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A Pragmatic and High-Performance Radiative Cooling Coating with Near-Ideal Selective Emissive Spectrum for Passive Cooling

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A Pragmatic and High-Performance Radiative Cooling Coating with Near-Ideal Selective Emissive Spectrum for Passive Cooling coatings Communication A Pragmatic and High-Performance Radiative Cooling Coating with Near-Ideal Selective Emissive Spectrum for Passive Cooling Mingxue Chen 1,2, Wenqing Li 1,2, Shuang Tao 1,2, Zhenggang Fang 1,2,*, Chunhua Lu 1,2,3,* and Zhongzi Xu 1,2,3 1 State Key Laboratory of Materials-Oriented Chemical Engineering, College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China; [email protected] (M.C.); [email protected] (W.L.); [email protected] (S.T.); [email protected] (Z.X.) 2 Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing Tech University, Nanjing 210009, China 3 Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, Nanjing 210009, China * Correspondence: [email protected] (Z.F.); [email protected] (C.L.) Received: 16 December 2019; Accepted: 24 January 2020; Published: 5 February 2020 Abstract: Radiative cooling is a passive cooling technology that can cool a space without any external energy by reflecting sunlight and radiating heat to the universe. Current reported radiative cooling techniques can present good outside test results, however, manufacturing an efficient radiative material which can be applied to the market for large-scale application is still a huge challenge. Here, an effective radiative cooling coating with a near-ideal selective emissive spectrum is prepared based on the molecular vibrations of SiOx, mica, rare earth silicate, and molybdate functional nanoparticles. The radiative cooling coating can theoretically cool 45 ◦C below the ambient temperature in the nighttime. Polyethylene terephthalate (PET) aluminized film was selected as the coating substrate for its flexibility, low cost, and extensive production. As opposed to the usual investigations that measure the substrate temperature, the radiative cooling coating was made into a cubic box to test its space cooling performance on a rooftop. Results showed that a temperature reduction of 4 0.5 C was obtained in the nighttime and 1 0.2 C was achieved in the daytime. Furthermore, ± ◦ ± ◦ the radiative cooling coating is resistant to weathering, fouling, and ultraviolet radiation, and is capable of self-cleaning due to its hydrophobicity. This practical coating may have a significant impact on global energy consumption. Keywords: radiative cooling coating; selective emissive spectrum; flexibility; low-cost; extensive production; hydrophobicity 1. Introduction Cooling, as one of the major end-uses of energy globally and a major driver of peak electricity demand, has attracted considerable attention in recent years. For example, air conditioning consumes about 15% of the total primary energy used by American commercial buildings and 70% of total electricity consumption in Saudi Arabia [1,2]. In India, residential buildings consume 45% of the energy to maintain the conditions of thermal comfort [3]. Therefore, passive cooling could have a significant impact on global energy consumption for its ability to cool without external energy. Efficient radiative cooling devices are expected to exhibit a thermal radiative peak within 8 to 13 µm, which is known as the atmospheric window, in order to transfer the heat directly to outer space [4–6]. Radiative cooling technologies have been extensively investigated in the last decades [7–9]. However, although most Coatings 2020, 10, 144; doi:10.3390/coatings10020144 www.mdpi.com/journal/coatings Coatings 2020, 10, x FOR PEER REVIEW 2 of 8 CoatingsHowever,2020 ,although10, 144 most technologies present outside test results, manufacturing a radiative cooling2 of 8 material which can be implemented into the complex urban environment of crowded buildings is technologiesstill a challenge. present outside test results, manufacturing a radiative cooling material which can be implementedIf an efficient into the radiative complex material urban environment can be applie ofd crowded to the market buildings for islarge-scale still a challenge. application (for example,If an buildings, efficient radiative curtains, material and tents), can the be following applied to characteristics the market for should large-scale be exhibited application [10,11]: (for (1) example,it has a near-ideal buildings, emittance curtains, andprofile, tents), (2) theit has following a good characteristicscooling effect,should (3) the besubstrate exhibited should [10,11 be]: (1)rigid it hasor flexible, a near-ideal (4) it emittancecan be put profile, into extensive (2) it has production, a good cooling and (5) effect, the (3)price the is substrate affordable. should be rigid or flexible,In this (4) itwork, can be an put efficient into extensive radiative production, cooling coatin andg with (5) the a near-ideal price is aff ordable.selective emissive spectrum is reported,In this work, which an can effi meetcient the radiative five characteristics cooling coating above with for a near-ideal market application. selective emissive Due to spectrumthe Si–O–Si is reported,asymmetric which vibrations can meet of SiO thex five (8–10 characteristics μm) [12], the above B–O stretching for market vibration application. of mica Due (10–11 to the μ Si–O–Sim) [13], the Si–O stretching vibration modulated by rare earth ions (10–12 μm) [14], and the Mo–O stretching asymmetric vibrations of SiOx (8–10 µm) [12], the B–O stretching vibration of mica (10–11 µm) [13], the Si–Omode stretching of molybdate vibration (12–13 modulated μm) [15], by rare the earth four ions functional (10–12 µ m)nanoparticles [14], and the were Mo–O embedded stretching in mode the ofpoly(vinylidene molybdate (12–13 fluoride-co-hexafluoropropene)µm) [15], the four functional nanoparticles (P(VDF-HFP)) were coating embedded to match in thewell poly(vinylidene with the entire fluoride-co-hexafluoropropene)atmosphere window (8–13 μm). (P(VDF-HFP)) The P(VDF-HFP) coating composite to match coat welling with exhibits the entire excellent atmosphere optical windowproperties (8–13 to meetµm). the The characteristics P(VDF-HFP) (1,2). composite To satisfy coating the exhibitscharacteristics excellent (3–5), optical PET propertiesaluminized to film meet is theselected characteristics as the coating (1,2). substrate To satisfy due the to characteristicsbeing flexible, cheap, (3–5), PETand aluminizedmass-manufactured, film is selected and its asability the coatingto be applied substrate to duevarious to being fields. flexible, The composite cheap, and coating mass-manufactured, was made into and a cubic its ability box to betest applied its space to variouscooling fields.performance The composite on a rooftop. coating It was is worth made intonoting a cubic that boxthe toradiative test its spacecooling cooling coating performance showed a ontemperature a rooftop. reduction It is worth of noting 4 ± 0.5 that °C in the the radiative nighttime. cooling Compared coating with showed the recently a temperature published reduction radiative of 4cooling0.5 Cmaterial in the nighttime. based on PET Compared substrates with [10], the th recentlye radiative published cooling radiative coating coolingproposed material in this based work ± ◦ onshows PET a substrates better cooling [10], performance, the radiative coolingand is easier coating to fabricate proposed at in a thismuch work lower shows price a for better commercial cooling performance,availability. and is easier to fabricate at a much lower price for commercial availability. 2.2. MaterialsMaterials andand MethodsMethods 2.1.2.1. PETPET AluminizedAluminized FilmFilm SubstrateSubstrate PETPET aluminizedaluminized filmfilm waswas purchasedpurchased fromfrom GuangdongGuangdong YongshengYongsheng PackagingPackaging MaterialsMaterials Co.Co. Ltd. (Guangzhou,(Guangzhou, China). China). ToTo ensure ensure su sufficientfficient tensile tensile strength, strength, PET PET aluminized aluminized film film with with a thickness a thickness of 50of µ50m μ wasm was selected. selected. The The surface surface features features of the of the PET PET aluminized aluminized film film are are shown shown in Figure in Figure S1a,b. S1a,b. 2.2.2.2. PreparationPreparation ofof thethe RadiativeRadiative CoolingCooling CoatingCoating TheThe preparationpreparation of of the the radiative radiative cooling cooling coating coating is illustrated is illustrated in Figure in Figure1. Firstly, 1. Firstly, 2 g of 2 P(VDF-HFP) g of P(VDF- rawHFP) material raw material (Sigma-Aldrich, (Sigma-Aldrich, Shanghai, Shanghai, China) was China) dissolved was indissolved 35 g acetone in 35 to g form acetone the P(VDF-HFP)to form the solution.P(VDF-HFP) Then, solution. 2 g deionized Then, water2 g deionized was added water into was the added solution into to the make solution a P(VDF-HFP)-acetone-water to make a P(VDF-HFP)- precursoracetone-water solution. precursor Next, solution. 0.2 g SiO Next,x (Aladdin 0.2 g SiO Chemicals,x (Aladdin Chemicals, Shanghai, China),Shanghai, 0.6 China), g mica 0.6 (Mineral g mica Products,(Mineral Products, Shijiazhuang, Shijiazhuang, China), 0.8 China), g rare earth 0.8 g silicate, rare earth and silicate, 0.2 g molybdate and 0.2 g nanoparticles molybdate nanoparticles were added towere the added precursor to the solution. precursor After solution. ball-milling After for ball-mil 24 h, theling nanoparticles for 24 h, the werenanoparticles randomly were dispersed randomly into thedispersed precursor into solution. the precursor Finally, solution. the disperse Finally, system the disperse was used system to produce was used the radiative
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