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Preliminary study of passive radiative cooling under Singapore's tropical climate
Solar Energy Materials & Solar Cells 206 (2020) 110270
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Preliminary study of passive radiative cooling under Singapore’s tropical climate Di Han, Bing Feng Ng *, Man Pun Wan School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore, 639798
A R T I C L E I N F O
A B S T R A C T
Keywords: Radiative cooling Solar irradiance Humidity Photonics Tropical climate
Sub-ambient cooling can be achieved through radiative coolers that selectively emit radiation within the at mospheric window (8–13 μm) to outer space and suppress absorption/emission of other wavelengths. This study explores the feasibility of adopting radiative cooling in the hot and humid climate of Singapore through both numerical and experimental approaches. A theoretical simulation based on the heat transfer balance is first proposed to obtain the cooling power of the radiative cooler considering different solar spectral irradiance and total water vapor column. The larger solar irradiance in Singapore, especially within the ultraviolet and visible light spectrum where the absorbance of the material is relatively high, could counteract its cooling effects. Moreover, the increased atmospheric radiation induced by higher humidity and temperatures in Singapore could worsen cooling performances of the radiative material. Next, experimental investigations were conducted by measuring the steady-state temperatures of two radiative coolers (photonic radiative cooler and enhanced specular reflector film) under three typical weather conditions in Singapore, namely clear, partly cloudy and cloudy skies. While both radiative coolers were unable to achieve daytime cooling performance on a clear day, the enhanced specular reflector (ESR) film with higher solar reflectance can reach sub-ambient temperatures on a cloudy day. When it comes to night-time, the steady-state temperature of the photonic radiative cooler and ESR film was about 3.5 � C and 5 � C lower than ambient, respectively.
1. Introduction The challenge faced by many developed and developing countries is in the rising energy demand requirements for heating and cooling of indoor spaces [1]. In Singapore, up to 20% of total energy consumption is used for the cooling and regulation of indoor environment through air-conditioning and mechanical ventilation systems [2]. Such active means of cooling consumes large amounts of electrical energy and the demands are elevated in the daytime under the tropical climate where solar radiation is particularly high. Therefore, it is essential to find a solution to reduce energy consumption for the cooling of indoor spaces. Among the various cooling options, passive radiative cooling [3–6] that requires no energy input offers an attractive solution to the challenge. Passive radiative cooling relies on the earth’s atmosphere, which is a mixture of gases including N2, O2, CO2 and water vapor [7]. These compounds create a semi-transparent medium that can emit, absorb and scatter radiation across a wide range of the electromagnetic spectrum. However, there is a specific window in the 8–13 μm range where the sky is apparently transparent. Hence, a radiative cooler with well-designed
spectrum can be brought below ambient temperatures by selectively emitting radiation within the atmospheric window to the outer space and suppressing absorption/emission of other wavelengths. At night, radiative cooling can be achieved, either through black body envelope that emits across the entire electromagnetic spectrum or through metamaterials that emit selectively within the atmospheric window. The cooling performances of white TiO2 paint (which has near blackbody emissivity in the entire thermal infrared range) was compared against black paint [8] and the results showed that their temperature difference was 11 � C at 0.5 kPa vapor pressure and 6 � C at 2 kPa vapor pressure. Catalanotti et al. [3] demonstrated that a thin film of TEDLAR (polyvinyl–fluoride plastic) coated on top of an evaporated aluminium substrate can achieve 12 � C below ambient. Additionally, Granqvist and Hjortsberg [9] measured a thin film of SiO evaporated on top of an aluminium substrate, which can achieve 14 � C below ambient and has 61 W/m2 cooling power under 12 � C night temperature and 90% relatively humidity conditions. While most experiments reported an average of 10 � C below ambient at night, the limits can be pushed to 40 � C below ambient for a well-insulated radiative cooler at the Atacama
* Corresponding author. E-mail addresses: [email protected] (D. Han), [email protected] (B.F. Ng), [email protected] (M.P. Wan). https://doi.org/10.1016/j.solmat.2019.110270 Received 14 July 2019; Received in revised form 29 October 2019; Accepted 30 October 2019 Available online 15 November 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.
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desert [10]. However, the temperature depression was only in the range of 1–6 � C below ambient under Thailand’s hot and humid night sky [11], where humidity played a significant role. In the day, solar radiation heats up the structure and 10% of solar absorption (0.3–2.5 μm) is sufficient to counteract the cooling effects of the radiative cooler [7]. Nevertheless, given that the wavelength of solar radiation is below that of the atmospheric window, a selective radiative cooler that reflects incident solar irradiance at wavelengths below 2.5 μm and emits radiation selectively in the atmospheric window can theoretically provide cooling effects in the day [12,13]. Recently, sig nificant progress has been made in the development of radiative coolers. In 2014, Raman et al. [14] achieved sub-ambient daytime radiative cooling for the first time, producing a temperature drop of 4.9 � C below ambient and cooling power of 40.1 W/m2 on a clear winter day in Cal ifornia. Utilizing engineered photonic materials with reflective surfaces, the cooler possessed strong thermal emission within 8–13 μm and had very high solar reflectance (97%). In 2016, Chen et al. [15] fabricated a selective emitter with layers of silicon nitride (Si3N4), amorphous silicon (Si) and aluminium (Al) on top of a Si wafer. The emitter recorded significant temperature reductions of 42 � C in winter and 37 � C in summer (both in California) with sun shade and vacuum chamber. One year later, Kou et al. [16] observed a sizeable cooling power of 127 W/m2 and a temperature difference of 8.2 � C under direct sunlight on a clear day in California with a polymer-silica-mirror. The mirror was fabricated by coating fused silica wafer with polydimethylsiloxane (PDMS) and silver film as a top layer and a back reflector, respectively. In 2018, Yang et al. [17] achieved record-high total solar reflectance (99%) for daytime radiative cooling with a dual-layer structure that is composed of a polytetrafluoroethylene (PTFE) on top of a silver film and a glass substrate. Most recently in 2019, Ao et al. [18] compared the performance of a specular and a diffuse surface for daytime radiative cooling, which revealed an equilibrium temperature of 2.5 � C and 1.5 � C lower than the surroundings when the solar intensity was about 430 W/m2 in Beijing. Apart from conventional materials, optimized conical-shaped multi-layered structures can theoretically also possess cooling power under direct sunlight [19,20]. To scale up the design and application of radiative cooling, polymer and fabric have proven to be feasible materials [21–23], together with cost-effective coatings for large area application [24–26]. It should be highlighted that the experiments conducted thus far are geographically situated in the temperate region with low humidity, solar radiation and minimal cloud cover. However, the potential of daytime radiative cooling under hot and humid climates has remained doubtful as the amount of solar radiation received and atmospheric water vapor can significantly influence the efficiency of radiative cooling [6,27]. A case in point as demonstrated in Hong Kong, radiative coolers are more effective in the night as compared to the day for cooling under hot and humid weather conditions [28]. Even with the same radiative cooler as Raman et al. [14] with vacuum chamber, daytime sub-ambient cooling performance could not be achieved. On a similar note, Bao et al. [25] fabricated a double-layer nanoparticle-based coating that comprise of a top reflective layer (TiO2) and bottom emitting layer (SiO2 or SiC). As the coating had 90.7% reflectance over the solar spectrum and 90.11% emittance within the atmospheric window, it could only achieve above ambient conditions (3–10 � C) in Shanghai where the temperature and relative humidity was around 30 � C and 70%, respectively. Studies on the effects of humidity on radiative cooling were also carried out recently. Suichi et al. [29] investigated the performance limit of a cooling device with alternating layers of SiO2 and poly (methyl methacrylate) (PMMA) on an Al mirror in the warm and humid envi ronment of Okayama, Japan. The device achieved about 2.8 � C above ambient (35 � C) with solar reflectivity of 89% and 72% emittance within the atmospheric window. The authors also calculated the averaged at mospheric transmittance and the resulting equilibrium temperature as a function of the precipitable water vapor. Likewise, Hossain and Gu [27] theoretically compared the cooling performance of radiative coolers in
two cities in Australia with different humidity levels. The results revealed poorer efficiency under lower atmospheric transmittance conditions. In addition, Hu et al. [30] simulated the impact of relatively humidity on the cooling performance of the integrated solar heating and radiative cooling collector. The authors observed a 62% decrease in cooling power when relative humidity is increased from 5% to 95%. Liu et al. [31] also performed a detailed investigation on the effects of total water vapor column (TWC) on radiative cooling performance through both simulations and experiments, which found that cooling power decreased by 86.6 W/m2 with increasing TWC. Evidently, humidity lowers atmospheric transmittance that increases downwelling atmo spheric radiation substantially. To recover cooling performances under humid climates, Wong et al. [32] suggested to integrate the cooler with an asymmetric electromagnetic transmission window, which can reflect downwelling atmospheric radiation. In addition, the performance of radiative coolers is sensitive to regional changes in solar intensity and a directional approach with angular confinement of solar radiation can potentially reduce the incoming solar energy [33,34]. With solar radiation and humidity playing critical roles in deter mining the performance of radiative cooling, the application of the technology becomes geographically specific. This paper is intended to make a preliminary study of passive radiative cooling under Singapore’s unique tropical climate that is highly influenced by its geographical location as a maritime country and latitude positioning close to the equator. A theoretical simulation based on the heat transfer balance is proposed to obtain cooling power of the radiative cooler with different solar spectral irradiance and total water vapor column. The effect of regional solar radiation and humidity in the daytime and the effect of humidity in the night on cooling performances are explained by modelling several climate conditions. In addition, experiments were performed to obtain steady-state temperatures of radiative coolers under three typical weather conditions in Singapore, namely clear, partly cloudy and cloudy. 2. Theoretical modelling 2.1. Heat transfer balance Consider a radiative cooler whose surface temperature is Ts under direct sunlight and exposed to the sky, the net cooling power Pcool can be calculated as: Pcool ðTs Þ ¼ Prad ðTs Þ
Patm ðTamb Þ
Psolar
Pconvþcond ðTs ; Tamb Þ
where the power radiated out by the radiative cooler surface is: Z π=2 Z ∞ sinθcosθ IB ðTs ; λÞεðλ; θÞdθdλ Prad ðTs Þ ¼ 2π 0
0
Here IB ðTs ; λÞ ¼ 2hc λ5
2
1 ehc=λkB Ts 1
(1)
(2)
is the spectral radiance of a blackbody by
Planck’s law at the surface temperature, where h is the Planck’s con stant, c is the velocity of light, λ is the wavelength, kB is the Boltzmann constant. εðλ; θÞ is the spectral and angular emissivity of the radiative cooler. The absorbed power from the downwelling atmospheric radiation can be expressed as: Z ∞ Z π=2 Patm ðTamb Þ ¼ 2π sinθcosθ IB ðTamb ; λÞεatm ðλ; θÞεðλ; θÞdθdλ (3) 0
0
2
where Tamb is the ambient temperature. IB ðTamb ; λÞ ¼ 2hc λ5
1 ehc=λkB Tamb 1
is the
spectral radiance of a blackbody by Planck’s law at ambient temperature Tamb . εatm ðλ; θÞ is the wavelength and angle-dependent emissivity of the atmosphere, which is related to the atmospheric transmittance tðλÞ in the zenith direction and given by εatm ðλ; θÞ ¼ 1 tðλÞ1=cos θ [9]. The incident solar irradiance absorbed by the radiative cooler is 2
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Fig. 1. Simulated solar spectral irradiance from SMARTS and (a) spectral reflectivity of the radiative cooler within the solar spectrum (b) corresponding spectral absorbance of the radiative cooler within UV and VIS spectrums.
Fig. 2. (a) Total solar irradiance in different regions. (b) Absorbed solar irradiance in different regions.
given by: Z ∞ Psolar ¼ εðλ; θsolar ÞIsolar ðλÞdλ 0
[12], 10% of solar absorption is already sufficient to counteract the cooling effects of the radiative cooler. To achieve meaningful daytime radiative cooling in practice, Raman et al. [14] indicated that average solar reflectivity must be more than 94%. However, their results were obtained from AM 1.5 global titled solar irradiance for the solar illu mination Isolar , which is representative of the average continental U.S. and mid-latitude area. For other areas, SMARTS (Simple Model of the Atmospheric Radiative Transfer of Sunshine) developed by NREL (Na tional Renewable Energy Laboratory) can be used to compute clear sky spectral irradiances [35]. To illustrate the difference in solar irradiance based on geographical location, three different locations, namely California, Hong Kong and Singapore were simulated using SMARTS with local input parameters in the configuration. As shown in Fig. 1 (a), total solar irradiance is highest at about 1001.8 W/m2 (Fig. 2 (a)) in Singapore, which is a tropical country situated close to the equator. The difference in spectral irradi ance between the three locations are particularly evident in the ultra violet and visible light wavelengths as shown in Fig. 1 (b). The ratio of the ultraviolet and visible light in the total solar irradiance is about 53% in Singapore as compared to that of 41% in California shown in Fig. 2 (a). As California is in the temperate region while Hong Kong is in the subtropical region, the same radiative cooler is expected to perform differently in the two cities. When the spectral irradiance is compared to the reflectivity of radiative coolers within the solar spectrum, the per formance of the coolers can be clearly observed. In Fig. 1, the reflectivity of a radiative cooler that is identical to the one used in Refs. [14,28] is
(4)
where Isolar represents the solar illumination. Eq. (4) does not have the angular integral when the radiative cooler is at a fixed angle. Lastly, the power loss due to the parasitic heat gain (convection and conduction) of the structure is: Pconvþcond ðTs ; Tamb Þ ¼ hc ðTamb
Ts Þ
(5)
where hc is the combined non-radiative heat transfer coefficient from the convective and conductive thermal contact of the structure with ambient air. In general, Eq. (1) is the net cooling power Pcool of the surface as a function of the surface temperature Ts and ambient temperature Tamb . If the value of Pcool is positive when Ts ¼ Tamb , the surface can produce cooling power, which indicates that the surface emits more energy to space than the absorbed radiation from the sunlight and atmosphere. The solution to Eq. (1) when Pcool ¼ 0 represents the steady-state tem perature Ts that is below ambient Tamb if the radiative cooler can pro duce cooling power. 2.2. Daytime solar radiation Solar radiation Psolar is an important factor affecting the performance of daytime radiative cooling and is large compared to atmospheric ra diation Patm and parasitic heat gain Pconvþcond in Eq. (1). In Rephaeli et al. 3
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Fig. 3. (a) Modelled atmospheric transmittance for three typical climates (the dotted lines indicate the range for the atmospheric window). (b) Simulated atmo spheric radiation absorbed by the radiative cooler with different ambient temperature and TWC.
downwelling atmospheric radiation is increased substantially according to εatm ðλ; θÞ in Eq. (3). Atmospheric water vapor content can be repre sented using the total water vapor column (TWC) that denotes the total gaseous water in a vertical column of atmosphere, which is influenced by both the local temperature and humidity. Here, the level of atmospheric transmittance is modelled using MODTRAN [36] with typical TWC values under three different climatic conditions as shown in Fig. 3 (a). TWC values of 1059.7 atm-cm, 3635.9 atm-cm and 5119.4 atm-cm are representative of mid-latitude winter, subtropical winter and tropical climates, respectively. As TWC in increased, atmospheric transmittance within the atmospheric window (8–13 μm) decreases from over 80% to below 60%. Also, the second atmospheric window (16–20 μm) as indicated in Ref. [29] is missing due to high levels of humidity. The atmospheric radiation absorbed by the radiative cooler can be calculated according to Eq. (3) and the results are shown in Fig. 3 (b). Evidently, the radiative cooler will absorb more downward atmospheric radiation with increase in humidity. In addition, the difference in absorbed energy between different humidity levels is enlarged with increased ambient temperature, which can be understood through Planck’s law in Eq. (3). The enhancement in absorbed atmo spheric radiation is about 18 W/m2 as TWC is increased from 1059.7 atm-cm to 5119.4 atm-cm when the ambient temperature is 0 � C. This difference is enlarged to 34.6 W/m2 when the ambient temperature is 40 � C. Compared to the cool and dry weather in mid-latitude winter, cooling performances of radiative coolers are restrained in hot and humid environments due to the increased absorbance of atmospheric radiation, for instance in Singapore where TWC levels are high at 5119.4 atm-cm. To illustrate the combined effects of solar radiation and humidity on cooling performances in the daytime, the net cooling power for the same reference and measured radiative coolers were calculated according to Eqs. (1)–(5) and shown in Fig. 4. The local solar spectral irradiance was obtained from SMARTS and the atmospheric transmittance calculated by MODTRAN using the local humidity and ambient temperature in three different regions. Meanwhile, the experimental data obtained in California and the cooling power calculated from our measured reflec tivity (case 2) of the same radiative cooler as in Ref. [14,28] can be seen in Fig. 4. The cross points of the curves with the vertical dashed line when Ts Tamb ¼ 0 are the net cooling power at the local ambient temperature. The cross points of the curves with the horizontal dashed line when Pcool ¼ 0 are the steady-state temperature of the radiative cooler. As observed, the simulated and experimental results match well in the simulated case of California under the cool and dry climate. How ever, when it comes to the hot and humid environment in Hong Kong, the cooling power is reduced to 11.37 W/m2. The cooling performance is
Fig. 4. Calculated cooling power in different regions and the experimental data in California in the daytime. (The simulated case in CA, HK and case 1 in SG are based on the reference emissivity in Ref. [14] while the simulated case 2 is based on the measured emissivity.)
shown. The reference reflectivity is obtained from Ref. [14] while measured reflectivity is obtained from an identical radiative cooler purchased from the same manufacturer as Ref [14] and measured using a UV-VIS-NIR spectrometer (PerkinElmer-Lambda 1050). As observed in Fig. 1 (b), the corresponding absorbance by Kirchhoff’s law increases rapidly as wavelength decreases within the ultraviolet (300–400 nm) and visible light (400–700 nm) range. On the other hand, the measured reflectivity (average value of 93% in infrared range) is smaller than the reference reflectivity (average value of 97% in infrared range) as shown in Fig. 1 (a). Using reference values from Ref. [14] for reflectivity to calculate the solar power absorbed by the radiative cooler in Eq. (4), solar power absorbed is no more than 40 W/m2 in Singapore as shown in Fig. 2 (b). However, when using measured reflectivity of the identical cooler that was purchased, the absorbed solar radiation is over 100 W/m2. As the radiative cooler will absorb more solar irradiance in Singapore than in Hong Kong and California, this results worsened cooling performance in Singapore. 2.3. Daytime humidity Another factor affecting daytime radiative cooling is humidity. In general, atmospheric water vapor content lowers the level of atmo spheric transmittance and as a result, heat is being trapped and 4
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Fig. 8. Cross-section schematic of the apparatus through the middle.
Fig. 5. Calculated cooling power for different total water vapor column with Tamb ¼ 30 � C and hc ¼ 6.9 W/m2K in the night-time.
Fig. 9. Image of the rooftop measurement setup.
Besides, when using the measured emissivity of the material in Singapore (case 2), the cooling power is the lowest due to the absorbed solar energy, especially within the UV and VIS range. By comparing the simulated case 1 and 2 (major difference in reflectivity in the solar spectrum) in Singapore under the same humidity condition, the domi nant effect of the absorbed solar radiation on the cooling performance can be revealed. This indicates the importance of material reflectivity in the performance of daytime radiative cooling.
Fig. 6. Measured reflectivity of two radiative coolers within solar spectrum. (The AM 1.5 solar spectrum is plotted for reference.)
2.4. Night-time humidity The effect of humidity on radiative cooling can be clearly illustrated in the nighttime using our measured emissivity when there is no solar radiation, as shown in Fig. 5. Here, the net cooling power is calculated at the ambient temperature of Tamb ¼ 30 � C, combined with non-radiative heat transfer coefficient of hc ¼ 6.9 W/m2K and atmospheric trans mittance with varying TWC values obtained from MODTRAN. In gen eral, there will be cooling effects for all humidity conditions in the night with the lack of solar radiation. When TWC is increased, the net cooling power decreases rapidly from 80.9 W/m2 to 19.1 W/m2 when TWC increased from 0 atm-cm to 8000 atm-cm. It can be observed that cool ing power is more affected at lower TWC and when TWC is increased, the two curves of TWC ¼ 6324.3 atm-cm and 8000 atm-cm almost overlap with each other. As for the steady-state temperature, the radi ative cooler can be cooled to 9.3 � C below ambient when the TWC ¼ 0 atm-cm. When TWC is 8000 atm-cm, the temperature reduc tion is only 2.1 � C. Particularly when TWC ¼ 5119.4 atm-cm, which corresponds to the humidity in Singapore, the simulation result shows a temperature of 3 � C below the ambient.
Fig. 7. Emissivity of two radiative coolers within mid-infrared wavelengths. (A typical atmospheric transmittance is plotted for reference.)
further reduced (a negative cooling power of 13.45 W/m2) under the hotter and more humid condition in Singapore, indicating that subambient temperatures cannot be achieved using emissivity parameters from the reference cooler in Ref. [14], denoted as case 1 in Fig. 4. 5
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Fig. 10. (a) Measured temperatures of four samples against ambient air temperature on a clear day. (b) Corresponding temperature difference. (c) Real-time hu midity, solar irradiance and image of sky.
3. Experimental investigation
multi-layer optical film technology to create a multilayer polymeric solar mirror with ultra-high reflectivity (about 98% across the visible spectrum). The thin film consists of 300 layers of polyethylene tere phthalate (PET) including other polyester and has an overall thickness of 65 μm. It was indicated [21] that the combination of ESR film and silver substrate could provide suitable solar and IR responses for radiative cooling. Here, the ESR film was cut to a size of 152 mm � 102 mm and then coated with 300 nm of silver using magnetron sputtering method. Meanwhile, two aluminum plate were cut to a size of 100 mm � 100 mm as reference. One was painted carbon black and the other polished as a mirror surface. The spectral reflectivity of the photonic radiative cooler and ESR film were measured by a UV-VIS-NIR spectrometer (PerkinElmer-Lambda 1050) with a snap-in integrating sphere within the wavelengths of 0.3–3 μm and a Fourier transform infrared spectrometer (PerkinElmer Spectrum Two FT-IR Spectrometer) using a specular reflectance acces sory with a calibrated gold coated mirror as a reflectance standard for
3.1. Experimental setup Two types of radiative cooler are prepared for the rooftop experi ment to study the feasibility of radiative cooling in Singapore. One cooler is of the same material as that proposed in Ref. [14] and demonstrated in Hong Kong [28]. The photonic device consists of seven alternating layers of hafnium dioxide (HfO2) and silicon dioxide (SiO2) with varying thickness. The top three layers that are thicker serves to emit infrared in the 8–13 μm wavelengths while the four thinner bottom layers deposited on 200 mm Ag coated Si wafer is supposed to provide a 97% solar reflection. However, our measured reflectivity is only about 93% in the infrared range as shown in Fig. 1(a). More details on the design and performance of the material can be found in Ref. [14]. Another cooler is made of the commonly used enhanced specular reflector (ESR) film developed by 3 M [37]. Vikuiti ESR film utilizes 6
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Fig. 11. (a) Measured temperatures of four samples against ambient air temperature on a partly cloudy day. (b) Corresponding temperature difference. (c) Real-time humidity, solar irradiance and image of sky.
the wavelength region between 3 and 20 μm. The reflectivity of ESR film is close to 99% in the VIS spectrum, which is better than that of photonic radiative cooler with about 90% as shown in Fig. 6. However, the reflectivity of ESR film drops significantly in the near-infrared wave lengths where solar irradiance is small while the photonic radiative cooler possesses higher stable reflectivity in this region. In the midinfrared wavelengths as shown in Fig. 7, both materials show selec tively strong emissivity within the atmospheric window. The average emissivity of ESR film is 92%, much higher than that of the photonic radiative cooler. Overall, both of photonic radiative cooler and ESR film are selective radiative cooling emitters which are strongly reflective over solar spectrum and strongly emissive in the atmospheric window. The schematic diagram of the experimental setup for both the pho tonic device and ESR on the rooftop is shown in Fig. 8. The radiative cooler is supported by polystyrene foam of the same cross-sectional area and of 60 mm thickness with low heat conductivity to minimise heat conduction. The polystyrene foam is covered with aluminum foil to prevent heating from incident thermal radiation and placed at the centre of the chamber, which is formed of clear acrylic and supported by a wooded frame of 218 mm height. The top surface of the wooden frame is covered with 12 μm low-density polyethylene film that works as a wind
shield and to minimise heat convection with the surrounding. Another layer of aluminum foil is placed on top of the low-density polyethylene film to reflect solar radiation and to form an aperture for the radiative cooler. The surface temperatures of the two radiative coolers and two reference plates are measured by a K-type self-adhesive thermocouple (SA3-K-120, Omega), which is attached to the centre of the bottom surface of the sample as shown in Fig. 8. A K-type thermocouple probe is placed inside a 12-ring solar radiation shield (Delta Ohm) with free air flow and no solar radiation to capture ambient temperatures in the surrounding of the apparatus as shown in Fig. 9. Similarly, the relative humidity is obtained from a combined temperature and humidity sensor with an accuracy of �3% placed inside another solar radiation shield. The sum of direct solar irradiance and diffuse irradiance (global radia tion) is measured by a secondary standard pyranometer (LP PYRA 10, Delta Ohm) with mV output, which is placed near the radiative cooler on top of the wooden frame. The spectral range is 283–2800 nm and the response time is smaller than 6 s. The above data is acquired using a NI DAQ module connected to the laptop and the sampling time interval is 20 s.
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Fig. 12. (a) Measured temperatures of four samples against ambient air temperature on a cloudy day. (b) Corresponding temperature difference. (c) Real-time humidity, solar irradiance and image of sky.
3.2. Experimental results and discussions
temperature of the photonic radiative cooler and ESR film, respectively. A negative value of Tamb Tc or Tamb Te in the daytime indicates a hotter photonic device or ESR temperature, respectively, as compared to ambient air. As observed, Tamb Tc can reach a minimum value of about 15 � C. Meanwhile, the value of Tamb Te in the day was mostly negative, except for some occasions when the solar irradiance is at a trough as a result of cloud cover. The temperature difference between the photonic radiative cooler and ESR film was enlarged with larger solar irradiance (a maximum difference of over 7 � C). Overall, the sur face temperatures of both radiative coolers were dominated by incident solar irradiance in the daytime while the material with higher solar reflectivity (ESR film) could reach lower temperatures. However, no cooling performance was found on a clear day in Singapore due to the effect of larger solar radiation and humidity as discussed in Sections 2.2 and 2.3. When it comes to night-time as shown in Fig. 10, both radiative coolers are able to achieve temperatures below ambient. Compared to the results in California and Hong Kong of approximately 7 � C and 6 � C lower than the ambient, respectively, the photonic device was only about 3.5 � C lower than ambient due to the higher humidity in
Both daytime and night-time radiative cooling performances were measured for the two different radiative coolers in three typical weather conditions in Singapore, namely clear, partly cloudy and cloudy. The weather condition is estimated in terms of the percentage of sky covered by clouds, which is a common practice in meteorology [38]. Due to the constantly evolving sky condition, average cloud coverage obtained from the local weather station is used. The surface temperature profile of different samples on a clear day is shown in Fig. 10 (a). In general, the radiative cooler identical to Refs. [14,28] and ESR film are unable to produce cooling below ambient in the daytime. As a result of higher solar reflectance, their temperatures are much lower than the black paint Al plate (close to 100 � C) and polished Al plate (close to 75 � C) when the solar irradiance is about 1017.87 W/m2. It can also be observed that the temperature of the ESR film is lower than that of the photonic radiative cooler with a higher average solar reflectivity, especially in the VIS spectrum. The corresponding temperature difference and real-time weather conditions can be found in Fig. 10 (b) (c), where Tc and Te represent the 8
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Singapore. The approximate value is also obtained from our simulation results for a steady-state temperature of 3 � C below the ambient when TWC ¼ 5119.4 atm-cm. On the other hand, the ESR film is able to ach ieve around 5 � C lower than ambient, which is almost similar in per formance as the black painted Al plate. This indicates strong emission within the atmospheric window and better cooling performance than the photonic device. Another indicator of cooler performance is the transition time in the late afternoon when the temperature difference between ambient air and radiative cooler changes from a negative to positive value. The value of Tamb Te would reach a positive value earlier than Tamb Tc . In addition, the corresponding solar irradiance during this transition time for the photonic device and ESR film is about 116 W/m2 and 300 W/m2, respectively, which also supports that ESR film has better cooling potential. The surface temperature profile of different samples and real-time weather conditions on a partly cloudy day are shown in Fig. 11. Dur ing daytime, the surface temperatures of the radiative coolers were mainly influenced by the solar irradiance. As compared to the clear day, there were more temperature fluctuations induced by intermittent cloud cover but there was also no sustained cooling below ambient during daytime. Overall, the temperature of the ESR film is still smaller than that of the photonic device in the daytime. As expected, the cooling performance would appear in the night-time and ESR film could produce larger cooling power. To further illustrate the effect of clouds cover on the cooling per formance, measurements were taken on a cloudy day and the corre sponding surface temperature profile and real-time weather conditions are shown in Fig. 12. On this day, after 3pm, the sky was totally covered with clouds, which can also be seen from the decreased solar irradiance and higher relative humidity in Fig. 12 (c). The corresponding solar irradiance at 3.30pm was about 250 W/m2, smaller than the 300 W/m2 on a clear day and the ESR film was cooled to below ambient. On the other hand, the temperature of the photonic radiative cooler dropped to below ambient at around 5 p.m. with the solar irradiance of 50 W/m2, smaller than the 116 W/m2 on a clear day. To some extent, cloud cover could improve the cooling performance of the radiative coolers by blocking away incident solar radiation, making daytime radiative cooling possible. On the other hand, cloud cover also prevents the transmission of emitted infrared radiation of the material, which suppresses the net cooling power in return. As the ESR film of high reflectivity can achieve daytime radiative cooling and much earlier than the photonic device, the reflectivity of the radiative cooling material should be high enough to achieve practical daytime radiative cooling, particularly within the UV and VIS spectrum.
partly cloudy and cloudy skies. Both radiative coolers were not able to achieve daytime cooling performance on a clear day due to high solar radiation and humidity. However, on a cloudy day, the ESR film with higher solar radiation reflectivity can reach sub-ambient temperatures. During the night, the steady-state temperature of the photonic device and ESR film was about 3.5 � C and 5 � C lower than the ambient, respectively. In summary, this preliminary study demonstrated the feasibility of passive radiative cooling in Singapore. Further improvements in the present cooler to successfully produce daytime cooling effects under Singapore’s climate can be achieved if the structure can be designed to reduce solar energy absorption within UV and VIS spectrum. 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 This study was funded by the Singapore Ministry of Education through grant no. 2018-T1-001-070 and supported through a start-up grant by Nanyang Technological University M4082022. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110270. References [1] L. P�erez-Lombard, J. Ortiz, C. Pout, A review on buildings energy consumption information, Energy Build. 40 (3) (2008) 394–398. [2] Building Construction Authority Singapore, Building Energy Benchmarking Report, 2014. [3] S. Catalanotti, V. Cuomo, G. Piro, D. Ruggi, V. Silvestrini, G. Troise, The radiative cooling of selective surfaces, Sol. Energy 17 (2) (1975) 83–89. [4] A.R. Gentle, G.B. Smith, Radiative heat pumping from the earth using surface phonon resonant nanoparticles, Nano Lett. 10 (2) (2010) 373–379. [5] A. Gentle, J. Aguilar, G. Smith, Optimized cool roofs: integrating albedo and thermal emittance with R-value, Sol. Energy Mater. Sol. Cells 95 (12) (2011) 3207–3215. [6] X. Lu, P. Xu, H. Wang, T. Yang, J. Hou, Cooling potential and applications prospects of passive radiative cooling in buildings: the current state-of-the-art, Renew. Sustain. Energy Rev. 65 (2016) 1079–1097. [7] M. Zeyghami, D.Y. Goswami, E. Stefanakos, A review of clear sky radiative cooling developments and applications in renewable power systems and passive building cooling, Sol. Energy Mater. Sol. Cells 178 (2018) 115–128. [8] B. Kimball, Cooling performance and efficiency of night sky radiators, Sol. Energy 34 (1) (1985) 19–33. [9] C. Granqvist, A. Hjortsberg, Radiative cooling to low temperatures: general considerations and application to selectively emitting SiO films, J. Appl. Phys. 52 (6) (1981) 4205–4220. [10] M.I. Ahmad, H. Jarimi, S. Riffat, Potentials of Nocturnal Cooling in Various Locations/Countries and Climatic Conditions, Nocturnal Cooling Technology for Building Applications, Springer, 2019, pp. 51–61. [11] J. Khedari, J. Waewsak, S. Thepa, J. Hirunlabh, Field investigation of night radiation cooling under tropical climate, Renew. Energy 20 (2) (2000) 183–193. [12] E. Rephaeli, A. Raman, S. Fan, Ultrabroadband photonic structures to achieve highperformance daytime radiative cooling, Nano Lett. 13 (4) (2013) 1457–1461. [13] K. Chen, M. Ono, S. Fan, L. Wei, Self-adaptive radiative cooling based on phase change materials, Opt. Express 26 (18) (2018) A777. [14] A.P. Raman, M.A. Anoma, L. Zhu, E. Rephaeli, S. Fan, Passive radiative cooling below ambient air temperature under direct sunlight, Nature 515 (7528) (2014) 540. [15] Z. Chen, L. Zhu, A. Raman, S. Fan, Radiative cooling to deep sub-freezing temperatures through a 24-h day–night cycle, Nat. Commun. 7 (2016) 13729. [16] J.-l. Kou, Z. Jurado, Z. Chen, S. Fan, A.J. Minnich, Daytime radiative cooling using near-black infrared emitters, ACS Photonics 4 (3) (2017) 626–630. [17] P. Yang, C. Chen, Z.M. Zhang, A dual-layer structure with record-high solar reflectance for daytime radiative cooling, Sol. Energy 169 (2018) 316–324. [18] X. Ao, M. Hu, B. Zhao, N. Chen, G. Pei, C. Zou, Preliminary experimental study of a specular and a diffuse surface for daytime radiative cooling, Sol. Energy Mater. Sol. Cells 191 (2019) 290–296.
4. Conclusion In this work, the performance of passive radiative cooling under Singapore’s tropical climate was investigated through both theoretical simulations and experiments. Through the simulations, the effects of solar radiation and humidity on the performance of radiative coolers were discussed. It was found that daytime cooling performance was mainly influenced by regional solar irradiance and humidity. The larger solar irradiance in Singapore, especially within the UV and VIS spectrum where the absorbance of the material is relatively high, could negate any cooling effects. Moreover, the increased atmospheric radiation induced by higher humidity and ambient temperature in Singapore could worsen the cooling performance. The net cooling power based on the heat transfer balance model showed that the radiative cooler could not pro duce cooling power in the daytime. In the night-time, cooling could be achieved and was mainly influenced by the total water vapor column. The calculated net cooling power decreased from 80.9 W/m2 to 19.1 W/ m2 with the increase of TWC from 0 atm-cm to 8000 atm-cm. A series of experimental investigations were conducted by measuring the steady-state temperatures of two radiative coolers and two reference Al plates in three typical weather conditions in Singapore, namely clear, 9
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Solar Energy Materials and Solar Cells 206 (2020) 110270 [28] C. Tso, K. Chan, C.Y. Chao, A field investigation of passive radiative cooling under Hong Kong’s climate, Renew. Energy 106 (2017) 52–61. [29] T. Suichi, A. Ishikawa, Y. Hayashi, K. Tsuruta, Performance limit of daytime radiative cooling in warm humid environment, AIP Adv. 8 (5) (2018), 055124. [30] M. Hu, B. Zhao, X. Ao, Y. Su, G. Pei, Parametric analysis and annual performance evaluation of an air-based integrated solar heating and radiative cooling collector, Energy 165 (2018) 811–824. [31] C. Liu, Y. Wu, B. Wang, C. Zhao, H. Bao, Effect of atmospheric water vapor on radiative cooling performance of different surfaces, Sol. Energy 183 (2019) 218–225. [32] R.Y. Wong, C. Tso, C.Y. Chao, B. Huang, M. Wan, Ultra-broadband asymmetric transmission metallic gratings for subtropical passive daytime radiative cooling, Sol. Energy Mater. Sol. Cells 186 (2018) 330–339. [33] B. Bhatia, A. Leroy, Y. Shen, L. Zhao, M. Gianello, D. Li, T. Gu, J. Hu, M. Solja�ci�c, E. N. Wang, Passive directional sub-ambient daytime radiative cooling, Nat. Commun. 9 (1) (2018) 5001. [34] B. Zhao, X. Ao, N. Chen, Q. Xuan, M. Hu, G. Pei, General strategy of passive subambient daytime radiative cooling, Sol. Energy Mater. Sol. Cells 199 (2019) 108–113. [35] SMARTS. https://www.nrel.gov/grid/solar-resource/smarts.html, 2019. [36] MODTRAN, available from: http://modtran.spectral.com/modtran_home, 2019. [37] 3M Vikuiti™ Enhanced Specular Reflector Film (ESR). [38] The National Weather Service obtained from. https://www.weather.gov /bgm/forecast_terms, 2019.
[19] M.M. Hossain, B. Jia, M. Gu, A metamaterial emitter for highly efficient radiative cooling, Adv. Opt. Mater. 3 (8) (2015) 1047–1051. [20] D. Wu, C. Liu, Z. Xu, Y. Liu, Z. Yu, L. Yu, L. Chen, R. Li, R. Ma, H. Ye, The design of ultra-broadband selective near-perfect absorber based on photonic structures to achieve near-ideal daytime radiative cooling, Mater. Des. 139 (2018) 104–111. [21] A.R. Gentle, G.B. Smith, A subambient open roof surface under the Mid-Summer sun, Advanced Science 2 (9) (2015) 1500119. [22] Y. Zhai, Y. Ma, S.N. David, D. Zhao, R. Lou, G. Tan, R. Yang, X. Yin, Scalablemanufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling, Science 355 (6329) (2017) 1062–1066. [23] Y. Peng, J. Chen, A.Y. Song, P.B. Catrysse, P.-C. Hsu, L. Cai, B. Liu, Y. Zhu, G. Zhou, D.S. Wu, Nanoporous polyethylene microfibres for large-scale radiative cooling fabric, Nat. Sustain. 1 (2) (2018) 105. [24] Z. Huang, X. Ruan, Nanoparticle embedded double-layer coating for daytime radiative cooling, Int. J. Heat Mass Transf. 104 (2017) 890–896. [25] H. Bao, C. Yan, B. Wang, X. Fang, C. Zhao, X. Ruan, Double-layer nanoparticlebased coatings for efficient terrestrial radiative cooling, Sol. Energy Mater. Sol. Cells 168 (2017) 78–84. [26] J. Mandal, Y. Fu, A.C. Overvig, M. Jia, K. Sun, N.N. Shi, H. Zhou, X. Xiao, N. Yu, Y. Yang, Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling, Science 362 (6412) (2018) 315–319. [27] M.M. Hossain, M. Gu, Radiative cooling: principles, progress, and potentials, Advanced Science 3 (7) (2016), 1500360.
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Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
Preliminary study of passive radiative cooling under Singapore’s tropical climate Di Han, Bing Feng Ng *, Man Pun Wan School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore, 639798
A R T I C L E I N F O
A B S T R A C T
Keywords: Radiative cooling Solar irradiance Humidity Photonics Tropical climate
Sub-ambient cooling can be achieved through radiative coolers that selectively emit radiation within the at mospheric window (8–13 μm) to outer space and suppress absorption/emission of other wavelengths. This study explores the feasibility of adopting radiative cooling in the hot and humid climate of Singapore through both numerical and experimental approaches. A theoretical simulation based on the heat transfer balance is first proposed to obtain the cooling power of the radiative cooler considering different solar spectral irradiance and total water vapor column. The larger solar irradiance in Singapore, especially within the ultraviolet and visible light spectrum where the absorbance of the material is relatively high, could counteract its cooling effects. Moreover, the increased atmospheric radiation induced by higher humidity and temperatures in Singapore could worsen cooling performances of the radiative material. Next, experimental investigations were conducted by measuring the steady-state temperatures of two radiative coolers (photonic radiative cooler and enhanced specular reflector film) under three typical weather conditions in Singapore, namely clear, partly cloudy and cloudy skies. While both radiative coolers were unable to achieve daytime cooling performance on a clear day, the enhanced specular reflector (ESR) film with higher solar reflectance can reach sub-ambient temperatures on a cloudy day. When it comes to night-time, the steady-state temperature of the photonic radiative cooler and ESR film was about 3.5 � C and 5 � C lower than ambient, respectively.
1. Introduction The challenge faced by many developed and developing countries is in the rising energy demand requirements for heating and cooling of indoor spaces [1]. In Singapore, up to 20% of total energy consumption is used for the cooling and regulation of indoor environment through air-conditioning and mechanical ventilation systems [2]. Such active means of cooling consumes large amounts of electrical energy and the demands are elevated in the daytime under the tropical climate where solar radiation is particularly high. Therefore, it is essential to find a solution to reduce energy consumption for the cooling of indoor spaces. Among the various cooling options, passive radiative cooling [3–6] that requires no energy input offers an attractive solution to the challenge. Passive radiative cooling relies on the earth’s atmosphere, which is a mixture of gases including N2, O2, CO2 and water vapor [7]. These compounds create a semi-transparent medium that can emit, absorb and scatter radiation across a wide range of the electromagnetic spectrum. However, there is a specific window in the 8–13 μm range where the sky is apparently transparent. Hence, a radiative cooler with well-designed
spectrum can be brought below ambient temperatures by selectively emitting radiation within the atmospheric window to the outer space and suppressing absorption/emission of other wavelengths. At night, radiative cooling can be achieved, either through black body envelope that emits across the entire electromagnetic spectrum or through metamaterials that emit selectively within the atmospheric window. The cooling performances of white TiO2 paint (which has near blackbody emissivity in the entire thermal infrared range) was compared against black paint [8] and the results showed that their temperature difference was 11 � C at 0.5 kPa vapor pressure and 6 � C at 2 kPa vapor pressure. Catalanotti et al. [3] demonstrated that a thin film of TEDLAR (polyvinyl–fluoride plastic) coated on top of an evaporated aluminium substrate can achieve 12 � C below ambient. Additionally, Granqvist and Hjortsberg [9] measured a thin film of SiO evaporated on top of an aluminium substrate, which can achieve 14 � C below ambient and has 61 W/m2 cooling power under 12 � C night temperature and 90% relatively humidity conditions. While most experiments reported an average of 10 � C below ambient at night, the limits can be pushed to 40 � C below ambient for a well-insulated radiative cooler at the Atacama
* Corresponding author. E-mail addresses: [email protected] (D. Han), [email protected] (B.F. Ng), [email protected] (M.P. Wan). https://doi.org/10.1016/j.solmat.2019.110270 Received 14 July 2019; Received in revised form 29 October 2019; Accepted 30 October 2019 Available online 15 November 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.
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desert [10]. However, the temperature depression was only in the range of 1–6 � C below ambient under Thailand’s hot and humid night sky [11], where humidity played a significant role. In the day, solar radiation heats up the structure and 10% of solar absorption (0.3–2.5 μm) is sufficient to counteract the cooling effects of the radiative cooler [7]. Nevertheless, given that the wavelength of solar radiation is below that of the atmospheric window, a selective radiative cooler that reflects incident solar irradiance at wavelengths below 2.5 μm and emits radiation selectively in the atmospheric window can theoretically provide cooling effects in the day [12,13]. Recently, sig nificant progress has been made in the development of radiative coolers. In 2014, Raman et al. [14] achieved sub-ambient daytime radiative cooling for the first time, producing a temperature drop of 4.9 � C below ambient and cooling power of 40.1 W/m2 on a clear winter day in Cal ifornia. Utilizing engineered photonic materials with reflective surfaces, the cooler possessed strong thermal emission within 8–13 μm and had very high solar reflectance (97%). In 2016, Chen et al. [15] fabricated a selective emitter with layers of silicon nitride (Si3N4), amorphous silicon (Si) and aluminium (Al) on top of a Si wafer. The emitter recorded significant temperature reductions of 42 � C in winter and 37 � C in summer (both in California) with sun shade and vacuum chamber. One year later, Kou et al. [16] observed a sizeable cooling power of 127 W/m2 and a temperature difference of 8.2 � C under direct sunlight on a clear day in California with a polymer-silica-mirror. The mirror was fabricated by coating fused silica wafer with polydimethylsiloxane (PDMS) and silver film as a top layer and a back reflector, respectively. In 2018, Yang et al. [17] achieved record-high total solar reflectance (99%) for daytime radiative cooling with a dual-layer structure that is composed of a polytetrafluoroethylene (PTFE) on top of a silver film and a glass substrate. Most recently in 2019, Ao et al. [18] compared the performance of a specular and a diffuse surface for daytime radiative cooling, which revealed an equilibrium temperature of 2.5 � C and 1.5 � C lower than the surroundings when the solar intensity was about 430 W/m2 in Beijing. Apart from conventional materials, optimized conical-shaped multi-layered structures can theoretically also possess cooling power under direct sunlight [19,20]. To scale up the design and application of radiative cooling, polymer and fabric have proven to be feasible materials [21–23], together with cost-effective coatings for large area application [24–26]. It should be highlighted that the experiments conducted thus far are geographically situated in the temperate region with low humidity, solar radiation and minimal cloud cover. However, the potential of daytime radiative cooling under hot and humid climates has remained doubtful as the amount of solar radiation received and atmospheric water vapor can significantly influence the efficiency of radiative cooling [6,27]. A case in point as demonstrated in Hong Kong, radiative coolers are more effective in the night as compared to the day for cooling under hot and humid weather conditions [28]. Even with the same radiative cooler as Raman et al. [14] with vacuum chamber, daytime sub-ambient cooling performance could not be achieved. On a similar note, Bao et al. [25] fabricated a double-layer nanoparticle-based coating that comprise of a top reflective layer (TiO2) and bottom emitting layer (SiO2 or SiC). As the coating had 90.7% reflectance over the solar spectrum and 90.11% emittance within the atmospheric window, it could only achieve above ambient conditions (3–10 � C) in Shanghai where the temperature and relative humidity was around 30 � C and 70%, respectively. Studies on the effects of humidity on radiative cooling were also carried out recently. Suichi et al. [29] investigated the performance limit of a cooling device with alternating layers of SiO2 and poly (methyl methacrylate) (PMMA) on an Al mirror in the warm and humid envi ronment of Okayama, Japan. The device achieved about 2.8 � C above ambient (35 � C) with solar reflectivity of 89% and 72% emittance within the atmospheric window. The authors also calculated the averaged at mospheric transmittance and the resulting equilibrium temperature as a function of the precipitable water vapor. Likewise, Hossain and Gu [27] theoretically compared the cooling performance of radiative coolers in
two cities in Australia with different humidity levels. The results revealed poorer efficiency under lower atmospheric transmittance conditions. In addition, Hu et al. [30] simulated the impact of relatively humidity on the cooling performance of the integrated solar heating and radiative cooling collector. The authors observed a 62% decrease in cooling power when relative humidity is increased from 5% to 95%. Liu et al. [31] also performed a detailed investigation on the effects of total water vapor column (TWC) on radiative cooling performance through both simulations and experiments, which found that cooling power decreased by 86.6 W/m2 with increasing TWC. Evidently, humidity lowers atmospheric transmittance that increases downwelling atmo spheric radiation substantially. To recover cooling performances under humid climates, Wong et al. [32] suggested to integrate the cooler with an asymmetric electromagnetic transmission window, which can reflect downwelling atmospheric radiation. In addition, the performance of radiative coolers is sensitive to regional changes in solar intensity and a directional approach with angular confinement of solar radiation can potentially reduce the incoming solar energy [33,34]. With solar radiation and humidity playing critical roles in deter mining the performance of radiative cooling, the application of the technology becomes geographically specific. This paper is intended to make a preliminary study of passive radiative cooling under Singapore’s unique tropical climate that is highly influenced by its geographical location as a maritime country and latitude positioning close to the equator. A theoretical simulation based on the heat transfer balance is proposed to obtain cooling power of the radiative cooler with different solar spectral irradiance and total water vapor column. The effect of regional solar radiation and humidity in the daytime and the effect of humidity in the night on cooling performances are explained by modelling several climate conditions. In addition, experiments were performed to obtain steady-state temperatures of radiative coolers under three typical weather conditions in Singapore, namely clear, partly cloudy and cloudy. 2. Theoretical modelling 2.1. Heat transfer balance Consider a radiative cooler whose surface temperature is Ts under direct sunlight and exposed to the sky, the net cooling power Pcool can be calculated as: Pcool ðTs Þ ¼ Prad ðTs Þ
Patm ðTamb Þ
Psolar
Pconvþcond ðTs ; Tamb Þ
where the power radiated out by the radiative cooler surface is: Z π=2 Z ∞ sinθcosθ IB ðTs ; λÞεðλ; θÞdθdλ Prad ðTs Þ ¼ 2π 0
0
Here IB ðTs ; λÞ ¼ 2hc λ5
2
1 ehc=λkB Ts 1
(1)
(2)
is the spectral radiance of a blackbody by
Planck’s law at the surface temperature, where h is the Planck’s con stant, c is the velocity of light, λ is the wavelength, kB is the Boltzmann constant. εðλ; θÞ is the spectral and angular emissivity of the radiative cooler. The absorbed power from the downwelling atmospheric radiation can be expressed as: Z ∞ Z π=2 Patm ðTamb Þ ¼ 2π sinθcosθ IB ðTamb ; λÞεatm ðλ; θÞεðλ; θÞdθdλ (3) 0
0
2
where Tamb is the ambient temperature. IB ðTamb ; λÞ ¼ 2hc λ5
1 ehc=λkB Tamb 1
is the
spectral radiance of a blackbody by Planck’s law at ambient temperature Tamb . εatm ðλ; θÞ is the wavelength and angle-dependent emissivity of the atmosphere, which is related to the atmospheric transmittance tðλÞ in the zenith direction and given by εatm ðλ; θÞ ¼ 1 tðλÞ1=cos θ [9]. The incident solar irradiance absorbed by the radiative cooler is 2
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Fig. 1. Simulated solar spectral irradiance from SMARTS and (a) spectral reflectivity of the radiative cooler within the solar spectrum (b) corresponding spectral absorbance of the radiative cooler within UV and VIS spectrums.
Fig. 2. (a) Total solar irradiance in different regions. (b) Absorbed solar irradiance in different regions.
given by: Z ∞ Psolar ¼ εðλ; θsolar ÞIsolar ðλÞdλ 0
[12], 10% of solar absorption is already sufficient to counteract the cooling effects of the radiative cooler. To achieve meaningful daytime radiative cooling in practice, Raman et al. [14] indicated that average solar reflectivity must be more than 94%. However, their results were obtained from AM 1.5 global titled solar irradiance for the solar illu mination Isolar , which is representative of the average continental U.S. and mid-latitude area. For other areas, SMARTS (Simple Model of the Atmospheric Radiative Transfer of Sunshine) developed by NREL (Na tional Renewable Energy Laboratory) can be used to compute clear sky spectral irradiances [35]. To illustrate the difference in solar irradiance based on geographical location, three different locations, namely California, Hong Kong and Singapore were simulated using SMARTS with local input parameters in the configuration. As shown in Fig. 1 (a), total solar irradiance is highest at about 1001.8 W/m2 (Fig. 2 (a)) in Singapore, which is a tropical country situated close to the equator. The difference in spectral irradi ance between the three locations are particularly evident in the ultra violet and visible light wavelengths as shown in Fig. 1 (b). The ratio of the ultraviolet and visible light in the total solar irradiance is about 53% in Singapore as compared to that of 41% in California shown in Fig. 2 (a). As California is in the temperate region while Hong Kong is in the subtropical region, the same radiative cooler is expected to perform differently in the two cities. When the spectral irradiance is compared to the reflectivity of radiative coolers within the solar spectrum, the per formance of the coolers can be clearly observed. In Fig. 1, the reflectivity of a radiative cooler that is identical to the one used in Refs. [14,28] is
(4)
where Isolar represents the solar illumination. Eq. (4) does not have the angular integral when the radiative cooler is at a fixed angle. Lastly, the power loss due to the parasitic heat gain (convection and conduction) of the structure is: Pconvþcond ðTs ; Tamb Þ ¼ hc ðTamb
Ts Þ
(5)
where hc is the combined non-radiative heat transfer coefficient from the convective and conductive thermal contact of the structure with ambient air. In general, Eq. (1) is the net cooling power Pcool of the surface as a function of the surface temperature Ts and ambient temperature Tamb . If the value of Pcool is positive when Ts ¼ Tamb , the surface can produce cooling power, which indicates that the surface emits more energy to space than the absorbed radiation from the sunlight and atmosphere. The solution to Eq. (1) when Pcool ¼ 0 represents the steady-state tem perature Ts that is below ambient Tamb if the radiative cooler can pro duce cooling power. 2.2. Daytime solar radiation Solar radiation Psolar is an important factor affecting the performance of daytime radiative cooling and is large compared to atmospheric ra diation Patm and parasitic heat gain Pconvþcond in Eq. (1). In Rephaeli et al. 3
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Fig. 3. (a) Modelled atmospheric transmittance for three typical climates (the dotted lines indicate the range for the atmospheric window). (b) Simulated atmo spheric radiation absorbed by the radiative cooler with different ambient temperature and TWC.
downwelling atmospheric radiation is increased substantially according to εatm ðλ; θÞ in Eq. (3). Atmospheric water vapor content can be repre sented using the total water vapor column (TWC) that denotes the total gaseous water in a vertical column of atmosphere, which is influenced by both the local temperature and humidity. Here, the level of atmospheric transmittance is modelled using MODTRAN [36] with typical TWC values under three different climatic conditions as shown in Fig. 3 (a). TWC values of 1059.7 atm-cm, 3635.9 atm-cm and 5119.4 atm-cm are representative of mid-latitude winter, subtropical winter and tropical climates, respectively. As TWC in increased, atmospheric transmittance within the atmospheric window (8–13 μm) decreases from over 80% to below 60%. Also, the second atmospheric window (16–20 μm) as indicated in Ref. [29] is missing due to high levels of humidity. The atmospheric radiation absorbed by the radiative cooler can be calculated according to Eq. (3) and the results are shown in Fig. 3 (b). Evidently, the radiative cooler will absorb more downward atmospheric radiation with increase in humidity. In addition, the difference in absorbed energy between different humidity levels is enlarged with increased ambient temperature, which can be understood through Planck’s law in Eq. (3). The enhancement in absorbed atmo spheric radiation is about 18 W/m2 as TWC is increased from 1059.7 atm-cm to 5119.4 atm-cm when the ambient temperature is 0 � C. This difference is enlarged to 34.6 W/m2 when the ambient temperature is 40 � C. Compared to the cool and dry weather in mid-latitude winter, cooling performances of radiative coolers are restrained in hot and humid environments due to the increased absorbance of atmospheric radiation, for instance in Singapore where TWC levels are high at 5119.4 atm-cm. To illustrate the combined effects of solar radiation and humidity on cooling performances in the daytime, the net cooling power for the same reference and measured radiative coolers were calculated according to Eqs. (1)–(5) and shown in Fig. 4. The local solar spectral irradiance was obtained from SMARTS and the atmospheric transmittance calculated by MODTRAN using the local humidity and ambient temperature in three different regions. Meanwhile, the experimental data obtained in California and the cooling power calculated from our measured reflec tivity (case 2) of the same radiative cooler as in Ref. [14,28] can be seen in Fig. 4. The cross points of the curves with the vertical dashed line when Ts Tamb ¼ 0 are the net cooling power at the local ambient temperature. The cross points of the curves with the horizontal dashed line when Pcool ¼ 0 are the steady-state temperature of the radiative cooler. As observed, the simulated and experimental results match well in the simulated case of California under the cool and dry climate. How ever, when it comes to the hot and humid environment in Hong Kong, the cooling power is reduced to 11.37 W/m2. The cooling performance is
Fig. 4. Calculated cooling power in different regions and the experimental data in California in the daytime. (The simulated case in CA, HK and case 1 in SG are based on the reference emissivity in Ref. [14] while the simulated case 2 is based on the measured emissivity.)
shown. The reference reflectivity is obtained from Ref. [14] while measured reflectivity is obtained from an identical radiative cooler purchased from the same manufacturer as Ref [14] and measured using a UV-VIS-NIR spectrometer (PerkinElmer-Lambda 1050). As observed in Fig. 1 (b), the corresponding absorbance by Kirchhoff’s law increases rapidly as wavelength decreases within the ultraviolet (300–400 nm) and visible light (400–700 nm) range. On the other hand, the measured reflectivity (average value of 93% in infrared range) is smaller than the reference reflectivity (average value of 97% in infrared range) as shown in Fig. 1 (a). Using reference values from Ref. [14] for reflectivity to calculate the solar power absorbed by the radiative cooler in Eq. (4), solar power absorbed is no more than 40 W/m2 in Singapore as shown in Fig. 2 (b). However, when using measured reflectivity of the identical cooler that was purchased, the absorbed solar radiation is over 100 W/m2. As the radiative cooler will absorb more solar irradiance in Singapore than in Hong Kong and California, this results worsened cooling performance in Singapore. 2.3. Daytime humidity Another factor affecting daytime radiative cooling is humidity. In general, atmospheric water vapor content lowers the level of atmo spheric transmittance and as a result, heat is being trapped and 4
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Fig. 8. Cross-section schematic of the apparatus through the middle.
Fig. 5. Calculated cooling power for different total water vapor column with Tamb ¼ 30 � C and hc ¼ 6.9 W/m2K in the night-time.
Fig. 9. Image of the rooftop measurement setup.
Besides, when using the measured emissivity of the material in Singapore (case 2), the cooling power is the lowest due to the absorbed solar energy, especially within the UV and VIS range. By comparing the simulated case 1 and 2 (major difference in reflectivity in the solar spectrum) in Singapore under the same humidity condition, the domi nant effect of the absorbed solar radiation on the cooling performance can be revealed. This indicates the importance of material reflectivity in the performance of daytime radiative cooling.
Fig. 6. Measured reflectivity of two radiative coolers within solar spectrum. (The AM 1.5 solar spectrum is plotted for reference.)
2.4. Night-time humidity The effect of humidity on radiative cooling can be clearly illustrated in the nighttime using our measured emissivity when there is no solar radiation, as shown in Fig. 5. Here, the net cooling power is calculated at the ambient temperature of Tamb ¼ 30 � C, combined with non-radiative heat transfer coefficient of hc ¼ 6.9 W/m2K and atmospheric trans mittance with varying TWC values obtained from MODTRAN. In gen eral, there will be cooling effects for all humidity conditions in the night with the lack of solar radiation. When TWC is increased, the net cooling power decreases rapidly from 80.9 W/m2 to 19.1 W/m2 when TWC increased from 0 atm-cm to 8000 atm-cm. It can be observed that cool ing power is more affected at lower TWC and when TWC is increased, the two curves of TWC ¼ 6324.3 atm-cm and 8000 atm-cm almost overlap with each other. As for the steady-state temperature, the radi ative cooler can be cooled to 9.3 � C below ambient when the TWC ¼ 0 atm-cm. When TWC is 8000 atm-cm, the temperature reduc tion is only 2.1 � C. Particularly when TWC ¼ 5119.4 atm-cm, which corresponds to the humidity in Singapore, the simulation result shows a temperature of 3 � C below the ambient.
Fig. 7. Emissivity of two radiative coolers within mid-infrared wavelengths. (A typical atmospheric transmittance is plotted for reference.)
further reduced (a negative cooling power of 13.45 W/m2) under the hotter and more humid condition in Singapore, indicating that subambient temperatures cannot be achieved using emissivity parameters from the reference cooler in Ref. [14], denoted as case 1 in Fig. 4. 5
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Fig. 10. (a) Measured temperatures of four samples against ambient air temperature on a clear day. (b) Corresponding temperature difference. (c) Real-time hu midity, solar irradiance and image of sky.
3. Experimental investigation
multi-layer optical film technology to create a multilayer polymeric solar mirror with ultra-high reflectivity (about 98% across the visible spectrum). The thin film consists of 300 layers of polyethylene tere phthalate (PET) including other polyester and has an overall thickness of 65 μm. It was indicated [21] that the combination of ESR film and silver substrate could provide suitable solar and IR responses for radiative cooling. Here, the ESR film was cut to a size of 152 mm � 102 mm and then coated with 300 nm of silver using magnetron sputtering method. Meanwhile, two aluminum plate were cut to a size of 100 mm � 100 mm as reference. One was painted carbon black and the other polished as a mirror surface. The spectral reflectivity of the photonic radiative cooler and ESR film were measured by a UV-VIS-NIR spectrometer (PerkinElmer-Lambda 1050) with a snap-in integrating sphere within the wavelengths of 0.3–3 μm and a Fourier transform infrared spectrometer (PerkinElmer Spectrum Two FT-IR Spectrometer) using a specular reflectance acces sory with a calibrated gold coated mirror as a reflectance standard for
3.1. Experimental setup Two types of radiative cooler are prepared for the rooftop experi ment to study the feasibility of radiative cooling in Singapore. One cooler is of the same material as that proposed in Ref. [14] and demonstrated in Hong Kong [28]. The photonic device consists of seven alternating layers of hafnium dioxide (HfO2) and silicon dioxide (SiO2) with varying thickness. The top three layers that are thicker serves to emit infrared in the 8–13 μm wavelengths while the four thinner bottom layers deposited on 200 mm Ag coated Si wafer is supposed to provide a 97% solar reflection. However, our measured reflectivity is only about 93% in the infrared range as shown in Fig. 1(a). More details on the design and performance of the material can be found in Ref. [14]. Another cooler is made of the commonly used enhanced specular reflector (ESR) film developed by 3 M [37]. Vikuiti ESR film utilizes 6
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Fig. 11. (a) Measured temperatures of four samples against ambient air temperature on a partly cloudy day. (b) Corresponding temperature difference. (c) Real-time humidity, solar irradiance and image of sky.
the wavelength region between 3 and 20 μm. The reflectivity of ESR film is close to 99% in the VIS spectrum, which is better than that of photonic radiative cooler with about 90% as shown in Fig. 6. However, the reflectivity of ESR film drops significantly in the near-infrared wave lengths where solar irradiance is small while the photonic radiative cooler possesses higher stable reflectivity in this region. In the midinfrared wavelengths as shown in Fig. 7, both materials show selec tively strong emissivity within the atmospheric window. The average emissivity of ESR film is 92%, much higher than that of the photonic radiative cooler. Overall, both of photonic radiative cooler and ESR film are selective radiative cooling emitters which are strongly reflective over solar spectrum and strongly emissive in the atmospheric window. The schematic diagram of the experimental setup for both the pho tonic device and ESR on the rooftop is shown in Fig. 8. The radiative cooler is supported by polystyrene foam of the same cross-sectional area and of 60 mm thickness with low heat conductivity to minimise heat conduction. The polystyrene foam is covered with aluminum foil to prevent heating from incident thermal radiation and placed at the centre of the chamber, which is formed of clear acrylic and supported by a wooded frame of 218 mm height. The top surface of the wooden frame is covered with 12 μm low-density polyethylene film that works as a wind
shield and to minimise heat convection with the surrounding. Another layer of aluminum foil is placed on top of the low-density polyethylene film to reflect solar radiation and to form an aperture for the radiative cooler. The surface temperatures of the two radiative coolers and two reference plates are measured by a K-type self-adhesive thermocouple (SA3-K-120, Omega), which is attached to the centre of the bottom surface of the sample as shown in Fig. 8. A K-type thermocouple probe is placed inside a 12-ring solar radiation shield (Delta Ohm) with free air flow and no solar radiation to capture ambient temperatures in the surrounding of the apparatus as shown in Fig. 9. Similarly, the relative humidity is obtained from a combined temperature and humidity sensor with an accuracy of �3% placed inside another solar radiation shield. The sum of direct solar irradiance and diffuse irradiance (global radia tion) is measured by a secondary standard pyranometer (LP PYRA 10, Delta Ohm) with mV output, which is placed near the radiative cooler on top of the wooden frame. The spectral range is 283–2800 nm and the response time is smaller than 6 s. The above data is acquired using a NI DAQ module connected to the laptop and the sampling time interval is 20 s.
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Fig. 12. (a) Measured temperatures of four samples against ambient air temperature on a cloudy day. (b) Corresponding temperature difference. (c) Real-time humidity, solar irradiance and image of sky.
3.2. Experimental results and discussions
temperature of the photonic radiative cooler and ESR film, respectively. A negative value of Tamb Tc or Tamb Te in the daytime indicates a hotter photonic device or ESR temperature, respectively, as compared to ambient air. As observed, Tamb Tc can reach a minimum value of about 15 � C. Meanwhile, the value of Tamb Te in the day was mostly negative, except for some occasions when the solar irradiance is at a trough as a result of cloud cover. The temperature difference between the photonic radiative cooler and ESR film was enlarged with larger solar irradiance (a maximum difference of over 7 � C). Overall, the sur face temperatures of both radiative coolers were dominated by incident solar irradiance in the daytime while the material with higher solar reflectivity (ESR film) could reach lower temperatures. However, no cooling performance was found on a clear day in Singapore due to the effect of larger solar radiation and humidity as discussed in Sections 2.2 and 2.3. When it comes to night-time as shown in Fig. 10, both radiative coolers are able to achieve temperatures below ambient. Compared to the results in California and Hong Kong of approximately 7 � C and 6 � C lower than the ambient, respectively, the photonic device was only about 3.5 � C lower than ambient due to the higher humidity in
Both daytime and night-time radiative cooling performances were measured for the two different radiative coolers in three typical weather conditions in Singapore, namely clear, partly cloudy and cloudy. The weather condition is estimated in terms of the percentage of sky covered by clouds, which is a common practice in meteorology [38]. Due to the constantly evolving sky condition, average cloud coverage obtained from the local weather station is used. The surface temperature profile of different samples on a clear day is shown in Fig. 10 (a). In general, the radiative cooler identical to Refs. [14,28] and ESR film are unable to produce cooling below ambient in the daytime. As a result of higher solar reflectance, their temperatures are much lower than the black paint Al plate (close to 100 � C) and polished Al plate (close to 75 � C) when the solar irradiance is about 1017.87 W/m2. It can also be observed that the temperature of the ESR film is lower than that of the photonic radiative cooler with a higher average solar reflectivity, especially in the VIS spectrum. The corresponding temperature difference and real-time weather conditions can be found in Fig. 10 (b) (c), where Tc and Te represent the 8
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Singapore. The approximate value is also obtained from our simulation results for a steady-state temperature of 3 � C below the ambient when TWC ¼ 5119.4 atm-cm. On the other hand, the ESR film is able to ach ieve around 5 � C lower than ambient, which is almost similar in per formance as the black painted Al plate. This indicates strong emission within the atmospheric window and better cooling performance than the photonic device. Another indicator of cooler performance is the transition time in the late afternoon when the temperature difference between ambient air and radiative cooler changes from a negative to positive value. The value of Tamb Te would reach a positive value earlier than Tamb Tc . In addition, the corresponding solar irradiance during this transition time for the photonic device and ESR film is about 116 W/m2 and 300 W/m2, respectively, which also supports that ESR film has better cooling potential. The surface temperature profile of different samples and real-time weather conditions on a partly cloudy day are shown in Fig. 11. Dur ing daytime, the surface temperatures of the radiative coolers were mainly influenced by the solar irradiance. As compared to the clear day, there were more temperature fluctuations induced by intermittent cloud cover but there was also no sustained cooling below ambient during daytime. Overall, the temperature of the ESR film is still smaller than that of the photonic device in the daytime. As expected, the cooling performance would appear in the night-time and ESR film could produce larger cooling power. To further illustrate the effect of clouds cover on the cooling per formance, measurements were taken on a cloudy day and the corre sponding surface temperature profile and real-time weather conditions are shown in Fig. 12. On this day, after 3pm, the sky was totally covered with clouds, which can also be seen from the decreased solar irradiance and higher relative humidity in Fig. 12 (c). The corresponding solar irradiance at 3.30pm was about 250 W/m2, smaller than the 300 W/m2 on a clear day and the ESR film was cooled to below ambient. On the other hand, the temperature of the photonic radiative cooler dropped to below ambient at around 5 p.m. with the solar irradiance of 50 W/m2, smaller than the 116 W/m2 on a clear day. To some extent, cloud cover could improve the cooling performance of the radiative coolers by blocking away incident solar radiation, making daytime radiative cooling possible. On the other hand, cloud cover also prevents the transmission of emitted infrared radiation of the material, which suppresses the net cooling power in return. As the ESR film of high reflectivity can achieve daytime radiative cooling and much earlier than the photonic device, the reflectivity of the radiative cooling material should be high enough to achieve practical daytime radiative cooling, particularly within the UV and VIS spectrum.
partly cloudy and cloudy skies. Both radiative coolers were not able to achieve daytime cooling performance on a clear day due to high solar radiation and humidity. However, on a cloudy day, the ESR film with higher solar radiation reflectivity can reach sub-ambient temperatures. During the night, the steady-state temperature of the photonic device and ESR film was about 3.5 � C and 5 � C lower than the ambient, respectively. In summary, this preliminary study demonstrated the feasibility of passive radiative cooling in Singapore. Further improvements in the present cooler to successfully produce daytime cooling effects under Singapore’s climate can be achieved if the structure can be designed to reduce solar energy absorption within UV and VIS spectrum. 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 This study was funded by the Singapore Ministry of Education through grant no. 2018-T1-001-070 and supported through a start-up grant by Nanyang Technological University M4082022. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110270. References [1] L. P�erez-Lombard, J. Ortiz, C. Pout, A review on buildings energy consumption information, Energy Build. 40 (3) (2008) 394–398. [2] Building Construction Authority Singapore, Building Energy Benchmarking Report, 2014. [3] S. Catalanotti, V. Cuomo, G. Piro, D. Ruggi, V. Silvestrini, G. Troise, The radiative cooling of selective surfaces, Sol. Energy 17 (2) (1975) 83–89. [4] A.R. Gentle, G.B. Smith, Radiative heat pumping from the earth using surface phonon resonant nanoparticles, Nano Lett. 10 (2) (2010) 373–379. [5] A. Gentle, J. Aguilar, G. Smith, Optimized cool roofs: integrating albedo and thermal emittance with R-value, Sol. Energy Mater. Sol. Cells 95 (12) (2011) 3207–3215. [6] X. Lu, P. Xu, H. Wang, T. Yang, J. Hou, Cooling potential and applications prospects of passive radiative cooling in buildings: the current state-of-the-art, Renew. Sustain. Energy Rev. 65 (2016) 1079–1097. [7] M. Zeyghami, D.Y. Goswami, E. Stefanakos, A review of clear sky radiative cooling developments and applications in renewable power systems and passive building cooling, Sol. Energy Mater. Sol. Cells 178 (2018) 115–128. [8] B. Kimball, Cooling performance and efficiency of night sky radiators, Sol. Energy 34 (1) (1985) 19–33. [9] C. Granqvist, A. Hjortsberg, Radiative cooling to low temperatures: general considerations and application to selectively emitting SiO films, J. Appl. Phys. 52 (6) (1981) 4205–4220. [10] M.I. Ahmad, H. Jarimi, S. Riffat, Potentials of Nocturnal Cooling in Various Locations/Countries and Climatic Conditions, Nocturnal Cooling Technology for Building Applications, Springer, 2019, pp. 51–61. [11] J. Khedari, J. Waewsak, S. Thepa, J. Hirunlabh, Field investigation of night radiation cooling under tropical climate, Renew. Energy 20 (2) (2000) 183–193. [12] E. Rephaeli, A. Raman, S. Fan, Ultrabroadband photonic structures to achieve highperformance daytime radiative cooling, Nano Lett. 13 (4) (2013) 1457–1461. [13] K. Chen, M. Ono, S. Fan, L. Wei, Self-adaptive radiative cooling based on phase change materials, Opt. Express 26 (18) (2018) A777. [14] A.P. Raman, M.A. Anoma, L. Zhu, E. Rephaeli, S. Fan, Passive radiative cooling below ambient air temperature under direct sunlight, Nature 515 (7528) (2014) 540. [15] Z. Chen, L. Zhu, A. Raman, S. Fan, Radiative cooling to deep sub-freezing temperatures through a 24-h day–night cycle, Nat. Commun. 7 (2016) 13729. [16] J.-l. Kou, Z. Jurado, Z. Chen, S. Fan, A.J. Minnich, Daytime radiative cooling using near-black infrared emitters, ACS Photonics 4 (3) (2017) 626–630. [17] P. Yang, C. Chen, Z.M. Zhang, A dual-layer structure with record-high solar reflectance for daytime radiative cooling, Sol. Energy 169 (2018) 316–324. [18] X. Ao, M. Hu, B. Zhao, N. Chen, G. Pei, C. Zou, Preliminary experimental study of a specular and a diffuse surface for daytime radiative cooling, Sol. Energy Mater. Sol. Cells 191 (2019) 290–296.
4. Conclusion In this work, the performance of passive radiative cooling under Singapore’s tropical climate was investigated through both theoretical simulations and experiments. Through the simulations, the effects of solar radiation and humidity on the performance of radiative coolers were discussed. It was found that daytime cooling performance was mainly influenced by regional solar irradiance and humidity. The larger solar irradiance in Singapore, especially within the UV and VIS spectrum where the absorbance of the material is relatively high, could negate any cooling effects. Moreover, the increased atmospheric radiation induced by higher humidity and ambient temperature in Singapore could worsen the cooling performance. The net cooling power based on the heat transfer balance model showed that the radiative cooler could not pro duce cooling power in the daytime. In the night-time, cooling could be achieved and was mainly influenced by the total water vapor column. The calculated net cooling power decreased from 80.9 W/m2 to 19.1 W/ m2 with the increase of TWC from 0 atm-cm to 8000 atm-cm. A series of experimental investigations were conducted by measuring the steady-state temperatures of two radiative coolers and two reference Al plates in three typical weather conditions in Singapore, namely clear, 9
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