Radiative Coupled Evaporation Cooling Hydrogel for Above-Ambient Heat Dissipation and Flame Retardancy

Abstract

By combining the merits of radiative cooling (RC) and evaporation cooling (EC), radiative coupled evaporative cooling (REC) has attracted considerable attention for sub-ambient cooling purposes. However, for outdoor devices, the interior heating power would increase the working temperature and fire risk, which would suppress their above-ambient heat dissipation capabilities and passive water cycle properties. In this work, we introduced a REC design based on an all-in-one photonic hydrogel for above-ambient heat dissipation and flame retardancy. Unlike conventional design RC film for heat dissipation with limited cooling power and fire risk, REC hydrogel can greatly improve the heat dissipation performance in the daytime with a high workload, indicating a 12.0 °C lower temperature than the RC film under the same conditions in the outdoor experiment. In the nighttime with a low workload, RC-assisted adsorption can improve atmospheric water harvesting to ensure EC in the daytime. In addition, our REC hydrogel significantly enhanced flame retardancy by absorbing heat without a corresponding temperature rise, thus mitigating fire risks. Thus, our design shows a promising solution for the thermal management of outdoor devices, delivering outstanding performance in both heat dissipation and flame retardancy.

1. Introduction

With the increasing power density in electronic devices, efficient heat dissipation has become a critical concern, as operating temperature significantly influences device performance [1, 2], particularly in outdoor equipment exposed to sunlight, such as base stations, transformers, and electronic billboards. Traditional cooling strategies, such as air-conditioners or fans, consume substantial amounts of electricity and cause refrigerant emissions [3, 4]. Additionally, these conventional approaches often result in elevated working temperatures under direct sunlight, especially during peak daylight hours [5, 6]. Consequently, driven by growing awareness of energy efficiency and environmental sustainability, there is an urgent need to develop passive heat dissipation technologies that are both environmentally friendly and energy-efficient [7–9].

Radiative cooling (RC) and evaporation cooling (EC) have been widely explored as energy-saving approaches for heat dissipation [10–12]. RC dissipates heat into outer space (~3 K) via the atmospheric long-wave infrared (LWIR) window, requiring materials with high thermal emittance (εLWIR) and high solar reflectance (Rsolar) [13–15]. Materials such as SiO2 [16, 17], hBN [18], PVDF-HFP [19, 20], and PDMS [21] possess minimal solar absorption but high intrinsic mid-infrared absorption, which, when combined with scattering structures [22, 23] (e.g., porous [17, 24], sphere [25, 26], hollow [27, 28], fiber [29, 30], core–shell [31]), enhance cooling performance. However, RC faces limitations, including thermal stability concerns at elevated temperatures due to polymer degradation [32, 33], and restricted cooling power (typically below 150 W m⁻2 at room temperature), which is further influenced by environmental factors like humidity, cloudiness, and rainfall [34]. EC utilizes the latent heat of water evaporation, providing high cooling power [35–37]. However, in outdoor daytime, water has an unavoidable solar absorption, which makes it difficult to cool down the outdoor device under sunlight. In addition, the passive water supply also limits its practical application since the additional active water replenishment structures will increase the cost and structural complexity [38].

Combining RC and EC, known as radiative evaporative cooling (REC), significantly enhances daytime cooling performance by simultaneously exploiting high solar reflectance, thermal emittance, and latent heat release [39–41]. Recent studies have focused primarily on sub-ambient REC applications [42, 43], which could achieve ~3.3 °C lower than pure RC [44]. While little attention was paid to the above-ambient heat dissipation application, the passive water supply at such a high working temperature is also a challenge. It is urgent to develop REC technology in the above-ambient heat dissipation to improve its stability under high working temperatures.