Improving Heat Transfer of Radiative Heat Sink-Integrated Electrocaloric Cooling

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. The use of hydrofluorocarbons (HFCs) as refrigerants in VCR contributes significantly to global warming, thousands of times more than carbon dioxide (CO₂) [1]. 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. Among them, electrocaloric (EC) cooling shows a high coefficient of performance (COP), low power consumption, and direct usage of electricity [2]. EC cooling utilizes an adiabatic temperature change (<i>ΔT</i>) of the refrigerant induced by an external electric field. However, the maximum cooling performance of EC cooling is restricted by the temperature change of the refrigerant. Previous attempts to overcome this limitation and maintain the heat sink cool involve introducing multiple heat exchanges [3]. Nonetheless, the complex structure leads to thermal losses at device interfaces and significant costs. On the other hand, passive radiative cooling exploits the radiative heat exchange between Earth (~ 300 K) and cold outer space (~ 3 K) through the atmospheric window (8-13 μm), enabling an object to maintain a 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 (0.28-2.5 μm) [4]. However, the low thermal conductivity of the porous structure hinders the dissipation of large heat fluxes, making them unsuitable for heat sink applications.<br/><br/>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 its temperature below the ambient air temperature even under the large heat flux introduced by the EC effect. The TCRC, composed of boron nitride sheet (BNNS) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) matrix, exhibits high thermal conductivity of 1.41 W/mK in the through-plane direction. 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 visible range (~ 99%) and high emittance in the atmospheric window (~ 95%). 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 system, a comparison is conducted among the R-iEC system, (i) a metal heat sink, (ii) EC device, and (iii) radiative cooler. While the metal heat sink and EC device experience heating from direct sunlight, the R-iEC system demonstrates an outstanding cooling heat flux of ~ 87 W/m² and cooling temperature of 7.3 K, overwhelming other samples.<br/><br/>[1] Shi, J. et al. Electrocaloric cooling materials and devices for zero-global-warming-potential, high-efficiency refrigeration. <i>Joule</i>, <b>3</b>, 1200-1225 (2019)<br/>[2] Ma, R. et al. Highly efficient electrocaloric cooling with electrostatic actuation. <i>Science</i>, <b>357</b>, 1130-1134 (2017)<br/>[3] Meng, Y. et al. A cascade electrocaloric cooling device for large temperature lift. <i>Nat. Energy</i>, <b>5</b>, 996-1002 (2020)<br/>[4] Mandal, J. et al. Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling. <i>Science</i>, <b>362</b>, 315-319 (2018)