Chip-scale Cooling Devices Based On Electrocaloric Effect

Open Access
Author:
Gu, Haiming
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 08, 2014
Committee Members:
  • Qiming Zhang, Dissertation Advisor
  • Qiming Zhang, Committee Chair
  • Jerzy Ruzyllo, Committee Member
  • Zhiwen Liu, Committee Member
  • Brent Craven, Committee Member
Keywords:
  • Electrocaloric Effect
  • Cooling Device
  • Chip Cooling
  • Chip refrigerator
Abstract:
The electrocaloric effect (ECE) is a temperature and entropy change of an insulating material when there is an electric field induced polarization change. Recently, giant ECE was discovered in a series of dielectric materials, such as Polyvinylidene fluoride (PVDF) based polymers and ferroelectric ceramics. The ECE induced adiabatic temperature change in these materials can be higher than 10 K which makes them attractive because they provide the unique opportunity to explore and develop solid state cooling devices which are compact while providing high cooling power density and high efficiency compare to other cooling techniques, such as vapor compression refrigeration(VCR), thermoelectric, thermoacoustic, etc. A few ECE refrigerators have been studied in previous publications. However, the performance of those devices are still not so attractive. Besides, most of the existing works are based on simulation, and very few work has prototype demonstration. In this thesis, two types of chip scale ECE based cooling devices with more advanced configurations have been proposed, studied, and experimentally demonstrated. One is named as ElectroCaloric Oscillatory Refrigerator (ECOR), which involves solid-state plates with anisotropic thermal conductivity as regenerators. The low thermal conductivity along the temperature gradient direction can reduce the heat conduction loss thus highly improve the performance of the device. Finite volume simulation shows that the temperature span (Tspan) of the refrigerator can be as large as around 50 K. The volumetric cooling power density Pc and coefficient of performance (COP) of the refrigerator was also studied. The maximum Pc can reach to 9.2 W/cm3, while the COP remains at 6.5, which is about 45% of the Carnot efficiency. A prototype of ECOR was experimentally demonstrated. The EC module with multilayer structure was fabricated with high energy electro irradiated poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer thin film. The regenerators with anisotropic thermal conductivity was realized by cutting thin slits in stainless steel plate and then refilled with epoxy. The testing results show that a maximum value of 6.6 K Tspan was obtained when the device was working at 1 Hz and with an electric field of 100 MV/m. The second proposed device configuration is based on a rotary structure. This refrigerator involves no external regenerators. Instead, the regeneration process is realized by direct heat exchange between EC materials. The 1-D simulation results show that the Tspan of the device is nearly proportional to the number of segments in one ring (Ns), and the ECE induced adiabatic temperature change (Delta TECE). Owing to the absence of external regenerators, the Pc of this device can reach 33 W/cm3. Meanwhile, the COP of the device keeps above 8.5, which is about 60% of Carnot efficiency. 3-D finite element simulation validates the 1-D simulation results. A preliminary prototype was fabricated to demonstrate the proposed concept. About 0.8 K Tspan was obtained across the device when Delta TECE is 0.4 K. Compare with the existing ECE cooling devices, the ECOR device in this thesis involves solid state regenerator with anisotropic thermal conductivity which can highly reduce the conduction heat loss and thus improve the performance of the device. In the rotary cooling device, novel regeneration process is designed so that no external regenerators are required which also highly improve the cooling power density and efficiency of the device. The ideas of both solid state regenerator with anisotropic thermal conductivity, and the novel regeneration process are the main contributions of this thesis and will stimulate the development of high performance ECE cooling devices in the future.