buildings

Review Implementation of Passive Technology in Buildings: A Review

Suhendri , Mingke Hu * , Yuehong Su * , Jo Darkwa and Saffa Riffat

Department of Architecture and Built Environment, University of Nottingham, Nottingham NG7 2RD, UK; [email protected] (S.); [email protected] (J.D.); saffa.riff[email protected] (S.R.) * Correspondence: [email protected] (M.H.); [email protected] (Y.S.)

 Received: 29 October 2020; Accepted: 21 November 2020; Published: 26 November 2020 

Abstract: Radiative cooling (RC) is attracting more interest from building engineers and architects. Using the sky as the heat sink, a radiative cooling material can be passively cooled by emitting heat to the sky. As a result of the development of material technology, RC research has been revived, with the aim of increasing the materials’ cooling power as well as finding reliable ways to utilize it in cooling for buildings. This review identifies some issues in the current implementation of RC technologies in buildings from an architectural point of view. Besides the technical performance of the RC technologies, some architectural aspects, such as integration with architectural features, aesthetic requirements, as well as fully passive implementations of RC, also need to be considered for building application. In addition, performance evaluation of a building-integrated RC system should begin to account for its benefit to the occupant’s health and comfort alongside the technical performance. In conclusion, this review on RC implementation in buildings provides a meaningful discussion in regard to the direction of the research.

Keywords: radiative cooling; architectural application; combination; passive design architecture

1. Introduction Global warming forces buildings to consume more energy for cooling. For buildings in urban areas, they also experience the so-called (UHI), and this increases the cooling demand even more in the warm and hot climate region [1]. Some locations that used to maintain their via also now need mechanical assistance. The International Energy Agency (IEA) reported that energy consumption for cooling has tripled during the last three decades [2]. Reliance of the energy generation on fossil fuel makes the cooling demand somewhat paradoxical, i.e., by cooling down our buildings, we made the earth even warmer [3]. Thus, passive cooling in buildings plays a crucial role in the environment [4]. Various passive cooling techniques have been developed by researchers and engineers. The general mechanism of passive cooling is actually by dissipating heat from buildings to the environmental heat sink [5]. The most commonly utilized mechanism is and evaporation. These two mechanisms mainly deal with ambient air or ground as the heat sinks. Nowadays, there is an emerging field of study in which is used as a means of cooling with the sky as the heat sink [6]. This mechanism is not new in nature, e.g., plants experienced this with the effect of dew and frost formation on their leaves [7]. In buildings, radiative cooling (RC) can be applied to building envelopes, especially those that acquire the highest sky view factor (SVF). Despite their potential, RC techniques have not been widely used in buildings. Challenges of RC application are well defined by researchers. The two most frequently mentioned challenges are technical and cost problems. Technical problems are related to low cooling power, sophisticated material

Buildings 2020, 10, 215; doi:10.3390/buildings10120215 www.mdpi.com/journal/buildings Buildings 2020, 10, 215 2 of 28 technology to produce the , complicated systems in implementations, durability, and maintenance issues [8]. Cost problems consist of high production cost and high installation cost [9,10]. There is also a problem regarding geographical constraints. Generally, the RC panel is highly dependent on climate and geographical conditions. Factors such as sky condition, wind speed, atmospheric particle, etc. strongly affect the performance of RC panels [5,7,11–13]. RC performs badly in humid conditions. Moreover, a problem also occurs in the form of a mismatch between the cooling demand and the supply. The highest cooling power of an RC panel occurs at night, while building occupants require cooling in the daytime [5,14]. From an architectural point of view, options for design are more crucial. RC panels should be placed on the side that is fully exposed to the sky, which already limits the design option of the building roof, or other smaller sky-facing building components [15]. Moreover, the roof-to-floor area ratio is significantly low for multi-story buildings. This again constraints the design option for architects [9]. Furthermore, structural considerations could also limit the design options [15]. In short, implementing passive RC in buildings does not appear to be beneficial for utility and architectural demand. RC power is not enough for a larger building. Moreover, the technology needs to be more building-integrated [5,7,16]. Many proposals for the implementation of RC in buildings are still in the stage of research and development, and they are mainly about the emitter material or the use of RC to assist active cooling technology [17]. These challenges remain further research topics in the RC field. Although there are several reviews on RC technologies, and some of their conclusions are cited above, to the authors’ best knowledge, only two reviews focused on the applications of RC in buildings. The first paper by Lu et al. [7] elaborates on the cooling power of RC materials, which back in 2016 seemed to be the main barrier for the adoption of RC in buildings. A recent review on a similar topic by Chen et al. [18] in 2020 updated the conditions on RC material explorations, which have been improved since 2016 but still require a real building application, not to mention a large-scale one. Chen et al. [18] suggested that the potential for real RC application in buildings might lie in RC combination with a heating, ventilation, and (HVAC) system. Moreover, the two reviews also mentioned that there is a need for study on the economic aspect of many prototype RC systems. However, one stakeholder that is also important for the application of RC in buildings, i.e., the architects, was excluded in the reviews. Architects should be taken into consideration as they implement RC concepts in their designs [19]. Thus, this review offers to fill in the gap by analyzing the current development of RC technology in buildings from an architectural point of view and proposes some possible research direction of passive RC application in buildings. This review does not only include works that directly implement RC in building, but also looks at some relevant papers on the technological development of RC. The review is arranged in six sections with the main content, besides the introduction and conclusions, describing RC principles, its state-of-the-art application in buildings, the architectural features that are involved in current applications, and an outlook for architectural application of passive RC.

2. Radiative Cooling Principles All solid surfaces radiate heat in the form of electromagnetic radiation, whose power is proportional to temperature and , and is distributed across the frequency spectrum. Thus, the term total emissive power is differentiated from spectral emissive power, where the former refers to energy emitted over the entire spectrum, while the latter suggests energy emitted at a specific wavelength interval. The spectral emissive power of an ideal blackbody is governed by Planck’s law as stated by Equation (1). 2 2πhc0 EBλ(T, λ) = (1) n2λ5[ehc0/nλkT 1] − EBλ(T, λ) is the spectral emissive power of a blackbody at a certain temperature T for a particular wavelength λ, h is Planck’s constant (6.626 10 34 J s), c is light speed in vacuum (2.998 108 m/s), n is × − · 0 × Buildings 2020, 10, x FOR PEER REVIEW 3 of 29

Buildings 2020, 10, 215 3 of 28 Buildings𝐸 2020(𝑇,, 10 𝜆) , isx FOR the spectralPEER REVIEW emissive power of a blackbody at a certain temperature T for a particular3 of 29 −34 8 wavelength λ, h is Planck’s constant (6.626 × 10 J∙s), 𝑐 is light speed in vacuum (2.998 × 10 m/s), 23 −123 nthe is refractivethe𝐸 refractive(𝑇, 𝜆) index is the index of spectral the of medium the emissive medium (1 for power (1 vacuum), for of vacuum), a blackbody and k is and Boltzmann’s at k ais certain Boltzmann’s constanttemperature constant (1.3807 T for (1.3807 10a particular− ×J K10− ). × · −34 8 wavelengthJ∙KFigure).1 Figureplots λ, theh1 isplots Planck’s distribution the distribution constant of blackbody (6.626 of blackbod × 10 emissive J∙sy emissive), 𝑐 power is light power for speed some for in some temperatures vacuum temperatures (2.998 against × 10 against m/s), the nwavelength.the is wavelength.the refractive It indicates It indexindicates of that the that emissionsmedium emissions (1 from forfrom vacuum), di difffferenterent blackbodyand blackbody k is Boltzmann’s temperatures temperatures constant peakedpeaked (1.3807 at different di ff×erent 10−23 J∙Kwavelengths.). Figure The 1 plots maximum the distribution wavelength of blackbod 𝜆λmax,, the they wavelengthemissive wavelength power in in which which for some a a blackbody blackbody temperatures at at a against certain thetemperature wavelength. T emits It indicates the maximum that emissions power fromof radiation, different can blackbody be calculated temperatures from Equation peaked (2), at differentwhich is wavelengths.obtained from The Equation maximum (1). When Whenwavelength applied applied 𝜆 to to ,terre terrestrialthe strialwavelength bodies bodies inwith with which a a temperature temperature a blackbody around around at a certain300 300 K, K, temperatureEquation (2) T gives emits λ𝜆max the ofmaximum 9.69.6 µµmm [[20].20 power]. of radiation, can be calculated from Equation (2), which is 2898 obtained from Equation (1). When applied λto terre= strial bodies with a temperature around 300 (2)K, max T Equation (2) gives 𝜆 of 9.6 µm [20].

Figure 1. Distribution of spectral emissive power of a blackbody for different temperatures, replotted from SpectralCal [21]. Figure 1. DistributionDistribution of of spectral spectral emissive emissive power power of of a a blackbody blackbody for for different different temperatures, temperatures, replotted fromfrom SpectralCal [21]. [21]. 2898 𝜆 = (2) 𝑇 The earth’s thermal radiation, which peaks 2898 at 9.6 µm,µm, is radiated to outer space through the 𝜆 = (2) atmosphere. Fortunately, Fortunately, the the earth’s earth’s atmosphere atmosphere is𝑇 isrelatively relatively transparent transparent for for thermal thermal radiation radiation at 8– at 8–13 µm, the wavelength in which the λmax of terrestrial radiation peaks. This band is called the 13 µm,The the earth’s wavelength thermal inradiation, which the which 𝜆 peaks of terrestrial at 9.6 µm, radiation is radiated peaks. to outerThis bandspace isthrough called the atmospheric window. This atmospheric window makes possible the cooling of the earth’s surface via atmosphere.atmospheric window.Fortunately, This the atmospheric earth’s atmosphere window makesis relatively possible transparent the cooling for of thermal the earth’s radiation surface at via 8– radiation in the direction of the sky. Figure2 superimposes the thermal radiation of a terrestrial body 13radiation µm, the in thewavelength direction inof thewhich sky. the Figure 𝜆 2 supeof terrestrialrimposes radiationthe thermal peaks. radiation This ofband a terrestrial is called body the with 300 K temperature on the value of atmospheric transmittance. atmosphericwith 300 K temperature window. This on atmosphericthe value of atmosphericwindow makes transmittance. possible the cooling of the earth’s surface via radiation in the direction of the sky. Figure 2 superimposes the thermal radiation of a terrestrial body with 300 K temperature on the value of atmospheric transmittance.

Figure 2. ThermalThermal radiation of average terrestrial temperature (T = 300300 K) K) lies lies within within the atmospheric window band of 8–13 µµmm [[22].22]. Figure 2. Thermal radiation of average terrestrial temperature (T = 300 K) lies within the atmospheric Passive RC techniques utilize this phenomenon. A typical RC panel places the emitter inside windowPassive bandRC techniques of 8–13 µm utilize[22]. this phenomenon. A typical RC panel places the emitter inside a fullya fully insulated insulated frame frame and and protects protects it itfrom from convective convective heat heat loss loss by by a a transparent transparent cover, cover, usually a polyethylenePassive RC film,film, techniques as illustrated utilize byby Figurethis phenomenon.3 3.. TheThe coolingcooling A powerpowertypical ( (RCQ𝑄net panel) of of a a radiative radiativeplaces the surface surface emitter is is insidedefined defined a fullyas netnet insulated heatheat transfertransfer frame from from and the the protects surface. surface. Ait surfaceAfrom surface convective exposed exposed to heat the to skytheloss absorbssky by absorbsa transparent radiation radiation from cover, thefrom sunusually the (Q sunsun a) polyethylene film, as illustrated by Figure 3. The cooling power (𝑄) of a radiative surface is defined as net heat transfer from the surface. A surface exposed to the sky absorbs radiation from the sun Buildings 2020, 10, 215 4 of 28

Buildings 2020, 10, x FOR PEER REVIEW 4 of 29 and the atmosphere (Qatm) and emits its radiation (Qrad). In addition to the radiative heat emitted or received by the surface, it also gains and loses heat from conduction and convection (Qcond+conv). (𝑄 ) and the atmosphere (𝑄 ) and emits its radiation (𝑄 ). In addition to the radiative heat Thus, the energy balance equation of a radiative surface can be written as Equation (3). emitted or received by the surface, it also gains and loses heat from conduction and convection (𝑄 ). Thus, the energy balance equation of a radiative surface can be written as Equation (3). Qnet = Q (T, λ) Qatm(T ) Qsun Q + (3) rad − amb − ± conv cond 𝑄 = 𝑄(𝑇,) 𝜆 − 𝑄(𝑇) − 𝑄 ± 𝑄 (3)

Figure 3. Figure 3. Illustration of typical radiativeradiative coolingcooling (RC)(RC) panel design andand heatheat transfertransfer onon thethe panel.panel. Detailed equations for each component of the above equation are summarized by Raman et al. [12]. Detailed equations for each component of the above equation are summarized by Raman et al. Qrad is the emissive power of an RC surface with area A at temperature T for the wavelength λ, [12]. 𝑄 is the emissive power of an RC surface with area A at temperature T for the wavelength as shown in Equation (4). The absorbed heat due to incident atmospheric thermal radiation (Qatm) at λ, as shown in Equation (4). The absorbed heat due to incident atmospheric thermal radiation ( 𝑄) ambient temperature Tamb is given by Equation (5). at ambient temperature 𝑇 is given by Equation (5). Z Z ∞ Qrad𝑄(T,(λ𝑇,) =) 𝜆 =A 𝐴 𝑑𝛺𝑐𝑜𝑠dΩcos θ dλE B𝜃 𝑑𝜆𝐸λ(T(𝑇,, λ)) 𝜆  𝜖(𝜆,(λ, θ) 𝜃) (4)(4) 0 Z Z ∞ Q (T ) = A dΩcos θ dλE (T, λ) (λ, θ)  (λ, θ) (5) atm𝑄amb(𝑇) = 𝐴 𝑑𝛺𝑐𝑜𝑠 B𝜃 𝑑𝜆𝐸λ(𝑇,) 𝜆 𝜖(𝜆, 𝜃)atm 𝜖(𝜆, 𝜃) (5) 0 where, where, π Z Z 2 dΩ = 2π dθsinθ (6) 𝑑𝛺=2𝜋0 𝑑𝜃𝑠𝑖𝑛 𝜃 (6) R In Equations (4) and (5), A represents the area of the emitter, and dΩ denotes angular integral overIn a hemisphere Equations (4) as and shown (5), in𝐴 represents Equation (6). the Moreover,area of the emitter,EBλ(T, λ and) represents 𝑑𝛺 denotes the spectral angular emissive integral powerover a ofhemisphere blackbody as at theshown emitter’s in Equation temperature (6). Moreover,T for the wavelength 𝐸(𝑇, 𝜆) representsλ as formulated the spectral by Equation emissive (1), whereaspower of (blackbodyλ, θ) is the at angle-dependent the emitter’s temperature emissivity of T thefor emitter.the wavelength For Equation λ as (5),formulated there is an by additional Equation emissivity,(1), whereasatm 𝜖(𝜆,(λ, θ) 𝜃) ,is which the angle-dependent is the angle-dependent emissivity emissivity of the ofemitter. the atmosphere. For Equation (5), there is an additionalFurther, emissivity, the absorbed 𝜖(𝜆, solar 𝜃), radiationwhich is the is formulated angle-dependent using Kirchoemissivityff’s radiationof the atmosphere. law as shown by EquationFurther, (7), wherethe absorbed(λ, θsun solar) is theradi emissivityation is formulated of the emitter using at Kirchoff’s the angle ofradiation the sun’s law position as shown(θsun by). InEquation Equation (7), (7),whereIAM 𝜖 1.5(𝜆,is 𝜃 solar) is intensitythe emissivity using of AM the 1.5emitter spectra. at the Conduction angle of the sun’s and convection position (𝜃 heat). transferIn EquationQcond (7),+conv IAM( 1.5T ,isT solaramb) betweenintensity theusing absorber AM 1.5/emitter spectra. with Conduction the surroundings and convection are also heat considered transfer 𝑄(𝑇, 𝑇 ) between the absorber/emitter with the surroundings are also considered in Equation (8), where hc is heat transfer coefficient, 𝑇 is ambient temperature, and 𝑇 is the surface temperature of the RC emitter. 𝑄 = 𝐴 𝑑𝜆𝜖(𝜆, 𝜃 ) 𝐼.(𝜆) (7) Buildings 2020, 10, 215 5 of 28

in Equation (8), where hc is heat transfer coefficient, Tamb is ambient temperature, and Temitter is the surface temperature of the RC emitter. Z ∞ Qsun = A dλ(λ, θsun) IAM 1.5(λ) (7) 0

Q + (T, T ) = A hc(T T ) (8) cond conv amb amb − emitter From the mathematical model of a typical RC emitter as shown above, factors for a successful RC emitter can be derived. Firstly, the RC emitter needs to be properly insulated from conduction and protected from unwanted convective loss. Secondly, the emitter must have high emissive power in the atmospheric window band. This also means the convection cover needs to be transparent in the same band to transmit the thermal radiation from the emitter. Thirdly, the emitter needs to reflect as much as possible the incident solar radiation to work in the daytime. Another important factor is the atmospheric or sky condition, i.e., a humid atmosphere limits the transparency of the atmospheric window. In other words, a clear sky is more beneficial to the RC emitter than an overcast sky.

3. Research on the Application of Radiative Cooling in Buildings Attempts to utilize RC in buildings can be traced back to the 1970s when Bartoli et al. [23] and Harrison and Walton [24] conducted experiments using two similar RC emitter designs with different materials, namely TEDLAR (a polyvinyl fluoride film) and TiO2 white paint as the emitter, respectively. In the same period, Givoni [25] proposed another design of a passive RC system that can provide heating and cooling for buildings. The field started to attract more attention from researchers during the 1990s. During the decade, besides explorations that focused on the cooling power of emitter materials [26–30], some proposed RC systems involving working fluid to extract the cooling and other elements such as thermal storage [31–33] and desiccant [34] to improve the system’s performance. Since then, the number of explorations of the application of RC in buildings has grown significantly. There are different classifications of RC technologies applied in buildings. Erell and Santamouris [15] classified RC technologies into two categories based on how the RC system is utilized, namely movable insulation and heat exchangers. In an RC system with movable insulation, the emitter, which was actually a , was protected from solar radiation at daytime and exposed to the sky at nighttime by turning the insulation off from the emitter [31,35]. In contrast, in an RC system with a , a working fluid, either water or air, was used as a medium to “carry the coldness” of the emitter to the building interior [16,36]. On the other hand, Zeyghami et al. [16] classified the RC technologies into two categories based on working time, namely nocturnal and diurnal. Nocturnal cooling consists of two general designs, i.e., a gray emitter which emits in the whole range of the wavelength, and a selective emitter which was designed to have emissivity higher or lower at a certain wavelength. Meanwhile, diurnal RC prefers to be equipped with selective emitter material and assisted with a cover shield. The classifications of RC technology that have been performed by researchers mark a historical or rather a sequential development of RC technology. In this review, we classify RC technologies based on the type of improvement carried out by researchers in order to obtain technology applicable to buildings. The first category is “material improvement”, which includes studies that focus on new materials or that enhance the current materials for emitters. The second category is “design improvement”, i.e., researchers tried to modify the panel configuration, design, or supporting element to improve the emitter performance. The last category is “combination with other technologies”, which includes applications of RC “to assist” or “with the assistance of” other technologies. Table1 summarizes the classifications of improvement strategies and the reported performance. Buildings 2020, 10, 215 6 of 28

Table 1. Improvement strategies for RC technology application in buildings.

Maximum Cooling Researcher Category Time Improvement Strategy Power/Minimum Temperature energy saving for cooling up to [37] Film-based emitter 26–49% Nocturnal [38] Film-based emitter 43 W/m2

[39] Photonic emitter 5 ◦C below ambient

[40] Photonic emitter 5.2 ◦C below ambient [41] Film-based emitter 120 W/m2 [42] Film-based emitter not available (N/A)

[43] Film-based emitter 2.5 ◦C below ambient

[8] Film-based emitter 4.2 ◦C below ambient [44] Film-based emitter 95.1 W/m2 [45]Diurnal Photonic emitter 110 W/m2

[46] Nanoparticle-based emitter 25.5 ◦C below ambient Emitter material improvement [47] Nanoparticle-based emitter 35 ◦C below ambient [48] Photonic emitter N/A [49] Photonic emitter 14.3 W/m2 [50] Photonic emitter 7.7 0.2 C below ambient ± ◦ [51] Film-based cover 23 W/m2 [52] Film-based cover 175 W/m2 [53]Nocturnal Film-based cover N/A [54] Film-based cover N/A

[55]Cover material improvement Diurnal Photonic cover 50 W/m2 Design aspect: water contact [56] 97.8 W/m2 with the emitter, insulation Design aspect: construction [11] 2.5 C below ambient material ◦ Design aspect: water contact [57] 52 W/m2 with the emitter [58] Design aspect: air 90 W/m2 Nocturnal Design aspect: insulation, [59] 2.5–4 C below ambient air duct ◦ [60] Design aspect: insulation 38 W/m2 [61] Design aspect: insulation N/A Design aspect: cover, [62] 1–6 ◦C below ambient Design improvement air duct 37 C below ambient through a [63] Design aspect: insulation ◦ 24 h cycle Design aspect: glazing and [64] 100 W/m2 Diurnal convection scheme

[65] Design aspect: orientation 7.4 ◦C below ambient

[66] Design aspect: appearance 3.9 ◦C below ambient Buildings 2020, 10, 215 7 of 28

Table 1. Cont.

Maximum Cooling Researcher Category Time Improvement Strategy Power/Minimum Temperature energy saving for cooling up [67] to 53% [30] Wall material 55.8 W/m2 Wall material and insulation, [68] 87 W/m2 cold water storage Solar Collector, phase [69] N/A change material (PCM) [70] (PV) 128.5 W/m2 [71] Solar Collector 50.3 W/m2 [72] Cold storage N/A [73] Solar Collector 27.3 W/m2 PV, heating, ventilation, and [74] air conditioning (HVAC) 123.9 W/m2 system [75] PV 41.7 W/m2 [76] HVAC N/A Solar Collector and PV, cold [77] 75 W/m2 water storage energy saving potential can reach [78] PCM 77% for low-rise buildings

[79] Desiccant 5.5–7 ◦C below ambient energy saving up to 7400 kWh per [34] Solar Collector, desiccant Nocturnal year [80] PV, solar collector, HVAC N/A

Combination energy saving for cooling up to [81] HVAC 46% for Radiant cooling, air [82] 18.1 W/m2 handling unit (AHU) [83] or HVAC 5–10W/m2 energy saving 10.22% compared to [84] Ground coupled conventional ground heat pump [85] PV, solar collector 72 W/m2 [86] HVAC N/A Thermosyphon and cold [87] energy saving for cooling up to 8% water storage [88] Solar Collector 36.61 W/m2 [32] 14.5 W/m2 [17] Photovoltaic thermal (PV/T) 65 W/m2 Thermosyphon and cold [89] N/A water storage

[25] Solar Collector 4–5 ◦C below ambient [90] Solar Collector 55.1 W/m2 [91] Solar Collector 40 W/m2 Thermosyphon and cold [33] 15 C below ambient water storage ◦ [31] 77.2 W/m2 Buildings 2020, 10, 215 8 of 28

Table 1. Cont.

Maximum Cooling Researcher Category Time Improvement Strategy Power/Minimum Temperature [92] PV 72.94 W/m2

[93] Solar Collector, HVAC 4.7 ◦C below ambient

[94] Water tank 8.4 ◦C temperature reduction [95] HVAC 16 W/m2 [96] HVAC more than 200 W/m2 daytime and nighttime cooling of the radiative sky cooling Thermoelectric, cold water [97] subsystem contribute to 55.0% storage tank and 45.0% of annual cold generation, respectively energy saving for cooling up to [98] Attic insulation 3.7–11.8 kWh/m2 Cold water storage, radiant [99] 12.5 W/m2 cooling in the room Diurnal energy saving for cooling up [100] PCM, thermosyphon to 25% energy saving for cooling up [101] PCM, wall material to 47.9% energy saving for cooling up [102] PCM, cold water storage to 10% 50 W/m2 on the wall, and [103] Wall material and insulation 120 W/m2 on the roof [104] Shading device reduces by up to 40% energy saving for cooling between [105] Glazing 40.9–63.4%

3.1. Material Improvement The development of nanomaterial technology has helped to increase the cooling power of RC materials. The material improvement involves two parts, namely emitter material and convection cover material. Detailed reviews of the emitter materials have been carried out by Zhao et al. [14] and Family and Menguc [106]. In the work of Zhao et al. [14], they categorize the material technologies for passive RC into four categories, i.e., a natural emitter, film-based emitter, nanoparticle-based emitter, and photonic emitter. Examples of these different emitters are shown in Figure4. In this review, only some material examinations that are relevant to the application of RC in buildings are included. There are at least three goals in the RC emitter material field of study, namely, improving cooling power in daytime [12], improving performance in humid conditions [44], and making a cost-efficient material [25,58]. Efforts on daytime RC were conducted using different approaches by material researchers, i.e., film-based emitters [8,41–44], nanoparticle-based emitters [46,47], and photonic emitters [45,48–50]. The engineered material must reflect most of the solar and atmospheric radiation and at the same time be able to produce thermal radiation in the specific atmospheric window band. The film-based emitter could produce all-day cooling between 2–9 ◦C for buildings on a typical sunny day in northern US latitudes [41]. The photonic emitter recorded a 110 W/m2 cooling power under direct sunlight [45]. Further, nanoparticle-based emitters in the works of Liu et al. [46] and Kim and Lenert [47] were also recorded at sub-ambient temperature at daytime, with 25.5 ◦C and 35 ◦C below ambient, respectively. Buildings 2020, 10, 215 9 of 28

Buildings 2020, 10, x FOR PEER REVIEW 8 of 29

(a) (b)

(c) (d)

FigureFigure 4.4. DifferentDifferent mechanism used used in in four four categories categories of of passive passive RC RC materials: materials: (a ()a natural) natural emitter, emitter, a amechanism mechanism found found in in Saharan Saharan silver silver ants ants [107] [107 ](repri (reprintednted from from Solar Solar Energy Energy Ma Materialsterials and and Solar Cells, 206,206, JeongJeong etet al.,al., DaytimeDaytime passivepassive radiativeradiative coolingcooling by ultra-emissive bio-inspired polymeric surface, 110296,110296, CopyrightCopyright (2020), (2020), with with permission permission from from Elsevier); Elsevier); (b )( film-basedb) film-based emitter emitter design design by Fan by Fan et al. et [44 al.] (reprinted[44] (reprinted from from Applied Applied Thermal Ther Engineering,mal Engineering, 165, 165, Fan Fan et al., et Yttria-stabilizedal., Yttria-stabilized zirconia zirconia coating coating for passivefor passive daytime daytime radiative radiative cooling cooling in humid in environment,humid environment, 114585, Copyright 114585, Copyright (2020), with (2020), permission with frompermission Elsevier); from ( cElsevier);) nanoparticle-based (c) nanoparticle-based emitter design emitter by design Huang by andHuang Ruan and [ 108Ruan] (reprinted [108] (reprinted from Internationalfrom International Journal Journal of Heat of and Heat Mass and Transfer, Mass Transfer, 104, Huang 104, and Huang Ruan, and Nanoparticle Ruan., Nanoparticle embedded double-layerembedded double-layer coating for daytimecoating radiativefor daytime cooling, radi 890–896,ative cooling, Copyright 890–896, (2017), Copyright with permission (2017), fromwith Elsevier);permission (d )from photonic Elsevier); emitter (d design) photonic by Gao emitter et al. [ 50design] (reprinted by Gao Solar et al. Energy [50] Materials(reprinted and Solar Solar Energy Cells, 200,Materials Gao et and al., Solar Approach Cells, to200, fabricating Gao et al., high-performance Approach to fabricating cooler high-performance with near-ideal emissive cooler with spectrum near- forideal above-ambient emissive spectrum air temperature for above-ambient radiative air cooling, temperature 110013, radiative Copyright cooling, (2019), 110013, with permission Copyright from(2019), Elsevier). with permission from Elsevier).

InThere the subtropicalare at least three climate, goals daytime in the RC RC emitter emitters material in the work field of Jeongstudy, etnamely, al. [49] improving give a remarkable cooling result,power whichin daytime is 7.2 [12],◦C belowimproving ambient performance at daytime. in hu However,mid conditions in the [44], region and of making tropical a climatecost-efficient with highermaterial , [25,58]. daytimeEfforts on RC da is hardlyytime RC achieved were [conducted39,40,44]. The using best di resultsfferent in approaches experimenting by withmaterial RC materialsresearchers, for humidi.e., film-based climate came emitters from [8,41–44], an enhanced nanoparticle-based specular reflector emitters (ESR) material [46,47], by and 3M [photonic109] that couldemitters be [45,48–50]. at sub-ambient The en temperaturegineered material on a very must humid reflect and most cloudy of the night solar [39 and,40 ].atmospheric radiation and atDespite the same many time scientists be able to pursuing produce higher thermal RC radiation power both in the in thespecific daytime atmospheric and humid window conditions, band. onlyThe film-based a few have emitter focused could on theproduce affordability all-day coolin of theg materials.between 2–9 For °C instance,for buildings Givoni on a [ typical25] and sunny Erell andday Etzionin northern [58] proposedUS latitudes cheaper [41]. RCThe emitter photonic options, emitter but recorded their examination a 110 W/m2 resulted cooling inpower a very under low coolingdirect sunlight power of [45]. RC Further, compared nano to theparticle-based other materials. emitters Current in the high-performance works of Liu et RCal. [46] materials and Kim are stilland expensiveLenert [47] to were produce also recorded and have at limited sub-ambient durability temperature [8]. at daytime, with 25.5 °C and 35 °C below ambient,In terms respectively. of convection cover materials, the spectral properties and durability are key issues. BenlattarIn the et subtropical al. [53,54] are climate, among daytime the first RC to modify emitters the in spectral the work properties of Jeong ofet convectional. [49] give cover. a remarkable Using a chemicalresult, which solution is 7.2 deposition °C below method, ambient they at createdaytime. a cadmium However, sulfide in the (CdS) region thin of film tropical that is climate transparent with forhigher humidity, radiation daytime in the 8–13RC isµ mhardly band. achieved They estimated [39,40,44]. a temperature The best results reduction in experimenting of 65 K between with the uncoveredRC materials nocturnal for humid emitter climate and came the covered from an one enhanced [54]. In anotherspecular study, reflector Naghshine (ESR) material and Saboonchi by 3M [109] [52] comparedthat could dibeff aterent sub-ambient thin film multilayertemperature structures on a very for humid RC convection and cloudy cover. night Among[39,40]. the 30 possible Despite many scientists pursuing higher RC power both in the daytime and humid conditions, only a few have focused on the affordability of the materials. For instance, Givoni,[25] and Erell and Etzion [58] proposed cheaper RC emitter options, but their examination resulted in a very low cooling Buildings 2020, 10, x FOR PEER REVIEW 9 of 29 Buildings 2020, 10, x FOR PEER REVIEW 9 of 29

power of RC compared to the other materials. Current high-performance RC materials are still power of RC compared to the other materials. Current high-performance RC materials are still expensive to produce and have limited durability [8]. expensive to produce and have limited durability [8]. In terms of convection cover materials, the spectral properties and durability are key issues. In terms of convection cover materials, the spectral properties and durability are key issues. Benlattar et al. [53,54] are among the first to modify the spectral properties of convection cover. Using Benlattar et al. [53,54] are among the first to modify the spectral properties of convection cover. Using a chemical solution deposition method, they create a cadmium sulfide (CdS) thin film that is a chemical solution deposition method, they create a cadmium sulfide (CdS) thin film that is transparent for infrared radiation in the 8–13 µm band. They estimated a temperature reduction of transparent for infrared radiation in the 8–13 µm band. They estimated a temperature reduction of Buildings65 K between2020, 10, the 215 uncovered nocturnal emitter and the covered one [54]. In another study, Naghshine10 of 28 65 K between the uncovered nocturnal emitter and the covered one [54]. In another study, Naghshine and Saboonchi [52] compared different thin film multilayer structures for RC convection cover. and Saboonchi [52] compared different thin film multilayer structures for RC convection cover. Among the 30 possible multilayers structures from a combination of 16 thin film materials, structures multilayersAmong the structures30 possible from multilayers a combination structures of from 16 thin a combination film materials, of 16 structures thin film materials, that involved structures cubic that involved cubic ZnS in their layers are better at protecting the RC emitter from parasitic heat loss ZnSthat in involved their layers cubic are ZnS better in their at protecting layers are thebetter RC at emitter protecting from the parasitic RC emitter heat lossfrom during parasitic the heat day loss and during the day and night. Their schematic thin film multilayer structure is shown in Figure 5. night.during Their the schematicday and thinnight. film Their multilayer schematic structure thin film is shown multilayer in Figure structure5. Moreover, is shown investigations in Figure for5. Moreover, investigations for durable alternatives of convection cover were conducted by Bathgate durableMoreover, alternatives investigations of convection for durable cover alternatives were conducted of convection by Bathgate cover and were Bosi conducted [51]. They by found Bathgate that and Bosi [51]. They found that zinc sulfide (ZnS) was the most promising material for the RC emitter and Bosi [51]. They found that zinc sulfide (ZnS) was the most promising material for the RC emitter zinccover, sulfide as shown (ZnS) in was Figure the most6. promising material for the RC emitter cover, as shown in Figure6. cover, as shown in Figure 6.

Figure 5. Multilayer structure thin film scheme of Naghshine and Saboonchi [52] (reprinted from FigureFigure 5. 5.Multilayer Multilayer structure structure thin thin film film scheme scheme of Naghshine of Naghshine and Saboonchiand Saboonchi [52] (reprinted [52] (reprinted from Opticsfrom Optics Communications, 410, Naghshine and Saboonchi, Optimized thin film coatings for passive Communications,Optics Communications, 410, Naghshine 410, Naghshine and Saboonchi, and Saboonch Optimizedi, Optimized thin film thin coatings film coatings for passive forradiative passive radiative cooling applications, 416–423, Copyright (2018), with permission from Elsevier). They coolingradiative applications, cooling applications, 416–423, Copyright 416–423, (2018),Copyright with (2018), permission with frompermis Elsevier).sion from They Elsevier). investigated They investigated 30 combinations of layers. 30investigated combinations 30 combinations of layers. of layers.

Figure 6. Typical RC panel design with insulation and convection cover [51] (reprinted from Solar FigureFigure 6. 6. Typical RC panel design with insulation and convection cover [51] (reprinted from Solar Energy MaterialsTypical RCand panel Solar designCells, 95/10, with insulationBathgate an andd Bosi, convection A robust cover convection [51] (reprinted cover material from Solar for Energy Materials and Solar Cells, 95/10, Bathgate and Bosi, A robust convection cover material for Energyselective Materials radiative and cooling Solar applications, Cells, 95/10, 2778–2785, Bathgate andCopyright Bosi, A (2011), robust with convection permission cover from material Elsevier). for selectiveselective radiative radiative cooling cooling applications, applications, 2778–2785,2778–2785, CopyrightCopyright (2011), with permission from from Elsevier). Elsevier).

3.2. Design Improvement Besides enhancements in RC materials, improvements in the design of RC systems have also been proposed. Most researchers focused on two main aspects of RC system design, namely emitter insulation, and emitter contact to the working fluid. The emitter’s insulation is one of the crucial elements in the roof-integrated RC systems designed by Dimoudi and Androutsopoulos [56] and Khedari et al. [62]. Craig et al. [110] went further by suggesting that improving the RC emitter’s insulation on the roof not only increases its performance, but by modifying the configuration of Buildings 2020, 10, x FOR PEER REVIEW 10 of 29

3.2. Design Improvement Besides enhancements in RC materials, improvements in the design of RC systems have also been proposed. Most researchers focused on two main aspects of RC system design, namely emitter insulation, and emitter contact to the working fluid. The emitter’s insulation is one of the crucial Buildingselements2020 in, 10the, 215 roof-integrated RC systems designed by Dimoudi and Androutsopoulos [56]11 ofand 28 Khedari et al. [62]. Craig et al. [110] went further by suggesting that improving the RC emitter’s insulation on the roof not only increases its performance, but by modifying the configuration of the the roof’s insulation, a conventional roof material could even be an RC emitter. Figure7 shows the roof’s insulation, a conventional roof material could even be an RC emitter. Figure 7 shows the roof- roof-integrated RC system and how the roof insulation was structured [56]. integrated RC system and how the roof insulation was structured [56].

Figure 7.7. Proposed configurationsconfigurations of insulation for roof—integratedroof—integrated solar collector—RC emitters by Dimoudi andand Androutsopoulos Androutsopoulos [56] (Adapted[56] (Adapted from Solar from Energy, Solar 80/ 8,Energy, Dimoudi and80/8, Androutsopoulos, Dimoudi and TheAndroutsopoulos, cooling performance The cooling of a performance radiator based of roofa radiator component, based 1039–1047,roof component, Copyright 1039–1047, (2006), withCopyright permission (2006), from with Elsevier). permission from Elsevier).

Regarding the insulation from convection between the emitter material and the cover shield, some researchersresearchers proposed proposed the the use use of vacuumof vacuum to minimize to minimize parasitic parasitic thermal thermal load toload the emitterto the emitter [60,63]. Chen[60,63]. et Chen al. [63 et] are al. the[63] first are tothe experiment first to experiment with a vacuum-enhanced with a vacuum-e RCnhanced emitter. RC Their emitter. design Their intends design to achieveintends daytimeto achieve RC daytime and succeeds RC and in succeeds obtaining in a maximumobtaining a of maximum 42 ◦C below of 42 ambient °C below under ambient intense under solar radiation.intense solar Tso radiation. et al. [60], Tso however, et al. [60], did however, not achieve did a not daytime achieve RC a e ffdaytimeect but couldRC effect deliver but nocturnalcould deliver RC innocturnal a more RC humid in a climatemore humid in Hong climate Kong. in Hong Their Kong design,. Their shown design, in Figure shown8, in could Figure provide 8, could a coolingprovide powera cooling of 38power W/m of2 at 38 night. W/m2 at night. A different different approach approach was was used used in inthe the cover cover design design by Falt by Faltet al. et [61,64]. al. [61 They,64]. proposed They proposed a triple- a triple-glazingglazing skylight skylight which features which features high absorptivity. high absorptivity. The gas blocks The gas the blocks infrared the part infrared of solar part radiation, of solar radiation,and, thus, and,reduces thus, heat reduces gain heatto the gain building’s to the building’s interior interiorand at night and atreleases night releases heat via heat radiation via radiation to the tosky. the The sky. novelty The novelty of their of design their design lies in lies the in middle the middle that could that could tilt, allowing tilt, allowing the formation the formation of a ofgap a gapbetween between the theglass glass and and the theskylight’s skylight’s edge. edge. This This ga gap,p, in inturn, turn, enables enables the the gas gas to to move move between between the upper and lower part of the skylight, thus, when the upper gas is cooled by nocturnal radiation, it is replaced byby thethe warmerwarmer lowerlower gas.gas. See FigureFigure9 9 for for thethe illustrationillustration of of the the design. design. Moreover, to utilize cooling from an RC emitter in the building, the most feasible way is by using a working fluid, which can either be a water-based or an air-based system. Furthermore, the water-based system is divided into an open system and a closed system. In the water-based open system, the cold storage water directly contacts the RC emitter, without any circulation of any working fluid. In the closed system, however, circulated water is used as a working fluid to deliver coldness either to storage or to a heat exchanger. The conceptual drawing of the typical water-based and air-based RC system is shown in Figure 10. The advantages and disadvantages of the water and air-based system are already summarized by Lu et al. [7] and Zhang et al. [81]. The lower installation cost and the simplicity of the system are among the advantages of the air-based system, while the water-based system is better in terms of cooling performance because water has a higher heat capacity than air. It is important to note that the effectiveness of the heat transfer between the emitter and the fluid has also been the focus of investigations for both the water- and air-based RC [11,57–59]. The investigations prescribed the optimum mass flow rate of the fluid to obtain the maximum cooling effect, as Hosseinzadeh and Buildings 2020, 10, 215 12 of 28

Taherian [57] indicated that the mass flow rate of the fluid is critical in achieving the best cooling performance of an RC emitter. Buildings 2020, 10, x FOR PEER REVIEW 11 of 29 Buildings 2020, 10, x FOR PEER REVIEW 11 of 29

FigureFigure 8. 8. 8. Design DesignDesign of of ofvacuum vacuum vacuum chamber chamber chamber as as insulation asinsulation insulation and and andpa partrt partof of the the of convective convective the convective cover cover for cover for the the forRC RC theemitter emitter RC emitter[60][60] (reprinted (reprinted [60] (reprinted from from Renewable fromRenewable Renewable Energy, Energy, Energy, 106, 106, 106,Tso Tso Tsoet et al., etal., al.,A A Afield field field investigation investigation investigation of of passive passivepassive radiative radiative coolingcooling under under Hong Hong Kong’s Kong’s climate,clim climate,ate, 52–61, 52–61, CopyrightCopyright Copyright (2017),(2017), (2017), withwith with permissionpermission permission fromfr fromom Elsevier).Elsevier). Elsevier).

Figure 9. Design of a triple-glazing skylight that can operate as an RC emitter by incorporating a high- Figure 9. DesignDesign of of a atriple-glazing triple-glazing skylight skylight that that can can operate operate as an as RC an emitter RC emitter by incorporating by incorporating a high- a absorptivity gas. RC occurs for the upper gas, while the lower gas obtains heat from the building’s high-absorptivityabsorptivity gas. gas.RC occurs RC occurs for the for theupper upper gas, gas, while while the the lower lower gas gas obtains obtains heat heat from from the the building’s interior. The middle glass then switches the cool and warm air in the upper and lower part of the interior. The The middle middle glass glass then then switches switches the the cool cool and and warm air in the upper and lower part of the glazing, enabling the cycle to continue [61] (reprinted from Building and Environment, 126, Falt et al., glazing, enabling the cycle to continue [61] [61] (reprinted from Building and En Environment,vironment, 126, Falt et al., al., Modified predator-prey algorithm approach to designing a cooling or insulating skylight, 331–338, ModifiedModified predator-prey algorithm approach to designingdesigning a cooling or insulatinginsulating skylight, 331–338, Copyright (2017), with permission from Elsevier). Copyright (2017), with permissionpermission from Elsevier).Elsevier).

OtherMoreover,Moreover, small to to utilize design utilize cooling cooling considerations from from an an RC wereRC emitter emitter also in studied,in the the building, building, such the as the themost most aesthetic feasible feasible way appearanceway is is by by using using of theaa working working emitter. fluid, fluid, The which appearancewhich can can either either of the be be RCa a water-base water-base emitter isdd or obviously or an an air-based air-based interesting system. system. for Furthermore, Furthermore, architects andthe the water- mightwater- acceleratebasedbased system system the implementationis is divided divided into into an an of open RCopen in system system building and and design. a a closed closed Lee system. system. et al. [ 66In In] the andthe water-based Sonwater-based et al. [111 open open] employed system, system, dithetheff erentcold cold storage techniquesstorage water water to directly directly create contacts coloredcontacts theemitters. the RC RC emi emi Bytter,tter, adding without without a photonicany any circulation circulation nanolayer of of any any in working working the order fluid. fluid. of metal–insulator–metalInIn the the closed closed system, system, however, however, (MIM) belowcirculated circulated the water emitter,water is is used Leeused etas as al.a a working working [66] could fluid fluid decorate to to deliver deliver their coldness coldness RC emitter. either either to storage or to a heat exchanger. The conceptual drawing of the typical water-based and air-based Theto storage MIM layersor to a consisted heat exchanger. of Ag-SiO The2-Ag, conceptual and a variation drawing of of the the colors typical was water-based achieved by and varying air-based the RC system is shown in Figure 10. The advantages and disadvantages of the water and air-based thicknessRC system of is the shown SiO2 layer. in Figure On the 10. other The hand,advantages Son et al.and [111 disadvantages] coated the emitter of the withwater silica-embedded and air-based perovskitesystemsystem are are toalready already color it.summarized summarized Figure 11 displays by by Lu Lu et et theal. al. [7] colored[7] and and Zhang RCZhang emitter et et al. al. [81]. by [81]. Son The The et lower al.lower [111 installation installation]. Both colored cost cost and RCand emittersthethe simplicity simplicity could of achieveof the the system system sub-ambient are are among among temperature the the advantag advantag duringeses of of thethe the daytime.air-based air-based system, system, while while the the water-based water-based systemsystem is is better better in in terms terms of of cooling cooling performance performance beca becauseuse water water has has a a higher higher heat heat capacity capacity than than air. air. It It isis important important to to note note that that the the effectiveness effectiveness of of the the heat heat transfer transfer between between the the emitter emitter and and the the fluid fluid has has alsoalso beenbeen thethe focusfocus ofof investigationsinvestigations forfor bothboth thethe water-water- andand air-basedair-based RCRC [11,57–59].[11,57–59]. TheThe investigationsinvestigations prescribed prescribed the the optimum optimum mass mass flow flow rate rate of of the the fluid fluid to to obtain obtain the the maximum maximum cooling cooling effect,effect, as as Hosseinzadeh Hosseinzadeh and and Taherian Taherian [57] [57] indicated indicated that that the the mass mass flow flow rate rate of of the the fluid fluid is is critical critical in in achievingachieving the the best best cooling cooling performance performance of of an an RC RC emitter. emitter. Buildings 2020, 10, x FOR PEER REVIEW 12 of 29

Buildings 2020, 10, 215 13 of 28

Buildings 2020, 10, x FOR PEER REVIEW 12 of 29 (a) (b)

(a) (c) (b)

Figure 10. Configuration of different ways to utilize cooling from the RC emitter: (a) water-based open system; (b) water-based closed system (c) air-based system [81] (reprinted from Applied Energy, 224, Zhang et al., Energy saving and economic analysis of a new hybrid radiative cooling system for single-family houses in the USA, 271–281, Copyright (2018), with permission from Elsevier).

Other small design considerations were also studied, such as the aesthetic appearance of the emitter. The appearance of the RC emitter is obviously interesting for architects and might accelerate the implementation of RC in building design. Lee et al. [66] and Son et al. [111] employed different techniques to create colored emitters. By adding a photonic nanolayer in the order of metal– (c) insulator–metal (MIM) below the emitter, Lee et al. [66] could decorate their RC emitter. The MIM layersFigureFigure consisted 10.10. ConfigurationConfiguration of Ag-SiO2-Ag, of didifferent andfferent a variation waysways toto utilizeutilize of the coolingcooling colors fromwasfrom achievedthethe RCRC emitter:emitter: by varying ((aa)) water-basedwater-based the thickness of theopenopen SiO system;system;2 layer. ((bb ))On water-basedwater-based the other closedclosed hand, system Son ( cet) air-based al. [111] system coated [81] [81 ]the (reprinted (reprinted emitter from from with Applied Applied silica-embedded Energy, Energy, perovskite224,224, ZhangZhang to color etet al.,al., it. EnergyEnergy Figure savingsaving 11 displays andand economiceconomic the colore analysisanalysisd RC ofof emitter aa newnew hybridhybr by Sonid radiativeradiative et al. [111]. coolingcooling Both systemsystem colored forfor RC emitterssingle-familysingle-family could achieve houseshouses sub-ambient inin thethe USA,USA, 271–281,271–281, temperature CopyrightCopyright during (2018),(2018), the withwith daytime. permissionpermission fromfrom Elsevier).Elsevier).

Other small design considerations were also studied, such as the aesthetic appearance of the emitter. The appearance of the RC emitter is obviously interesting for architects and might accelerate the implementation of RC in building design. Lee et al. [66] and Son et al. [111] employed different techniques to create colored emitters. By adding a photonic nanolayer in the order of metal– insulator–metal (MIM) below the emitter, Lee et al. [66] could decorate their RC emitter. The MIM layersFigure consisted 11. Samples of Ag-SiO of colored 2-Ag, daytime and a variation RC emitter of researched the colors by was Son achieved et al. [111] [111] by(reprinted varying from the Nano thickness of theEnergy, SiO2 Sonlayer.Son et et al., Onal., Colored theColored other emitters emitters hand, with with silica-embeddedSon silicaet al.-embedded [111] perovskite coated perovskite nanocrystalsthe emitter nanocrystals forwith effi cient silica-embeddedfor daytimeefficient perovskiteradiativedaytime to radiative cooling,color it. 105461,cooling, Figure Copyright 105461,11 displays Copyright (2020), the with colore (2020), permissiond with RC emitterpermission from Elsevier).by fromSon etElsevier). al. [111]. Both colored RC emitters could achieve sub-ambient temperature during the daytime. 3.3. Combination with Other Technologies Application of RC in buildings often appears in combinations with other cooling technologies. TheseThese combinationscombinations can can be be categorized categorized as as active active systems systems and and passive passive systems. systems. Many Many of the of water-based the water- RCbased systems RC systems are active are systems, active systems, i.e., assisted i.e., byassisted a pump by to a circulate pump to the circulate water. The the passivewater. water-basedThe passive system is found where a thermosyphon mechanism drives the flow of the fluid. Air-based systems, water-based system is found where a thermosyphon mechanism drives the flow of the fluid. Air- onbased the systems, other hand, on the are other more hand, often are found more as often a passive found system. as a passive Some system. strategies Some involve strategies a fan involve as the Figure 11. Samples of colored daytime RC emitter researched by Son et al. [111] (reprinted from Nano activea fan as part the of active an air-based part of RC an to air-based maintain RC the airflowto maintain to the the RC airflow panel. Ato detailed the RC comparisonpanel. A detailed for the Energy, Son et al., Colored emitters with silica-embedded perovskite nanocrystals for efficient precedentcomparison of for RC the combinations precedent of is RC shown combinations in Table2. is shown in Table 2. daytime radiative cooling, 105461, Copyright (2020), with permission from Elsevier).

3.3. Combination with Other Technologies Application of RC in buildings often appears in combinations with other cooling technologies. These combinations can be categorized as active systems and passive systems. Many of the water- based RC systems are active systems, i.e., assisted by a pump to circulate the water. The passive water-based system is found where a thermosyphon mechanism drives the flow of the fluid. Air- based systems, on the other hand, are more often found as a passive system. Some strategies involve a fan as the active part of an air-based RC to maintain the airflow to the RC panel. A detailed comparison for the precedent of RC combinations is shown in Table 2. Buildings 2020, 10, 215 14 of 28

Table 2. Comparison of different means of architectural implementation of RC.

Researcher Architectural Feature Means of Implementation Combination [25] Air-based Solar Collector [31] Air-based Thermal mass Passive system [30] Air-based Thermal mass [98] Air-based Attic Insulation [94] Water-based open system Water tank [93] Air-based Solar Collector, HVAC [83] Air-based Fan or HVAC [79] Air-based Desiccant [34] Air-based Solar Collector, Desiccant [80] Air-based PV, Solar Collector, HVAC [74] Air-based PV, HVAC [75]N/A PV [76] Water-based closed system HVAC [77] Water-based closed system Solar Collector and PV, cold water storage [78] Water-based closed system PCM Roof [81] Water-based closed system HVAC [82] Water-based closed system Radiant cooling, HVAC [84] Active system Water-based closed system Ground coupled heat pump [85] Water-based closed system PV, Solar Collector [86] Water-based closed system HVAC [87] Water-based closed system Thermosyphon and cold water storage [32] Water-based closed system Heat pipe [17] Water-based closed system PV, Solar Collector [89] Water-based closed system Thermosyphon and cold water storage [90] Water-based closed system Solar Collector [33] Water-based closed system Thermosyphon and cold water storage [95] Water-based closed system HVAC [96] Water-based closed system HVAC [99] Water-based closed system Cold water storage [102] Water-based closed system PCM, Cold Water Storage [88] Air-based Solar Collector N/A [92]N/A PV [103] Passive system Air-based Wall material and insulation Roof and Wall [112] Passive or active Air or water-based Temperature-regulating module [113] Passive system Air-based Trombe wall [67] Passive system Air-based Trombe wall [100] Passive system Water-based closed system PCM wall Wall [101] Passive system Water-based closed system PCM wall [68] Active system Water-based closed system Wall material and insulation, cold water storage [105] Passive system Air-based Glazing material Window or skylight [114] Passive system Air-based Glazing material

3.3.1. Active System Specifically, the solar collector [25,34,77,90], photovoltaics (PV) [17,76,77,85], air conditioning (AC) [81,83,86,95,96], and cold water storage [33,89,99,102] are among the frequently studied combinations for the active systems. Givoni [25] is among the first to utilize a solar collector panel as an RC panel. The strategy is to utilize the absorber of the solar collector during the day as an emitter at night. This so-called dual-functional collector is further developed using more advanced techniques and materials [17,88,90]. Spectral-selective coating on the solar thermal absorber was used, as well as a low-density polyethylene (LDPE) film as the cover, replacing the glass cover in the conventional solar thermal collector. The latest results by Hu et al. [90] produce 55.1 W/m2 cooling power at night. Their design is illustrated in Figure 12. Buildings 2020, 10, x FOR PEER REVIEW 14 of 29

Architectural Researcher Means of Implementation Combination Feature Water-based [102] PCM, Cold Water Storage closed system [88] Air-based Solar Collector N/A [92] N/A PV [103] Passive system Air-based Wall material and insulation Roof and Wall Passive or Air or water- Temperature-regulating [112] active based module [113] Passive system Air-based Trombe wall [67] Passive system Air-based Trombe wall Water-based [100] Passive system PCM wall closed system Wall Water-based [101] Passive system PCM wall closed system Wall material and Water-based [68] Active system insulation, cold water closed system storage [105] Window or Passive system Air-based Glazing material [114] skylight Passive system Air-based Glazing material

3.3.1. Active System Specifically, the solar collector [25,34,77,90], photovoltaics (PV) [17,76,77,85], air conditioning (AC) [81,83,86,95,96], and cold water storage [33,89,99,102] are among the frequently studied combinations for the active systems. Givoni [25] is among the first to utilize a solar collector panel as an RC panel. The strategy is to utilize the absorber of the solar collector during the day as an emitter at night. This so-called dual-functional collector is further developed using more advanced techniques and materials [17,88,90]. Spectral-selective coating on the solar thermal absorber was used, as well as a low-density polyethylene (LDPE) film as the cover, replacing the glass cover in the Buildingsconventional2020, 10, 215 solar thermal collector. The latest results by Hu et al. [90] produce 55.1 W/m2 cooling15 of 28 power at night. Their design is illustrated in Figure 12.

FigureFigure 12. Cross-section 12. Cross-section schematic schematic drawing drawing ofof photo-thermalphoto-thermal and and RC RC (PTRC) (PTRC) design design by Hu by et Hu al.et [90] al. [90] (reprinted(reprinted from from Renewable Renewable Energy, Energy, 139, 139, Hu Hu et et al., al., Experimental study study on on a hybrid a hybrid photo-thermal photo-thermal and and radiativeradiative cooling cooling collector collector using using black black acrylic acrylic paintpaint as as the the panel panel coating, coating, 1217–1226, 1217–1226, Copyright Copyright (2019), (2019), with permission from Elsevier). with permission from Elsevier). Combining PV with RC was initially visualized in the design of Harbeman House by Saitoh and Combining PV with RC was initially visualized in the design of Harbeman House by Saitoh and Fujino [77], a so-called sustainable house proposal that attempted to integrate various sustainable Fujino [77], a so-called sustainable house proposal that attempted to integrate various sustainable technologies in the house. A more persistent study on the possible application of the PV-RC technologies in the house. A more persistent study on the possible application of the PV-RC combination Buildings 2020, 10, x FOR PEER REVIEW 15 of 29 is conducted by Zhao et al. [70,75,92,115]. Using photonic material, they develop several strategies in PV-RCcombination ranging from is conducted nocturnal by Zhao to diurnal et al. [70,75,92, cooling.115]. In termsUsing ofphotonic building material, energy they consumption, develop several PV-RC can bestrategies more beneficial in PV-RC because ranging the from combined nocturnal electricity to diurnal and coolingcooling. energy In terms resulting of building from theenergy system is moreconsumption, than the output PV-RC energy can frombe more PV beneficial alone [74 ,because75]. Both the RC combined combinations electricity with and solar cooling thermal energy and PV can beresulting divided from into twothe typessystem when is more installed than onthe the ou roof,tput i.e.,energy similar from orientation PV alone or [74,75]. opposite Both orientation. RC Withcombinations similar orientation, with solar the thermal researchers and PV placed can be the divided RC panel into ontwo the types same when side installed of the roofon the as roof, the solar i.e., similar orientation or opposite orientation. With similar orientation, the researchers placed the collector or PV, normally the sun-facing side [17,70,88,90]. In contrast, the opposite orientation used RC panel on the same side of the roof as the solar collector or PV, normally the sun-facing side the opposite[17,70,88,90]. side In of contrast, the roof the to reduceopposite solar orientation heat gain used to the the opposite emitter side [75, 77of, 85the]. roof to reduce solar Furthermore,heat gain to the RC emitter is also [75,77,85]. commonly used to assist HVAC systems. Usually, the emitter is used to provide chilledFurthermore, water forRC theis also cooling commonly coil of used AC, to enabling assist HVAC the systemsystems. to Usually, be more the energy emitter effi is cientused to [95 ,96]. The designprovide by chilled Jeong water et al. [for95 ],the for cooling instance, coil usedof AC, two enabling types ofthe cooling system coils, to be conventionalmore energy coolingefficient coils and RC-supplied[95,96]. The design cooling by coils,Jeong thus,et al. RC[95], acted for instance, as a supplementary used two types cooling of cooling supplier. coils, conventional The system was claimedcooling to be coils able and to RC-supplied reduce cooling cooling energy coils, consumptionthus, RC acted byas a 35%. supplementary Anothervariant cooling supplier. of the RC-HVAC The systemsystem came was from claimed Zhang to etbe al.able [81 to], reduce who added cooling a energy cold water consumption storage by to 35%. stock Another cooling variant energy of from the RC. FigureRC-HVAC 13 displays system the schematiccame from diagramZhang et ofal. an[81], RC-assisted who added HVAC a cold system.water storage to stock cooling energy from RC. Figure 13 displays the schematic diagram of an RC-assisted HVAC system.

Figure 13. Schematic of a typical RC-assisted HVAC system [81] (reprinted from Applied Energy, 224, Figure 13. Schematic of a typical RC-assisted HVAC system [81] (reprinted from Applied Energy, Zhang et al., Energy saving and economic analysis of a new hybrid radiative cooling system for single- 224, Zhang et al., Energy saving and economic analysis of a new hybrid radiative cooling system for family houses in the USA, 271–281, Copyright (2018), with permission from Elsevier). single-family houses in the USA, 271–281, Copyright (2018), with permission from Elsevier). 3.3.2. Passive System In terms of the passive system, RC has been combined with more diverse techniques such as wall-mounted RC [68,98,103], phase change material (PCM) [100,101], thermal mass [30,31], and the Trombe wall [67,113]. Oliveti et al. [103] attempted to include thermal radiation from the wall to the sky to the overall heat exchange model of a wall. Yong et al. [68] went further than developing a mathematical model by proposing an RC system mounted on the wall. Their system is a dual- functional solar collector that can provide heating in winter and cooling in summer, as shown in Figure 14. However, the system is an active system, involving pumps to circulate water to be stored in the cold and hot water storage. The fully passive implementations of a wall-mounted RC were performed by Shen et al. [100] and He et al. [101]. Their designs are quite similar in principle, using the thermosyphon method to extract cooling from the wall and storing the heat in a PCM (see Figure 15). In terms of cooling performance, the wall-mounted dual-functional heating–cooling emitter was predicted to be able to reduce building energy consumption by 47.9% [68]. Buildings 2020, 10, 215 16 of 28

3.3.2. Passive System In terms of the passive system, RC has been combined with more diverse techniques such as wall-mounted RC [68,98,103], phase change material (PCM) [100,101], thermal mass [30,31], and the Trombe wall [67,113]. Oliveti et al. [103] attempted to include thermal radiation from the wall to the sky to the overall heat exchange model of a wall. Yong et al. [68] went further than developing a mathematical model by proposing an RC system mounted on the wall. Their system is a dual-functional solar collector that can provide heating in winter and cooling in summer, as shown in Figure 14. However, the system is an active system, involving pumps to circulate water to be stored in the cold and hot water storage. The fully passive implementations of a wall-mounted RC were performed by Shen et al. [100] and He et al. [101]. Their designs are quite similar in principle, using the thermosyphon method to extract cooling from the wall and storing the heat in a PCM (see Figure 15). In terms of cooling performance, the wall-mounted dual-functional heating–cooling emitter was predicted to be able toBuildings reduce 2020 building, 10, x FOR PEER energy REVIEW consumption by 47.9% [68]. 16 of 29

(a) (b)

(c)

FigureFigure 14. Structure 14. Structure of solar of solar absorber absorber that serves,that serves, i.e., twoi.e., functionstwo functions for heating for heating and coolingand cooling as designed as by Yongdesigned et al. [by68 ]:Yong (a) designet al. [68]: without (a) design an airwithout gap foran air insulation gap for insulation purposes; purposes; (b) design (b) withdesign an with air gapan for air gap for an area with nighttime ambient temperature that is low enough to be used as cooling, an area with nighttime ambient temperature that is low enough to be used as cooling, nocturnal cooling nocturnal cooling thus provided by both the sky and the surroundings; (c) schematic diagram thus provided by both the sky and the surroundings; (c) schematic diagram showing the mechanism in showing the mechanism in which the dual-functional system works—red line for heating, blue line which the dual-functional system works—red line for heating, blue line for cooling (reprinted from for cooling (reprinted from Renewable Energy, 74, Yong et al., Performance analysis on a building- Renewableintegrated Energy, solar heating 74, Yong and et cooling al., Performance panel, 627–632, analysis Copyright on a (2015), building-integrated with permission from solar Elsevier). heating and cooling panel, 627–632, Copyright (2015), with permission from Elsevier). Buildings 2020, 10, 215 17 of 28 Buildings 2020, 10, x FOR PEER REVIEW 17 of 29 Buildings 2020, 10, x FOR PEER REVIEW 17 of 29

Figure 15. Dual-functional RC-PCM wall design by He et al. [101]. During the daytime, the absorbed Figure 15. Dual-functional RC-PCM wall design by He et al. [101]. During the daytime, the absorbed Figureheat is 15.storedDual-functional by PCM and RC-PCMis later released wall design at night by Hevia etRC, al. thus, [101]. the During temperature the daytime, of the the room absorbed can be heat is stored by PCM and is later released at night via RC, thus, the temperature of the room can be heatkept iscomfortable, stored by PCM and andthe PCM is later can released “recharge” at night (reproduced via RC, thus,from theEnergy temperature and Buildings, of the room199, He can et kept comfortable, and the PCM can “recharge” (reproduced from Energy and Buildings, 199, He et beal., kept Experimental comfortable, study and on thethe performance PCM can “recharge” of a novel (reproduced RC-PCM-wall, from 297–310, Energy Copyright and Buildings, (2019), with 199, al., Experimental study on the performance of a novel RC-PCM-wall, 297–310, Copyright (2019), with Hepermission et al., Experimental from Elsevier). study on the performance of a novel RC-PCM-wall, 297–310, Copyright (2019), permission from Elsevier). with permission from Elsevier). Whereas the aforementioned researchers used PCM to regulate the heat gain and loss in the wall Whereas the aforementioned researchers used PCM to regulate the heat gain and loss in the wall or buildingWhereas enclosure, the aforementioned some researchers researchers have used attempted PCM to regulate to use theinsulation heat gain and and thermal loss in themass wall to or building enclosure, some researchers have attempted to use insulation and thermal mass to or building enclosure, some researchers have attempted to use insulation and thermal mass to regulate regulateregulate heat transfer from anan RCRC emitteremitter to to the the bu building.ilding. Etzion Etzion and and Erell Erell [31] [31] mentioned mentioned at at least least two two heat transfer from an RC emitter to the building. Etzion and Erell [31] mentioned at least two functions functionsfunctions of thermal mass oror otheotherr typestypes of of thermal thermal storage storage strategies strategies when when combined combined with with nocturnal nocturnal of thermal mass or other types of thermal storage strategies when combined with nocturnal RC for a RCRC for a building. Firstly, thermalthermal massmass cancan absorbabsorb thethe excessive excessive heat heat received received by by the the RC RC emitter emitter building. Firstly, thermal mass can absorb the excessive heat received by the RC emitter during the duringduring the daytime. Secondly, itit maintainsmaintains thethe coo coolingling rate rate of of the the RC RC emitte emitter rto to a adesired desired rate, rate, thus, thus, daytime. Secondly, it maintains the cooling rate of the RC emitter to a desired rate, thus, heat does not heatheat does not dissipate rapidly frfromom thethe building,building, and and the the RC RC emitter emitter becomes becomes steady. steady. Thus, Thus, Etzion Etzion dissipate rapidly from the building, and the RC emitter becomes steady. Thus, Etzion and Erell [31] andand Erell [31] examined the bestbest locationlocation forfor placplacinging thermal thermal mass. mass. They They found found that that thermal thermal mass mass examined the best location for placing thermal mass. They found that thermal mass should be placed shouldshould be placed on the roofroof or,or, inin moremore generalgeneral terms, terms, should should be be closely closely coupled coupled with with the the radiative radiative on the roof or, in more general terms, should be closely coupled with the radiative emitter [31]. emitteremitter [31]. Furthermore,Furthermore, LiuLiu etet al.al. [ [112][112]112] alsoalalsoso developed developeddeveloped aa temperature-regulatingtemperature-regu temperature-regulatinglating module modulemodule (TRM) (TRM)(TRM) for forfor solar solarsolar heatingheatingheating and and RC. RC. TheThe TRMTRM consistsconsists ofof polyethylene polyethylene film filmfilm as asas the thethe convection convection cover, cover,cover, a a aporous porousporous RC RCRC material, material,material, ananan aluminum aluminumaluminum sheet,sheet, andand aa solarsolarsolar absorberabsorber (Figure(Figure 16).16 16).). The TheThe layer layerlayer order orderorder was waswas reversed reversedreversed for forfor heating heatingheating mode. mode.mode. TheTheThe TRM TRM maintained maintained aa maximummaximum indoorindoorindoor temperaturetemperature of of 27.5 27.5 °C◦ °CC in inin the thethe hottest hottesthottest days daysdays of ofof summer summersummer and andand 252525◦ °C°CCfor for somesome hourshours onon winterwinter days. days.days. TheThe heatingheating and and cooling cooling provided providedprovided by byby the thethe TRM TRMTRM correspond correspondcorrespond to toto 42.4%42.4%42.4% saving saving inin thethe electricityelectricity bill. bill.bill.

((a)) (b(b) )

Figure 16. Cont. Buildings 2020, 10, 215 18 of 28 Buildings 2020, 10, x FOR PEER REVIEW 18 of 29

(c)

Figure 16. Temperature-regulatingTemperature-regulating module module by by Liu Liu et et al. al. [112]: [112 ]:(a) ( acooling) cooling mode; mode; (b) ( bheating) heating mode; mode; (c) (whenc) when applied applied on onthe the roof roof (repro (reproducedduced from from Energy Energy Conversion Conversion and and Management, Management, 205, 205, Liu Liu et et al., Research on the performance of radiative cooling andand solar heating coupling module to direct control indoorindoor temperature,temperature, 112395,112395, CopyrightCopyright (2020),(2020), withwith permissionpermission fromfrom Elsevier).Elsevier). 4. Architectural Features of Current Radiative Cooling Systems 4. Architectural Features of Current Radiative Cooling Systems As the previous section summarizes, RC for buildings has been prototyped in very diverse As the previous section summarizes, RC for buildings has been prototyped in very diverse ways. ways. Design alternatives are even numerous when RC is combined with other cooling technologies. Design alternatives are even numerous when RC is combined with other cooling technologies. Nevertheless, analysis of the RC systems from an architectural point of view should be conducted Nevertheless, analysis of the RC systems from an architectural point of view should be conducted before it is widely accepted by the architectural community as one of the promising passive design before it is widely accepted by the architectural community as one of the promising passive design strategies for sustainable buildings [19]. One way of doing so is by analyzing the precedents of strategies for sustainable buildings [19]. One way of doing so is by analyzing the precedents of architectural features involved in the proposals of passive applications of RC. As compiled in Table2, architectural features involved in the proposals of passive applications of RC. As compiled in Table some building components or architectural features that have been involved in passive RC systems are 2, some building components or architectural features that have been involved in passive RC systems revealed. Theoretically, the roof is the best location to place an RC emitter compared to other building are revealed. Theoretically, the roof is the best location to place an RC emitter compared to other envelopes. However, architects might want more flexibility in their design, and few researchers have building envelopes. However, architects might want more flexibility in their design, and few applied RC in the wall and façade. These researches, although very few in number, offer alternatives in researchers have applied RC in the wall and façade. These researches, although very few in number, architectural implementation. offer alternatives in architectural implementation. 4.1. Roof 4.1. Roof Installing the RC emitter on the roof is the simplest and most promising way. Besides its highest sky viewInstalling factor the compared RC emitter to the on wallthe roof or other is the building simplest components, and most promising the roof way. is also Besides a common its highest place forsky buildingview factor service compared installations. to the wall Available or other roof-integrated building components, passive the RC roof systems is also consist a common of both place air- andfor building water-based service systems. installations. The roof Available water-based roof-integrated RC is an open passive system RC which systems is quite consist similar of both to a roofair- pondand water-based design [94,116 systems.] (Figure The 17 roof). Disadvantages water-based RC of the is an roof open water-based system which RC are is morequite orsimilar less theto a same roof aspond the design roof pond, [94,116] such (Figure as diffi 17).culty Disadvantages in waterproofing of the theroof roof, water-based additional RC load are more to the or roof less structure, the same andas the maintenance roof pond, ofsuch the as cleanliness difficulty ofin thewaterproofin water. It cang the also roof, only additional be installed load on to top the of roof flat roofsstructure, and aandffects maintenance the accessibility of the of cleanliness the roof for of other the water. uses [ 117It can]. also only be installed on top of flat roofs and affectsMoreover, the accessibility the roof of air-based the roof system for other off ersuses more [117]. techniques. The most straightforward use of an RC emitter was firstly proposed by Etzion [31], where the RC emitter is attached to a concrete roof slab. By this design, the cooling effect of the RC emitter is absorbed by the thermal mass of the roof slab and in turn transmitted to the room. Another air-based roof system uses an air channel to extract the cooling from the RC emitter [25,98]. By using an air channel attached to an RC emitter, cooling is provided by means of cool airflow from the air channel instead of convection of the interior air with the building envelops. This is arguably better for the distribution of the cool and fresh air. Figure 18 shows one example of how the air channel was used to extract cooling from the RC emitter [98]. In the design, the air was used for heat removal in the attic, although it can be further explored for the room’s heat removal as well. Buildings 2020, 10, x FOR PEER REVIEW 19 of 29

Buildings 2020, 10, 215 19 of 28

Buildings 2020, 10, x FOR PEER REVIEW 19 of 29

Figure 17. Water-based open system RC on the roof of a building [116] (reproduced from Renewable Energy, 29/11, Cheikh and Bouchair, Passive cooling by evapo-reflective roof for hot dry climates, 1877–1886, Copyright (2004), with permission from Elsevier).

Moreover, the roof air-based system offers more techniques. The most straightforward use of an RC emitter was firstly proposed by Etzion [31], where the RC emitter is attached to a concrete roof slab. By this design, the cooling effect of the RC emitter is absorbed by the thermal mass of the roof slab and in turn transmitted to the room. Another air-based roof system uses an air channel to extract the cooling from the RC emitter [25,98]. By using an air channel attached to an RC emitter, cooling is provided by means of cool airflow from the air channel instead of convection of the interior air with the building envelops. This is arguably better for the distribution of the cool and fresh air. Figure 18 Figure 17. Water-based open system RC on the roof of a building [116] (reproduced from Renewable showsFigure one 17. exampleWater-based of how open the system air channel RC on thewas roof used of ato building extract [cooling116] (reproduced from the fromRC emitter Renewable [98]. In Energy, 29/11, Cheikh and Bouchair, Passive cooling by evapo-reflective roof for hot dry climates, the Energy,design, 29the/11, air Cheikh was used and for Bouchair, heat removal Passive in cooling the attic, by evapo-reflective although it can roof be further for hot dryexplored climates, for the 1877–1886, Copyright (2004), with permission from Elsevier). room’s1877–1886, heat removal Copyright as (2004),well. with permission from Elsevier). Moreover, the roof air-based system offers more techniques. The most straightforward use of an RC emitter was firstly proposed by Etzion [31], where the RC emitter is attached to a concrete roof slab. By this design, the cooling effect of the RC emitter is absorbed by the thermal mass of the roof slab and in turn transmitted to the room. Another air-based roof system uses an air channel to extract the cooling from the RC emitter [25,98]. By using an air channel attached to an RC emitter, cooling is provided by means of cool airflow from the air channel instead of convection of the interior air with the building envelops. This is arguably better for the distribution of the cool and fresh air. Figure 18 shows one example of how the air channel was used to extract cooling from the RC emitter [98]. In the design, the air was used for heat removal in the attic, although it can be further explored for the room’s heat removal as well.

(a)

(b)

FigureFigure 18. 18.Air-based Air-based RC RC system system on theon roofthe usingroof using an air channelan air channel to utilize to the utilize cooling the [98 cooling]: (a) illustration [98]: (a) illustration of how the system works to remove heat from the attic; (b) schematic of the air channel of how the system works to remove heat from the attic; (b) schematic of the air channel and the RC paneland (reproducedthe RC panel from (reproduced Energy and fr Buildings,om Energy 203, and(a D.) Buildings, Zhao et al., 203, Roof-integrated D. Zhao et al., radiative Roof-integrated air-cooling systemradiative to achieveair-cooling cooler system attic forto achieve building cooler energy attic saving, for building 109453, Copyrightenergy saving, (2019), 109453, with permissionCopyright from(2019), Elsevier). with permission from Elsevier).

4.2. Wall The second most appropriate architectural feature to place the RC emitter on is the wall. The wall has the advantage of providing a large surface when compared to the roof or other building envelopes. As with the roof RC, the wall RC also appeared in two systems, air-based and water-based systems. (b) It is worth noting that most of the existing proposals on passive RC systems mounted on the wall are dual-functionalFigure 18. Air-based (heating–cooling) RC system modules. on the Forroof instance,using an theair air-basedchannel to wall utilize RC systemthe cooling is a combination[98]: (a) with Trombeillustration wall, of how which the was system developed works to by remove Sameti heat and from Kasaeian the attic; [67 (]b and) schematic consists of of the a glassair channel cover and and the RC panel (reproduced from Energy and Buildings, 203, D. Zhao et al., Roof-integrated radiative air-cooling system to achieve cooler attic for building energy saving, 109453, Copyright (2019), with permission from Elsevier). Buildings 2020, 10, x FOR PEER REVIEW 20 of 29

4.2. Wall The second most appropriate architectural feature to place the RC emitter on is the wall. The wall has the advantage of providing a large surface when compared to the roof or other building envelopes. As with the roof RC, the wall RC also appeared in two systems, air-based and water-based Buildingssystems.2020 It ,is10 worth, 215 noting that most of the existing proposals on passive RC systems mounted 20on of the 28 wall are dual-functional (heating–cooling) modules. For instance, the air-based wall RC system is a combination with Trombe wall, which was developed by Sameti and Kasaeian [67] and consists of a a thermal mass located directly behind the glass. The thermal mass function is to collect the sunlight glass cover and a thermal mass located directly behind the glass. The thermal mass function is to entering the façade during heating mode and dissipate the heat to the night sky during cooling time. collect the sunlight entering the façade during heating mode and dissipate the heat to the night sky The glass cover is open during the heating days to protect it from solar radiation and closed during the during cooling time. The glass cover is open during the heating days to protect it from solar radiation heating nights to prevent radiative heat loss. The reverse is applied for cooling days. Nevertheless, and closed during the heating nights to prevent radiative heat loss. The reverse is applied for cooling it is important to note that an RC-Trombe wall system has some features that can affect its performance days. Nevertheless, it is important to note that an RC-Trombe wall system has some features that can such as external glazing material, vents geometry and position, thermal storage, and Trombe wall affect its performance such as external glazing material, vents geometry and position, thermal area [118,119]. storage, and Trombe wall area [118,119]. For the water-based RC wall, the system is accompanied by PCM to store the coldness and uses For the water-based RC wall, the system is accompanied by PCM to store the coldness and uses thermosyphon phenomena to extract it from the emitter, as described in Section3 (see Figure 15)[ 101]. thermosyphon phenomena to extract it from the emitter, as described in Section 3 (see Figure 15) A similar design to that of He et al. [101] was also tested by Shen et al. [100]. Compared to the testing [101]. A similar design to that of He et al. [101] was also tested by Shen et al. [100]. Compared to the room with a brick wall, the cooling load in the RC-PCM wall room was 42% and 25% lower at ideal testing room with a brick wall, the cooling load in the RC-PCM wall room was 42% and 25% lower and moderate conditions. Furthermore, there are concerning factors that affect the performance of an at ideal and moderate conditions. Furthermore, there are concerning factors that affect the RC-PCM wall system, i.e., the parasitic heat loss due to outdoor wind. Their system was not equipped performance of an RC-PCM wall system, i.e., the parasitic heat loss due to outdoor wind. Their with a convection cover, thus, the effect of wind speed was significant. The implementation of the system was not equipped with a convection cover, thus, the effect of wind speed was significant. The RC-PCM wall in the testing room can be seen in Figure 19. implementation of the RC-PCM wall in the testing room can be seen in Figure 19.

Figure 19. The drawing of an experimental room for an RC-PCM wall by Shen et al. [100]. The RC- Figure 19. The drawing of an experimental room for an RC-PCM wall by Shen et al. [100]. The RC-PCM PCM wall was mounted on the south wall. The measurement data from this room were compared wall was mounted on the south wall. The measurement data from this room were compared with with a conventional brick wall room (reprinted from Applied Thermal Engineering, 176, Shen et al., a conventional brick wall room (reprinted from Applied Thermal Engineering, 176, Shen et al., InvestigationInvestigation onon thethe thermalthermal performanceperformance ofof thethe novelnovel phasephase changechange materialsmaterials wallwall withwith radiativeradiative cooling,cooling, 115479,115479, CopyrightCopyright (2020),(2020), withwith permissionpermission fromfrom Elsevier).Elsevier).

4.3.4.3. Other Building Components

4.3.1.4.3.1. Glazing Material OpeningsOpenings onon the the building envelope are are the sourcethe source of solar of fenestrationsolar fenestration into the into building’s the building’s interior. Variousinterior. glazing Various materials glazing havematerials been have developed been developed to reduce their to reduce transmissivity their transmissivity in the solar and in the infrared solar bands.and infrared Furthermore, bands. researchersFurthermore, intended researchers to also inte maximizended to thermal also maximize radiation thermal of the glazing radiation material of the in theglazing atmospheric material windowin the atmospheric band. With window this strategy, band. the With glazing this strategy, materials the not glazing only reduce materials heat gainnot only but alsoreduce produce heat gain cooling butfor also the produce building. cooling Two prototypes, for the building. namely Two transparent prototypes, film namely and coating transparent to be added film onand top coating of glazing to be materials, added on have top recently of glazing been materials, developed have [105 recently,114,120]. been Currently, developed the transparent [105,114,120]. RC materialsCurrently, are the only transparent studied RC for materials skylight application, are only studied as shown for skylight in Figure application, 20. Future as development shown in Figure for transparent RC film or coating might be evaluated for window application. Buildings 2020, 10, x FOR PEER REVIEW 21 of 29 Buildings 2020, 10, x FOR PEER REVIEW 21 of 29 Buildings20. Future2020 , 10development, 215 for transparent RC film or coating might be evaluated for window21 of 28 20.application. Future development for transparent RC film or coating might be evaluated for window application.

Figure 20. A transparent RC emitter used on a skylight to provide daylighting as well as passive Figure 20. A transparent RC emitter used on a skylight to provide daylighting as well as passive Figurecooling 20. for A buildings transparent [114] RC (reproduced emitter used from on Solar a skylight Energy to Materials provide daylightingand Solar Cells, as well 213, as Ziming passive et cooling for buildings [114] (reproduced from Solar Energy Materials and Solar Cells, 213, Ziming et al., coolingal., Low-cost for buildings radiative [114] cooling (reproduced blade coating from withSolar ultrahigh Energy Materials visible light and transmittance Solar Cells, 213, and Ziming emission et Low-cost radiative cooling blade coating with ultrahigh visible light transmittance and emission within al.,within Low-cost an “atmospheric radiative cooling window”, blade 110563, coating Copyrigh with ultrahight (2020), visible with permissi light transmittanceon from Elsevier). and emission withinan “atmospheric an “atmospheric window”, window”, 110563, 110563, Copyright Copyrigh (2020),t with(2020), permission with permissi fromon Elsevier). from Elsevier). 4.3.2. Paints 4.3.2. Paints Recently, paintpaint waswas proposedproposed asas aa meansmeans toto actact asas aa scalablescalable RCRC emitter.emitter. It is well known by Recently, paint was proposed as a means to act as a scalable RC emitter. It is well known by researchersresearchers ofof passivepassive RCRC thatthat thethe currentlycurrently availableavailable technologiestechnologies areare notnot yetyet scalablescalable andand feasiblefeasible researchers of passive RC that the currently available technologies are not yet scalable and feasible forfor buildingbuilding use.use. Considering paint as a maturemature technologytechnology andand alwaysalways certainlycertainly usedused inin buildingsbuildings for building use. Considering paint as a mature technology and always certainly used in buildings (either(either forfor roof,roof, walls,walls, oror otherother partsparts ofof aa building),building), forfor MandalMandal etet al.al. [[121],121], anan RCRC paintpaint mightmight bebe thethe (either for roof, walls, or other parts of a building), for Mandal et al. [121], an RC paint might be the answer toto the problem ofof scalabilityscalability ofof RCRC technologies.technologies. For them, the material technologies have the answer to the problem of scalability of RC technologies. For them, the material technologies have the capabilitycapability toto developdevelop aa scalablescalable and and e effectiveffective RC RC paint. paint. The The current current development development of of cool cool roof roof coating coating is capability to develop a scalable and effective RC paint. The current development of cool roof coating anis an example example of theof the success success of materialof material technologies technologies to enhance to enhance paint paint performance. performance. However, However, they they also is an example of the success of material technologies to enhance paint performance. However, they highlightedalso highlighted some generalsome general challenges challenges for the developmentfor the development of RC paint of RC besides paint the besides technical the di technicalfficulties. also highlighted some general challenges for the development of RC paint besides the technical Thedifficulties. challenges The arechallenges the assessment are the assessment of geographical of geographical conditions conditions in which RC in which paint benefitsRC paint the benefits most, difficulties. The challenges are the assessment of geographical conditions in which RC paint benefits asthe well most, as theas well examination as the examination of the effect of of the pollution, effect of dirt, pollution, and dust dirt, on theand durabilitydust on the and durability performance and the most, as well as the examination of the effect of pollution, dirt, and dust on the durability and ofperformance the paint. of the paint. performance of the paint. Furthermore, RC paint can also be seenseen inin thethe perspectiveperspective ofof thethe aestheticaesthetic appearanceappearance of RCRC Furthermore, RC paint can also be seen in the perspective of the aesthetic appearance of RC surfaces.surfaces. Currently, Currently, research on this aspect is scarce.scarce. The study conducted by Lee et al. [[66]66] is oneone surfaces. Currently, research on this aspect is scarce. The study conducted by Lee et al. [66] is one example ofof thethe attemptattempt to to answer answer the the aesthetic aesthetic appearance appearance of of an an RC RC emitter emitter (see (see Figure Figure 21). 21). Research Research on example of the attempt to answer the aesthetic appearance of an RC emitter (see Figure 21). Research RCon RC paint paint might might promote promote the the progress progress on aestheticon aesthetic studies studies of RC of surfaces.RC surfaces. on RC paint might promote the progress on aesthetic studies of RC surfaces.

Figure 21. Schematic diagram showing the potential role of RC paints for buildings [121] (reprinted Figure 21. Schematic diagram showing the potential role of RC paints for buildings [121] (reprinted Figurefrom Joule, 21. Schematic 4/7, Mandal diagram et al., showing Paints theas a potential Scalable role and of Effective RC paints Radiative for buildings Cooling [121] Technology (reprinted from for from Joule, 4/7, Mandal et al., Paints as a Scalable and Effective Radiative Cooling Technology for Joule,Buildings, 4/7, Mandal 1350–1356, et al., Copyright Paints as (2020), a scalable with and permission Effective Radiativefrom Elsevier). Cooling Technology for Buildings, Buildings, 1350–1356, Copyright (2020), with permission from Elsevier). 1350–1356, Copyright (2020), with permission from Elsevier).

Buildings 2020, 10, 215 22 of 28

5. Outlook for Architectural Application of Passive Radiative Cooling The previous sections have discussed the development of RC technologies and how they have been applied to reduce the cooling energy of buildings. From an architectural perspective, some issues regarding the implementation of passive RC in buildings arise from the discussions.

In the range of the current RC power and the challenge to overcome the mismatch in cooling • supply and demand, studies on the application of RC in buildings can search for an efficient RC system or effective storage mechanism. Additionally, exploration of the combination of RC with other passive or active cooling techniques should be continued and even extended, because in this way, the disadvantages of RC technologies can be compensated by the advantages of other cooling techniques. Moreover, in terms of exploration of potential RC combinations, there might be other passive design strategies in architecture, outside the cooling techniques, that have not yet been taken into consideration by RC researchers, such as natural ventilation and daylighting strategies. Therefore, a review on the type of strategies of passive design architecture that are suitable for combination with RC is still outstanding. Regarding architectural aspects, there are many considerations neglected by current RC studies. • The roof may have an advantage in regard to the sky view factor, but another building element, such as the wall, may offer advantages in surface area as well as design flexibility. Additional façade elements on the wall, such as a shading device, secondary skin, cladding, and window, are potential locations for the RC emitter. In addition, the aesthetic aspect is also important. Thus, research on transparent and colored RC materials or even RC paints would encourage more flexibility in the architectural application. Following the notions on architectural aspects, another important point arises, that is, the lack of • research on the integration of RC systems in building design. Observations of the implementation of the RC system into real buildings should be introduced. The design process of such an observation and the observation itself might reveal some influential details that have not yet been considered. Most of, if not all, the investigations of RC in buildings have focused on reducing cooling energy. • Besides, the benefit of the RC system, if working ideally, may lead to healthy and comfortable buildings. This area of study, namely the contribution of passive RC in creating thermal comfort for building occupants as well as its further effect on health (and productivity in the working space), will eventually arise. Lastly, two general factors should not be forgotten, namely, the durability of the radiative material • and the cost of the material. Since there are many studies still in the lab scale, these factors have not been calculated by many researchers. Nevertheless, these two aspects can be determinant in terms of real application. Architects and building owners usually prefer to directly know the cost of installation, saving potential, and payback period of the implemented RC systems. Full life-cycle analysis of the system can also be an object of study by researchers in the field.

6. Conclusions The RC research field was revived by the development of new materials. There have been many high-performance RC materials that resulted from the experimentations. The present challenge in this field is to provide scalable and durable RC materials. Besides these two purposes, research on colored and transparent RC materials could also widen the application of RC in buildings. Likewise, pursuing RC paints might be an alternative way to create scalable and colorful emitters, and thus could attract more attention from the architectural community. Furthermore, the available RC materials have been implemented in various RC module designs, as their utilization to reduce cooling energy demand for buildings has also been conducted. Such efforts can continue to be pursued with emphasis on the combination of RC with other passive design strategies. The combination is not limited only to other passive cooling techniques but could also be carried out with natural ventilation, heat storage, Buildings 2020, 10, 215 23 of 28 daylighting, etc. In addition to this, the designs of building-integrated RC should begin to look at building components other than the roof to be the place for installation. Only a handful of building features have been involved in the current explorations such as walls and skylights. Another research direction for the application of RC in buildings is the evaluation of the RC performance in terms of the occupant’s health and comfort. The two indicators could be supplementary to the current performance evaluation, i.e., cooling power or energy saving. This is especially relevant when the RC is combined with other passive design strategies, which may require multi-perspective performance evaluation. At the latter stage, a life-cycle analysis of a building-integrated RC system could also be included. Nevertheless, the efforts to apply RC in buildings need to be more integrated into the architectural design. One way of achieving this is by implementation of the currently available RC materials or panels to a real building, which can be an existing building or a newly constructed building. This type of case study using real building would necessitate a design integration and could uncover some unanticipated aspects of building-integrated RC. Moreover, the uncovered aspects will be further examined in future studies in the field.

Author Contributions: Conceptualization, S. and M.H.; writing—original draft preparation, S.; writing—review and editing, S., M.H., Y.S., J.D. and S.R.; visualization, S. All authors have read and agreed to the published version of the manuscript. Funding: This review is supported with a PhD studentships funded by Indonesia Endowment Fund for Education (Lembaga Pengelola Dana Pendidikan), Ministry of Finance, Republic of Indonesia, reference number S-2401/LPDP.4/2019 and H2020 Marie Skłodowska-Curie Actions-Individual Fellowships (842096). Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature

RC radiative cooling PCM phase change material PV photovoltaics PV/T photovoltaics thermal AC air conditioning HVAC heating, ventilation, and air conditioning TRM temperature-regulating module LDPE low-density polyethylene MIM metal-insulator-metal ESR enhanced specular reflector 2 1 E λ spectral emissive power (W m µm ) B · − · − T temperature (K) Tamb ambient temperature (K) Temitter emitter temperature(K) A emitter’s area (m2) λ wavelength λ (µm) λmax the wavelength in which a blackbody emits maximum radiation (µm) h Planck’s constant (6.626 10–34 J s) × · c speed of light in vacuum (2.998 108 m s 1) 0 × · − n refractive index (1 for vacuum) 23 1 k Boltzmann’s constant (1.3807 10 J K− ) × − · Qnet cooling power of the RC emitter (W) Qsun radiation from the sun (W) Qatm atmospheric radiation (W) Qcond+conv parasitic heat loss from the RC emitter due to conduction and convection (W) 2 1 hc heat transfer coefficient (W m K− ) · − ·  emissivity of the emitter atm emissivity of the sky Buildings 2020, 10, 215 24 of 28

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