Solid-State ACs Promise Efficiency, But Scientists Have Doubts

After three years of record-breaking heat, this one is set to be yet another scorcher. Air-conditioning? Not going anywhere.

The International Energy Agency projects that the number of AC units will triple by 2050. That's good for health—one Lancet study estimated that AC prevented nearly 200,000 premature deaths in 2019 alone—but bad for the planet. Artificial chill already accounts for 7% of global electricity use and 3% of greenhouse-gas emissions, and if improperly disposed of, the units can leak refrigerants with more global-warming potential than carbon dioxide.

Feeling the heat, a number of scientists and startups are hoping to amp up solid-state cooling, which is currently used at a small scale for things like mini fridges, EV batteries, and some high-end gaming computers. Traditional ACs transfer heat by using a compressor and a fan to circulate a refrigerant and turn it from liquid to gas. Solid-state systems, on the other hand, move heat through conductive materials like gadolinium and bismuth telluride—which could theoretically cool spaces and surfaces with fewer messy side effects.

The catch is whether they can match the efficiency of conventional AC. "One of the key questions that remain is why are the solid-state coolers not as efficient as typical thermodynamic cycles?" says Pramod Reddy, a professor of mechanical engineering at the University of Michigan who studies heat transfer. Research and pilot programs are underway to test a range of approaches.

Brooklyn-based Mimic Systems uses thermo­electric cooling, which passes a current through semiconductive materials to shift heat from one side to another. Its room-scale climate control system is being piloted in an apartment in Vancouver. The German company Magnotherm is set to test its system, which relies on a magneto­caloric setup that transfers heat by magnetizing and demagnetizing materials, in a chain of supermarkets.

A team in Hong Kong has announced that its elastocaloric device, whose material heats and cools as it expands and contracts, can dip below 0 °C. And the UK’s Barocal is betting on barocaloric systems, which change temperature in response to shifts in pressure. But experts, especially in thermoelectrics, have doubts about how well any solid-­state scheme can compete.

For most modern HVAC systems, the coefficient of performance (COP) is 3, explains Jeff Snyder, a professor at Northwestern University who studies electrical and thermal conductivity. That essentially means the system moves three units of heat for every unit of energy that goes into it. Thermoelectrics in particular tend to have a much lower performance at high levels of temperature change, Snyder says, which means they’re best suited for niche uses such as cooling the back of a car seat.