absorption fridges are the kind you find in an RV. Krasnow explains why: they run on propane and have “no internal moving parts, no valves, no pumps.” Inside it’s one sealed volume, basically a very cleverly bent tube.

The diagrams for these things are a tangle of arrows. Krasnow wanted to watch the arrows move. “So, I wanted to make a clear one just so we could actually see what was flowing inside the refrigerator.”

Glass was the first idea. A standard ammonia fridge runs at about 15 bar, which he puts at “200 and something PSI,” with dozens of passageways and flammable hydrogen inside. That’s a lot of glassblowing to hold that much pressure.

Then he found the Einstein-Szilard design. Albert Einstein and Leo Szilard invented it, and the Chan and McCulloch paper Krasnow posted with his files cites their 1928 and 1930 work. It uses three fluids at a single pressure. In their paper, butane is the refrigerant, ammonia is the inert gas and water is the absorbent. The system “has no mechanical moving parts and can be driven by heat alone.”

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Krasnow needed a refrigerant that boils near room temperature, separates from water by weight, leaves ammonia and the plastic alone, and preferably won’t burn. Add up those requirements, he says, and “we basically end up with one chemical”: perfluoropentane. Later in the video he repeats why: “we picked perfluoropentane because it would not be flammable.”

The heat goes in through metal. “To get the heat from the alcohol flame into the ammonia liquid inside this refrigerator, I machined a copper slug.” He soldered it to a piece of copper-clad circuit board, so the thin copper seals the joint without carrying much heat into the plastic around it.

The printed parts sealed with just a wash and cure. Krasnow expected to spend his time on gaskets and clearances and instead spent it keeping air bubbles from blocking the tiny channels. The only thing pushing fluid around is the bubble pump in the boiler, which is not much force.

The plastic is working near its limit. Krasnow says the build pushes “the limits of what that clear resin” should handle, with the boiler at 75 degrees C. The parts are thick enough, with enough air around them, that warping wasn’t much of a problem.

Ammonia also eats copper. It forms a blue copper complex, which is why commercial absorption fridges aren’t built with copper tubing. For a short experiment, Krasnow used copper heat sinks inside anyway. He’d hoped the blue would show how much ammonia was in the water, but the color barely changes.

His design files are on GitHub: Fusion 360 and STEP models, flow diagrams, and temperature logger data. The README sums it up as a tabletop design with “60W input from an alcohol burner.”

Krasnow says he has no plans to commercialize it. He does think the absorber, where the liquid has to spread thin over thick copper fins, is where the next gains are. How much colder than 5 degrees below the room can a printed fridge get?