Snapshots from Physics History

November 11, 1930: Patent granted for Einstein-Szilard Refrigerator

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Einstein and Szilárd’s patent application

Albert Einstein is best known to the general public for devising the world’s most famous equation: E=mc2. But his contributions to physics extend over a 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 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, and served in the Austro-Hungarian Army during World War I. After the war, he returned to university, studying physics under Einstein and Max Planck, among others. 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 in the 19th century. 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.

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 U.S. Patent Office compared in significance 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 Bern 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.

When they met, Einstein was a world-famous physicist, thanks to his work on relativity, while Szilard was a graduate assistant at the University of Berlin. The impetus for the two men’s collaboration on a refrigerator occurred in 1926, when news broke of the death of a family in Berlin. The family perished  as a result of toxic gas fumes from 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. They 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. 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.

One of the components they 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.

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.

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, 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. But 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. 

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 built a prototype as part of a project to develop more robust appliances, and a former graduate student at Georgia Tech, Andy Delano, also built a prototype of one of Einstein and Szilard’s designs. Yet another team at Cambridge University is experimenting with cooling via magnetic fields.


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