Several years ago, researchers at the Polytechnic University of Catalonia and the University of Cambridge performed a series of simple experiments that could have huge implications for cooling and refrigeration.
They placed plastic crystals of neopentyl glycol—a common chemical used to produce paints and lubricants—into a chamber, added oil, and cranked down a piston. As the fluid compressed and applied pressure, the temperature of the crystals rose by around 40 ˚C.
It was the largest temperature shift ever recorded from placing materials under pressure, at least when the findings were published in a Nature Communications paper last year. And alleviating the pressure has the opposite effect, cooling the crystals dramatically.
The research team said the results highlight a promising approach to replacing traditional refrigerants, potentially delivering “environmentally friendly cooling without compromising performance.” Such advances are crucial, since increasing wealth, growing populations, and rising temperatures could triple energy demands from indoor cooling by 2050 without major technological improvements, the International Energy Agency projects.
The temperature change in the materials was comparable to those that occur in the hydrofluorocarbons that drive cooling in standard air conditioning systems and refrigerators. Hydrofluorocarbons, however, are powerful greenhouse gases.
The work is based on a long-known phenomenon, familiar if you’ve ever stretched a balloon and touched it to your lips, in which so-called caloric materials release heat when placed under pressure or stressed. Submitting certain materials to magnetic and electrical fields, or some combination of these forces, also does the trick in some cases.
Scientists have been developing magnetic refrigerators based on these principles for decades, though they tend to require large, powerful, and expensive magnets. But researchers are making considerable strides in the field, according to a review paper in Science on Thursday, written by Xavier Moya and N.D. Mathur, materials scientists at the University of Cambridge who worked on the experiments described above.
Research teams are pinpointing numerous caloric materials that undergo large temperature shifts and putting them to work in prototype heating and cooling devices, the authors note. Materials and devices that can release and transfer large amounts of heat using electricity, strain, and pressure—approaches that only really took off starting a little more than a decade ago—are already catching up with the performance achieved through decades of work in magnet-based cooling devices.
In addition to reducing the need for hydrofluorocarbons, the hope is the technology could eventually be more energy efficient than standard cooling devices, given the heat released relative to the amount of energy needed to drive the change. A critical difference with this technology is that the materials remain in a solid state, while traditional refrigerants, like hydroflurocarbons, work by shifting between gas and liquid phases.
Triggering a phase change
Here’s how the technology works:
Many materials exhibit small temperature changes under certain forces. But researchers have been hunting for materials that undergo large shifts, ideally from as little added energy as possible. Among other materials, certain metal alloys have shown promising results under strain; some ceramics and polymers respond well to electrical fields; and inorganic salts and rubber look promising for pressure.
The forces or fields line up the atoms or molecules within the materials in more orderly ways, bringing about a phase change similar to what occurs when free-flowing water molecules turn into compact ice crystals. (In the case of caloric materials, however, the phase change occurs while the materials remain in a solid state, though one that is more rigid.) This process releases enough latent heat to account for the energy difference between the two states. When the materials revert back as the forces are released, it produces a temperature decrease that can then be exploited for cooling.
This isn’t very different from how cooling devices work today: they decompress hydrofluorocarbons to the point that they switch from a liquid to a gas. But this solid-state cooling approach can be far more energy efficient, at least in part because you don’t have to move the molecules nearly as far to bring about the phase change, says Jun Cui, a senior scientist with Ames Laboratory.
Moving into the market
The key to delivering competitive commercial devices is identifying affordable materials that undergo large temperature shifts, easily revert back, withstand extended cycles of these changes without breaking down (commercial refrigerators can run for millions of cycles), and aren’t expensive.
Certain materials and use cases are getting close to reaching the commercial market, says Ichiro Takeuchi, a materials scientist at the University of Maryland. He launched a company to produce cooling devices from materials that respond to stress about a decade ago, called Maryland Energy & Sensor Technology.
His research group developed a prototype cooling device that compresses and releases tubes made from nickel titanium to induce heating and cooling. Water running through the tubes absorbs and dissipates heat during the initial phase, and the process then runs in reverse to chill water that can be used to cool a container or living space.
The company plans to produce a wine cooler, which doesn’t require the same cooling power as a large refrigerator or window AC unit, as an initial product, using an unspecified but less expensive material.
Moya, one of the authors of the Science paper, cofounded his own startup about a year and half ago. Barocal, based in Cambridge, England, has developed a prototype heat pump relying on plastic crystals that are “related to neopentyl glycol but better,” he says.
All told, a dozen or so startups have been formed to commercialize the technology, and a number of existing companies, including Chinese home appliances giant Haier and Astronautics Corporation of America, have explored its potential as well.
Cui expects we’ll see some of the first commercial products based on materials that change temperature in response to force and stress within the next five to 10 years, but he says it will likely take years longer for prices to become competitive with standard cooling products.
Update: This story was updated the clarify the timing of the neopentyl glycol experiments.