The story of Einstein's refrigeratorS

We tend to think of Albert Einstein has a highfalutin theoretical physics guru, but the physicist also worked on much more everyday tasks...like developing an energy-efficient refrigerator. Allow Jennifer Ouellette from Cocktail Party Physics to explain.

Ah, innovation! What would we do without this driver of new technology and new consumer markets? Science is the breeding ground for said technological creativity, and even those scientists who focus primarily on curiosity-driven basic research — such as theoretical physics — often find their curiosity piqued by the challenge of finding a solution to a real-world problem.

Take, for example, Albert Einstein, best known to the general public for devising the world's most famous equation: E=mc2. But his contributions to physics extend over an impressively broad range of topics, including Brownian motion, the photoelectric effect, special and general relativity, and stimulated emission, which led to the development of the laser. Less well known, even among physicists, is his work with Leo Szilard to develop an energy efficient absorption refrigerator with no moving parts.

Szilard was born in Budapest, Hungary in 1898, the son of a civil engineer. In 1916, he enrolled as an engineering student at Budapest Technical University, but his education was interrupted the following year, when he joined the Austro-Hungarian Army. After the war, he attended the Institute of Technology in Berlin — not so much by choice, as because of "racial quotas" (he fled Berlin in 1933 to escape Nazi persecution) — where he met Albert Einstein and Max Planck. Szilard earned his doctorate in physics in 1922, and he and Einstein became close friends.

His dissertation was in thermodynamics, and in 1929 he published a seminal paper, "On the Lessening of Entropy in a Thermodynamic System by Interference of an Intelligent Being" – part of an ongoing attempt by physicists to better understand the "Maxwell's Demon" thought experiment first proposed by James Clerk Maxwell. It contained a description of "Szilard's engine," a hypothetical heat engine that violates the second law of thermodynamics by continuously turning the heat energy of its environment into work.

This was the example the Spousal Unit featured in a blog post just before the Thanksgiving holiday, about a new Maxwell's-Demon-type experiment conducted by Shoichi Toyabe and collaborators in Japan, that appeared recently in Nature. Alas, as with many things in the nuanced field of thermodynamics, the result was misinterpreted in several press accounts as converting information into energy. Per the Spousal Unit: "That's not quite right - it's more like using information to extract energy from a heat bath." And he cited Szilard's Engine to illustrate the difference:

Consider two pistons with the same number of gas particles inside, with the same total energy. But the top container is in a low-entropy state with all the gas on one side of the piston; the bottom container is in a high-entropy state with the gas equally spread out.You see the difference - from the top configuration we can extract useful work by simply allowing the piston to expand. In the process, the total energy of the gas goes down (it cools off). But in the bottom piston, nothing's going to happen. There's just as much energy inside there, but we can't get it out because it's in a high-entropy state.

In 1929, Leó Szilárd used a similar setup to establish an amazing result: the connection between energy and information. The connection is not that "information carries energy"; if I tell you some information about gas particles in a box, that doesn't change their total energy. But it does help you extract that energy. Effectively, learning more information lowers the entropy of the gas. That's a loosey-goosey statement, [we love that the Spousal Unit says stuff like "loosey-goosey"] because there is more than one way to define "entropy"; but one reasonable definition is that the entropy is a measure of the information you don't have about a system. (In the piston above, we know more about the gas in the low-entropy setup, since we have a better idea of where it is localized.)

The story of Einstein's refrigeratorS

Anyway, the point is, that early dissertation work of Leo's proved useful when it came time to design a new kind of refrigerator. Szilard had a knack for invention, applying for patents for an x-ray sensitive cell and improvements to mercury vapor lamps while still a young scientist. He also filed patents for an electron microscope, as well as the linear accelerator and the cyclotron, all of which have helped revolutionize physics research. Szilard's most important contribution to 20th century physics was the neutron chain reaction, first conceived in 1933. In 1955, he and Enrico Fermi received a joint patent on the first nuclear reactor, which the US Patent Office compared in significant to the patents issued for the telegraph and telephone in the 19th century.

Einstein wasn't a stranger to the patent process, either, having worked as a patent clerk in Berlin as a young man. He later received a patent with a German engineer named Rudolf Goldschmidt in 1934 for a working prototype of a hearing aid. A singer of Einstein's acquaintance who suffered hearing loss provided the inspiration for the invention.

The impetus for the two men's collaboration on a refrigerator occurred in 1926, when newspapers reported the tragic death of an entire family in Berlin, due to toxic gas fumes that leaked throughout the house while they slept, the result of a broken refrigerator seal. Such leaks were occurring with alarming frequency as more people replaced traditional ice boxes with modern mechanical refrigerators which relied on poisonous gases like methyl chloride, ammonia and sulfur dioxide as refrigerants. Einstein was deeply affected by the tragedy, and told Szilard that there must be a better design than the mechanical compressors and toxic gases used in the modern refrigerator. Together they set out to find one.

And now, a brief primer on how your refrigerator works. One of the neat things about thermodynamics is that if you can create a large enough differential — for example, a big difference in temperature between two compartments — you've got yourself a handy energy source to tap into should the need arise. Refrigerators work on a simple concept known as the Carnot cycle. Gas — usually ammonia or freon these days, not the toxic gases more common during Einstein's era — is pressurized in a chamber, said pressure causes that gas to heat up, this heat is then dissipated by coils on the back of the appliance, and the gas condenses into a liquid. It's still highly pressurized, sufficiently so that the liquid flows through a hole to a second low-pressure chamber.

That abrupt change in pressure makes the liquid ammonia boil and vaporize into a gas again, also dropping its temperature — thereby keeping your perishable foodstuffs nicely chilled. The cold gas gets sucked back into the first chamber, and the entire cycle repeats ad infinitum — or at least as long as the appliance is plugged in. That's always the catch, you see. The refrigerator is not a truly "closed system": it gets a constant influx of energy from the wall outlet that enables it to operate continuously. Left on its own, without that crucial influx, and the interior would cease to be nicely chilled, and all the food therein would perish.

To address the toxic gas concerns, Einstein and Szilard focused their attention on absorption refrigerators, in which a heat source – in that time, a natural gas flame – is used to drive the absorption process and release coolant from a chemical solution, instead of a mechanical compressor. An earlier version of this technology had been introduced in 1922 by Swiss inventors, and Szilard found a way to improve on their design, drawing on his expertise in thermodynamics. His heat source drove a combination of gases and liquids through three interconnected circuits.

The story of Einstein's refrigeratorS

They still needed some version of a Carnot cycle. Anyone who lives at high altitudes (Denver residents, we're looking at you!) knows that water boils at lower temperatures when the air pressure is lower, as is the case in the Mile-High City. (Air pressures are higher at sea level.) The Einstein-Szilard fridge exploited this effect, using just pressurized ammonia, butane and water, with no need for electricity to operate the appliance (depending on your choice of heat source), and no moving parts — thereby eliminating the possibility of seal failure.

One side contained a flask filled with butane (the evaporator), which was then injected by a new vapor (the ammonia) just above the butane, creating that all-important differential. This would decrease the boiling temperature, and as the liquid water boiled off, it sapped energy from its surroundings — chilling the compartment in the process.

One of the components the two physicists designed for their refrigerator was the Einstein-Szilard electromagnetic pump, which had no moving parts, relying instead on generating an electromagnetic field by running alternating current through coils. The field moved a liquid metal, and the metal, in turn, served as a piston and compressed a refrigerant. The rest of the process worked much like today's conventional refrigerators.

Einstein and Szilard needed an engineer to help them design a working prototype, and they found one in Albert Korodi, who first met Szilard when both were engineering students at the Budapest Technical University, and were neighbors and good friends when both later moved to Berlin.

The German company A.E.G. agreed to develop the pump technology, and hired Korodi as a full-time engineer. But the device was noisy due to cavitation as the liquid metal passed through the pump. One contemporary researcher said it "howled like a jackal," although Korodi claimed it sounded more like rushing water. Korodi reduced the noise significantly by varying the voltage and increasing the number of coils in the pump. Another challenge was the choice of liquid metal. Mercury wasn't sufficiently conductive, so the pump used a potassium-sodium alloy instead, which required a special sealed system because it is so chemically reactive.

Despite filing more than 45 patent applications in six different countries, none of Einstein and Szilard's alternative designs for refrigerators ever became a consumer product, although several were licensed, thereby providing a tidy bit of extra income for the scientists over the years. And the Einstein/Szilard pump proved useful for cooling breeder reactors. The prototypes were not energy efficient, and the Great Depression hit many potential manufacturers hard. But it was the introduction of a new non-toxic refrigerant, freon, in 1930 that spelled doom for the Einstein/Szilard refrigerator. The economics supported the freon-based mechanical compressor technology, and that's what most folks still use today.

Interest in their designs has revived in recent years, fueled by environmental concerns over climate change and the impact of freon and other chlorofluorocarbons on the ozone layer, as well as the need to find alternative energy sources. In 2008, a team at Oxford University led by Malcolm McCulloch (an electrical engineer who is passionate about green technologies) built a prototype as part of a project to develop more robust appliances. They modified the design slightly, replacing the types of gases used, in hopes of quadrupling the efficiency of Einstein and Szilard's original design. McCulloch is also toying with the notion of using a solar-powered heat pump to make the appliance even more energy efficient.

Meanwhile, other scientists at rival Cambridge University have explored cooling via magnetic fields, with no need for adding extra energy, in yet another modified design of the Einstein-Szilard fridge. "Ours works in a similar way (to freon fridges) but instead of using a gas we use a magnetic field and a special metal alloy," project manager Neil Wilson told Green Optimistic in 2008. "When the magnetic field is next to the alloy, it's like compressing the gas, and when the magnetic field leaves, it's like expanding the gas. This effect can be seen in rubber bands — when you stretch the band it gets hot and when you let the band contract it gets cold."

And finally, a former graduate student at Georgia Tech, Andy Delano, also built a prototype of one of Einstein and Szilard's designs as part of his master's and doctoral thesis work. "Literally, you heat one end and the other gets cold," Delano explained at the time. He researched the refrigeration cycle and modeled it on a computer, using his own money to build the prototype. His then-roommate just happened to be majoring in civil engineering, and helped weld the prototype together, while his brother (another Georgia Tech alum) drew on his industrial design degree to create an animated version of Einstein and Szilard's original patent diagram, bringing the movement of the various fluids to life. Delano's version used electric resistance heaters as the heat source, mostly for convenience, but a small gas burner or solar energy sources could also be used.

It took months for Delano and his partners to finish building the prototype, but good news — it worked right off the bat. Well, almost: at first, he got ice, so he tweaked the mix of chemicals to get a chill, not not an outright freeze. It does bode well for the possibility of creating a refrigerator/freezer combination in the future — although that future is probably still pretty far off. Still, all these prototypes are further proof of principle that Einstein and Szilard were clearly onto something — they were just 70-odd years too soon.

[Comic below: Saturday Morning Breakfast Cereal. If you're not reading the strip regularly, why not?]

The story of Einstein's refrigeratorS

This post originally appeared on Cocktail Party Physics.