Communications of the ACM
New Horizons in Thermoelectric Cooling
Peltier-style thermoelectric modules rely on a current passing through a juncture of dissimilar materials to transfer thermal energy from one side to the other.
Credit: Stemulate.org
A discovery made in 1834 is providing the foundation for new devices for cooling devices, spaces, and people.
The coolers spring from the realization by French physicist Jean Peltier that passing a current through the junction of two dissimilar conductors will cause one to get colder, the other warmer. A little over 100 years later, Soviet physicist Abram Ioffe proposed using the principle to make a solid-state refrigerator.
By the early 2000s, thermoelectric coolers were well-established in certain market niches, thanks to their lack of moving parts, small size, and flexible form factors.
According to Mark Logan, vice president for business development at Wappingers Falls, NY-based manufacturer Solid State Cooling Systems, there are upwards of 60,000 thermoelectric-based coolers currently operating. However, he notes, their inefficiency at cooling larger volumes, compared to standard vapor-compression refrigerators, have limited their applications.
Several trends, both technological and political, have come together recently to expand the potential of thermoelectric cooling. One is the development of new materials and semiconductor technologies. Researchers at Rensselaer Polytechnic Institute, for example, have developed a faster and cheaper way of making the semiconductors required for thermoelectric cooling, while others at The Pennsylvania State University (Penn State) Materials Research Institute are working with new combinations of materials to push the temperature changes achievable to new levels.
Austin, TX-based semiconductor manufacturer Sheetak is having similar success with devices embodying a different structure altogether. "Most people make devices that are like a series connection of p-type and n-type elements," says Uttam Ghoshal, Sheetak CEO and president, who founded the company with funding from the U.S. Defense Department's Advanced Research Projects Agency-Energy (ARPA-E). "We took another look at the topology itself. We have made another type of layout, a 3D (three-dimensional) mix and match in a very different structure. We call it 'pi morph.'"
Image credit: quantumtunnel.wordpress.com.
According to Ghoshal, the result of the new layout is an increase of at least 25% in the temperature differential that can be achieved, as well as increased efficiency at lower differentials. "We have filed patent applications and will be selling the chips online starting in June," Ghoshal says. "The higher-performance chips will be sold to refrigerator OEMs."
In addition to improved device performance, Ghoshal cites advances in insulator technologies, such as aerogels, as contributing to the increase appeal of thermoelectric-based coolers. "Right now, if you compare them with traditional vapor compressors, traditional thermoelectrics are like a third of the efficiency, and we are about two-thirds."
Because the thermoelectric devices are cheaper than compressors, manufacturers can invest in a thermoelectric system and spend the money saved on advanced insulation, resulting in a refrigerator with the same Energy Star energy efficiency rating and the same selling price as a standard refrigerator.
Environmental concerns also make solid-state coolers more appealing. Air conditioning and refrigeration both mostly rely on vapor-compression technology, which not only require a great deal of power, but use refrigerants that contain or produce greenhouse gases.
The pressure to come up with greenhouse-gas-free cooling technology is leading multinational refrigerator manufacturers to look for alternative solutions, says Ghoshal, "And when it comes to small-scale refrigerators, there's not much other than thermoelectrics."
"If you look in Europe or Japan right now, there is no question that this is a very big deal," says Logan. "It was growing in the U.S., but with the change of administration, we have seen less interest in that topic in the past year."
A Growing Market
Solid State Cooling Systems manufactures thermal management solutions for laboratory, medical, and other scientific applications. "All the applications we serve need some sort of precise temperature control," explains vice president of sales Kevin Carswell, "and we typically control to plus or minus 0.05° C or better." In a typical configuration, the target device is mounted to a plate which is cooled by fluid kept at a constant temperature by their system.
Carswell and Logan say the use of thermoelectrics in scientific devices is growing. One example is the low-light charge-coupled device (CCD) cameras used as single-photon counters for astonomy; Logan says scientists will "run the CCD array at maybe -100° C" to reduce the need to count background photons.
In addition to precision, Logan says, the company's devices also provide significant energy savings. "When we go into the semiconductor market, we'll save about 80% to 85% of the electricity of compressor-based systems for the same cooling, because the thermoelectrics use only the power required to get the cooling. We can respond within seconds to a change in heat load, and when they go to idle, we ramp right down."
Ghoshal says the new pi morph devices are enabling Sheetak to explore previously infeasible applications. "So far thermoelectrics have been relegated to small systems that can keep cold things cold pretty well, but now we're talking about deep freezing to -70° C. We weren't able to do this until about two years ago."
What about cooling people? Can thermoelectric devices reduce some of the need for air conditioning as well? The Climate Ribbon from Tempronics is intended to do just that. Tempronics was founded in 2007 to develop technology for maintaining a nanometer of separation between the two sides of a Peltier module, which would inhibit the "backflow" in which the warmed side contributes heat back to the cooled side—part of the reason thermoelectric devices can be inefficient. While that research was ongoing, the company started to look for ways to monetize thermoelectrics.
"Whether you have one or 1,000 of the chips, they have the same efficiency. We developed a way of spreading them out over a large area, and on soft surfaces they can flex," says Tempronics founder and chief technology officer Tarek Makansi. "We found that the old thermoelectric chips that have been around for 50 years are very inexpensive and would actually work extremely well in this new architecture." The company has just signed an agreement with Lear Corporation, a global supplier of automotive seating, and is looking for other OEMs to incorporate its technology. (The devices can be used to warm as well as cool, by reversing the current.)
Because the Tempronics cushions work by conduction rather than convection—cooling a person through contact with the skin rather than cooling the air around them—they are more efficient. "If you want to air-condition a room to 75° when it's hot out, the air coming out of the vents needs to be around 55°," Makansi explains. "But we can cool the skin down by 20°, and it feels cold. We can cool a person well by using 60 watts of power." Distributing the chips over a wide area also helps with the backflow issue, since the heat isn't concentrated and channels in the cushion allow airflow to carry the heat away.
"We're still not as efficient as compressors for very large cooling applications," Makansi acknowledges. "You won't see us cooling batteries in Tesla cars, or datacenters in large rooms."
Jake Widman is a San Francisco, CA-based freelance writer focusing on connected devices and other Smart Home and Smart City technologies.
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