Colored radiative cooling: How to balance color display and radiative cooling performance

Highlights

• A colored radiative cooler with a metal-dielectric-metal structure is proposed.

• The competing role between color display and radiative cooling power is revealed.

• Lightness plays a more dominant role than chroma and hue for colored radiative cooling.

Abstract

Radiative cooling, which aims at cooling objects by radiating heat into the outer space through the atmosphere window, has garnered wide interests since it is a passive refrigeration mode without extra energy consumption. However, most of the proposed structures, to maximize the cooling performance, are in white color, which may limit their practical applications due to some aesthetic consideration. To reveal the competing role between color display and radiative cooling performance, we take a metal-dielectric-metal (MDM) colored radiative cooler structure as representative to explore how the structure parameters influence the cooling performance and color display respectively, and clarify the key parameters for the desired performance. Moreover, the reflectance of the colored radiative cooler is identified as peak- and valley-types to clarify the color display performance. The Spearman rank order correlation coefficients between radiative cooling performance and the CIE-LCH color space parameters (lightness, chroma, and hue) are calculated, revealing that the lightness plays a dominant role. The present study is expected to reveal the competing role of radiative cooling power and color display, and trigger the practical applications of radiative cooling technologies.

Introduction

Radiative cooling, which aims at radiating heat directly into the sky at ~3 K through the atmosphere windows, has garnered increasing attentions due to zero-energy input, great cooling potential, and widespread applications [1]. Taking the extremely low-temperature outer space as the cold source, objects on the earth at ~300 K can theoretically radiate heat at huge amount according to the Stefan-Boltzmann law. Although this working principle has been proposed for about 60 years, early radiative cooling technologies only works at nighttime for the reason that the solar energy input will easily compensate the cooling power in the daytime. Note that the typical solar intensity is around 1000 W/m², while the radiative cooling power is around 100 W/m². Even though, early nighttime experiments had demonstrated that a surface could be cooled below the ambient temperature by 10–15 °C, which fuel the subsequent researchers with confidence to explore the daytime radiative cooling technologies [2]. The daytime radiative cooling remains to be a grand challenge, but researchers have been sparing no effort to explore the possible materials and structures to maximize the reflectance in the solar spectrum (0.3–2.5 μm) and emissivity at the atmosphere window (8–13 μm) simultaneously. It is until recent decade that daytime radiative cooling technologies made a great breakthrough thanks to the advance of thermal photonic designs and micro/nano-fabrication techniques. In the first seminal demonstration of daytime radiative cooling, a photonic solar reflector and a thermal emitter was fabricated, which reflects 97% of incident sunlight and exhibits selective emissivity at the atmosphere window simultaneously, and cool to 4.9 °C below the ambient temperature [3]. Later on, many approaches have been proposed for daytime radiative cooling ranging from photonic design, particle-based matrix, composite, textile, etc [[4], [5], [6], [7], [8], [9]]. As for practical applications, these radiative cooling structures, films, and coatings are usually mounted externally for better radiative heat transfer and higher cooling power. However, a new problem arises that the radiative cooler always appears in white color so as to reflect the incident solar light as much as possible, but white colors are not always welcome in some scenarios for the aesthetic reasons. Therefore, the colored radiative cooler (CRC) is more demanded with more practical and broader applications.

To achieve CRC, two strategies were proposed basically [10]. The first one is the dye rendering strategy, which directly applies dye materials onto the surface of the traditional cooler. For example, Chen et al. combined a layer of commercial paints with a porous P(VDF-HFP) or titanium dioxide (TiO₂)/polymer composite membrane to demonstrate paintable bilayer coatings [11]. Although this strategy can be applied in flexible materials like coating and fabric, the superlayer dye will absorb not only the radiation energy caused by color display in visible band, but also part of radiation energy in infrared band, which will greatly weaken the cooling performance. The second strategy is structural color strategy, which enables color decoration of the cooler through structural design [[12], [13], [14]]. In principle, an object's color can be calculated by the incident radiation spectrum, the reflectance of the object in visible band, and the color matching function of human eye. These three factors represent the process of light projected onto the surface of object, light reflected into human's eyes by the surface, and optical signal converted to electrical signal by human's cones. While the radiation spectrum of sunlight and the color matching function of human eye are fixed and untunable, only through altering visible reflectance of object can one manipulate the color display. Based on this principle, the CRC is realized by designing specific visible reflectance which is based on multi-band emissivity regulation. Compared to the dye rendering strategy, structural color strategy with a complicated and subtle structure can be designed elaborately to avoid extra energy absorbed in infrared waveband, thus enabling better cooling performance. However, it yet suffers from complicated structure design, difficulty and cost in manufacture as well as poor ductility. Lee et al. designed a multi-layer CRC which is the first one based on photonic structure to achieve a good color for CRC with yellow, cyan and magenta colors. When convective heat transfer coefficient h = 6 W/(m²·K), the cooler could get a 3.9 °C lower temperature than the ambient temperature on average [15]. Sheng et al. presented a CRC with no angular dependence by adopting optical Tamm resonance, which got a better cooling effect of 5–6 °C below ambient when h = 6 W/(m²·K) [16]. Li et al. predicted the theoretic limit temperature of radiative cooling and heating corresponding to several kinds of colors and carried out a pink CRC and heater. However, it still lacked practical structures for other colors, and the theoretical potentials of cooling power for CRC remain unsolved [17].

Here, we take a metal-dielectric-metal (MDM) [18,19] colored radiative cooler structure as representative to explore how the structural parameters influence the cooling performance and color display respectively. Different roles of the parameters in color display module are identified through compressive parameter exploration. Moreover, the reflectance spectra of the CRC are categorized into peak- and valley-types, whose characteristics are analyzed thoroughly. Angle dependency of color exhibited by the proposed structure is also studied. Finally, the correlations between radiative cooling power and CIE-LCH color space parameters (lightness, chroma, and hue) are also calculated to offer an intuitional relation between color display and radiative cooling performance.