Focus: Two Types of Cooling Require Different Designs
Thermoelectric devices can be used to remove heat from hot objects, like computer components or batteries. However, these so-called Peltier coolers are typically optimized for keeping a cold object cold, which is a different thermodynamics problem. Researchers have now developed a new design principle for thermoelectric devices that is tailored for computer and battery applications. As a demonstration, they constructed a cooling system with two types of metal-based thermoelectric materials. Their device reached a performance level about three times higher than traditional semiconductor-based Peltier coolers.
Commercial Peltier coolers are often small, portable refrigeration elements, such as beverage coolers or camping fridges. As in all refrigeration applications, the goal is to maintain a relatively cold temperature in an object whose surroundings are at room temperature. To accomplish this type of cooling, heat has to be drawn from cold to hot—opposite the natural direction. The situation is different for so-called “active cooling,” where the aim is to accelerate the natural heat flow from a hot object to relatively cool surroundings.
In designing thermoelectric cooling devices, engineers choose materials based on a figure of merit called zT. A high zT is desirable as it means that a lot of heat is carried by electric currents through the material. In addition, high zT materials have a small thermal conductivity, which helps prevent heat from flowing back through the device into the cold object. But when the object is hot, heat flows out naturally, and a large thermal conductivity is better. “When you are trying to drain away heat from a computer, zT is the wrong parameter to optimize,” says Joseph Heremans from Ohio State University in Columbus.
Heremans’ team considered a general cooling situation in which a hot object is placed in contact with a thermoelectric device surrounded by a relatively cold reservoir. A voltage is applied to the device, which causes heat-carrying charge carriers (electrons or holes) to flow from the hot side to the cold side. To optimize cooling, the researchers derived a new figure of merit that they call the effective thermal conductivity. This parameter is the sum of the normal (passive) thermal conductivity and an “active” thermal conductivity that only turns on when a voltage is applied.
The researchers then searched for materials that had large effective thermal conductivities. They hit upon two types of materials: magnon-drag metals and Kondo-effect metals. In magnon-drag metals, the conducting electrons interact with magnons, which are collective excitations of spin in magnetic materials. This “drag” interaction means that each electron carries with it some extra heat supplied by magnons. Similarly, in Kondo-effect metals, the electrons interact strongly with each other, which boosts the energy (or heat) associated with each conducting electron (or hole).
Using a magnon-drag metal (cobalt) and a Kondo-effect metal (cerium-palladium), Heremans’ team constructed a Peltier cooler and placed it between hot and cold reservoirs. They tested the device both in active mode and in passive mode (no current). With a current of 5 amperes, the device drained out roughly 100 milliwatts more heat from the hot reservoir than it did without a current. In terms of thermal conductivity, the passive mode registered 40 watts per meter-kelvin, whereas the active mode reached about 1000 watts per meter-kelvin, depending on the hot-cold temperature difference. Heremans says that such a “dual-mode” cooler could be advantageous for a CPU. When the number of computing processes is low, the cooler could work in passive mode, but it could switch to active whenever a burst of CPU activity occurs.
Geoff Wehmeyer, a mechanical engineering professor at Rice University in Texas, finds the new work intriguing. “Rather than using the thermoelectric material as a refrigerator," he says, "the authors show that they can use it as an unusually efficient heat spreader.” He says the change in perspective “flips the script” on the familiar rules for designing thermoelectric devices. And he expects the work to inspire more research attempting to optimize thermoelectric cooling systems.
This research is published in Physical Review Applied.
Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.