Electrocaloric polymer serves as pocket-size cooling device
A 6-mm-thick flexible band-aid-sized device can cool a battery by 8°C. The device, reported in a recent issue of Science, could lead to personal coolers people could carry in their pockets during exercise, energy-efficient cooling for mobile and wearable electronics, and panels that can cool individuals instead of the whole house to save energy, the technology’s developers say.
Currently air-conditioners and refrigerators rely on the compression of liquid or vapor refrigerants. These systems consume a lot of energy and are bulky and complex. Solid-state cooling systems based on thermoelectric materials such as bismuth antimony telluride ceramics are a more compact alternative, but these materials are expensive and require a fairly large amount of electrical energy to remove heat.
More recently, researchers have sought to build efficient, compact cooling devices based on the electrocaloric effect, observed in certain dielectric materials. Applying an electric field lines up the material’s charged molecules, increasing its temperature. The material cools when the field is removed. The electrocaloric effect has been proposed recently as a way to make efficient, compact coolers.
Poly(vinylidene fluoride) (PVDF)–based ferroelectric polymers and ceramics such as lead zirconium titanate are well-known electrocaloric materials. But practical cooling devices have been hard to make using these because they require heat to be carried from the cold end (heat source) to the hot end (heat sink). This can be done by using an electric motor or by pumping fluids, both of which increase the size and complexity of the system while reducing efficiency. “There is no solid-state cooling technology that works well and efficiently today,” says Qibing Pei, materials science and engineering professor at University of California, Los Angeles.
Pei and his colleagues have come up with a design that uses electrostatic force to rapidly move a thin polymer film between the hot and cool ends. They chose the electrocaloric polymer poly(vinylidene fluoride- ter-trifluoroethylene-ter-chlorofluoroethylene) because it shows a large temperature change of 12°C.
The 7 cm × 3 cm × 0.6 cm device is simple in design. The researchers put a carbon nanotube film between two polymer films and coated the top and bottom of the stack with carbon nanotube electrodes. Then they sandwiched this stack—with 6-mm-thick spacers at each end—so that it is suspended diagonally between polyimide sheets coated with conductive silver nanowire films.
Electric field applied between the silver nanowire and carbon nanotube electrodes creates an electrostatic force that pulls the polymer stack toward the silver nanowires. To operate the device, an electric field is applied alternately between the top and bottom electrodes. At the top electrode (heat sink) a voltage is applied to the polymer stack so it gets hot and dumps heat to the sink. The voltage is cut at the bottom electrode (heat source) so the polymer cools and absorbs heat from the source.
“The [electrocaloric] polymer is both the refrigerant, which generates temperature change, and the pump, which efficiently transfers the refrigerant between the cold and hot ends,” write Qiming Zhang and Tian Zhang of The Pennsylvania State University (Penn State) in a perspective article accompanying the report.
The device could cool a smartphone battery by more than 8°C within seconds. The researchers are now working on stacking multiple cooling devices that would allow further cooling.
Because they are compact, low-noise, and efficient, electrocaloric coolers could open novel applications, the Penn State researchers write. “One could imagine wearable cooling bandages that replace ice bags for injury treatment and cooling of biologic tissues and organs; compact air conditioners and thermal management systems placed on office desks or integrated into chairs for localized climate control; and electronic devices such as labs-on-a-chip.”
Read the abstract in Science.