Dynamic impact of climate on the performance of daytime radiative cooling materials

https://doi.org/10.1016/j.solmat.2020.110426Get rights and content

Highlights

  • Radiative cooling materials have better performance in hot and arid climates.

  • Most radiative cooling materials exhibit the greatest response to changes in ambient radiation.

  • Higher ambient air temperatures correspond to larger sub-ambient temperature of the surfaces.

  • An AEMT window is a promising solution to improve the performance of radiative coolers under humid conditions.

Abstract

By strongly reflecting solar radiation and being highly emissive within the atmospheric window, daytime radiative coolers can achieve sub-ambient temperature under direct sunlight. Radiative cooling performance is strongly coupled to specific climatic conditions since cooling efficiency is strongly affected by ambient air temperature, wind speed, and solar and ambient radiation intensity. In this paper, using a well-validated thermal model, the cooling performance of three radiative cooling materials with varying optical properties was evaluated under three distinct and representative climates. This analysis permits us to better understand the sensitivity of daytime radiative cooling materials to different climatic conditions, present strategies for selecting the ideal spectral properties of materials and investigate how to enhance cooling performance under adverse climatic conditions. It is shown that radiative cooling materials have better performance in hot and arid climates. Most radiative cooling materials exhibit the greatest response to changes in ambient radiation. Higher ambient air temperatures correspond to larger sub-ambient temperature of the surfaces, but this change is lower than that of the corresponding air temperature. Furthermore, by coupling a special optical grating window onto the surface of a radiative cooler, cooling performance can be significantly enhanced by asymmetrically reflecting incoming radiation but permitting outgoing emission. While an ideal material that only emits in the atmospheric window wavelengths presents the best performance under a large range of solar radiation, ambient radiation, and air temperature, the broadband ideal emitter exhibits higher cooling potential when coupled with the optical grating window.

Introduction

The heat island phenomenon along with global climate change combine to increase the ambient temperature of cities, as well as the frequency, duration, and intensity of heat waves [1]. The overheating of cities exacerbates energy problems, deteriorates thermal comfort conditions, causes health issues for vulnerable populations, and leads to huge economic losses [2]. Space cooling dominated by the use of air conditioning accounts for 2.9% and 6.7% of the total energy consumption of residential and commercial buildings worldwide, respectively, and a very serious increase is expected by 2050 [3]. In some cities, like Hong Kong, air conditioning constitutes over 40% of the energy consumption of commercial buildings [4]. Air conditioning is responsible for important local and global environmental problems like the heating of the ambient air caused by releasing additional anthropogenic heat in addition to the ozone depletion caused by its refrigerants. Conversely, passive cooling technologies like the employment of reflective materials, fluorescent surfaces, and thermochromic devices can cool the space with only minimal additional energy requirements and without presenting side effects to the environment [[5], [6], [7], [8], [9], [10]]. Passive cooling is also possibly implemented together with energy conversion devices, like PV panels [11], phase change materials [[12], [13], [14]] and hybrid power system [15], to further enhance the energy saving efficiency [16]. Daytime radiative cooling is one of these passive cooling techniques and is an extensively researched field with very high potential [17,18].

Due to the existence of the atmospheric window, mainly in the 8–13 μm wavelength range (Fig. 1), surfaces can dissipate heat through the atmosphere to outer space. Night time radiative cooling has been proven to work in the past [19]. Surfaces with high solar reflectivity mainly in the 0.3–2.5 μm range, and emissivity close to 1 in the 8–13 μm range, can be possibly cooled to sub-ambient temperature. Daytime sub-ambient cooling has not been observed until recent years, as nanotechnology and photonic structures have developed [20].

In 2014, a thermal photonic cooler was tested in Stanford, California [20]. While being exposed to solar radiation exceeding 850 W/m2, it reached a surface temperature of 4.9 °C below the ambient air temperature. With a thin polyethylene cover acting as the convective shield, its cooling power was 40.1 W/m2 at ambient air temperature. Also, in Stanford during the winter [21], using a direct sunlight shading device as well as a vacuum chamber to eliminate non-radiative heat transfer, a maximal reduction of 42 °C below the ambient air temperature occurred when a surface was exposed to peak solar radiation. In Pasadena, California [22], a polymer-coated fused silica layer together with a near-ideal solar reflector was tested on a roof in 2017. Covered by a polyethylene film, an average sub-ambient temperature of 8.2 °C was reported under direct sunlight. Also, in 2017, a scalable-manufactured polymer was demonstrated to have a cooling power of 93 W/m2 under direct sunshine on clear autumn days in Cave Creek, Arizona [23]. A feedback-controlled system was used to keep the surface temperature the same as that of the ambient air, which enabled the direct exposure of the surface to the surrounding air. In 2019, Li et al. engineered a cellulose nanofiber which is a mechanically strong radiative cooling material and can have continuous sub-ambient cooling during both day and night [24]. For all these coolers mentioned above, the experimental results agreed well with their theoretical predictions.

The above-mentioned successful field demonstrations of daytime radiative cooling were performed mostly in arid American cities with an average precipitable water vapor (PWV), a key parameter for humidity evaluation, between 0 and 20 mm during the measuring month, as shown in Fig. 2. However, for regions with a more humid climate, the predicted high cooling performance has not been achieved. The widely accepted explanation is that high humidity reduces the transmittance of the atmospheric window, leading to higher overall ambient radiation intensity. For example, a cooler with an average solar reflectance of 0.89 and emissivity in the atmospheric window of 0.72 was tested under warm and humid conditions in Okayama, Japan [25]. It was predicted to achieve a surface temperature close to 1.3 °C below the ambient temperature when the PWV was set to 1 mm. Despite having a convective shield, the cooler was measured to be 2.8 °C warmer than the ambient. The typical value of PWV is around 20 mm in early-summer days in Japan, and this high humidity may cause the discrepancy in results between simulation and experiment. Similar disagreement was reported in Ref. [26]. A double-layer coating designed to provide daytime radiative cooling was tested in mid-September in Shanghai and 5–11.5 °C sub-ambient temperature was predicted under a humidity of 2% and cloud cover close to zero. However, the measured temperature of the cooler was 3–10 °C higher than the ambient temperature. This disagreement was attributed to the influence of high humidity, which was 60%, and the high cloud coverage, which was close to 0.5, during the experiment. In a recent study, a radiative cooling material was tested in Phoenix, USA (Midlatitude, arid); New York, USA (Midlatitude, coastal); and Chattogram, Bangladesh (Tropical, coastal) respectively [27]. A sub-ambient temperature of 6 °C at noon was reported in warm and arid conditions in Phoenix, and it reached 5 °C below the ambient temperature in a cold afternoon in New York. When the surface was tested under a foggy and hazy winter sky in Chattogram, the sub-ambient temperature decreased to 3 °C. Fig. 2 presents the average PWV of the above analyzed cities during the measuring month extracted from Meteonorm 7.3 [28]. These values are presented together with the geographic positions and altitudes of the cities. PWV influences the transmittance of the atmospheric window and thus changes the overall intensity of longwave radiation and the radiative balance of the surface.

To investigate the influence of the main climatic factors on the cooling performance of daytime radiative cooling systems, a detailed sensitivity analysis was carried out to investigate the influence of air temperature as well as solar and ambient radiation on the efficiency of reference radiative coolers. These three climatic parameters were selected for analysis given that a) they have the highest impact on the efficiency of radiative cooling materials and b) they present very significant spatial and climatic variability. As noted, wind speed is also a contributory climatic factor which affects the convective heat transfer coefficient and varies among different climates. However, as the influence of this coefficient on cooling performance has been widely studied and well researched [21,22,[29], [30], [31]], it will not be further explored in this paper. As the convection is always pulling the surface temperature towards the ambient air temperature, the common conclusion in these researches is that when surface temperature is higher than that of the ambient air, the larger the convective heat transfer coefficient, the better the cooling performance; when the surface temperature is lower than the ambient air temperature, larger convective heat transfer coefficient leads to worse cooling performance. It is also indicated in Ref. [22] that the superior cooling performance of narrowband emitter which only emits in 8–14 μm is greatly weakened when convection is strong.

This paper aims to a) analyze and emphasize the significance of taking specific climatic conditions into consideration when designing and assessing the performance of daytime radiative cooling devices, b) investigate the sensitivity of different advanced daytime radiative cooling materials to changes in the main climatic parameters, and c) present technological solutions and strategies to select the ideal spectral properties for a material and enhance its cooling performance under adverse climatic conditions.

Section snippets

Methodology

To assess and investigate the cooling performance of radiative cooling devices under different climatic conditions, the following research methodology was proposed, as shown in Fig. 3. In the first step, a spectral numerical simulation model based on the dynamic thermal balance of the radiative coolers was developed. The model was designed to calculate the surface temperature of the cooling devices according to specific environmental settings and spectral material properties. At the second

Performance of the three materials under the selected climatic conditions

The calculated performances of the three radiative cooling materials under the three selected climatic conditions are presented in Fig. 7. As shown, climate has a very significant impact on the materials’ cooling performance. For example, the temperature of material 1, at 14:00 in Sydney and Alice Springs, was about 2.5 K higher than the ambient air temperature and about 1.5 K below the ambient temperature, respectively. In parallel, the surface temperature of material 2 in Sydney and Alice

Conclusions

This is the first study analyzing the sensitivity of the performance of daytime radiative coolers to various and adverse climatic conditions. The results presented in this paper imply that climate conditions are extremely important factors in the design of radiative cooling materials since a radiative cooler that performs well in an arid climate could perform poorly in a humid climate. This inconsistency in performance is mainly caused by the differences in ambient radiation and ambient

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

We thank Dr Angus Gentle from University of Technology Sydney for providing help in the measurement using Newport Oriel 80350 FTIR. A special acknowledgement goes to the China Scholarship Council (CSC) who supports the PhD project of Jie Feng and Kai Gao.

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