STRUCTURAL MATERIALS A radiative cooling structural material Tian Li 1 *, Yao Zhai 2 *, Shuaiming He 1 *, Wentao Gan 1 , Zhiyuan Wei 3 , Mohammad Heidarinejad 4 †, Daniel Dalgo 4 , Ruiyu Mi 1 , Xinpeng Zhao 2 , Jianwei Song 1 , Jiaqi Dai 1 , Chaoji Chen 1 , Ablimit Aili 2 , Azhar Vellore 5 , Ashlie Martini 5 , Ronggui Yang 2,6 , Jelena Srebric 4 , Xiaobo Yin 2,3 ‡, Liangbing Hu 1 ‡ Reducing human reliance on energy-inefficient cooling methods such as air conditioning would have a large impact on the global energy landscape. By a process of complete delignification and densification of wood, we developed a structural material with a mechanical strength of 404.3 megapascals, more than eight times that of natural wood. The cellulose nanofibers in our engineered material backscatter solar radiation and emit strongly in mid-infrared wavelengths, resulting in continuous subambient cooling during both day and night. We model the potential impact of our cooling wood and find energy savings between 20 and 60%, which is most pronounced in hot and dry climates. B uildings account for more than 40% of the total energy demand and 70% of electricity use in the United States, leading to an annual national energy bill of more than $430 billion. Heating and cooling accounts for ~48% of this energy use, making it the largest individual energy expense (1). In general, cooling is more challenging than heating because of the second law of thermodynamics (2). As a re- sult, passive radiative cooling has become attract- ive for improving building energy efficiencies by providing a perpetual path to dissipate heat from these structures through the atmospheric transparent window into the ultracold universe with zero energy consumption. Nocturnal radia- tive cooling has been investigated on pigmented paints, dielectric coating layers, metallized poly- mer films, and even organic gases because of their intrinsic thermal emission properties (2–6). Daytime radiative cooling is more challenging, as natural high-infrared emissive materials also tend to absorb visible wavelengths, though advances include using precision-designed nanostruc- tures (7, 8) or hybrid optical metamaterials (9) to tailor material spectrum responses for con- tinuous cooling. However, it remains a challenge to both manufacture and apply these structures at the size and scale required for construction purposes. Wood has been used for thousands of years and has emerged as an important sustainable building material to potentially replace steel and concrete because of its economic and envi- ronmental advantages (10). We engineered wood by complete delignification followed by mechan- ical pressing to render a structural material (Fig. 1, A and B) with daytime subambient cool- ing effects (figs. S1 to S8). We used scanning electron microscopy (SEM) to show that the wood exhibits multiscale cellulose fibers or fiber bundles (Fig. 1C and figs. S9 to S11). Our cooling wood is composed of cellulose nanofibers par- tially aligned in the tree’s growth direction (Fig. 1D and fig. S11); these fibers are nonabsorbing in the visible range (figs. S12 to S15). The mul- tiscale fibers and channels (fig. S16) function as randomized and disordered scattering elements for an intense broadband reflection at all visible wavelengths (Fig. 1E and figs. S17 and S18). Mean- while, the molecular vibration and stretching of cellulose in cooling wood facilitate strong emission in the infrared (Fig. 1F). The heat flux emitted by the cooling wood exceeds the ab- sorbed solar irradiance, resulting in passive sub- ambient radiative cooling for both day and night. The delignified and mechanically pressed wood also delivers mechanical strength and tough- ness that are, respectively, ~8.7 and 10.1 times the strength and toughness of natural wood. These findings establish cooling wood as a mul- tifunctional structural material that may pro- vide a path for improving the energy efficiency of buildings. The largely disordered mesoporous cellulose structures render the cooling wood extremely hazy. A reflective, hazy surface can effectively scatter incident light in a hemispherical solid angle, which is particularly desirable for building applications to avoid visual discomfort caused RESEARCH Li et al., Science 364, 760–763 (2019) 24 May 2019 1 of 4 1 Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742, USA. 2 Department of Mechanical Engineering, University of Colorado Boulder, Boulder, CO 80309, USA. 3 Materials Sciences and Engineering Program, University of Colorado Boulder, Boulder, CO 80309, USA. 4 Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA. 5 Department of Mechanical Engineering, University of California, Merced, Merced, CA 95340, USA. 6 School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, P.R. China. *These authors contributed equally to this work. †Present address: Department of Civil, Architectural, and Environmental Engineering, Illinois Institute of Technology, Chicago, IL 60616, USA. ‡Corresponding author. Email: [email protected] (L.H.); [email protected] (X.Y.) B A G D C F E m n 0 0 5 m μ 0 0 2 m c 5 Fig. 1. Cooling wood demonstrates passive daytime radiative cooling. Photos of a board of (A) natural wood and (B) cooling wood. (C) SEM image of the cooling wood showing the aligned wood channels. (D) SEM image of partially aligned cellulose nanofibers of the cooling wood. (E) Schematic showing the wood structure strongly scattering solar irradiance. (F) Schematic of infrared emission by molecular vibration of the cellulose functional groups. (G) Setup of the real-time measurement of the subambient cooling performance of the cooling wood. on January 29, 2021 http://science.sciencemag.org/ Downloaded from
Transcript
STRUCTURAL MATERIALS
A radiative cooling structural materialTian Li1*, Yao Zhai2*, Shuaiming He1*, Wentao Gan1, Zhiyuan Wei3,Mohammad Heidarinejad4†, Daniel Dalgo4, Ruiyu Mi1, Xinpeng Zhao2, Jianwei Song1,Jiaqi Dai1, Chaoji Chen1, Ablimit Aili2, Azhar Vellore5, Ashlie Martini5,Ronggui Yang2,6, Jelena Srebric4, Xiaobo Yin2,3‡, Liangbing Hu1‡
Reducing human reliance on energy-inefficient cooling methods such as airconditioning would have a large impact on the global energy landscape. By a processof complete delignification and densification of wood, we developed a structuralmaterial with a mechanical strength of 404.3 megapascals, more than eight timesthat of natural wood. The cellulose nanofibers in our engineered material backscattersolar radiation and emit strongly in mid-infrared wavelengths, resulting in continuoussubambient cooling during both day and night. We model the potential impact ofour cooling wood and find energy savings between 20 and 60%, which is mostpronounced in hot and dry climates.
Buildings account for more than 40% of thetotal energy demand and 70% of electricityuse in the United States, leading to anannual national energy bill of more than$430 billion. Heating and cooling accounts
for ~48% of this energy use, making it the largestindividual energy expense (1). In general, coolingis more challenging than heating because ofthe second law of thermodynamics (2). As a re-sult, passive radiative cooling has become attract-ive for improving building energy efficienciesby providing a perpetual path to dissipate heatfrom these structures through the atmospherictransparent window into the ultracold universewith zero energy consumption. Nocturnal radia-tive cooling has been investigated on pigmentedpaints, dielectric coating layers, metallized poly-mer films, and even organic gases because oftheir intrinsic thermal emission properties (2–6).Daytime radiative cooling ismore challenging, asnatural high-infrared emissivematerials also tend
to absorb visible wavelengths, though advancesinclude using precision-designed nanostruc-tures (7, 8) or hybrid optical metamaterials (9)to tailor material spectrum responses for con-tinuous cooling. However, it remains a challengeto both manufacture and apply these structuresat the size and scale required for constructionpurposes.Wood has been used for thousands of years
and has emerged as an important sustainablebuilding material to potentially replace steeland concrete because of its economic and envi-ronmental advantages (10). We engineered woodby complete delignification followed by mechan-ical pressing to render a structural material(Fig. 1, A and B) with daytime subambient cool-ing effects (figs. S1 to S8). We used scanningelectron microscopy (SEM) to show that thewood exhibits multiscale cellulose fibers or fiberbundles (Fig. 1C and figs. S9 to S11). Our coolingwood is composed of cellulose nanofibers par-
tially aligned in the tree’s growth direction (Fig.1D and fig. S11); these fibers are nonabsorbingin the visible range (figs. S12 to S15). The mul-tiscale fibers and channels (fig. S16) function asrandomized and disordered scattering elementsfor an intense broadband reflection at all visiblewavelengths (Fig. 1E and figs. S17 and S18). Mean-while, the molecular vibration and stretchingof cellulose in cooling wood facilitate strongemission in the infrared (Fig. 1F). The heat fluxemitted by the cooling wood exceeds the ab-sorbed solar irradiance, resulting in passive sub-ambient radiative cooling for both day and night.The delignified and mechanically pressed woodalso delivers mechanical strength and tough-ness that are, respectively, ~8.7 and 10.1 timesthe strength and toughness of natural wood.These findings establish cooling wood as a mul-tifunctional structural material that may pro-vide a path for improving the energy efficiencyof buildings.The largely disordered mesoporous cellulose
structures render the cooling wood extremelyhazy. A reflective, hazy surface can effectivelyscatter incident light in a hemispherical solidangle, which is particularly desirable for buildingapplications to avoid visual discomfort caused
RESEARCH
Li et al., Science 364, 760–763 (2019) 24 May 2019 1 of 4
1Department of Materials Science and Engineering, Universityof Maryland, College Park, MD 20742, USA. 2Department ofMechanical Engineering, University of Colorado Boulder,Boulder, CO 80309, USA. 3Materials Sciences andEngineering Program, University of Colorado Boulder,Boulder, CO 80309, USA. 4Department of MechanicalEngineering, University of Maryland, College Park, MD 20742,USA. 5Department of Mechanical Engineering, University ofCalifornia, Merced, Merced, CA 95340, USA. 6Schoolof Energy and Power Engineering, Huazhong University ofScience and Technology, Wuhan 430074, P.R. China.*These authors contributed equally to this work. †Present address:Department of Civil, Architectural, and Environmental Engineering,Illinois Institute of Technology, Chicago, IL 60616, USA.‡Corresponding author. Email: [email protected] (L.H.);[email protected] (X.Y.)
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Fig. 1. Cooling wood demonstrates passive daytime radiative cooling. Photos of a board of (A) natural wood and (B) cooling wood. (C) SEM imageof the cooling wood showing the aligned wood channels. (D) SEM image of partially aligned cellulose nanofibers of the cooling wood. (E) Schematicshowing the wood structure strongly scattering solar irradiance. (F) Schematic of infrared emission by molecular vibration of the cellulose functionalgroups. (G) Setup of the real-time measurement of the subambient cooling performance of the cooling wood.
by strong specularly reflected light (11). We showthe reflection haze spectra of the cooling woodwith an incident angle of 8°, demonstrating thatthematerial has an extremely high reflection hazeof 96% on average (fig. S17). The high, diffusivereflectance in the solar radiation range leads tothe bright whiteness of the cooling wood (Fig. 2A)(12). The higher reflection when the incomingpolarization direction is along the fiber align-ment direction is attributable to the strongscattering (fig. S18). We investigated the emis-sivity spectra of the cooling wood in the infraredrange from 5 to 25 mm, i.e., covering the spec-troscopically important wavelength range forroom-temperature blackbodies (Fig. 2B). Thecooling wood exhibits high emissivity (close tounity) in the infrared range, emitting stronglyat all angles and radiating a net heat flux throughthe atmospheric transparency window (8 to13 mm) to the cold sink of outer space in the formof infrared radiation. Thus, the cooling wood isblack in the infrared range, a marked differencefrom its appearance in the solar spectrum, whereit is white (i.e., simultaneously displaying a lackof absorption and high reflectivity). The infraredemissivity spectrum response shows negligibleangular dependence (from 0° to 60°). The aver-age emissivity across the atmospheric window isalso greater than 0.9 for emission angles between±60° (Fig. 2C), indicating a stable emitted heatflux when the cooling wood is aimed at differentangles in relation to the sky, as it would be in prac-tical applications. Figure S19 shows the Fouriertransform infrared absorbance of the coolingwood. The strong emission from 8 to 13 mm ismainly contributed by the complex infrared emis-sion of OH association and C–H, C–O, and C–O–Cstretching vibrations between 770 and 1250 cm−1
(11). The cellulose exhibits the strongest infraredabsorbance byOHandC–Ocentered at ~1050 cm−1
(9 mm) (11), which coincidently lies in the at-mospheric transparency window (13). The highemissivity across the rest of the infrared spec-trum results in radiative heat exchange betweenthe cooling wood and the atmosphere (such as inthe second atmospheric window between 16 and25 mm), which further increases the overall radi-ative cooling flux when the surface temperatureis close to that of the ambient environment (14).We demonstrated the subambient radiative
cooling performance of the cooling wood duringboth day and night over 24-hour continuousthermal measurement in Cave Creek, Arizona(33°49′32″ N, 112°1′44″W; 585-m altitude). Wetested two sets of cooling wood, 200 mm by200 mm in size, in two thermal boxes in parallelto monitor the subambient radiative cooling tem-perature directly as well as the cooling powerwith the assistance of a feedback-controlledheating system (Fig. 2D) (9). We elevated thetwo thermal boxes 1.2 m over the sunlight-shadedground to avoid heat conducted from the groundto the boxes and overestimation of the thermalcouples for ambient temperature measurement(fig. S3B). We found that the cooling wood hadradiative cooling powers of 63 and 16 W/m2
during the night and daytime (between 11 a.m.and 2 p.m.), respectively, leading to an averagecooling power of 53 W/m2 over the 24-hourperiod. We measured the steady-state radiativecooling temperature of the cooling wood syn-chronously in the second box, in which theKapton heater was turned off. The cooling woodexhibits a radiative cooling temperature belowambient during both night and daytime (Fig. 2E).The average below-ambient temperature was>9°C during the night and >4°C duringmidday(between 11 a.m. and 2 p.m.). Both the naturalwood and the cooling wood exhibit similar ther-mal conductivities between their top and bottom
surfaces (fig. S20), and these values are higherthan that of thermal insulation wood (15) becauseof the densified structure created by mechan-ical pressing. We observed the scattered cloudsduring the measurement, which slightly reducedthe net radiative cooling effects (16). In addition,we used fluorosilane treatment, which can beused to make the wood superhydrophobic witha water contact angle of ~150° (fig. S21) andfurther improves the weatherability and protectsthe cooling wood from water condensate.The cooling wood is also mechanically stron-
ger and tougher than natural wood because ofthe larger interaction area between exposed hy-droxyl groups of the aligned cellulose nanofibersin the growth direction after lignin removal(Fig. 3A) (17). The cooling wood demonstrates atensile strength as high as 404.3 MPa, which is~8.7 times that of natural wood. An improvedtoughness of 3.7MJ/m3was also observed, whichis 10.1 times that of natural wood (Fig. 3B).We observed a simultaneous enhancement inmechanical toughness (fig. S22), which is desir-able in structural material design (17–19). Weattributed this to the energy dissipation enabledby repeated hydrogen-bond formation and/orbreaking at themolecular scale in the delignifiedand mechanically pressed material.The ratio of mechanical strength to weight is
a critical parameter in buildings, especially be-cause of cost considerations (20). The specifictensile strength of the cooling wood reaches upto 334.2 MPa cm3/g (Fig. 3C), surpassing thatof most structural materials, including Fe–Mn–Al–C steel, magnesium, aluminum alloys, andtitanium alloys (21–23). The mechanical scratchhardness of the cooling wood also shows greatimprovement compared with that of the un-treated natural wood. As characterized by a linearreciprocating tribometer (fig. S23), the scratch
Li et al., Science 364, 760–763 (2019) 24 May 2019 2 of 4
Fig. 2. Optical characterization and thermalmeasurement of cooling wood. (A) Absorptionof the natural and cooling wood in the solar spectrum.(B) Infrared emissivity spectra of the cooling woodbetween 5 and 25 mm at different emission angles.(C) Polar distribution of the average emissivityacross the atmospheric window of the coolingwood. (D) Schematic of the thermal box used tocharacterize the radiative cooling power and coolingtemperature. PE, polyethylene. (E) Twenty-four–hour continuous measurement of the 200 mm–
by–200 mm cooling wood. (Top) Measurement inBox one: Direct measurement of the radiativecooling power of the cooling wood. The heater wason, and a feedback control program maintained thewood temperature at the same temperatureas the ambient environment. At this condition, theheating power is the same as the total radiativecooling power because all other heat fluxes are zerobecause of the zero-temperature difference.(Middle) Measurement in Box two: Steady-statetemperature of the cooling wood. (Bottom)Temperature difference between the ambientsurroundings and the cooling wood.
hardness of the cooling wood reaches up to175.0 MPa in direction C, which is 8.4 timesthat of natural wood (Fig. 3, D and E). Comparedwith natural wood, the scratch hardness of thecooling wood also increased by a factor of 5.7and 6.5 in directions A and B, respectively. Theflexural strength of cooling wood is ~3.3 timesas high as that of natural wood (fig. S24, A toC). The axial compressive strength of the cool-ing wood is also much higher than that of nat-ural wood. The cooling wood shows a high axialcompressive strength of 96.9 MPa, which is 3.2times as high as that of natural wood (fig. S24, Dto F). Cooling wood also exhibits a toughnessthat is 5.7 times as high as that of natural wood(fig. S24, G and H).The cooling wood is superior to natural wood
for building efficiency applications in terms ofcontinuous cooling capability and mechanicalstrength (Fig. 3F). The properties of coolingwood, including continuous subambient cooling,high mechanical strength, bulk structure, lowdensity, sustainability, and bulk fabrication pro-cess, make it attractive as a structural materialwhen compared with other radiative coolingmaterials (7–9, 24–27). Raman et al. (7) demon-strated a photonic approach to meet the strin-gent demands of high thermal emission in themid-infrared and strong solar reflection usingseven alternating layers of HfO2 and SiO2 of vary-ing thicknesses. However, the material is difficultto execute at the scale required for buildings.Another metamaterial thin film was demon-strated to have the potential for scalable manu-facturing (9) but cannot be used as a structuralcomponent. The influence on radiative coolingperformance from local weather conditions, in-cluding wind speed, precipitable water, andcloud cover, has been investigated on large-scaleradiative cooling metamaterial and systems (16).Durability for long-term outdoor applicationsmust be considered if the cooling wood is to beutilized as a structural material on the externalsurfaces of buildings in the future. Surface treat-ment methods could improve the resistivity ofthe cooling wood against water (28), fire (29),ultraviolet exposure (30), and biological factors(31) to satisfy the need for long-term outdoordurability.The combination of the visible white (i.e., high
solar reflectance) and infrared black (i.e., highinfrared emissivity) properties of the coolingwood leads to a highly efficient radiative cool-ing material (Fig. 4, A and B). The mechanicalstrength also allows the cooling wood to be usedas both roof and siding material without othermechanical support. We used EnergyPlus ver-sion 8 and the parameters listed in table S1 tomodel the potential energy savings of usingcooling wood on exterior surfaces (wall sidingand roofing membranes) of buildings. Our en-ergy model accounts for a total heat balance onboth the internal and external building enclosuresurfaces, the heat transfer through the buildingenclosures, and heat sources and sinks, such asinternal loads generated by equipment, occu-pants, and lighting. This modeling is governed
by energy-balance equations for both the outsideand inside surfaces of the building, as shown intable S2, which are solved simultaneously. Todetermine an annual rate of energy consump-tion, we solved the governing equations itera-tively with an hourly time step over a year. Theinternal boundary conditions used an indoor airtemperature set point of 24°C, and the externalboundary conditions used hourly weather datafor a typical meteorological year (32). These mod-els use ray tracing for all components of radiativeheat transfer, including direct and indirect fluxes,and fluxes reflected from both the ground andsurrounding building surfaces.The building models that we used in this
study are midrise apartment buildings acrossthe United States, based on data from old (builtbefore 1980) and new (built after 2004) struc-tures provided by the U.S. Department of En-ergy Commercial Reference Buildings database(33). This building type is the most suitableamong the reference buildings because of theimportance of weather-related loads on the to-tal building energy consumption (34). The en-ergy modeling process established a baselineenergy-consumption pattern for these old andnew buildings and then modified the wall sid-ing and roof membrane material properties onthe basis of the cooling-wood performance topredict an energy-consumption pattern (figs. S25and S26).
Sixteen cities in the United States were selectedfor this study: Albuquerque (NM), Atlanta (GA),Austin (TX), Boulder (CO), Chicago (IL), Duluth(MN), Fairbanks (AK), Helena (MT), Honolulu(HI), LasVegas (NV), LosAngeles (CA),Minneapolis(MN), New York City (NY), Phoenix (AZ), SanFrancisco (CA), and Seattle (WA) (35). Thesecities are representative of all U.S. climate zones,allowing us to extend the results of this study tothe entire country. The modified building mod-els use cooling wood in place of common woodsiding, which is a layer of the roofing and sidingassembly, to determine the passive cooling powergenerated as a result of the local weather.Wedetermined the total cooling energy-saving
patterns for the selected 16 cities and the percentsavings relative to the baseline (Fig. 4, C and D).The midrise apartments built before 1980 andafter 2004 are end members for assessing theenergy savings, and buildings built in betweenwill be between these two bounds.We found thatan average of ~35% in cooling energy savings canbe obtained for old midrise apartment buildings,and an average of ~20% can be obtained for newmidrise apartments (Fig. 4E).The energy savings from the installation of
the cooling wood on the exterior surface ofthese buildings show that, on average for old andnew midrise apartments, Austin (22.9 MJ/m2),Honolulu (28.2 MJ/m2), Las Vegas (21.1 MJ/m2),Atlanta (17.1 MJ/m2), and Phoenix (32.1 MJ/m2)
Li et al., Science 364, 760–763 (2019) 24 May 2019 3 of 4
Fig. 3. Cooling wood as a multifunctional structural material. (A) Schematics showing theorigin of the high mechanical strength from the molecular bonding of the aligned cellulosenanofibers. The (B) tensile strength and (C) specific ultimate strength of the cooling wood arecompared with those of natural wood and some common metals and alloy (21–23). (D andE) Scratch-hardness characterization of the natural wood and the cooling wood in three differentdirections. A, B, and C denote directions parallel, perpendicular, and at a 45° angle to the treegrowth direction, respectively. (F) Performance comparison of cooling wood and natural wood.Error bars in (C) and (D) indicate measurement variations among the samples.
would have the highest energy savings amongthe selected 16 cities. Phoenix had the highestpotential cooling savings because of its hot anddry climate. Therefore, cities in the Southwestmay be the most suitable for the installationof this material to reduce energy consumptionfor cooling. However, if the cooling wood re-mains exposed during the winter months, theheating energy cost would subsequently increase.The offset of the increased heating energy costsand a more detailed analysis of the overall en-ergy savings can be found in fig. S26. We pre-dicted the cooling energy savings of midrisebuildings extended for all U.S. cities on thebasis of local climate zones. The results showthat cities with hot and dry climates have thelargest potential cooling energy savings. Theenergy-savings effect of cooling wood has the po-tential to relax the energy load associated withconditioning indoor spaces that accounts for31% of the total building primary energy con-sumption (36). We also evaluated the effect ofneighboring structures on the energy perform-ance (figs. S27 to S30). Surrounding buildingsdecrease the cooling energy demand of the build-ing covered with cooling wood because of theshading that the surrounding structures provide.Therefore, the potential cooling energy savings
obtained by using cooling wood changes, onaverage, from 35% for an isolated building to51% for the highest urban density in pre-1980buildings and changes from 21 to 39% for post-2004 buildings.We developed a multifunctional, passive radi-
ative cooling material composed of wood thatcan be fabricated by using a scalable bulk processto engineer its spectral response. The coolingwood exhibits superior whiteness, which orig-inates from the low optical loss of the cellulosefibers and the material’s disordered photonicstructure. The energy emitted within the infra-red range of the cooling wood overwhelms theamount of solar energy received. We confirmedthis cooling effect by real-time temperature mea-surements of natural and cooling-wood samples,in which the materials were exposed to the sky.Additionally, cooling wood is 8.7 times as strongas and 10.1 times as tough as natural wood. Theintrinsic lightweight nature of the cooling woodhas a specific strength three times that of widelyused Fe–Mn–Al–C structural steel. This multi-functional, scalable cooling-wood material holdspromise for future energy-efficient and sus-tainable building applications, enabling a sub-stantial reduction in carbon emission and energyconsumption.
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ACKNOWLEDGMENTS
Funding: This project is not directly funded. L.H. and T.L. acknowledgethe support of the A. James & Alice B. Clark Foundation and theA. James School of Engineering at the University of Maryland. X.Y.acknowledges the support of the Gordon and Betty Moore Foundation.Author contributions: T.L., Y.Z., and S.H. contributed equally to thiswork. L.H., T.L., Y.Z., S.H., and X.Y. designed the experiments. T.L.,S.H., W.G., R.M., J.So., J.D., C.C., A.V., and A.M. performed the materialpreparation and characterization as well as mechanical measurementsand analysis. Y.Z., Z.W., X.Z., A.A., X.Y., and R.Y. contributed to thethermal and optical measurement and analysis. M.H., D.D., and J.Sr.performed the modeling for building efficiency. Y.Z., Z.W., and T.L. wentto Arizona for field tests. L.H., T.L., Y.Z., and X.Y. collectively wrote themanuscript. Competing interests: L.H., T.L., and S.H. are the inventorson a patent currently pending at the international stage (WO 2019/055789; filed 14 September 2018). All the other authors declare that theyhave no competing interests. Data and materials availability: All dataare available in the manuscript or the supplementary materials.
SUPPLEMENTARY MATERIALS
science.sciencemag.org/content/364/6442/760/suppl/DC1Materials and MethodsSupplementary TextFigs. S1 to S30Tables S1 and S2References (37–39)
27 July 2018; resubmitted 8 November 2018Accepted 22 April 201910.1126/science.aau9101
Li et al., Science 364, 760–763 (2019) 24 May 2019 4 of 4
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Cooling Energy (MJ/m2)
Fig. 4. Modeling energy savings by installing cooling-wood panels on roofing andexternal siding of midrise apartment buildings. (A) When used as a building material,the cooling wood exhibits high solar reflectance and high infrared emissivity. (B) Photo of a5-cm-thick piece of cooling wood. (C) Total cooling energy savings per year and (D) percentageamong all 16 cities. (E) Average cooling energy savings and percentage among all 16 cities.(F) Total predicted cooling energy savings of midrise buildings extended for all U.S. citiesbased on local climate zones.
, this issue p. 760Sciencewould be of particular value in hot and dry climates.cooling savings of their wood for 16 different U.S. cities, which suggested savings between 20 and 50%. Cooling wooddelignification and re-pressing to create a mechanically strong material that also cools passively. They modeled the
engineered a wood throughet al.Passive radiative cooling materials are engineered to do this extremely well. Li One good way to reduce the amount of cooling a building needs is to make sure it reflects away infrared radiation.
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