A surprising set of materials could soon help make more efficient air conditioners that don’t overtax the electrical grid on hot days.
As extreme heat continues to shatter records around the globe, electricity demand for air conditioning is expected to triple in the next few decades—an increase of about 4,000 terawatt-hours between 2016 and 2050, according to the International Energy Agency, or roughly the same electricity demand as the entire US electrical grid in 2022.
That’s why the race to build more efficient air conditioners has become increasingly urgent. While some companies are focused on improving existing designs, others are looking to entirely new systems that use materials called desiccants. These systems could cool more efficiently, even in extreme heat and humidity, reducing stress on the grid.
A typical air conditioner cools indoor spaces by pumping a refrigerant around in a cycle and through heat exchangers, soaking up heat from the inside air and releasing it outside. (Heat pumps work in the same way, either running the opposite direction, or in reversible systems that can both heat and cool.)
That approach, called vapor compression, is over 100 years old, and the basic design hasn’t changed much since its invention, says Ankit Kalanki, a manager in the carbon-free buildings program at the Rocky Mountain Institute, a nonprofit energy think tank. Pumping refrigerant around and compressing it enough to shuttle heat outside requires a lot of energy, especially when temperatures are very high.
Vapor compression systems also deal with humidity and heat together, which is another drawback. Keeping a building comfortable has a lot to do with maintaining a low humidity environment, Kalanki says, but air conditioners must cool down air to pull moisture out of it. Without a designated system to tackle humidity, he says, buildings are often “over-cooled,” which can add a huge energy burden.
Systems that tackle dehumidification and cooling separately could keep building temperatures comfortable with less energy and allow for more flexibility in different environments. And a growing number of startups are looking to desiccants to accomplish just that.
Hot take
Desiccants are materials that suck up moisture. The silica beads in those little packets that accompany new purses and shoes are a type of desiccant, designed to keep products dry as they’re shipped around the world.
Other types of desiccants could be added to existing designs for air conditioners, absorbing water from the air and cutting down on the energy required to keep rooms comfortable. Transaera, an MIT spinout founded in 2018, is developing a hybrid system that uses a type of material called metal organic frameworks. Adding the materials to vapor compression-based air conditioners could allow the company’s system to use 35% less energy than average models, according to Transaera CEO Sorin Grama.
But other companies are looking to use desiccants in cooling systems that would replace traditional air conditioners altogether. Florida-based startup Blue Frontier, for example, is using liquid desiccants to build cooling systems. The key ingredient is different from the silica beads in shoe packaging, but the comparison is a common one—“We get that a lot,” says Matt Tilghman, the company’s co-founder and chief technology officer.
Rather than small silica beads, Blue Frontier’s cooling technology relies on a salt solution that’s so concentrated, it can pull moisture from the air.
Here’s how Blue Frontier’s cooling system works: first, a stream of air passes through a channel and over a thin layer of desiccant, which pulls moisture out of the air. Next, the now-dry air goes through an evaporative cooling step, which lowers the temperature of the air (basically the same way sweat cools your skin).
In the evaporative cooling step, the air is split into two streams. One runs past a thin layer of water, which absorbs energy and drops the air’s temperature. That cooler, humid air is used to cool a metal surface, which in turn sucks heat out of the other stream of still-dry air. The humid air gets funneled outside, and the cool, dry air is blown into the building.
Evaporation is an efficient cooling method, one that’s employed in low-cost devices called air coolers (also referred to as swamp coolers or evaporative coolers), which can use 80% less electricity than standard air conditioners. These devices usually add moisture to the air to cool it, which only works when starting with dry air, so their use is typically limited to dry environments, like the southwestern US.
By pairing evaporative cooling with desiccants, Blue Frontier’s system can work in virtually any climate, Tilghman says. Its operations can be tweaked to handle changes in the weather or in the thermostat set point, altering the balance between cooling and dehumidifying, which could help unlock further efficiency gains. The company’s approach should be able to cut annual electricity use by a total of between 50% and 80% compared with a conventional air conditioning system, depending on the environment, Tilghman says.
