The Future of Cool

Today’s refrigeration technology reliably keeps your holiday leftovers in the safety zone and stops the summer swelter at the front door, but its potential effect on Earth’s climate—not to mention your electric bill—can leave you hot under the collar.

In a development that may open the door to the next big step in refrigeration and air-conditioning technology, scientists at the University of Maryland have developed a 3D-printed cooling material that is highly efficient and eco-friendly and sidesteps durability issues previous researchers have faced. Their study was published last week in Science.

Although vapor compression cooling has dominated the multi-billion dollar refrigeration and HVAC markets for over 150 years, it has plateaued where efficiency is concerned, while using chemical refrigerants with high global-warming potential.

Meanwhile, an alternative known as solid-state elastocaloric cooling has been under development for the last decade and is a front-runner to replace vapor compression cooling. The new technology works by applying mechanical stress—squeezing, for instance—to materials to cause them to absorb and release latent heat. That allows cooling without harmful chemicals and requiring potentially much less power.

The problem, however, is that the repeated stress on the elastocaloric materials (often shape-memory alloys) can cause material fatigue and eventual failure.

To that end, an international team of collaborators led by Ichiro Takeuchi, a professor of materials science and engineering at Maryland, has developed a more durable elastocaloric cooling material using a blend of nickel and titanium, forged using a 3D printer, that is potentially more efficient than current technology. It’s also completely “green” and can be quickly scaled up for use in commercial devices.

Takeuchi has been developing the technology for a decade and received the UMD Outstanding Invention of the Year for this research in 2010.

“The key to this innovation that is fundamental, but not often discussed, is that materials fatigue—they wear out,” said Takeuchi. “This is a problem when people expect their refrigerators to last for a decade, or longer. So, we addressed the problem in our study.”

The team tested its creation heavily over a four-month period, and it still maintained its integrity.

“Some known elastocaloric materials start showing degradation in cooling behavior after just hundreds of cycles. To our surprise, the new material we synthesized showed no change after 1 million cycles,” said Huilong Hou, a postdoctoral researcher and the first author of the work.

The international team included scientists at the U.S. Department of Energy Ames Laboratory, where 3D printing was carried out, and researchers from the Colorado School of Mines, who helped investigate the internal structure of the printed materials.

“In this field of alternative cooling technologies, it’s very important to work on both the materials end, as well as the systems end; we are fortunate to have a highly qualified team of experts at UMD College Park to work on both ends,” Takeuchi said. “It’s only when these two efforts closely align that you make rapid progress, which our team was able to do.”