Electrocaloric Material Makes Solid-State Fridge Scalable

Refrigerant-free refrigerators would be portable and efficient

3 min read

Illustration of a person approaching a refrigerator. The background is snowflakes.

Many of today’s refrigerators and air conditioners have a fundamental flaw. Most coolers operate by vapor compression, relying on a fluid to absorb heat and wick it away. Vapor compression tech is cheap and proven, but it’s also inefficient and about as downsizable as a 1950s vacuum-tube computer. Plus, its workhorse fluids—in particular, hydrofluorocarbons (HFCs)—often enter the atmosphere as potent greenhouse gases.

Fortunately, there are a few solid-state alternatives to vapor compression that avoid these problems. More than just cleaning up refrigerators’ acts, the alternatives could create cooling devices in miniature, small enough to fit in a pocket. One such alternative relies on solid materials that change temperature under an electric field: what scientists call the electrostatic effect.

Researchers have now created arguably the most successful demonstration yet of an electrocaloric component. Relying on a ceramic multilayer capacitor, this regenerative heat exchanger (a.k.a. regenerator) features a difference in temperature more than 50 percent greater than any electrocaloric that preceded it.

“This is really something of interest because the technology we are using is intrinsically scalable.”
—Emmanuel Defay, Luxembourg Institute of Science and Technology

When an electric field courses through an electrocaloric material, the material reacts by warming up; when the field vanishes, the material cools back down. The trick is to switch on the electric field, hold it on, force the resulting heat to radiate away, and then switch off the field—encouraging the material to chill to depths lower than its original temperature.

Researchers have known about the electrocaloric effect for more than half a century, but for most of that time, they could not do much with it. Until well into the 21st century, no one could coerce an electrocaloric material into a temperature difference of more than 10 ºC.

Then, in the late 2010s, researchers discovered they could boost an electrocaloric material’s potency by fashioning it into a multilayer capacitor. “If we put the right material in there, then we could have access to a larger [change in temperature],” says Emmanuel Defay, a materials scientist at the Luxembourg Institute of Science and Technology (LIST).

Defay and colleagues in Luxembourg and at Japan’s Murata Manufacturing set their eyes on one particular ceramic: a perovskite, lead scandium tantalate (PST). They created a regenerator from stacked layers of PST submerged in silicone oil.

First, the regenerator’s electric field activates, and its PST heats up, as electrocalorical materials do. A syringe pump pushes the silicone oil one way through the stack, which absorbs heat from the PST and creates a hot end on the far side. The electric field deactivates, and the stack cools down; the pump pushes the oil back through the stack to the other, cold end, and the PST absorbs some of its heat. Repeating this process over and over creates a regenerative cycle.

When their LIST and Murata researchers first published their regenerator results in Science in 2020, it exhibited a record 13 °C difference between its hot and cold ends. After several years of tinkering with the regenerator’s design, Defay and his colleagues have increased that delta to another record, 20.9 °C.

Moreover, they measured a cooling power of 4.2 watts. That figure may be orders of magnitude less than a garden-variety vapor-compression refrigerator, but in the world of electrocalorics, the authors say it’s a 15-fold improvement over any material that came before.

Defay is optimistic about the future. “This is really something of interest because the technology we are using is intrinsically scalable,” Defay says. “We can scale it because those elements we are using are already commercialized for other purposes.”

But before a warming world can begin fighting heat waves with electrocaloric coolers, the regenerator’s builders will have to further iterate their design. For one thing, none of the present ceramics’ key elements are appealing for mass production. Lead is toxic; scandium is prohibitively expensive; tantalum is a conflict material in Central Africa and, Defay says, best avoided.

Additionally, the silicone oil that the researchers use to absorb heat is actually a relatively bad thermal conductor. Defay would prefer to use water, which could do the job far more effectively. Unfortunately, water—or any electrically conductive fluid like it—will short-circuit the regenerator. So Defay says for the present purposes they were forced to use a poor substitute. However, he adds, if the regenerator could be redesigned to be waterproofed, the problem would be solved. “We could then increase the cooling power by one order of magnitude,” Defay says.

Electrocalorics are not the only technology that could create solid-state refrigerators. On the reverse of the electromagnetic coin are magnetocaloric materials, which change temperature under a magnetic field. In fact, in 2014, other researchers built a magnetocaloric refrigerator capable of on the order of a hundred times as much cooling power as even the present record-shattering electrocaloric regenerator.

Defay and colleagues published their record regenerator results in Science earlier this month.
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