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Wood Composite as an Energy Efficient Building Material: Guided Sunlight Transmittance and Effective Thermal Insulation

Tian Li, Mingwei Zhu, Zhi Yang, Jianwei Song, Jiaqi Dai, Yonggang Yao, Wei Luo, Glenn Pastel, Bao Yang, and Liangbing Hu*

Dr. T. Li, Prof. M. Zhu, J. Song, J. Dai, Y. Yao, Dr. W. Luo, G. Pastel, Prof. L. HuDepartment of Materials Science and EngineeringUniversity of Maryland College ParkCollege Park, MD 20742, USAE-mail: [email protected]. Yang, Dr. W. Luo, Prof. B. YangDepartment of Mechanical EngineeringUniversity of Maryland College ParkCollege Park, MD 20742, USA

DOI: 10.1002/aenm.201601122

Department of Energy. And windows play a key role in energy management within buildings. For the first time, we have dem-onstrated a transparent wood composite as the building material to efficiently harvest sunlight to provide consistent and uniform indoor lighting. The vertically aligned transparent wood fibers in natural wood exhibit an efficient visible light guiding effect with a large forward to back scat-tering ratio. When used as window or roof, the transparent wood can effectively guide sunlight into the house. Unique optical properties including an extreme optical haze (>95%) in the broadband range and a high transmittance (>85%) lead to a uniform and comfortable indoor ambient lighting without a glare effect in buildings. The transparent wood composite also has much better thermal insulation than glass with at least three times lower thermal conductivity. Greenhouse gas emission from residential and commercial sectors

can mainly be attributed to the energy use of buildings. The application of our energy efficient transparent wood building material can yield substantial energy savings with associated reductions in greenhouse gas emission. The wood based trans-parent composites can find a range of potential applications in next-generation energy efficient buildings.

2. Introduction

As promoted by the U.S. Department of Energy (DOE), energy consumption of buildings is required to reduce 20% by 2020, and 50% as the long-term goal.[1] Energy used for lighting and thermal comfort contributes to more than 50% of the total energy consumption in residential and commercial build-ings.[2] Consequently, conserving air conditioning and lighting usage especially during daytime can yield substantial savings. Sunlight is the best, most natural light for most daily living needs. Glass is the most commonly used material for sun-light harvesting. However, glass windows suffer from the fol-lowing problems. First, glass often creates shadowing effects and discomfort glare.[3–5] To create efficient, uniform, and consistent indoor lighting inside the building, the light har-vesting window needs to yield effective directional scattering

Among many other requirements, energy efficient building materials require effective daylight harvesting and thermal insulation to reduce electricity usage and weatherization cost. The most commonly used daylight harvesting mate-rial, glass, has limited light management capability and poor thermal insula-tion. For the first time, transparent wood is introduced as a building material with the following advantages compared with glass: (1) high optical transpar-ency over the visible wavelength range (>85%); (2) broadband optical haze (>95%), which can create a uniform and consistent daylight distribution over the day without glare effect; (3) unique light guiding effect with a large forward to back scattering ratio of 9 for a 0.5 cm thick transparent wood; (4) excellent thermal insulation with a thermal conductivity around 0.32 W m−1 K−1 along the wood growth direction and 0.15 W m−1 K−1 in the cross plane, much lower than that of glass (≈1 W m−1 K−1); (5) high impact energy absorption that eliminates the safety issues often presented by glass; and (6) simple, scalable fabrication with reliable performance. The demonstrated transparent wood composite exhibits great promise as a future building material, especially as a replacement of glass toward energy efficient building with sustainable materials.

1. Broader Context

Energy efficient building is required to consume less energy than before in indoor lighting and weatherization. Effective and consistent sunlight harvesting can substantially reduce electrical usage while promoting natural and comfortable indoor lighting. Meanwhile, effective thermal insulation of the building material could ultimately pay for itself through cost savings in air conditioning usage. Lighting and air con-ditioning account for more than ≈50% of the total energy used in buildings in the United States, according to the U.S.

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including (1) a high transparency over visible range and (2) a large scattering effect in the forward direction. Current strategies used to realize directional scattering often involve complex nanostructures based on Mie scat-tering or other resonant scattering effects where the size of the nanostructures needs to be finely tuned.[6] Consequently, such tech-niques show limited capability for large-scale commercial applications. Second, due to the intrinsic high thermal conductivity of glass, one-third of the energy used to heat or cool the building is lost through inefficient glass windows.[7] Third, glass is highly brittle and shatters upon sudden impact, which can lead to severe safety issues.

In contrast to glass, wood is a natural thermal insulator with excellent mechanical strength, which has been used as a structural material for houses and cabins for thousands of years. The wood cell lumens are naturally grown in the vertical direction for the trans-port of water and nutrients for the photo-synthesis process,[8,9] which are composed of cellulose and hemicellulose[10–13] with lignin inside.[8] However, natural wood is not trans-parent due to light absorbing lignin and microsized scattering cell lumens. In this work, we demonstrate the application of the transparent wood as an energy efficient light harvesting building material with the following advantages. First, our transparent wood can efficiently harvest sunlight with a broadband trans-mittance of >85%. Thanks to the extremely high haze (≈95%) of transparent wood, the indoor illumination can be kept uni-form and consistent. Second, the transparent wood exhibits a directional forward scattering effect, which can be used to effec-tively guide sunlight into the building. Third, wood cells present large phonon resistance with multiple boundaries. The thermal conductivity along and across the wood channels is measured to be as low as 0.32 and 0.15 W m−1 K−1, respectively. When used as a transparent building material, the wood composite can provide better thermal insulation than standard glass and help reduce air conditioning usage. Furthermore, our transparent wood shows high impact absorption capability. When subjected to a sudden impact, the microchannels with infiltrated polymer absorb and disperse the energy, helping keep the wood from shattering. Figure 1 is an illustration of using transparent wood as sunlight harvesting rooftop. The transmitted light intensity distribution is insensitive to the direction of the sun, keeping the indoor light consistent throughout the day. The conductive heat flow can also be reduced with a more consistent indoor temperature. The transparent wood used as a window or rooftop material could ultimately pay for itself by providing cost savings in lighting and air conditioning energy usage inside the house.

Figure 2a is a scanning electron microscopy (SEM) image of a natural wood block (basswood used in this work). Transparent wood composites were fabricated by selectively removing lignin and subsequently filling the index matching polymer.[14–16] The cell walls are composed of cellulose and hemicellulose with a refractive index around 1.5. As seen in the figure, the wood cells

are naturally aligned along the direction of growth. The aniso-tropic, open lumens in wood blocks allow for both fast lignin removal and polymer infiltration to form a transparent wood composite with a well-preserved microstructure.[14] After the polymer infiltration, the refractive index mismatch between wood cell walls and the polymer inside the cells is greatly reduced. As a result, the wood composite exhibits a high transmittance. Figure S1 (Supporting Information) shows the top view and the side view of the polymer infiltrated wood cell lumens. Note that there are many suitable choices for infiltration polymers as long as the refractive index is close to 1.5 and the material has a low viscosity. With the small refractive index mismatch between the cellulose and the epoxy, light can propagate along the growth direction while the wood cells are functioning as lossy waveguides with a diameter ranging from tens to hundreds of micrometers depending on the species of natural wood. In order to show the light propagation in transparent wood, we have used the DJ532-10 (Thorlabs Inc.), a 532 nm green single mode laser, as the incoming light source with a spot size of 200 μm. The beam is incident from the right hand side with a 45° input angle and is indicated by the arrow in Figure 2a,b. A wood block with a large thickness of 1.4 cm was used so that the propagation of the beam inside the wood block can be clearly observed. As can be seen in the top view of the wood composite, Figure 2b, the beam quickly diverges after reaching the top surface of the wood and then propagates along the wood channel direction. The bright laser light is well directed, indicating an efficient guiding effect. Figure S2 (Supporting Information) shows the side view and the top view of the transparent wood block with the incoming green laser beam illuminating perpendicularly, at a 45° and 70° angle, respectively. As shown, light confinement inside the wood is mainly determined by the wood channel alignment direction instead of the incident light angle.



Figure 1. A house with a transparent wood rooftop can achieve a comfortable lighting condi-tion and more constant ambient temperature. The transparent wood efficiently harvests and guides the sunlight along the wood cell growth direction. The transmitted light is largely scat-tered in the forward direction to create a comfortable and uniform lighting. The low thermal conductivity of transparent wood helps to reduce the conductive heat flow and maintain a con-stant temperature inside. The application of transparent wood as the light harvesting building material not only reduces energy bill, but also promotes more comfortable living conditions.

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The densely packed and vertically aligned channels of the transparent wood function as cylindrical broadband waveguides with high propagation scattering losses. This unique light management capability of the transparent wood cells results in a macroscopic light propagation effect with a large hazi-ness. The optical properties including haze, forward transmit-tance, and backward reflection are carefully investigated and summarized in Figure 2c. An integrated sphere was used to measure the optical properties. The results show that the trans-parent wood exhibits a high transmittance around 90% and a simultaneously high optical haze around 95%. By taking an averaging 90% transmittance and ≈10% reflection within the wavelength range from 500 to 1100 nm, a directional forward to back scattering ratio as high as 9 was obtained. For compar-ison, nanostructures including nanocones and nanospheres are often used in order to achieve directional scattering under the

light management schemes using Mie scattering.[6,17–19] How-ever, the spectral response is usually sensitive to wavelength and the forward to back scattering ratio is often smaller than what is exhibited by transparent wood cells. While exhibiting a high transmittance, the haze of the transparent wood can exceed 95% which is likely due to the scattering of the vertically propagating light by microstructural roughness. This is funda-mentally different from the scattering of haze paper in which fibers are mainly oriented in the planar direction and perpen-dicular to the light propagating direction.[20,21] Furthermore, the overall transparency for the wood composite is comparable to standard glass, plastic, and cellulose-based nanopaper[22–24] confirming the effectiveness of our developed procedure for transparent wood composites. Figure 2d shows a schematic of the single mode laser at a tilted angle incident on a trans-parent wood sample with the transmitted light pattern captured



Figure 2. a) An SEM image of the wood microstructure. After being made transparent, the microchannels of wood are well-preserved, which function as microsized waveguides with high propagation loss for incoming light. b) Top view of the guided light propagation in a thick transparent wood sample. The single mode 532 nm laser beam with a spot size of 200 μm is incident at a 45° angle to the 1.4 cm wood block. c) A 0.5 cm thick transparent wood window exhibits high transmittance and low reflectance with effective broadband forward scattering in the visible wavelength range as well as extremely high optical haze. This unique light management capability renders the transparent wood efficient in guiding the incoming light while at the same time largely scattering the light in the forward direciton. d) The transmitted beam pattern of the 45° incident laser beam. The pattern of the transmitted beam does not show any obvious divergence from a 2D Guassian distribution, owning to the combined results of the effective forward guiding effect and high optical haze.

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on the screen. Interestingly, the beam intensity does not show notable deviation from a standard Gaussian distribution. Light management plays a crucial role[17,25–28] in the effort to improve the overall conversion efficiency of solar cells and light emitting diodes (LEDs). These transparent wood composites, with their unique light management capability, can potentially serve as effective transparent coating or substrate materials for building integrated photovoltaic.[29,30]

Figure 3a compares the haziness of transparent wood, haze paper, and typical soda-lime glass. Besides the high transmit-tance, haze of the transparent wood composite reaches 95% and is much higher than that of the ultrahigh haze nano-paper, which exhibits a typical haze value of ≈60%.[29,31,32] In order to demonstrate the performance of a transparent wood window as an efficient daylight harveting building material with high haze and high transmittance (Figure 3a), we have built a wooden house model with a transparent wood roof 8 cm × 12 cm, as shown in Figure S3 (Supporting Information). Sources of glare include the morning and evening positions of the sun, ice, reflective surfaces on cars, highly polished floors, and the windows of nearby buildings. Glare can interfere with the clarity of a visual image. When used for daily applications, the transparent wood is shown to provide an effective anti-glaring effect. When looking through the transparent wood composite, glare is completely removed while a more uni-form brightness is obtained as demonstrated in Figure 3b. In Figure 3c, we compare the effectivness of using soda-lime glass and transparent wood, respectively, as a light harvesting roof and test both designs in the model. A solar simulator from Oriel

Instruments-Newport was used as the white light source. The model house is purposely made high so that the effect of wood roof can be more pronouncedly observed. When incorporating a light harvesting building material into the house model, uni-form indoor illumination is observed. In comparison to a glass rooftop, the high haze and high transparency of the wood com-posite result in maximized sunlight harvesting of the building and a much consistent light distribution over the course of the day. A calibrated Si detector from Thorlabs was used to eval-uate the light distribution inside the house model. Six different spots were selected and marked as 1–6 for the glass top house and the transparent wood top house, respectively. The results are shown in Figure 3d. The maximum light intensity inside the glass roof house is 12.3 mW cm−2 while the minimum light intensity is only 0.35 mW cm−2, making the illumination non-uniformity more than 35 times. On the contrary, for the house with our transparent wood rooftop, the light intensity differ-ence between brightest corner (4.9 mW cm−2) over the darkest corner (2.1 mW cm−2) is only 2.3 times. Thus, the transparent wood building material is experimentally shown to be an effec-tive solution to save indoor lighting energy and to provide uni-form illumination with enhanced visual comfort and privacy protection owning to its intrinsic haziness.

Besides the requirement for daylight harvesting and mechan-ical strength, transparent building materials also need to meet the requirements for climate protection. Building materials for providing enhanced thermal insulation is therefore highly desir-able.[33] Effective insulation retards the flow of heat through the building shell and provides a structural barrier between the



Figure 3. a) Graph of transmittance percentage versus haze percentage of standard glass, transparent paper, and transparent wood. The haze of trans-parent wood is the highest around 95%. Haze paper generally exhbits a haze around 60%.[29,31,32] Glass exhibits limited light management capability with the lowest value of haze. b) Photographic evidence of the problematic glaring effect with glass in comparison to the uniform and comfortable lighting through transparent wood. c) Photographic evidence of the uniform light distribution inside the house model when using the transparent wood as daylight haversting rooftop in comparison of using glass. d) Compared with glass, the transparent wood rooftop house shows a much more uniform light distribution.

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house and outside environment. If well insulated, the house stays warmer in the winter and cooler in the summer. The walls of most residential and commercial buildings are generally well insulated with materials such as wood and composite foam.[33] However, transparent building materials such as glass have a much higher thermal conductivity which results in higher heat flow than the surrounding materials and an overall reduction in thermal insulation of the building. Thermal insulation from windows is particularly important[34] since thermal bridging across transparent windows and roofs that are made of glass can reduce energy effi-ciency and allow condensation. Current strat-egies to reduce heat loss through windows such as multiple layer glazing are often costly and can add significant weight. On the other hand, wood is a natural insulator with air pockets in the cell structure.[35,36] As shown in Figure 4a, the transparent wood composite provides a high resistance to phonon trave-ling in the wood fiber microstructure. The radial heat travelling pathway yields an even larger phonon scattering effect than that in the axial direction. The anisotropic thermal properties of the transparent wood can be attributed to the alignment of wood cells, which has been well-preserved after lignin removal and polymer infiltration. As can be seen in Figure 4b, we measured a thermal conductivity of around 0.32 W m−1 K−1 in the axial direction and 0.15 W m−1 K−1 in the radial direction, comparable to the thermal conductivity of original basswood.[35] Inter-estingly, a bulk polymer block (the same polymer that has been infiltrated into wood) shows a higher thermal conductivity of around 0.53 W m−1 K−1. The resulting lower thermal conductivity of transparent wood is likely due to the high phonon resistance across the wood cell walls (mainly cellulose and hemicellulose) and the multiple inter-faces phonon scattering effect. In contrast,

glass (Fisher Scientific Microscopic Glass) has a much higher thermal conductivity measured to be ≈1.0 W m−1 K−1 (Figure 4b), proving the transparent wood more effective in reducing conductive heat flow.

Besides its extreme light management capability, the mechanical properties of the transparent wood composite also need to be investigated. Glass has presented significant safety concerns when used as a building block for residential and commercial struc-tures. When it undergoes a sudden impact such as flying debris, an earthquake, or even sudden movement of the occupants, glass can break and spray shattered pieces in all directions. Sometimes, glass can have sudden and spontaneous failure caused by edge or surface damage which propagates through

creep loads. The breaking of glass requires immediate mainte-nance and attention, since the shattered glass presents severe safety issues. On the other hand, wood can withstand higher impact owning to the Van der Waals interactions between the cellulose and the energy absorbing polymer infiltrated micro-structure. Figure 5a shows the resulting morphology of glass and transparent wood after fracture due to a sudden hit from a dropping sharp object. The glass shattered immediately into pointy pieces while our shock-resistant transparent wood stays



Figure 4. a) An illustration of the radial and axial heat transport in transparent wood. The multiple interfaces between cellulose and infiltrated polymer leads to high phonon resistance. b) The measured thermal conductivities of standard glass, epoxy, axial and radial direction of our transparent wood. The transparent wood exhibits anisotropic thermal properties due to the naturally aligned wood microstrucutre.

Figure 5. a) The impact test of a piece of standard glass and a transparent wood composite of similar thickness. The glass shatters upon the sudden impact (upper photo) while the trans-parent wood only shows a dent on the surface (lower photo). b) The strain–stress curve of transparent wood compared with glass. Glass is highly brittle exhbiting a linear strain–stress relation while transparent wood composite is ductile, reaching a strain level two orders higher than that of glass before breaking (Table S1, Supporting Information). c) Photographic evi-dence that the transparent wood sample is water-resistant. Samples exhibit no obvious change after 72 h immersion in water.

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intact, which is highly desirable as a safe, hassle-free, and anti-shatter, transparent building material. Glass is fairly rigid, but can be brittle as well.[37] When glass is under a load it can only accommodate stress to a relatively low level and then suddenly fails. The failure can be sudden and spectacular. Once a crack starts there is little within its structure to stop it from propa-gating. Consequently, glass exhibits a linear curve in strain and stress curve[38] as shown in Figure 5b. In contrast, the trans-parent wood possesses a much higher strain of 6%, more than two orders higher than that of standard soda-lime glass.[38] This substantial increase in ductility is highly desirable for the application as a structural material. Even after breaking upon a sudden impact, the transparent wood is only bent and split instead of shattering into multiple sharp pieces. For commer-cial application as a building material, the transparent wood is also required to be water-resistant. We have immersed the transparent wood sample in water as shown in Figure 4c. After 72 h, the sample is intact without any shape distortion or any degradation in mechanical and optical properties. The SEM observation of the epoxy filled wood walls (Figure S4, Supporting Information) and the mechanical properties of the transparent wood (Figure S5, Supporting Information) after 72 h water immersion have also been carried out. The results show that water has negligible effect on the properties of the transparent wood potentially due to the encapsulation of the polymer component.

3. Conclusion

In this work, we demonstrate the application of transparent wood as an energy efficient building material. We show that the transparent wood exhibits a high transmittance comparable to glass and a unique directional forward scattering effect with an extremely high haze around 95%. The transparent wood can thus be used to reduce the daytime lighting energy usage by efficiently guiding the sunlight into the house while providing uniform and consistent illumination throughout the day. Our transparent wood also has a much lower thermal conductivity compared with glass, making it a better thermally insulating building material with a lower carbon footprint. In addition, the polymer infiltrated wood microstructure exhibits high impact absorption capability with a high ductility, therefore eliminating the safety issues often presented by glass. The transparent wood block can potentially be fabricated as a simple and scal-able process, rendering it a promising candidate to improve energy management in buildings.

4. Experimental SectionMaterials and Chemicals: In this work, basswood was used for the

fabrication of transparent wood. The chemicals for selectively removing lignin were NaOH and Na2SO3 (Sigma-Aldrich). The infiltrated polymer is AeroMarine Epoxy #300/21 with low viscosity and negligible shrinkage after curing.

Transparent Wood Fabrication: The lignin of wood was selectively removed by immersing the wood into a solution of diluted boiling NaOH (2.5 mol L−1, in DI water) and Na2SO3 (0.4 mol L−1, in DI water) for 3 h, followed by immersion in boiling H2O2 (2.5 mol L−1, in DI water) solution for 2–3 h until the yellow color of the wood was completely

gone. The resulting white wood was subsequently immersed in the as-prepared liquid epoxy resin for thorough infiltration. The transparent wood composite was obtained after a complete solidification of epoxy.

Optical Property Characterization: A UV–vis Spectrometer Lambda 35 (PerkinElmer, USA) with an integrated sphere was used to measure the optical properties including haze, forward transmittance, and backward reflection. A 532 nm (green) single mode laser DJ532-10 (Thorlabs Inc.) was used as the incoming light source with a spot size of around 200 μm. The beam was collimated first before varying the angle incident on the transparent wood block. The transparent wood has a dimension of width × height × length = 1.4 cm × 3.0 cm × 3.5 cm. The 2D light intensity distribution of the transmitted light after the transparent wood block was characterized with a calibrated Si detector S-130C from Thorlabs. The light guiding effect of the house model with a transparent wood rooftop was tested under the solar simulator from Newport. A Xenon lamp was used as the white light source with an illumination area of 6 cm in diameter.

Thermal Conductivity Characterization: In order to evaluate the thermal conductivity of transparent wood composite, a steady-state method was applied. The transparent samples were cut to a strip dimension with 6 mm width and 30 mm length. A heat sink and an electric heater were attached to opposite ends of the wood-epoxy samples and two fine-gage, K-type thermocouples were placed at a distance of L, to measure the temperature difference ΔT along the heat flow direction. To minimize convective heat loss, the samples were placed in a vacuum chamber with a heat shield. A LabVIEW program from National Instrument was employed to monitor and record data of the sample temperature and electrical power of heater. Once a steady-state is reached, the thermal conductivity of the sample can be determined by applying Fourier’s

Law: = ∆sk

Q LA T

where Qs is the heat flowing through the samples, L is

the distance between two thermocouples, A is the cross-section area of the sample through which the power flows, and ΔT is the temperature difference measured by thermocouples. In the ideal case, all the power supplied to the heater flows through the sample and into the heat sink, so that there is no heat loss through other means, therefore the heat flowing across any cross section is constant. However, in real measurements, heat loss is inevitable through radiation, convection, and heat conduction of electric wires, so the power flowing through the sample Qs can be written as: Qs = Qinput − Qloss, where Qloss is the power lost by radiation, heat conduction through the connection leads, and convection. The uncertainty of the thermal conductivity measurement along the film samples is predicted to be about 10%.

Mechanical Property Characterization: The transparent wood composite was sectioned into test samples about 5 cm long, 1 cm wide, and 3 mm thick for mechanical tests. A Tinius Olsen H5KT tester was used to carry out the stress–strain measurements for the samples.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsT.L. and M.Z. contributed equally to this work. The authors acknowledge the support of the Maryland NanoCenter, including the AIMLab and the Fablab.

Received: May 28, 2016Revised: July 6, 2016

Published online:

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