Solenoid Actuating Electrocaloric Cooling Device with Relaxor Ferroelectric Polymer

Solid-state cooling based on caloric effects, such as the magnetocaloric, elastocaloric, and electrocaloric effects, has been developed to substitute for conventional vapor compression-based cooling using the greenhouse gas as a refrigerant. Among the various solid-state cooling systems, in recent years, electrocaloric (EC) cooling has gained considerable attention because of its high energy efficiency, low cost, direct operation of electricity, and feasibility of application at mass production. The EC effect leads to an adiabatic temperature change resulting from sudden entropy change of dipole polarization induced by an external electric field in a dielectric material. Through recent studies, it is discovered that ferroelectric polymers containing poly(vinylidene fluoride) (PVDF) show a large EC effect. The implementation of an EC cooling system using the polymer has challenging issues: pumping and transferring the heat between a heat source and a heat sink. It is needed that an actuator unit can move the EC material from the heat source to the heat sink. Various actuators or heat exchange units are suggested in previous research, however, they require high voltage and complex structures.<br/>In this work, the [P(VDF-TrFE-CFE)] is used as an EC material with a solenoid actuator. The terpolymer film (EC film) is fabricated through a solution process and annealed to enhance crystallinity. The enhanced crystallinity is confirmed by XRD analysis, and the thickness of the film observed by the SEM image is 40 nm. After, single-walled carbon nanotubes (CNT) are coated onto the top and bottom of the EC film as an electrode layer to apply the electric field. The induced adiabatic temperature change of the EC material is ~ 2.2 K with the applied electric field of 50 MV/m. To ensure conformal thermal contact and reduce a loss of heat between the EC film and heat source/sink, the film is sandwiched in a thin metal plate with thermal grease. The formed metal/EC film/metal (MEM) structure, EC stack, serves as a solid-state refrigerant.<br/>The solenoid actuation mechanism is controlled by a small voltage, (~10 V). The push operation requires the control voltage and the pull operation doesn’t require the control voltage, only a return spring is required. The small voltage can oscillate the EC stack with no degradation, such as parasitic heating, due to the weight of the EC stack being much less than the solenoid's maximum force. For a successful cooling operation, the actuation cycle of the EC stack must be correlated with the EC effect cycle. In other words, the cycle is as follows: (i) Solenoid actuator pushes the EC stack toward heat sink. (ii) The applied electric field induces the EC heating of the EC stack. (iii) The heat transfer occurs from the EC stack to the heat sink until the thermal equilibrium. (iv) Solenoid actuator pulls the EC stack toward the heat source. (v) The removed electric field induces the EC cooling of the EC stack. (vi) The heat transfer occurs from the heat source to the EC stack until the thermal equilibrium. These six steps repeat continuously. As both solenoid actuation and EC effect are controlled by the square form electric signal, it is convenient to match the actuation cycle with the EC effect cycle. Consequently, through the combining of EC stack and solenoid actuator, the maximum temperature lift of 1.5 K is achieved under 50 MV/m with the no-load condition. Finally using this solenoid actuation, we expect that actuating with electrically stable, sustainable, flexible to structural limitation by the tunning of the stroke length is possible.