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Yttria-stabilized zirconia coating for passive daytime radiative cooling in humid environment
Applied Thermal Engineering 165 (2020) 114585
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Yttria-stabilized zirconia coating for passive daytime radiative cooling in humid environment ⁎
Jianshu Fana, Ceji Fua, , Tairan Fub, a b
T
⁎
LTCS and Department of Mechanics and Engineering Science, College of Engineering, Peking University, Beijing 100871, China Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China
H I GH L IG H T S
Yttria-Stabilized Zirconia coating for passive daytime radiative cooling. • Proposed improved passive daytime radiative cooling under high humidity. • Achieved mechanisms for improved radiative cooling. • Elucidated • Achieved more stable net cooling power subjected to changing humidity.
A R T I C LE I N FO
A B S T R A C T
Keywords: Passive daytime radiative cooling Solar irradiation Atmospheric transparency window Humidity Yttria-stabilized zirconia
Passive daytime radiative cooling is a promising new field for solving the energy shortage all over the world. The performance of daytime radiative cooling is limited by several factors such as non-radiative heat transfer, outdoor-environment erosion and atomsphere humidity. We propose in this work a 8 wt% yttria stabilized zirconia (8YSZ) coated SiO2 (glass)/Ag radiative cooler for improving daytime radiative cooling performance. Numerical results show that the proposed radiative cooler can achieve a cooling power of 95.1 W/m2 under direct AM1.5 solar irradiation and dry environment. In addition, the cooler temperature can drop by 10.3 ℃ below the ambient under direct solar irradiation and 1 m/s wind speed. We also evaluate the cooling performance of the proposed structure under different humidity and it is remarkable to find that the 8YSZ coated radiative cooler can boost the net cooling power under high humidity, compared to that without 8YSZ coating. This proposed structure may have potential practical applications in daytime radiative cooling due to its fantastic corrosion resistance, anti-degeneration and adaptability to high as well as rapidly changing humidity.
1. Introduction Passive radiative cooling that does not need external input of electricity is an appealing strategy for energy saving in the 21st century and it has drawn much attention [1–9]. In particular, achieving effective cooling power during the daytime is the most attractive merit of this technology since the cooling demand peaks in the daytime hours [1]. It is also promising to be alternative to the conventional active cooling systems such as air conditioners and the efficiency of outdoor energy conversion devices like solar cells and thermoelectric converters can be improved if they are made to fulfill the function of radiative cooling as well [2,3,5,10,11]. In principle, passive radiative cooling relies on the cooler to radiate heat out to the cold outer space as much as possible through the infrared (IR) atmospheric transparency windows while absorb almost zero of the solar irradiation [6]. Hence, the emissivity of
⁎
the cooler surface should be spectrally-selective. In the past several years, different engineered structures [1,6,7,12–19] have been proposed for the spectrally-selective radiative cooler, among which scalable structures such as the polymer-glass bead composite [16], the polymer (PDMS)-coated glass wafer [1], and the hierarchically porous polymer coating [19] are the typically attractive cases. However, the properties of many polymers have been proved to be denatured outdoors such as the degeneration, yellowing and oxidation under longtime direct sunshine [20] or degraded elasticity of PDMS under timevarying temperature conditions [21]. Furthermore, the non-radiative heat transfer effect would also reduce the net cooling power of the cooler at sub-ambient temperatures [22]. Therefore, it is meaningful to propose a simple daytime radiative cooler structure that is corrosionresistant and can improve the radiative cooling performance under direct solar irradiation. Thermal barrier coatings (TBCs) are low-
Corresponding authors. E-mail addresses: [email protected] (C. Fu), [email protected] (T. Fu).
https://doi.org/10.1016/j.applthermaleng.2019.114585 Received 6 July 2019; Received in revised form 3 October 2019; Accepted 24 October 2019 Available online 25 October 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
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coating on its top side and an Ag film at its bottom side. The 8YSZ coating can tailor effectively the emissivity of the cooler in the 1st and the 2nd atmospheric transparency windows, while the Ag film acts as a reflector. The layer thicknesses of the 8YSZ coating, the glass slab and the Ag film are labeled in Fig. 1 as d1, d2 and d3, respectively. In this work, the optical constants of 8YSZ are taken as those obtained by Nychka et al. [27], while the optical properties of SiO2 (glass) are from the tabulated values in Palik’s handbook [28]. The refractive indices of 8YSZ and glass, and their corresponding extinction coefficients varying with wavelength are shown in Fig. 2(a) and (b), respectively. It can be seen that the refractive index of glass remains almost constant while its extinction coefficient is very small in the solar spectrum. In the mid-infrared region, however, the refractive index of glass becomes highly dependent on wavelength and its extinction coefficient is larger by three to five orders of magnitude than that in the solar spectrum. Therefore, a thin glass film can be highly emissive in the mid-infrared region while is transparent in the solar spectrum. This spectrally-selective emissivity and transmissivity of a glass film makes it a suitable candidate for use in passive daytime radiative cooling [1,16]. The optical constants of 8YSZ are quite similar to those of glass and a thin 8YSZ film can be highly emissive in the 1st atmospheric transparency window. However, the extinction coefficient of 8YSZ is so large in the 2nd atmospheric transparency window that a thin 8YSZ film can be highly reflective in this region. For the dielectric function of Ag, the Drude model is adopted [29]
thermal conductivity ceramics which have been extensively applied in advanced turbine components, such as aircraft engines and ground gas turbines, for achieving increased efficiency, performance and durability [23]. From the 1970s to the present, 8 wt% yttria stabilized zirconia (8YSZ) has been the most widely used material for TBC on account of its low density, low thermal conductivity, good thermal shock resistance and fantastic corrosion resistant properties [24]. In this work, we present the design and performance analysis of a passive daytime radiative cooler made of a SiO2 (glass) layer coated with a 8YSZ film on the top and a silver (Ag) film at the bottom as a reflector. Humidity is one the most important factor that affects significantly the performance of radiative cooling and has been the focus of some recent studies [5,25]. Total precipitable water (TPW) is a critical parameter for quantifying humidity of the atmosphere, which is defined as the depth of water in a column of the atmosphere if it is totally precipitated as rain. Potential promising applications of passive daytime radiative cooling may be found in the typical densely-populated and industrial areas around 39° north latitude, where located are many metropolises such as Rome, Beijing, San Francisco, New York and Madrid, etc. In these areas the NCEP-NCAR Reanalysis climite data [26] show that the averaged TPW on the ground mostly ranges from 1 mm to 20 mm. However, most studies, including experimental and numerical analyses, on passive daytime radiative cooling are under dry envioronment (TPW < 10 mm). Suichi et al. [25] showed that while the transmissivity of the atmosphere in the 1st atmospheric transparency window (8–13 μm) decreases slightly due to the increase of humidity, vast decrease of it is found in the 2nd atmospheric transparency window (16–25 μm). Therefore, the cooling power of radiative coolers having high emissivity not only in the 1st atmospheric transparency window, but also in the 2nd one [1,16] may drop dramastically once humidity is increasing. Humidity change with varying weather conditions could bring about synchronous change of the cooling power, which could lower the stability of the radiative cooling perfoarmance. Particularly, Chen et al. [7] found that the radiative cooler can be heated up by the absorbed radiative energy from the atmosphere if it has a high emissivity in the 2nd atmospheric transparency window. Here, we evaluate the cooling performance of the proposed 8YSZ/SiO2/Ag structure under different humidity and show that such a radiative cooler can operate quite stably even under high humidity fluctuation conditions and adding the 8YSZ coating could lead to great improvement in the cooling power when the humidity is high.
ωp2
ε (ω) = ε∞ −
ω2
+ iγω
(1)
where the high-frequency constant ε∞ = 3.4 , the plasma frequency ωp = 1.39 × 1016 rad/s, the scattering rate γ = 2.7 × 1013 rad/s. In the present study, this Drude model is used for wavelength of λ > 1.9 μm and the optical constants of Ag are modified to the measured data by Johnson and Christy [30] when λ ⩽ 1.9 μm, according to Yang et al. [31]. The energy balance in passive daytime radiative cooling is schematically shown in Fig. 3. The net radiative cooling power under direct solar irradiation measures the performance of the proposed cooler, which can be calculated as [6]:
Pcool (T ) = Prad (T ) − Patm (Tamb) − PSun − Pcond + conv
(2)
where Prad (T) represents the power emitted by the cooler at temperature T and is expressed as [6]: 2. Structural design and theoretical analysis
Prad (T ) = A The proposed structure for passive daytime radiative cooling is shown in Fig. 1, which consists of a SiO2 (glass) slab with an 8YSZ
∫ dΩ cos θ ∫0
∞
dλIBB (T , λ ) ε (λ, θ)
(3)
where A is the surface area of the cooler, θ is the incident angle and π /2 dθ sin θ denotes the integration over a hemisphere.
∫ dΩ = 2π ∫0
2hc 2
1
IBB (T , λ ) = λ5 hc /(λkB T ) is the spectral intensity of a blackbody at −1 e temperature T, with h Planck’s constant, c the speed of light in vacuum, λ the wavelength and kB Boltzmann’s constant. ε (λ, θ) represents the spectral directional emissivity of the cooler and can be expressed asε (λ, θ) = 0.5[εTE (λ, θ) + εTM (λ, θ)], withεTE (λ, θ) andεTM (λ, θ) being the spectral directional emissivity for transverse electric (TE) and transverse magnetic (TM) waves, respectively. The thermal irradiance from the atmosphere that is absorbed by the cooler is expressed as [6]:
Patm (Tamb) = A
∫ dΩ cos θ ∫0
∞
dλIBB (Tamb, λ ) ε (λ, θ) εatm (λ, θ)
(4)
where Tamb is the ambient air temperature and is assumed 300 K in this work. εatm (λ, θ) denotes the spectral directional emissivity of the atmosphere and is given by εatm (λ, θ) = 1 − t (λ )1/cos θ with t(λ) the atmospheric transmissivity in the zenith direction [2]. The solar irradiance that is absorbed by the cooler is written as Psun and can be calculated by [6]:
Fig. 1. Schematic cross-section of the proposed structure for passive daytime radiative cooling. 2
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Fig. 2. (a) Refractive indices and (b) extinction coefficients of 8YSZ and glass varying with wavelength.
Pcond + conv (T , Tamb) = Ahc (Tamb − T )
(6)
where hc is the combined heat transfer coefficient by conduction and convection. Here in our simulations, the emissivity is calculated with a spectral resolution of 0.1 nm and an angular resolution of 0.5 degree, which results in convergence of the radiative cooling power to be within 0.5% of relative accuracy. 3. Results and discussion Before we investigate the radiative cooling power of the proposed structure, we first calculate the structure’s spectral directional absorptivity for incidence of TE and TM plane waves at different incident angles. The spectral directional emissivity of the structure is then obtained according to Kirchhoff’s law, i.e., the spectral directional emissivity is equal to the spectral directional absorptivity. After that, the spectral hemispherical emissivity of the structure can be obtained by integrating the averaged spectral directional emissivity for TE and TM waves over the hemisphere. The scattering matrix method [33] is adopted in the calculations and the values of the layer thicknesses d2 and d3 are set as 10 μm and 200 nm. But, for comparison, the value of d1 is taken as 0 (no 8YSZ coating), 10 and 20 μm, respectively. For brevity, the structure with 10-μm-thick 8YSZ coating is named Case Ⅰ while that with 20-μm-thick 8YSZ coating is termed Case Ⅱ in this work. Fig. 4(a) and (b) show the calculated spectral directional absorptivity of Case I for incidence of TE and TM plane waves at three different incident angles, respectively. Very low absorptivity in the solar spectrum and very high absorptivity in the 1st atmospheric transparency window is clearly seen for incidence of both TE and TM waves at each of the
Fig. 3. Schematic of the energy balance in passive daytime radiative cooling.
Psun = A
∫0
∞
dλε (λ, 0) IAM1.5 (λ )
(5)
where IAM1.5 (λ ) is the solar spectral intensity obtained from the AM1.5 Global Tilt Spectrum of the ASTM G-173 data [32]. Here, we assume θsun = 0o , which corresponds to the cooler facing directly to the sun. The last term on the right hand side of Eq. (2) represents the heat transfer between the cooler and the ambient air by conduction and convection and can be expressed as
Fig. 4. The spectral directional absorptivity of the structure corresponding to Case I for incidence of (a) TE plane wave and (b) TM plane wave at three different incident angles. 3
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Fig. 5. Spectral hemispherical emissivity of the proposed structure for different thicknesses of the 8YSZ coating. Also shown are the spectral hemispheric emissivity of an ideal cooler, the AM 1.5 solar Global Tilt Spectrum, the spectral hemispheric emissivity of the atmosphere at TPW = 1 mm.
transfer coefficient of hc ≈ 6 and 12 W m−2 K−1, respectively. We calculate the net cooling power of the structures of Case Ⅰ and Case Ⅱ as well as an ideal radiative cooler under direct solar irradiation, assuming that the ambient temperature is at 300 K and the TPW is equal to 1 mm. The results are presented in Fig. 6(a)– (c) for the non-radiative heat transfer coefficient hc taken as 0, 6 and 12 W/m2 K, respectively. It can be seen that the net cooling power decreases monotonically with the temperature of the cooler, and when the cooler is in thermal equilibrium with the ambient, its net cooling power of 95.1 W/m2 and 91.7 W/m2 can be achieved for Case I and Case II, respectively, accounting for 77% and 74% of the net cooling power of the ideal cooler, i.e., 123.3 W/m2, under the same conditions. Furthermore, the temperature drop of the structure below the ambient at zero net cooling power is obtained as 10.3 ℃ (6.2 ℃) for Case I and 9.6 ℃ (5.9 ℃) for Case II, respectively, when hc is equal to 6 (12) W/m2 K, compared to the temperature drop of 15.1 ℃ (8.7 ℃) for the ideal radiative cooler. Therefore, the proposed structure is shown to have outstanding passive radiative cooling performance. The effect of layer thicknesses of 8YSZ and SiO2 on the net cooling power of the structure under direct solar irradiation is also studied, assuming that the structure is in thermal equilibrium with the ambient air. In the calculation, the smallest layer thicknesses of 8YSZ and SiO2 are both set as 1 μm. The result is shown in Fig. 7, where only the positive values of the net cooling power are retained and the small sawtooth-like features on the border of zero net cooling power come from data interpolation. The inset displays the locally enlarged plot of the net cooling power for 8YSZ layer thickness varying from 1 to 20 μm and SiO2 layer thickness varying from 1 to 50 μm. It can be seen that the net cooling power first increases with both the layer thicknesses of 8YSZ and SiO2 and its maximum appears around 10 μm for both layer thicknesses. The net cooling power decreases with further increase of the layer thicknesses. This figure provides important guidance for maximizing the net cooling power by choosing appropriately the layer thicknesses of 8YSZ and SiO2. In order to explore the proposed cooler’s cooling capacity under relative high humidity, we calculate the net cooling power of the structure without 8YSZ coating, Case Ⅰ and Case Ⅱ as a function of TPW from 1 to 20 mm, assuming that the temperature of the structure is the same as that of ambient air at 300 K. From the data in the Earthscan [37] and using suitable least-squares-estimate variants [38], the solar irradiance can be shown to range from 680 to 1050 W/m2 with AM 1.5 under different pollution and weather conditions. Therefore, our calculations are based on the AM1.5 ASTM G-173 standard solar irradiance (1000.4 W/m2) and a smaller solar irradiance of 700 W/m2 that has also been considered by Zhao et al. [5]. The atmospheric
specified incident angles. Particularly, the absorptivity in the 2nd atmospheric transparency window is much lower than that in the 1st atmospheric transparency window. Note that the highly oscillating feature of the absorptivity curve for wavelength extended up to the 1st atmospheric transparency window is due to the effect of wave interference in the 8YSZ and glass films. However, the wave interference effect is not observed in the 2nd atmospheric transparency window owing to the fact that the large extinction coefficient of 8YSZ gives rise to a much smaller penetration depth than the coating thickness in this region. In addition, the high absorptivity peak in the ultraviolet region comes from large absorption in the Ag film which though is highly reflective in the visible and the infrared regions. The spectral hemispherical emissivity of Cases I and II is shown and compared to the case without 8YSZ coating in Fig. 5, in which the highly oscillating feature of the emissivity curve due to the effect of wave interference has been smoothened by integration over the hemisphere. One can see that adding the 8YSZ coating can result in the emissivity increasing slightly in the solar spectrum but increasing significantly in the 1st atmospheric transparency window. In other words, though adding the 8YSZ coating results in more solar irradiance to be absorbed, more thermal energy can be emitted to the outer space by the structure through the 1st atmospheric transparency window and the emission is larger for Case II than for Case I. In contrast, due to its highly reflective property, adding the 8YSZ coating causes the emissivity to drop dramatically in the 2nd atmospheric transparency window, which can benefit passive radiative cooling in many situations and will be discussed below. For comparison, the emissivity of an ideal radiative cooler, which is equal to unity in the 1st atmospheric transparency window but is zero elsewhere [6], is added in Fig. 5. Also, for highlighting the distribution of the structure’s emissivity, the AM 1.5 solar Global Tilt Spectrum and the spectral hemispherical emissivity of the atmosphere εatm (λ ) at TPW equal to 1 mm [2] are superimposed in Fig. 5 as well. Here, εatm (λ ) is calculated based on the atmospheric transmissivity in the zenith direction using ATRAN – an online software provided by the SOFIA Science Center [34,35]. In the calculation, input of the observatory latitude and zenith angle of observation is set to 39° N and 0°, respectively. Around 39° N latitude, averaged altitude of many metropolises such as Rome, Beijing, San Francisco and New York is about 30 m [36]. Thus, input of the observatory altitude is set as 30 m. Now we investigate the net cooling power of the radiative cooler as a function of the temperature difference between the cooler and the ambient, considering the effect of non-radiative heat transfer. It has been shown by Eden Rephaeli et al. [12] that the presence of 1 m/s and 3 m/s wind speed would result in a combined non-radiative heat 4
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Fig. 6. Net radiative cooling power varying with the temperature difference T − Tamb for the structures of Cases I and II and an ideal radiative cooler under AM1.5 solar irradiation and the non-radiative heat transfer coefficient hc set as (a) 0 W/m2 K, (b) 6 W/m2 K and (c) 12 W/m2 K. Tamb is taken as 300 K and TPW = 1 mm.
transparency windows at different values of humidity are calculated and the results are presented in Fig. 8(a). It can be clearly seen that as TPW changes from 1 mm to 10 mm and to 20 mm, the averaged transmissivity of the 1st atmospheric transparency window decreases slowly from 0.885 to 0.833 and to 0.789, while that of the 2nd atmospheric transparency window drops drastically from 0.426 to 0.098 and to 0.032. It means that the 2nd atmospheric transparency window would be closed with the increase of humidity. Similar change of the emissivity in the 1st and the 2nd atmospheric windows with the increase of humidity can also be found in the work by Suichi et al. [25]. The effects of humidity and solar irradiance on the net cooling power of the above-mentioned three structures under direct solar irradiation are shown in Fig. 8(b). We can see that the net cooling power of each structure under solar irradiance of 700 W/m2 is larger than that under solar irradiance of 1000.4 W/m2. Specifically, when TPW = 1 mm, the net cooling power is 95.1 W/m2 for Case Ⅰ and 91.7 W/m2 for Case II at 1000.4 W/m2 solar irradiance, which increases respectively to 104.3 W/m2 103.5 W/m2 at 700 W/m2 solar irradiance. The increase of net cooling power comes from less absorbed power from the sun as the solar irradiance decreases. In addition, increase of the net cooling power is by 13% for Case II, higher than that (10%) for Case I, owing to the fact that the emissivity is higher in 1st atmospheric window for Case II than for Case I. Thus, more heat can be removed to the outer space by thermal radiation for Case II.
Fig. 7. The net cooling power varying with the layer thicknesses of 8YSZ and SiO2 for the structure in thermal equilibrium with the ambient air.
transmissivity is adopted from ATRAN [34,35]. Note that the range of TPW considered represents those of the areas including South Europe, North America and East Asia near the 39 degrees northern latitude. The averaged transmissivities of the 1st and the 2nd atmospheric 5
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Fig. 8. (a) The average atmospheric transmissivity in the 1st and the 2nd transparency windows; (b) The net cooling power of the structures with and without 8YSZ coating under 700 W/m2 and 1000 W/m2 solar irradiance at ambient temperature of 300 K as a function of TPW.
under a solar irradiance of 700 W/m2, the net cooling power for Case Ⅰ only declines from 104.3 W/m2 to 77.5 W/m2 when TPW increases from 1 mm to 20 mm. Similarly, the net cooling power decline for Case Ⅱ is only from 103.5 W/m2 to 75 W/m2. In other words, even at TPW = 20 mm, the net cooling power for Case Ⅰ and Case Ⅱ can still maintain at 74% and 73% of the corresponding values at TPW = 1 mm. Similar results are found for the cases when the solar irradiance is at 1000.4 W/m2. Finally, we have to admit that though promising, the technology of passive daytime radiative cooling still faces many challenges in applications, since the net cooling power decreases with the surface temperature of the cooler and heat transfer by conduction and convection affects adversely the cooling performance if the ambient air is warmer than the cooler. In view of these problems, some new active daytime radiative cooling strategy has recently been proposed by combining a spectrally selective surface to a photovoltaic-powered air conditioning system where heat is rejected actively from the surface by radiation and convection [39].
In addition, Fig. 8(b) shows that adding the 8YSZ coating can improve the net cooling power of the structure under high humidity conditions. When TPW = 1 mm, the structure without 8YSZ coating achieves a net cooling power of 97.7 W/m2 at 1000.4 W/m2 solar irradiance and 104.5 W/m2 at 700 W/m2 solar irradiance, both of which show better performance than Cases Ⅰ and Ⅱ under the same conditions. However, the structure having 8YSZ coating outperforms that without 8YSZ coating in humid environment. When TPW = 20 mm, the net cooling power for Cases Ⅰ and Ⅱ at 700 W/m2 solar irradiance is increased by 20% and 16%, respectively, compared to case without 8YSZ coating. Similarly, increase of 19% and 10% is achieved respectively for Cases Ⅰ and Ⅱ at solar irradiance of 1000.4 W/m2. The mechanisms for the net cooling power enhancement can be found from the emissivity curves shown in Fig. 5. For the case without 8YSZ coating, the ultralow solar absorptivity and the high emissivity in the 1st and the 2nd atmospheric windows contribute to the large net cooling power in dry environment. As the humidity increases, decrease of the atmospheric transmissivity in the 1st and the 2nd atmospheric transparency windows results in less heat being radiated to the outer space from the structure without 8YSZ coating. However, even shutdown of the 2nd atmospheric transparency window does not influence the performance of the structures of Cases Ⅰ and Ⅱ a lot since the emissivity is much lower in the 2nd atmospheric transparency window for these two cases. Furthermore, improved emissivity in the 1st atmospheric transparency window for Cases I and II results in more heat being radiated to the outer space, compared to the case without 8YSZ coating. Another important advantage of the structure having 8YSZ coating is that it can achieve relatively more stable radiative cooling performance under large fluctuating humidity. Without 8YSZ coating, large and rapid change of humidity can result in the net cooling power of the structure changing in large magnitude, which may cause damage to the radiative cooling systems consisting of multiple modules. It may influence stable output voltage of thermoelectric device [3] and affect stable output volume of below-the-ambient water [5]. Furthermore, Chen et al. [7] found that the cooler will be heated up by the absorbed radiative energy from the warmer atmosphere (Tamb > Tcooler) if its emissivity is high in the 2nd atmospheric transparency window. The structure with 8YSZ coating, however, can maintain more stable cooling performance when the humidity of the environment is subjected to large and rapid change, since its emissivity is higher in the 1st atmospheric window but is much lower in the 2nd atmospheric window, compared to the case without 8YSZ coating. For example,
4. Conclusion In summary, we propose a planar tri-layer structure made of an 8YSZ coating, a SiO2 slab and a Ag film for passive daytime radiative cooling. The performace of the proposed radiaitve cooler is numerically investagted under different ambient conditions and direct solar irradiation. The results indicate that a net cooling power of 95.1 W/m2 can be achieved under direct AM1.5 solar irradiation and dry environment. Passive radiative cooling can result in dropping of the cooler temperature to around 10 ℃ and 6 ℃ below the ambient, considering the effect of wind at 1 m/s and 3 m/s, respectively. Furthermore, the cooling performances of the proposed structure under different humidity are evaluated and the results show that the proposed structure can improve the net cooling power under high humidity conditions, compared to the case without 8YSZ coating. As a consequence, the proposed radiative cooler can maintain more stable performance than the case without 8YSZ coating when humidity is in large fluctuation. The results in this work demonstrate that 8YSZ may be the potential coating material for radiative cooling and the proposed structure is promising for practical passive daytime radiative cooling applications in environment of high as well as rapidly changing humidity.
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Declaration of Competing Interest
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None. Acknowledgment J. Fan and C. Fu were supported by the National Natural Science Foundation of China (No. 51576004). T. Fu was supported by the National Natural Science Foundation of China (No. 51576110). References [1] J. Kou, Z. Jurado, Z. Chen, S. Fan, A.J. Minnich, Daytime radiative cooling using near-black infrared emitters, ACS Photon. 4 (2017) 626–630. [2] C.G. Granqvist, A. Hjortsberg, Radiative cooling to low temperatures: general considerations and application to selectively emitting SiO films, J. Appl. Phys. 52 (1981) 4205–4220. [3] E. Mu, Z. Wu, Z. Wu, X. Chen, Y. Liu, X. Fu, Z. Hu, A novel self-powering ultrathin TEG device based on micro/nano emitter for radiative cooling, Nano Energy 55 (2019) 494–500. [4] X. Wu, C. Fu, Radiative Cooling by Using a Slab of Hexagonal Boron Nitride, in: Int. Heat Transf. Conf. Digit. Libr., 2018. [5] D. Zhao, A. Aili, Y. Zhai, J. Lu, D. Kidd, G. Tan, X. Yin, R. Yang, Subambient cooling of water: toward real-world applications of daytime radiative cooling, Joule 3 (2019) 111–123. [6] A.P. Raman, M.A. Anoma, L. Zhu, E. Rephaeli, S. Fan, Passive radiative cooling below ambient air temperature under direct sunlight, Nature 515 (2014) 540. [7] 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. [8] 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. [9] 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. [10] L. Long, Y. Yang, L.P. Wang, Simultaneously enhanced solar absorption and radiative cooling with thin silica micro-grating coatings for silicon solar cells, Sol. Energy Mater. Sol. Cells 197 (2019) 19–24. [11] M. Hu, B. Zhao, X. Ao, Y. Su, G. Pei, Numerical study and experimental validation of a combined diurnal solar heating and noctural radiative cooling collector, Appl. Therm. Eng. 145 (2018) 1–13. [12] E. Rephaeli, A. Raman, S. Fan, Ultrabroadband photonic structures to achieve highperformance daytime radiative cooling, Nano Lett. 13 (2013) 1457–1461. [13] L. Zhu, A. Raman, S. Fan, Color-preserving daytime radiative cooling, Appl. Phys. Lett. 103 (2013) 223902. [14] M.M. Hossain, B. Jia, M. Gu, A metamaterial emitter for highly efficient radiative cooling, Adv. Opt. Mater. 3 (2015) 1047–1051. [15] B. Zhao, M. Hu, X. Ao, N. Chen, G. Pei, Radiaitve cooling: a review of fundamentals, materials, applications, and prospects, Appl. Energy 236 489–5 (2019) 13.
7
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Yttria-stabilized zirconia coating for passive daytime radiative cooling in humid environment ⁎
Jianshu Fana, Ceji Fua, , Tairan Fub, a b
T
⁎
LTCS and Department of Mechanics and Engineering Science, College of Engineering, Peking University, Beijing 100871, China Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China
H I GH L IG H T S
Yttria-Stabilized Zirconia coating for passive daytime radiative cooling. • Proposed improved passive daytime radiative cooling under high humidity. • Achieved mechanisms for improved radiative cooling. • Elucidated • Achieved more stable net cooling power subjected to changing humidity.
A R T I C LE I N FO
A B S T R A C T
Keywords: Passive daytime radiative cooling Solar irradiation Atmospheric transparency window Humidity Yttria-stabilized zirconia
Passive daytime radiative cooling is a promising new field for solving the energy shortage all over the world. The performance of daytime radiative cooling is limited by several factors such as non-radiative heat transfer, outdoor-environment erosion and atomsphere humidity. We propose in this work a 8 wt% yttria stabilized zirconia (8YSZ) coated SiO2 (glass)/Ag radiative cooler for improving daytime radiative cooling performance. Numerical results show that the proposed radiative cooler can achieve a cooling power of 95.1 W/m2 under direct AM1.5 solar irradiation and dry environment. In addition, the cooler temperature can drop by 10.3 ℃ below the ambient under direct solar irradiation and 1 m/s wind speed. We also evaluate the cooling performance of the proposed structure under different humidity and it is remarkable to find that the 8YSZ coated radiative cooler can boost the net cooling power under high humidity, compared to that without 8YSZ coating. This proposed structure may have potential practical applications in daytime radiative cooling due to its fantastic corrosion resistance, anti-degeneration and adaptability to high as well as rapidly changing humidity.
1. Introduction Passive radiative cooling that does not need external input of electricity is an appealing strategy for energy saving in the 21st century and it has drawn much attention [1–9]. In particular, achieving effective cooling power during the daytime is the most attractive merit of this technology since the cooling demand peaks in the daytime hours [1]. It is also promising to be alternative to the conventional active cooling systems such as air conditioners and the efficiency of outdoor energy conversion devices like solar cells and thermoelectric converters can be improved if they are made to fulfill the function of radiative cooling as well [2,3,5,10,11]. In principle, passive radiative cooling relies on the cooler to radiate heat out to the cold outer space as much as possible through the infrared (IR) atmospheric transparency windows while absorb almost zero of the solar irradiation [6]. Hence, the emissivity of
⁎
the cooler surface should be spectrally-selective. In the past several years, different engineered structures [1,6,7,12–19] have been proposed for the spectrally-selective radiative cooler, among which scalable structures such as the polymer-glass bead composite [16], the polymer (PDMS)-coated glass wafer [1], and the hierarchically porous polymer coating [19] are the typically attractive cases. However, the properties of many polymers have been proved to be denatured outdoors such as the degeneration, yellowing and oxidation under longtime direct sunshine [20] or degraded elasticity of PDMS under timevarying temperature conditions [21]. Furthermore, the non-radiative heat transfer effect would also reduce the net cooling power of the cooler at sub-ambient temperatures [22]. Therefore, it is meaningful to propose a simple daytime radiative cooler structure that is corrosionresistant and can improve the radiative cooling performance under direct solar irradiation. Thermal barrier coatings (TBCs) are low-
Corresponding authors. E-mail addresses: [email protected] (C. Fu), [email protected] (T. Fu).
https://doi.org/10.1016/j.applthermaleng.2019.114585 Received 6 July 2019; Received in revised form 3 October 2019; Accepted 24 October 2019 Available online 25 October 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
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coating on its top side and an Ag film at its bottom side. The 8YSZ coating can tailor effectively the emissivity of the cooler in the 1st and the 2nd atmospheric transparency windows, while the Ag film acts as a reflector. The layer thicknesses of the 8YSZ coating, the glass slab and the Ag film are labeled in Fig. 1 as d1, d2 and d3, respectively. In this work, the optical constants of 8YSZ are taken as those obtained by Nychka et al. [27], while the optical properties of SiO2 (glass) are from the tabulated values in Palik’s handbook [28]. The refractive indices of 8YSZ and glass, and their corresponding extinction coefficients varying with wavelength are shown in Fig. 2(a) and (b), respectively. It can be seen that the refractive index of glass remains almost constant while its extinction coefficient is very small in the solar spectrum. In the mid-infrared region, however, the refractive index of glass becomes highly dependent on wavelength and its extinction coefficient is larger by three to five orders of magnitude than that in the solar spectrum. Therefore, a thin glass film can be highly emissive in the mid-infrared region while is transparent in the solar spectrum. This spectrally-selective emissivity and transmissivity of a glass film makes it a suitable candidate for use in passive daytime radiative cooling [1,16]. The optical constants of 8YSZ are quite similar to those of glass and a thin 8YSZ film can be highly emissive in the 1st atmospheric transparency window. However, the extinction coefficient of 8YSZ is so large in the 2nd atmospheric transparency window that a thin 8YSZ film can be highly reflective in this region. For the dielectric function of Ag, the Drude model is adopted [29]
thermal conductivity ceramics which have been extensively applied in advanced turbine components, such as aircraft engines and ground gas turbines, for achieving increased efficiency, performance and durability [23]. From the 1970s to the present, 8 wt% yttria stabilized zirconia (8YSZ) has been the most widely used material for TBC on account of its low density, low thermal conductivity, good thermal shock resistance and fantastic corrosion resistant properties [24]. In this work, we present the design and performance analysis of a passive daytime radiative cooler made of a SiO2 (glass) layer coated with a 8YSZ film on the top and a silver (Ag) film at the bottom as a reflector. Humidity is one the most important factor that affects significantly the performance of radiative cooling and has been the focus of some recent studies [5,25]. Total precipitable water (TPW) is a critical parameter for quantifying humidity of the atmosphere, which is defined as the depth of water in a column of the atmosphere if it is totally precipitated as rain. Potential promising applications of passive daytime radiative cooling may be found in the typical densely-populated and industrial areas around 39° north latitude, where located are many metropolises such as Rome, Beijing, San Francisco, New York and Madrid, etc. In these areas the NCEP-NCAR Reanalysis climite data [26] show that the averaged TPW on the ground mostly ranges from 1 mm to 20 mm. However, most studies, including experimental and numerical analyses, on passive daytime radiative cooling are under dry envioronment (TPW < 10 mm). Suichi et al. [25] showed that while the transmissivity of the atmosphere in the 1st atmospheric transparency window (8–13 μm) decreases slightly due to the increase of humidity, vast decrease of it is found in the 2nd atmospheric transparency window (16–25 μm). Therefore, the cooling power of radiative coolers having high emissivity not only in the 1st atmospheric transparency window, but also in the 2nd one [1,16] may drop dramastically once humidity is increasing. Humidity change with varying weather conditions could bring about synchronous change of the cooling power, which could lower the stability of the radiative cooling perfoarmance. Particularly, Chen et al. [7] found that the radiative cooler can be heated up by the absorbed radiative energy from the atmosphere if it has a high emissivity in the 2nd atmospheric transparency window. Here, we evaluate the cooling performance of the proposed 8YSZ/SiO2/Ag structure under different humidity and show that such a radiative cooler can operate quite stably even under high humidity fluctuation conditions and adding the 8YSZ coating could lead to great improvement in the cooling power when the humidity is high.
ωp2
ε (ω) = ε∞ −
ω2
+ iγω
(1)
where the high-frequency constant ε∞ = 3.4 , the plasma frequency ωp = 1.39 × 1016 rad/s, the scattering rate γ = 2.7 × 1013 rad/s. In the present study, this Drude model is used for wavelength of λ > 1.9 μm and the optical constants of Ag are modified to the measured data by Johnson and Christy [30] when λ ⩽ 1.9 μm, according to Yang et al. [31]. The energy balance in passive daytime radiative cooling is schematically shown in Fig. 3. The net radiative cooling power under direct solar irradiation measures the performance of the proposed cooler, which can be calculated as [6]:
Pcool (T ) = Prad (T ) − Patm (Tamb) − PSun − Pcond + conv
(2)
where Prad (T) represents the power emitted by the cooler at temperature T and is expressed as [6]: 2. Structural design and theoretical analysis
Prad (T ) = A The proposed structure for passive daytime radiative cooling is shown in Fig. 1, which consists of a SiO2 (glass) slab with an 8YSZ
∫ dΩ cos θ ∫0
∞
dλIBB (T , λ ) ε (λ, θ)
(3)
where A is the surface area of the cooler, θ is the incident angle and π /2 dθ sin θ denotes the integration over a hemisphere.
∫ dΩ = 2π ∫0
2hc 2
1
IBB (T , λ ) = λ5 hc /(λkB T ) is the spectral intensity of a blackbody at −1 e temperature T, with h Planck’s constant, c the speed of light in vacuum, λ the wavelength and kB Boltzmann’s constant. ε (λ, θ) represents the spectral directional emissivity of the cooler and can be expressed asε (λ, θ) = 0.5[εTE (λ, θ) + εTM (λ, θ)], withεTE (λ, θ) andεTM (λ, θ) being the spectral directional emissivity for transverse electric (TE) and transverse magnetic (TM) waves, respectively. The thermal irradiance from the atmosphere that is absorbed by the cooler is expressed as [6]:
Patm (Tamb) = A
∫ dΩ cos θ ∫0
∞
dλIBB (Tamb, λ ) ε (λ, θ) εatm (λ, θ)
(4)
where Tamb is the ambient air temperature and is assumed 300 K in this work. εatm (λ, θ) denotes the spectral directional emissivity of the atmosphere and is given by εatm (λ, θ) = 1 − t (λ )1/cos θ with t(λ) the atmospheric transmissivity in the zenith direction [2]. The solar irradiance that is absorbed by the cooler is written as Psun and can be calculated by [6]:
Fig. 1. Schematic cross-section of the proposed structure for passive daytime radiative cooling. 2
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Fig. 2. (a) Refractive indices and (b) extinction coefficients of 8YSZ and glass varying with wavelength.
Pcond + conv (T , Tamb) = Ahc (Tamb − T )
(6)
where hc is the combined heat transfer coefficient by conduction and convection. Here in our simulations, the emissivity is calculated with a spectral resolution of 0.1 nm and an angular resolution of 0.5 degree, which results in convergence of the radiative cooling power to be within 0.5% of relative accuracy. 3. Results and discussion Before we investigate the radiative cooling power of the proposed structure, we first calculate the structure’s spectral directional absorptivity for incidence of TE and TM plane waves at different incident angles. The spectral directional emissivity of the structure is then obtained according to Kirchhoff’s law, i.e., the spectral directional emissivity is equal to the spectral directional absorptivity. After that, the spectral hemispherical emissivity of the structure can be obtained by integrating the averaged spectral directional emissivity for TE and TM waves over the hemisphere. The scattering matrix method [33] is adopted in the calculations and the values of the layer thicknesses d2 and d3 are set as 10 μm and 200 nm. But, for comparison, the value of d1 is taken as 0 (no 8YSZ coating), 10 and 20 μm, respectively. For brevity, the structure with 10-μm-thick 8YSZ coating is named Case Ⅰ while that with 20-μm-thick 8YSZ coating is termed Case Ⅱ in this work. Fig. 4(a) and (b) show the calculated spectral directional absorptivity of Case I for incidence of TE and TM plane waves at three different incident angles, respectively. Very low absorptivity in the solar spectrum and very high absorptivity in the 1st atmospheric transparency window is clearly seen for incidence of both TE and TM waves at each of the
Fig. 3. Schematic of the energy balance in passive daytime radiative cooling.
Psun = A
∫0
∞
dλε (λ, 0) IAM1.5 (λ )
(5)
where IAM1.5 (λ ) is the solar spectral intensity obtained from the AM1.5 Global Tilt Spectrum of the ASTM G-173 data [32]. Here, we assume θsun = 0o , which corresponds to the cooler facing directly to the sun. The last term on the right hand side of Eq. (2) represents the heat transfer between the cooler and the ambient air by conduction and convection and can be expressed as
Fig. 4. The spectral directional absorptivity of the structure corresponding to Case I for incidence of (a) TE plane wave and (b) TM plane wave at three different incident angles. 3
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Fig. 5. Spectral hemispherical emissivity of the proposed structure for different thicknesses of the 8YSZ coating. Also shown are the spectral hemispheric emissivity of an ideal cooler, the AM 1.5 solar Global Tilt Spectrum, the spectral hemispheric emissivity of the atmosphere at TPW = 1 mm.
transfer coefficient of hc ≈ 6 and 12 W m−2 K−1, respectively. We calculate the net cooling power of the structures of Case Ⅰ and Case Ⅱ as well as an ideal radiative cooler under direct solar irradiation, assuming that the ambient temperature is at 300 K and the TPW is equal to 1 mm. The results are presented in Fig. 6(a)– (c) for the non-radiative heat transfer coefficient hc taken as 0, 6 and 12 W/m2 K, respectively. It can be seen that the net cooling power decreases monotonically with the temperature of the cooler, and when the cooler is in thermal equilibrium with the ambient, its net cooling power of 95.1 W/m2 and 91.7 W/m2 can be achieved for Case I and Case II, respectively, accounting for 77% and 74% of the net cooling power of the ideal cooler, i.e., 123.3 W/m2, under the same conditions. Furthermore, the temperature drop of the structure below the ambient at zero net cooling power is obtained as 10.3 ℃ (6.2 ℃) for Case I and 9.6 ℃ (5.9 ℃) for Case II, respectively, when hc is equal to 6 (12) W/m2 K, compared to the temperature drop of 15.1 ℃ (8.7 ℃) for the ideal radiative cooler. Therefore, the proposed structure is shown to have outstanding passive radiative cooling performance. The effect of layer thicknesses of 8YSZ and SiO2 on the net cooling power of the structure under direct solar irradiation is also studied, assuming that the structure is in thermal equilibrium with the ambient air. In the calculation, the smallest layer thicknesses of 8YSZ and SiO2 are both set as 1 μm. The result is shown in Fig. 7, where only the positive values of the net cooling power are retained and the small sawtooth-like features on the border of zero net cooling power come from data interpolation. The inset displays the locally enlarged plot of the net cooling power for 8YSZ layer thickness varying from 1 to 20 μm and SiO2 layer thickness varying from 1 to 50 μm. It can be seen that the net cooling power first increases with both the layer thicknesses of 8YSZ and SiO2 and its maximum appears around 10 μm for both layer thicknesses. The net cooling power decreases with further increase of the layer thicknesses. This figure provides important guidance for maximizing the net cooling power by choosing appropriately the layer thicknesses of 8YSZ and SiO2. In order to explore the proposed cooler’s cooling capacity under relative high humidity, we calculate the net cooling power of the structure without 8YSZ coating, Case Ⅰ and Case Ⅱ as a function of TPW from 1 to 20 mm, assuming that the temperature of the structure is the same as that of ambient air at 300 K. From the data in the Earthscan [37] and using suitable least-squares-estimate variants [38], the solar irradiance can be shown to range from 680 to 1050 W/m2 with AM 1.5 under different pollution and weather conditions. Therefore, our calculations are based on the AM1.5 ASTM G-173 standard solar irradiance (1000.4 W/m2) and a smaller solar irradiance of 700 W/m2 that has also been considered by Zhao et al. [5]. The atmospheric
specified incident angles. Particularly, the absorptivity in the 2nd atmospheric transparency window is much lower than that in the 1st atmospheric transparency window. Note that the highly oscillating feature of the absorptivity curve for wavelength extended up to the 1st atmospheric transparency window is due to the effect of wave interference in the 8YSZ and glass films. However, the wave interference effect is not observed in the 2nd atmospheric transparency window owing to the fact that the large extinction coefficient of 8YSZ gives rise to a much smaller penetration depth than the coating thickness in this region. In addition, the high absorptivity peak in the ultraviolet region comes from large absorption in the Ag film which though is highly reflective in the visible and the infrared regions. The spectral hemispherical emissivity of Cases I and II is shown and compared to the case without 8YSZ coating in Fig. 5, in which the highly oscillating feature of the emissivity curve due to the effect of wave interference has been smoothened by integration over the hemisphere. One can see that adding the 8YSZ coating can result in the emissivity increasing slightly in the solar spectrum but increasing significantly in the 1st atmospheric transparency window. In other words, though adding the 8YSZ coating results in more solar irradiance to be absorbed, more thermal energy can be emitted to the outer space by the structure through the 1st atmospheric transparency window and the emission is larger for Case II than for Case I. In contrast, due to its highly reflective property, adding the 8YSZ coating causes the emissivity to drop dramatically in the 2nd atmospheric transparency window, which can benefit passive radiative cooling in many situations and will be discussed below. For comparison, the emissivity of an ideal radiative cooler, which is equal to unity in the 1st atmospheric transparency window but is zero elsewhere [6], is added in Fig. 5. Also, for highlighting the distribution of the structure’s emissivity, the AM 1.5 solar Global Tilt Spectrum and the spectral hemispherical emissivity of the atmosphere εatm (λ ) at TPW equal to 1 mm [2] are superimposed in Fig. 5 as well. Here, εatm (λ ) is calculated based on the atmospheric transmissivity in the zenith direction using ATRAN – an online software provided by the SOFIA Science Center [34,35]. In the calculation, input of the observatory latitude and zenith angle of observation is set to 39° N and 0°, respectively. Around 39° N latitude, averaged altitude of many metropolises such as Rome, Beijing, San Francisco and New York is about 30 m [36]. Thus, input of the observatory altitude is set as 30 m. Now we investigate the net cooling power of the radiative cooler as a function of the temperature difference between the cooler and the ambient, considering the effect of non-radiative heat transfer. It has been shown by Eden Rephaeli et al. [12] that the presence of 1 m/s and 3 m/s wind speed would result in a combined non-radiative heat 4
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Fig. 6. Net radiative cooling power varying with the temperature difference T − Tamb for the structures of Cases I and II and an ideal radiative cooler under AM1.5 solar irradiation and the non-radiative heat transfer coefficient hc set as (a) 0 W/m2 K, (b) 6 W/m2 K and (c) 12 W/m2 K. Tamb is taken as 300 K and TPW = 1 mm.
transparency windows at different values of humidity are calculated and the results are presented in Fig. 8(a). It can be clearly seen that as TPW changes from 1 mm to 10 mm and to 20 mm, the averaged transmissivity of the 1st atmospheric transparency window decreases slowly from 0.885 to 0.833 and to 0.789, while that of the 2nd atmospheric transparency window drops drastically from 0.426 to 0.098 and to 0.032. It means that the 2nd atmospheric transparency window would be closed with the increase of humidity. Similar change of the emissivity in the 1st and the 2nd atmospheric windows with the increase of humidity can also be found in the work by Suichi et al. [25]. The effects of humidity and solar irradiance on the net cooling power of the above-mentioned three structures under direct solar irradiation are shown in Fig. 8(b). We can see that the net cooling power of each structure under solar irradiance of 700 W/m2 is larger than that under solar irradiance of 1000.4 W/m2. Specifically, when TPW = 1 mm, the net cooling power is 95.1 W/m2 for Case Ⅰ and 91.7 W/m2 for Case II at 1000.4 W/m2 solar irradiance, which increases respectively to 104.3 W/m2 103.5 W/m2 at 700 W/m2 solar irradiance. The increase of net cooling power comes from less absorbed power from the sun as the solar irradiance decreases. In addition, increase of the net cooling power is by 13% for Case II, higher than that (10%) for Case I, owing to the fact that the emissivity is higher in 1st atmospheric window for Case II than for Case I. Thus, more heat can be removed to the outer space by thermal radiation for Case II.
Fig. 7. The net cooling power varying with the layer thicknesses of 8YSZ and SiO2 for the structure in thermal equilibrium with the ambient air.
transmissivity is adopted from ATRAN [34,35]. Note that the range of TPW considered represents those of the areas including South Europe, North America and East Asia near the 39 degrees northern latitude. The averaged transmissivities of the 1st and the 2nd atmospheric 5
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Fig. 8. (a) The average atmospheric transmissivity in the 1st and the 2nd transparency windows; (b) The net cooling power of the structures with and without 8YSZ coating under 700 W/m2 and 1000 W/m2 solar irradiance at ambient temperature of 300 K as a function of TPW.
under a solar irradiance of 700 W/m2, the net cooling power for Case Ⅰ only declines from 104.3 W/m2 to 77.5 W/m2 when TPW increases from 1 mm to 20 mm. Similarly, the net cooling power decline for Case Ⅱ is only from 103.5 W/m2 to 75 W/m2. In other words, even at TPW = 20 mm, the net cooling power for Case Ⅰ and Case Ⅱ can still maintain at 74% and 73% of the corresponding values at TPW = 1 mm. Similar results are found for the cases when the solar irradiance is at 1000.4 W/m2. Finally, we have to admit that though promising, the technology of passive daytime radiative cooling still faces many challenges in applications, since the net cooling power decreases with the surface temperature of the cooler and heat transfer by conduction and convection affects adversely the cooling performance if the ambient air is warmer than the cooler. In view of these problems, some new active daytime radiative cooling strategy has recently been proposed by combining a spectrally selective surface to a photovoltaic-powered air conditioning system where heat is rejected actively from the surface by radiation and convection [39].
In addition, Fig. 8(b) shows that adding the 8YSZ coating can improve the net cooling power of the structure under high humidity conditions. When TPW = 1 mm, the structure without 8YSZ coating achieves a net cooling power of 97.7 W/m2 at 1000.4 W/m2 solar irradiance and 104.5 W/m2 at 700 W/m2 solar irradiance, both of which show better performance than Cases Ⅰ and Ⅱ under the same conditions. However, the structure having 8YSZ coating outperforms that without 8YSZ coating in humid environment. When TPW = 20 mm, the net cooling power for Cases Ⅰ and Ⅱ at 700 W/m2 solar irradiance is increased by 20% and 16%, respectively, compared to case without 8YSZ coating. Similarly, increase of 19% and 10% is achieved respectively for Cases Ⅰ and Ⅱ at solar irradiance of 1000.4 W/m2. The mechanisms for the net cooling power enhancement can be found from the emissivity curves shown in Fig. 5. For the case without 8YSZ coating, the ultralow solar absorptivity and the high emissivity in the 1st and the 2nd atmospheric windows contribute to the large net cooling power in dry environment. As the humidity increases, decrease of the atmospheric transmissivity in the 1st and the 2nd atmospheric transparency windows results in less heat being radiated to the outer space from the structure without 8YSZ coating. However, even shutdown of the 2nd atmospheric transparency window does not influence the performance of the structures of Cases Ⅰ and Ⅱ a lot since the emissivity is much lower in the 2nd atmospheric transparency window for these two cases. Furthermore, improved emissivity in the 1st atmospheric transparency window for Cases I and II results in more heat being radiated to the outer space, compared to the case without 8YSZ coating. Another important advantage of the structure having 8YSZ coating is that it can achieve relatively more stable radiative cooling performance under large fluctuating humidity. Without 8YSZ coating, large and rapid change of humidity can result in the net cooling power of the structure changing in large magnitude, which may cause damage to the radiative cooling systems consisting of multiple modules. It may influence stable output voltage of thermoelectric device [3] and affect stable output volume of below-the-ambient water [5]. Furthermore, Chen et al. [7] found that the cooler will be heated up by the absorbed radiative energy from the warmer atmosphere (Tamb > Tcooler) if its emissivity is high in the 2nd atmospheric transparency window. The structure with 8YSZ coating, however, can maintain more stable cooling performance when the humidity of the environment is subjected to large and rapid change, since its emissivity is higher in the 1st atmospheric window but is much lower in the 2nd atmospheric window, compared to the case without 8YSZ coating. For example,
4. Conclusion In summary, we propose a planar tri-layer structure made of an 8YSZ coating, a SiO2 slab and a Ag film for passive daytime radiative cooling. The performace of the proposed radiaitve cooler is numerically investagted under different ambient conditions and direct solar irradiation. The results indicate that a net cooling power of 95.1 W/m2 can be achieved under direct AM1.5 solar irradiation and dry environment. Passive radiative cooling can result in dropping of the cooler temperature to around 10 ℃ and 6 ℃ below the ambient, considering the effect of wind at 1 m/s and 3 m/s, respectively. Furthermore, the cooling performances of the proposed structure under different humidity are evaluated and the results show that the proposed structure can improve the net cooling power under high humidity conditions, compared to the case without 8YSZ coating. As a consequence, the proposed radiative cooler can maintain more stable performance than the case without 8YSZ coating when humidity is in large fluctuation. The results in this work demonstrate that 8YSZ may be the potential coating material for radiative cooling and the proposed structure is promising for practical passive daytime radiative cooling applications in environment of high as well as rapidly changing humidity.
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Applied Thermal Engineering 165 (2020) 114585
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Declaration of Competing Interest
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