Scientists are testing a new kind of air-conditioning, and the promise is cleaner cooling without refrigerants, but the big question is whether solid-state systems can ever match the efficiency of the ACs people already use

A new class of solid-state coolers aims to ditch the compressor. The efficiency gap is still real, experts say.

If you’re reading this from an air-conditioned room, you’re in good company, and that is not going to change any time soon. The International Energy Agency predicts that the number of AC units worldwide will triple by 2050. That’s good news for public health, too, as a 2021 report from the Lancet Countdown found that air conditioning saved close to 200,000 early deaths in 2019 alone. But it’s not good news for the climate. Artificial cooling already accounts for 7% of global electricity use and 3% of greenhouse gas emissions. If improperly disposed of, units can leak refrigerant, which can have a much higher global warming potential than carbon dioxide.

A growing number of scientists and startups believe they have a workaround: solid-state cooling.


What does ‘solid-state’ really mean
Traditional AC units use a compressor and fan to force a refrigerant through a cycle that changes it from liquid to gas and back again, transferring heat out of a room. Solid-state systems eliminate that. Instead, they transfer heat using conductive materials like gadolinium and bismuth telluride, materials that are already used on a small scale in mini fridges, EV batteries, and high-end gaming PCs. That means, in theory, fewer leaks, fewer moving parts, and less mechanical wear to cool spaces and surfaces.

The catch, according to Pramod Reddy, a mechanical engineering professor at the University of Michigan who studies heat transfer, is nobody has completely figured out why these systems keep underperforming compared to conventional AC.

Four different bets on how to get there
Some companies and research teams are trying out different versions of the concept. Brooklyn-based Mimic Systems uses thermoelectric cooling, where a current running through semiconductor materials forces heat from one side to the other, and its room-scale system is being tested in a Vancouver apartment. Germany’s Magnotherm is planning to test a magnetocaloric system, which heats and cools materials by magnetizing and demagnetizing them, inside a chain of supermarkets. A Hong Kong team has built an elastocaloric device that cools by stretching and compressing materials and claims it can reach below 0°C. And the UK’s Barocal is backing barocaloric systems, which change temperature in response to pressure changes.

Why the experts aren't fully sold yet
But even with all the buzz, questions remain, particularly among the researchers who study thermoelectric specifically. According to Jeff Snyder, a professor at Northwestern University who studies electrical and thermal conductivity, most modern HVAC systems have a coefficient of performance, or COP, of around 3, meaning they move three units of heat for every unit of energy input. Snyder says thermoelectric systems tend to perform much worse as the temperature difference increases, which is why they are currently better suited to niche jobs like cooling the back of a car seat rather than an entire house.

But efficiency isn't the only thing that matters, according to Lindsay Rasmussen, a manager at Third Derivative, Rocky Mountain Institute's climate tech accelerator, which is supporting both Magnotherm and Mimic. According to the EPA-cited industry standard reported by American Standard, the majority of U.S. air conditioners use a refrigerant called R-410A that has a global warming potential more than 2,000 times that of carbon dioxide. Solid-state systems also have fewer moving parts, which could make them more durable over time.

Rasmussen says the real test is not COP in a lab, but rather long-term energy draw compared with conventional units. Mimic claims its room-scale system should be comparable to a typical AC’s annual energy use. Elastocaloric and barocaloric room-scale prototypes, she says, are still likely two to three years away.

So is this the future of staying cool?
Maybe not the entire future, but maybe a meaningful chunk of it. As one expert tracking the space put it, even a small foothold in a market this big adds up. If solid-state coolers can grab a five percent share of the billions of AC units installed worldwide over the next few decades, that's still a huge amount of cooling delivered without a pound of refrigerant leaking into the atmosphere.
For now, though, your existing window unit isn't going anywhere, and neither is the science needed to replace it.