Improving Heat Transfer of Radiative Heat Sink-Integrated Electrocaloric Cooling

Abstract

Although vapor-compression refrigeration (VCR) has achieved high efficiency, reliability, and relatively compact size, a global consensus demands the development of novel cooling technology beyond the VCR. Hydrofluorocarbons (HFCs), which are mainly used as refrigerants for the VCR, promote global warming typically thousands of times more than carbon dioxide (CO2). In recent years, solid-state refrigeration methods utilizing the caloric effect, such as magnetocaloric and electrocaloric effects, as well as passive radiative cooling, have emerged as promising solutions to address this issue. These approaches offer advantages such as the absence of compressors and vapor refrigerants. Among them, electrocaloric (EC) cooling shows a high coefficient of performance (COP), low power consumption, and direct usage of electricity. EC cooling utilizes an adiabatic temperature change (ΔT) of the refrigerant induced by an external electric field. The maximum cooling performance of this method is restricted by the temperature change of the refrigerant. In previous work, to overcome the limitation, multiple heat exchanges based on the cascade structure, multilayer capacitor, and active regenerator are used to maintain the heat sink cool down. However, the complex structure of these approaches leads to thermal losses at device interfaces and requires significant costs. Passive radiative cooling, which exploits the radiative heat exchange between Earth (~ 300 K) and cold outer space (~ 3 K) through the atmospheric window (8-13 μm), enables an object to maintain its temperature below ambient temperature without external energy consumption. Conventional radiative coolers consist of a porous structure to achieve high radiative cooing performance by strongly scattering the solar spectrum. However, the low thermal conductivity of the porous hinders the dissipation of large heat fluxes. Consequently, although these systems exhibit high cooling performance as passive radiative coolers, they are not suitable for heat sink applications. In this study, we propose a radiative heat sink-integrated electrocaloric cooling (R-iEC) system to improve heat transfer and overcome the limitations within a unit heat exchange cycle. By incorporating the thermally conductive radiative cooler (TCRC) as a heat sink on the EC cooling device, the heat sink maintains the temperature below the ambient air temperature even under the large heat flux introduced by the EC effect. The TCRC, which consists of boron nitride sheet (BNNS) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) matrix, which exhibits high through-plane thermal conductivity of 1.41 W/mK. This thermal conductivity is 13 times higher than that of the pure PVDF-HFP matrix (0.12 W/mK). Additionally, the TCRC demonstrates high reflectance in the sunlight (~ 99 %) and high emittance in the atmospheric window (~ 95 %). The EC refrigerant is composed of a composite film of Poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) and Poly (vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (PVDF-TrFE-CFE). Under an electric field of 50 MV/m, the EC refrigerant achieves an adiabatic temperature change of 3 K. The EC refrigerant absorbs heat from heat sink and ejects heat through the heat sink by fluctuating from the heat source to the heat sink through electrostatic actuation. Consequently, the temperature span of the EC device is 1.6 K at the switching frequency of 0.2 Hz. To demonstrate the enhanced heat transfer and cooling performance of the R-iEC cooling system, a comparison is conducted among the R-iEC system, (i) a metal heat sink, (ii) EC device, and (iii) radiative cooler. As a result of outdoor field test, the R-iEC cooling system exhibits the largest cooling heat flux (~ 87 W/m2) and cooling temperature (7.3 K, below ambient air temperature) under direct sunlight.