A structural radiative cooling material

Buildings account for more than 40% of total energy demand and 70% of electricity consumption in the United States, resulting in an annual national energy bill of more than $ 430 billion. Heating and cooling account for about 48% of this energy consumption, making it the largest single energy expenditure (1). In general, cooling is more difficult than heating because of the second law of thermodynamics (2). As a result, passive radiative cooling has become attractive to improve the energy efficiency of buildings by providing a perpetual path to dissipate the heat of these structures through the atmospheric transparent window into the ultra-cold, non-energy consuming universe. # 39; energy. Nocturnal cooling has been studied on pigmented paints, dielectric coating layers, metallized polymeric films and even organic gases because of their intrinsic thermal emission properties (2–6). Daytime radiative cooling is more difficult, as natural emitting materials in the high infrared also tend to absorb visible wavelengths, although advances include the use of precisely designed nanostructures (7, 8) or hybrid optical metamaterials (9) to adapt the material spectrum responses to continuous cooling. However, the manufacture and application of these structures to the dimensions and scale required for construction remains a challenge.

The wood has been used for thousands of years and has proven to be a durable and important building material to potentially replace steel and concrete because of its economic and environmental benefits (ten). We designed the wood by complete delignification followed by mechanical pressing to obtain a structural material (Fig. 1, A and B) with diurnal subambient cooling effects (Figs S1 to S8). We used scanning electron microscopy (SEM) to show that the wood has cellulose fibers or bundles of fibers at several scales (Fig 1C and Figs S9 to S11). Our cooling wood is composed of cellulose nanofibers partially aligned in the direction of tree growth (Fig. 1D and Fig. S11); these fibers are not absorbent in the visible (Figs S12 to S15). Multiscale fibers and channels (Fig. S16) function as disordered and disordered scattering elements for intense broadband reflection at all visible wavelengths (Figs 1E and Figs S17 and S18). At the same time, molecular vibrations and stretching of cellulose in the cooling wood facilitate high emission in the infrared (Fig. 1F). The heat flux emitted by the cooling wood exceeds the solar irradiance absorbed, resulting in passive ambient radiative cooling, both day and night. Delignified and mechanically pressed wood also offers strength and toughness of 8.7 and 10.1 times, respectively, the strength and toughness of natural wood. These results show that cooling wood is a multifunctional structural material that can improve the energy efficiency of buildings.

The largely messy mesoporous cellulosic structures make the cooling wood extremely cloudy. A reflective and cloudy surface can effectively disperse the incident light in a hemispherical solid angle, which is particularly desirable for construction applications to avoid any visual discomfort caused by specularly reflected light (11). We show the reflective veil spectra of the wood being cooled with an angle of incidence of 8 °, demonstrating that the material has an extremely high reflection of 96% on average (Fig S17). The high diffusion reflectance in the solar radiation range leads to the bright whiteness of the cooled wood (Fig. 2A) (12). The higher reflection when the incoming polarization direction lies along the direction of alignment of the fiber is attributable to the high scattering (Figure S18). We have studied the emissivity spectra of infrared cooling wood ranging from 5 to 25 μm, that is, covering the spectroscopically important wavelength range for black bodies to the ambient temperature (Fig. 2B). The cooling wood has a high emissivity (close to the unit) in the infrared range, emitting strongly from all angles and emitting a net heat flow through the atmospheric transparency window (8 to 13 μm) to the cold well. outdoor space in the form of infrared. radiation. Thus, the cooling wood is black in the infrared range, a marked difference from its appearance in the solar spectrum, where it is white (i.e. it simultaneously exhibits a lack of absorption and a high reflectivity). The spectral response of emissivity in the infrared shows a negligible angular dependence (from 0 ° to 60 °). The average emissivity across the atmospheric window is also greater than 0.9 for emission angles of ± 60 ° (Fig. 2C), indicating a stable emitted heat flux when the wood in Cooling is directed at different angles from the sky, as it would be in practical applications. Figure S19 shows the Fourier transform infrared absorbance of the cooled wood. The high emission of 8 to 13 μm results mainly from the complex infrared emission of the OH association and the stretching vibrations C – H, C – O and C – O – C between 770 and 1250 cm.^-1 (11). Cellulose has the strongest infrared absorbance by OH and C – O centered at about 1050 cm^-1 (9 μm) (11), which happens to be in the window of atmospheric transparency (13). The high emissivity on the rest of the infrared spectrum results in a radiative heat exchange between the wood being cooled and the atmosphere (as in the second atmospheric window between 16 and 25 μm), which further increases the cooling flow. overall radiative temperature when the surface temperature is close to that of the surrounding environment (14).

We demonstrated the radiative coolant performance of cooling wood day and night on a continuous 24-hour thermal measurement at Cave Creek, Arizona (33 ° 49'32 "N, 112 ° 1'44" W; d & # 39; altitude). . We tested two 200mm x 200mm sets of cooling wood in two parallel heat boxes to directly monitor the sub-ambient radiative cooling temperature as well as the cooling power with the help of 39, a feedback heating system (Fig. 2D) (9). We elevated the two thermal boxes 1.2 m from the shaded ground by the sun's rays in order to avoid the heat of the ground towards the housings and the overestimation of the thermal torques for the measurement of the ambient temperature (fig S3B). We found that the cooling wood had radiative cooling powers of 63 and 16 W / m² during the night and day (between 11 am and 2 pm), resulting in an average cooling power of 53 W / m² over the 24 hour period. We measured the radiative cooling temperature in the equilibrium state of the cooling wood synchronously in the second box, in which the Kapton heating was off. The cooling wood has a radiative cooling temperature below room temperature during night and day (Fig 2E). The average temperature below room temperature was> 9 ° C during the night and> 4 ° C around noon (between 11:00 and 14:00). Natural wood and cooling wood have similar thermal conductivities between their upper and lower surfaces (Fig S20) and these values ​​are higher than thermal insulation wood (15) because of the densified structure created by mechanical pressing. We observed scattered clouds during the measurement, which slightly reduced the net effects of radiative cooling (16). In addition, we have used a fluorosilane treatment, which can be used to make superhydrophobic wood with a water contact angle of ~ 150 ° (Fig S21), further improves weather resistance and protects the cooling wood. condensate water.

Cooling wood is also more mechanically strong than natural wood because of the greater surface area of ​​interaction between the exposed hydroxyl groups of the cellulose nanofibers aligned in the growth direction after lignin removal (Fig. 3A). (Fig. 3A).17). The cooling wood has a tensile strength of up to 404.3 MPa, about 8.7 times that of natural wood. Improved toughness of 3.7 MJ / m³ was also observed, 10.1 times that of natural wood (Fig. 3B). We observed a simultaneous increase in mechanical toughness (Fig. S22), which is desirable in the design of structural materials (17–19). We attributed this to the energy dissipation made possible by the repeated formation of hydrogen bonds and / or molecular scale failure in the delignified and mechanically squeezed material.

The ratio of mechanical strength to weight is a critical parameter in buildings, particularly because of cost considerations (20). The specific tensile strength of the cooling wood reaches up to 334.2 MPa cm³/ g (Fig. 3C), surpassing that of most structural materials, including Fe – Mn – Al – C steel, magnesium, aluminum alloys and titanium alloys (21–23). The mechanical scratch hardness of the cooling wood also shows a significant improvement over that of untreated natural wood. According to a linear reciprocating tribometer (Figure S23), the scratch hardness of cooled wood reaches 175.0 MPa in the C direction, 8.4 times that of natural wood (Figure 3, D and E). Compared to natural wood, the scratch hardness of the cooling wood was also multiplied by 5.7 and 6.5 in directions A and B, respectively. The flexural strength of cooling wood is about 3.3 times that of natural wood (Fig S24, A to C). The axial compressive strength of the cooling wood is also much higher than that of natural wood. The cooling wood has a high axial compressive strength of 96.9 MPa, 3.2 times that of natural wood (Fig S24, D to F). Cooling wood also has a 5.7 times higher tenacity than natural wood (Figures S24, G and H).

Cooling wood is superior to natural wood for building efficiency applications in terms of continuous cooling capacity and mechanical strength (Fig 3F). The properties of the cooling wood, including continuous sub-ambient cooling, high mechanical strength, bulk structure, low density, durability and bulk manufacturing process, make it a structurally attractive material compared to other materials. radiative cooling materials (7–9, 24–27). Raman et al. (7) demonstrated a photonic approach to meet the stringent requirements of high thermal emission in the middle infrared and strong solar reflections using seven alternating layers of HfO₂ and SiO₂ of varying thicknesses. However, the equipment is difficult to execute on the scale required for buildings. It has been demonstrated that another thin film of metamaterial could be manufactured in an evolutionary way (9), but can not be used as a structural component. The influence of local weather conditions on radiative cooling performance, including wind speed, water precipitation, and cloud cover, has been studied on metamaterials and large scale radiative cooling systems. (16). Durability for long-term outdoor applications must be taken into account if the cooling wood is to be used in the future as a structural material on the external surfaces of buildings. Surface treatment methods could improve the resistivity of cooling wood against water (28), Fire (29), exposure to ultraviolet (30) and biological factors (31) to meet the need for long-term outdoor durability.

The combination of the properties of visible white (ie, high solar reflectance) and infrared black (ie, high infrared emissivity) of the cooling wood leads to a highly efficient radiative cooling material ( Fig. 4, A and B). The mechanical strength also allows the use of cooling wood as roofing material and coating without further mechanical support. We used EnergyPlus version 8 and the parameters listed in Table S1 to model the potential energy savings from using cooling wood on exterior surfaces (wall coverings and roofing membranes) of buildings. Our energy model allows for a total heat balance on the internal and external surfaces of the building, heat transfer across the building and heat sources and sinks, such as internal loads generated by the equipment , occupants and lighting. This modeling is governed by energy balance equations for the exterior and interior surfaces of the building, as shown in Table S2, which are solved simultaneously. To determine an annual rate of energy consumption, we solved the governance equations iteratively with one hour time step over a year. The internal boundary conditions used an indoor air temperature setpoint of 24 ° C and the external boundary conditions used hourly meteorological data for a typical meteorological year (32). These models use ray tracing for all components of radiative heat transfer, including direct and indirect fluxes, as well as reflective fluxes from both the ground and the surfaces of surrounding buildings.

The building models we used in this study are mid-rise apartment buildings across the United States, based on data from old structures (built before 1980) and new ones (built after 2004) provided by the US Department of Energy's reference building database (33). This type of building is the most appropriate among reference buildings because of the importance of weather-related loads on the total energy consumption of the building (34). The energy modeling process established a baseline energy consumption model for these old and new buildings, and then modified the properties of wall and roof cladding materials based on the performance of the cooling wood to provide a model of energy consumption. energy consumption (Figs S25 and S26). ).

Sixteen cities in the United States were selected for this study: Albuquerque (New Mexico), Atlanta (GA), Austin (TX), Boulder (CO), Chicago (IL), Duluth (MN), Fairbanks (AK), Helena (MT). Honolulu (HI), Las Vegas (NV), Los Angeles (CA), Minneapolis (MN), New York (NY), Phoenix (AZ), San Francisco (CA) and Seattle (WA) (35). These cities are representative of all US climate zones, allowing us to extend the results of this study to the whole country. Modified building models use cooling wood instead of the common wood siding, which is a layer of the roof and cladding, to determine the passive cooling power generated by local weather conditions.

We determined the total cooling energy savings models for the 16 cities selected and the percentage savings compared to the baseline scenario (Fig. 4, C and D). Mid-rise apartments built before 1980 and after 2004 are the last elements to evaluate energy savings, and buildings built between these two buildings will be located between these two limits. We found that cooling energy savings of about 35% on average can be achieved for older mid-rise apartment buildings and about 20% on average for newer ones. mid-rise apartments (Fig. 4E).

The energy savings resulting from the installation of the cooling wood on the exterior surface of these buildings show that Austin (22.9 MJ / m²), Honolulu (28.2 MJ / m)²), Las Vegas (21.1 MJ / m)²), Atlanta (17.1 MJ / m)²) and Phoenix (32.1 MJ / m²) would have the highest energy savings among the 16 selected cities. Phoenix had the highest cooling economy potential because of its hot, dry climate. Therefore, the southwestern cities might be the most appropriate for the installation of this material to reduce the energy consumption for cooling. However, if the cooling wood remains exposed during the winter months, the cost of heating energy would increase thereafter. Figure 1 illustrates the offset of the increase in heating energy costs and a more detailed analysis of overall energy savings. S26. We predicted the cooling energy savings of large medium-sized buildings for all US cities based on local climate zones. The results show that cities with hot and dry climates offer the greatest potential savings of cooling energy. The energy saving effect of wood cooling has the potential to mitigate the energy load associated with interior space conditioning, which accounts for 31% of the building's total primary energy consumption (36). We also evaluated the effect of neighboring structures on energy performance (Figs S27 to S30). The surrounding buildings decrease the cooling energy demand of the building covered with cooling wood due to the shading of the surrounding structures. As a result, the potential savings in cooling energy achieved using cooling wood ranged from an average of 35% for an isolated building to 51% for the highest urban density in pre-1980 buildings and 21% for residential buildings. 39% for buildings after 2004.

We have developed a multifunctional passive radiative cooling material composed of wood that can be fabricated using a scalable bulk process to design its spectral response. The cooling wood has a superior whiteness, which results from the low optical loss of the cellulose fibers and the disordered photonic structure of the material. The energy emitted in the infrared range of the cooling wood exceeds the amount of solar energy received. We confirmed this cooling effect by real-time temperature measurements of natural wood samples and cooling woods in which the materials were exposed to the sky. In addition, cooling wood is 8.7 times stronger and 10.1 times stronger than natural wood. The intrinsic lightness of the cooling wood has a specific resistance three times higher than that of the widely used Fe – Mn – Al – C structural steel. This multifunctional and scalable wood-based cooling material is promising for future sustainable and energy-efficient building applications, enabling a substantial reduction in carbon emissions and energy consumption.

Thanks: Funding: This project is not funded directly. L.H. and T.L. acknowledge the support of the A. James & Alice B. Clark Foundation and the A. James School of Engineering at the University of Maryland. X.Y. recognizes the support of the Gordon and Betty Moore Foundation. Author contributions: T.L., Y.Z. and S.H. also contributed to this work. 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 material preparation and characterization, as well as measurements and mechanical analyzes. Y.Z., Z.W., X.Z., A.A., X.Y. and R.Y. contributed to thermal and optical measurements and analyzes. M.H., D.D., and J.Sr. performed modeling for the efficiency of the building. Y.Z., Z.W. and T.L. went to Arizona for field tests. L.H., T.L., Y.Z. and X.Y. collectively wrote the manuscript. Competing interests: L.H., T.L., and S.H. are the inventors of an internationally patented patent (WO 2019/055789, filed September 14, 2018). All other authors state that they have no competing interests. Availability of data and materials: All data is available in the manuscript or in additional documents.