The name Albert Einstein has become a byword for genius – and rightly so. The German physicist’s theories of Special and General Relativity fundamentally reshaped our understanding of the uni-verse to a degree not seen since Isaac Newton, while his groundbreaking 1905 papers on Brownian Motion and the Photoelectric Effect laid the groundwork for modern atomic and quantum theory. Yet despite his status as a giant of theoretical physics, one thing Einstein is not commonly associat-ed with is hands-on mechanical innovation. It may thus come as a surprise that Einstein actually held over 50 patents for practical devices ranging from hearing aids, cameras, gyrocompasses, and even a stylish vest with an adjustable elasticated waistband. But the vast majority of Einstein’s pa-tents covered an altogether unexpected device: a new kind of refrigerator.

The origins of Einstein’s long-forgotten foray into practical mechanics lay in a tragic accident. In 1926 Einstein read a newspaper article about a Berlin family who had died in their sleep when their refrigerator sprung a leak. Refrigerators at the time ran on a variety of refrigerants such as Ammonia, Methyl Chloride, and Sulfur Dioxide, all of which were highly toxic or even explosive. Moved by this tragedy, Einstein set out to design a safer refrigeration system. To aid him in this en-deavour he enlisted the help of former student Leo Szilard, an acclaimed Hungarian physicist who in 1933 would invent the concept of the nuclear chain reaction. They proved to be an ideal team, Szilard having written his doctoral dissertation on the thermodynamics of fluctuating systems and Einstein having worked for many years as a clerk at the Swiss Patent Office in Bern, granting him intimate knowledge of the patent system. However, the presence of Einstein’s name on the patent applications raised more than a few eyebrows, with several patent clerks writing back to confirm that this was indeed the Einstein.

In total Einstein and Szilard were granted 45 patents covering three different refrigerator designs. Their first design was an improvement on the vapour-absorption cycle invented in 1922 by Swe-dish engineers Baltzar von Platen and Carl Munters. Unlike regular vapour-compression refrigera-tors which use a mechanical compressor, Einstein and Szilard’s system had no moving parts and was completely sealed, eliminating the need rotary seals and other components that could potential-ly leak. The refrigerator used a complicated circuit to circulate a combination of water, ammonia, and butane refrigerants and was powered from the outside by a gas flame, much like the propane refrigerators used by many campers and RV-ers.

But while this is the best-known of the Einstein-Szilard refrigerators, it is the pair’s second design which most excited Einstein. Dubbed the Volks-Kühlschrank or “people’s refrigerator”, this was an ingeniously simple device that could be powered by water pressure from an ordinary household tap. Water running through a Venturi generated a mild vacuum, which caused methanol stored in a tank to evaporate. This vapour then absorbed heat from the food compartment before dissolving in the outgoing water, carrying the heat with it. While this process eventually consumed the Methanol, requiring it to be regularly topped up, Einstein reasoned that Methanol was cheap enough for this to not be an issue. Indeed, Einstein had high hopes for this design, which he believed could bring reli-able refrigeration into the homes of even the poorest families.

The pair’s third design was by far their most technologically advanced, but also their most impracti-cal. This system was largely identical to a regular vapour-compression fridge except that instead of a mechanical compressor it used a series of powerful electromagnets to pump liquid metal through a circuit. The liquid metal was then used to compress regular refrigerant gases. At first the pair used liquid Mercury, but when this proved insufficiently conductive they switched to a liquid Sodium-Potassium alloy, which was not only highly corrosive but had a nasty habit of bursting into flames or even exploding on contact with air and water. Despite this rather significant hazard, however, as the system was completely sealed Einstein and Szilard were confident this would not be a serious problem.

Yet despite the ingenuity of these designs, in the end very little came of the Einstein-Szilard refrig-erators. In 1927 Swedish Electrolux bought the absorption refrigerator patents for around $750 (about $11K today), but this was mainly to retain control on their existing Munters/von Platen pa-tents; the Einstein-Szilard design was never incorporated into any commercial refrigerator. Further-more, while more efficient than other absorption cycles, the system was far less efficient than regu-lar vapour-compression fridges, and the discovery in 1930 of the non-toxic, non-flammable refrig-erant Freon nullified the safety advantages of the Einstein-Szilard design.

The ingenious water-powered refrigerator also turned out to be a dead end. Not only did Methanol turn out to be far more expensive than Einstein had anticipated, but water pressure in German apartments was notoriously weak and inconsistent. And while the electromagnetic pump was an impressive technological achievement, its sodium-potassium working fluid was far too dangerous for household use. It was also loud: in the words of Szilard’s friend, Hungarian Physicist Denis Ga-bor, the prototype “howled like a jackal.” Interestingly given Einstein’s more famous work in phys-ics, the Einstein-Szilard pump did eventually find a niche use in certain types of nuclear reactors, which use liquid sodium-potassium as a coolant.

While Einstein’s practical inventions never made as much of an impact as his theoretical work, 90 years since it was patented the Einstein-Szilard absorption refrigerator may be poised to make a comeback. An improved version called the Isobar is currently under development by UK inventor William Broadway for transporting vaccines in developing countries with limited electrical infra-structure – an application that would have delighted the famously humanitarian Einstein.