In Russia just after the Second World War, in what is now St. Petersburg, a physicist named Abram Ioffe pointed out that thermoelectric materials could be used to make a solid state refrigerator with no moving parts. This ignited a worldwide research frenzy, according to Penn State theoretical physicist Jerry Mahan.
Major labs around the world, including General Electric, Philips in Holland, Westinghouse, Bell, RCA, and IBM, all got busy and measured every known material. Although there were many niche applications, for instance for small, portable coolers or for silent-running refrigeration aboard the space station, those researchers never found a material efficient enough at cooling to replace the refrigerator in your kitchen, Mahan said. The most popular thermoelectric material, bismuth telluride, has been used commercially for 60 years in applications that required reliability and compact size more than efficiency. It’s still used today.
It was not until the 1990s that funding agencies again began to look into solid state refrigeration. The renewed interest arose from new combinations of materials made out of as many as three, four, or even five different elements, widening the search for better electrocaloric materials. In 2004, based on thermodynamic theory and molecular structure consideration, Penn State electrical engineer Qiming Zhang predicted a temperate change of more than 15 K in polymer films, at temperatures near 100 degree C. In 2006, a thin-film perovskite material was shown to produce a temperature change of 10 K, about 10X higher than previous materials, but at temperatures above 200 degrees C.
Theorists, including Mahan, have doubts that an electrocaloric device will ever be able to equal the efficiency of the household refrigerator that relies on the compression of gases, but experimentalists continue to look for (and find) better and better materials. One of those leading the hunt is Penn State’s Qiming Zhang, recognized as one of the world leaders in electrocaloric materials.
“I would have to disagree with the theorists,” Zhang remarks. “When you compare the mechanical loss from a typical refrigerator, an electrical field is so much easier to apply than mechanical force. You have to use electricity to drive the motor and the motor compresses the air. In solid state refrigeration, you apply an electric field directly to the refrigerant and change the temperature. You eliminate the middle stage.”
Plus, he says, you can make use of something called the regeneration process. When the refrigerant changes from a hot to a cold temperature it will release heat. When the opposite effect occurs and the refrigerant changes from cold to hot, the released heat can be used to heat another process, which will save energy.
In 2008, Zhang showed that a large electrocaloric effect could be realized at room temperature in a ferroic polymer with a temperature change of 12 degrees C. Ferroic polymers are ones that show a spontaneous electrical polarization.
In ferroic materials there are units of charge called dipoles that are linked to the lattice structure of the material. A change to the lattice can create an electric charge and the addition of charge can change the structure of the material. This is the basis of piezoelectricity, the familiar property used in ultrasound imaging. Some materials also have a spontaneous electrical polarization based on changes of temperature. This is the pyroelectric effect, used in night vision glasses
“In a good electrocaloric material you have a lot of random dipoles (molecules with separate positive and negative charge). When you add an electric field, the dipoles line up. You can use this process to cool or heat things,” says Zhang. “It is something like what happens when fluid becomes solid. It goes from a random structure to a crystal structure.”
For a long time, people didn’t pay much attention to the disordered state of the dipoles but instead were looking for materials that had high long-range polarity. It turns out that a very disordered material that can be induced into order quickly is a key to a large electrocaloric effect. Another way to put it is that the material should have a low dielectric constant; that is, a low capacity to hold opposite charges. Polymers have a lower dielectric constant than ceramics, such as barium tatinate. They also have a higher breakdown strength than ceramics.
“The same concept can be applied to looking for ceramics that could be good electrocaloric materials,” Zhang offers. “By combining those two things, a low dielectric constant and a high breakdown field, you can modify the ceramics to make good electrocalorics.”
Zhang’s own research on electrocalorics covers three fields – polymers, ceramics, and a combination of the two called ceramic-polymer composites that can combine the best features of the other two types of material. Right now the biggest advantage to ceramics is that there are multilayer ceramic manufacturing technologies available that are well understood and inexpensive. For that reason, Zhang expects to see ceramic electrocaloric devices reach the marketplace first, though he believes it will take time and resources to reach a point where solid state refrigeration replaces today’s vapor compression cooling.
“A lot of people still question if this will work out. The giant magnetocaloric effect (involving magnetic materials) was discovered in the 1990s, and a few years ago, they have demonstrated a working device. For electrocaloric cooling, there is still no working device. Right now, we just want to make a simple working electrocaloric cooling device we can set on a table and run with a battery or power from a wall outlet.”
Zhang compares the current state of electrocaloric devices to the early stages of liquid crystal displays, which were developed in the 1980s. At first the liquid crystals were slow and only useful for handheld calculators. When they were first tried on television screens, colors disappeared if you shifted your viewing angle. Now, huge flat screen panels using liquid crystals are common. Electrocaloric materials are like that, he believes.
“If we can design a material properly, the way they did with liquid crystals, I think we would have something. From the basic material and device point of view, I think that electrocaloric cooling is the future.”
If Zhang is right, it will be because solid state refrigeration requires no compressor that can be bulky and heavy, and no greenhouse gases that can leak into the atmosphere. If they could replace all air conditioning and refrigeration, they would be tapping into a 100 billion dollar industry. For now, he is envisioning compact personal air conditioners that would be useful if they could change the temperature of the surrounding by 10 degrees or less. Another idea, which he is discussing with the National Renewable Energy Laboratory, is to use compact solid state air conditioners for use in the sleeping cabs of long-distance trucks so that they can turn off their engines when they sleep, thereby saving huge amounts of diesel fuel.
“Once we demonstrate something that’s useful, we can capture the interest of many companies,” Zhang says. “The overall consensus of the electrocaloric research community is that we really need to put a device on the table and let people put their hands on it to feel significant cooling power.”
This winter (before December), Zhang will received funding of just under $3 million from DARPA, the agency for futuristic devices for the military, to fund a project to integrate electrocaloric materials with thermoelectric materials for a hybrid solid state cooling system.
“Our goal is to produce a product in two to three years,” he says optimistically.
Contact Qiming Zhang, Distinguished Professor of electrical engineering and professor of materials science and engineering at firstname.lastname@example.org
The electrocaloric effect is a coupling of electrical and thermal properties that results in a temperature change in response to an externally applied electric field.