Smart Windows
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1. Perovskite Smart Windows: The Light Manipulator in Energy Efficient Buildings, Advanced Materials, 2306423.
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S. Liuc, Y. Du, R. Zhang, H. Hea, A. Pan, T.C. Hoc, Y. Zhuc, Y. Li, H.L. Yip, Alex K.Y. Jen, C.Y. Tso*
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https://doi.org/10.1002/adma.202306423
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Abstract: Uncontrolled sunlight entering through windows contributes to substantial heating and cooling demands in buildings, which leads to high energy consumption from the buildings. Recently, perovskite smart windows have emerged as innovative energy-saving technologies, offering the potential to adaptively control indoor solar heat gain through their impressive sunlight modulation capabilities. Moreover, harnessing the high-efficiency photovoltaic properties of perovskite materials, these windows have the potential to generate power, thereby realizing more advanced windows with combined light modulation and energy harvesting capabilities. This review summarizes the recent advancements in various chromic perovskite materials for achieving light modulation, focusing on both perovskite structures and underlying switching mechanisms. The discussion also encompasses device engineering strategies for smart windows, including the improvement of their optical and transition performance, durability, combination with electricity generation, and the evaluation of their energy-saving performance in building applications. Furthermore, the challenges and opportunities associated with perovskite smart windows are explicated, aimed at stimulating more academic research and advancing their pragmatic implementation for building energy efficiency and sustainability.
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2. Study of a Passive Radiative Cooling Coating on Chemical Storage Tanks for Evaporative Loss Control, Renewable Energy 211 326-335.
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Y. Du, S. Liuc, X. Chen, Y. Li, T.C. Hoc, L.C. Chaoa, C.Y. Tso*, 2023.
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https://pubs.acs.org/doi/10.1021/acsami.3c11706?ref=pdf
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Abstract: Transparent wood (TW) has emerged as a sustainable alternative to conventional glass as an energy-efficient window glazing material owing to its exceptional optical transparency and superior mechanical and thermal performances. However, it is challenging to develop the TW-based colorswitching smart windows with both high optical performance and
mechanical strengths. In this work, an optically switchable and mechanically robust perovskite-coated thermochromic transparent wood (PTTW) is developed for use as smart windows to achieve an effective solar modulation and thermal management. PTTW exhibits a substantial solar modulation ability Δτsol of 21.6% and a high clear-state luminous transmittance τlum of 78.0%, which enable an efficient thermal regulation while ensuring high visual clarity. PTTW also offers enhanced mechanical properties (i.e., tensile strength σtens = 71.4 MPa and flexural strength σflex = 93.1 MPa) and improved thermal properties [i.e., thermal conductivity K = 0.247 W/(m·K) and heat capacity C = 1.69 J/(g·°C)] compared to glass-based smart windows, as well as excellent performance stability (i.e., 200 heating−cooling cycles), manifesting its applicability in real building scenarios. In addition, PTTW also demonstrates a remarkable thermal-regulating performance (i.e., 5.44 °C indoor air temperature regulation) and an energy-saving potential (i.e., 12.9% heating, ventilation, and air conditioning energy savings) in Hong Kong. Overall, this study contributes to the progression toward energy-efficient and sustainable buildings.
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3. A Novel Solar-based Human-centered Framework to evaluate Comfort-Energy Performance of Thermochromic Smart Windows with Advanced Optical Regulation, Energy and Buildings, 278 112638.
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Y.D. He, S. Liu, C.Y. Tso*, 2023.
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https://doi.org/10.1016/j.enbuild.2022.112638
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Abstract: Thermochromic windows provide a promising and practical solution to lower building air conditioning energy via automatically reducing window-through solar radiation which imposes large cooling loads on air conditioning systems. The conventional framework for evaluating thermochromic windows assumes that the solar radiation passing through smart windows is immediately absorbed by indoor air, which ignores the deviated occupant comfort by indoor solar radiation and leads to a biased comfort-energy performance of thermochromic windows. This study proposes a novel solar-based human-centered framework to evaluate the comfort-energy performance of hydrogel thermochromic smart windows (HTSWs) via integrating human-solar modelling, solar-based indoor set-point correction, and building simulations. The comfort-energy performance of HTSWs are evaluated with different warm climates and compared to that of normal windows. The main results indicate that, the conventional framework, which ignores solar radiation on indoor human bodies, underestimates the discomfort time duration of indoor occupants by 55.0% and 75.2% when evaluating normal windows and HTSWs, respectively. Under the human-centered framework, with solar-based set-point correction, indoor occupants with HTSWs regulating solar radiation will be comfortable during more than 95% of working hours; and HTSWs reduce nearly 50% of the discomfort hours compared to normal windows. In addition, the average energy saving potential of HTSWs under the human-centered framework is extended by 35.0% compared to that under the conventional framework. To be specific, the air conditioning energy savings of HTSWs under the conventional framework are 9.9%–20.3% (average 14.5%); under the human-centered framework, the energy savings are 13.5%–25.6% (average 19.5%, which is 35% higher than that under the conventional framework). The considerably improved comfort-energy performance of HTSWs under the human-centered framework will further contribute to the wide adoption of HTSWs.
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4. Regional Applicability of Thermochromic Windows based on Dynamic Radiation Spectrum, Renewable Energy 196 15-27.
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Y. Shen, T. Luo, Y. Zhang, C.Y. Tso*, N. Zhang, J. Xie, J. Liu, P. Xue, 2022.
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https://doi.org/10.1016/j.renene.2022.06.135
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Abstract: Thermochromic (TC) window has been developed as an effective approach to realize building energy saving by regulating solar transmittance according to temperature stimuli; however, the non-uniform transmission and absorption of the solar spectrum result in a significant color shift and temperature increase. This study investigates the radiometric, photometric and colorimetric performances of fourdifferent TC windows with an experiment-verified model. The simulation and calculation are conducted under measured spectrum conditions in Beijing; subsequently, a comprehensive evaluation method for daylight-thermal environment method is conducted. The results show that all selected TC windows possess an energy efficiency between 3.0% and 7.9%, improve daylight availability from 2.8% to 18.8%, and reduce discomfort hours by up to 111 h. Two hydrogel TC windows improve the quality of dynamic
transmitted daylight by nearly 7% considering correlated color temperature and color rendering index; the HETS hydrogel TC window is found to be the most suitable for Beijing. Finally, to increase the application potential of TC windows in Beijing, several material properties are desired, including high transition temperature with high solar modulation capability, high and uniform visible light transmittance, and low emissivity. This process for the optimization of climate-responsive material can be further adopted in other areas
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5. Near-Infrared-Activated Thermochromic Perovskite Smart Windows, Advanced Science, 2106090. (Featured as Inside Front Cover).
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S Liu, Y. Li, Y Wang, K.M. Yu, B.L. Huang and C.Y. Tso*, 2022.
https://doi.org/10.1002/advs.202106090
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Abstract: Perovskite-based thermochromic smart windows that can change color have attracted much interest. However, the high transition temperature (>45 °C in air) hinders their practical application. Herein, a near-infrared (NIR) activated thermochromic perovskite window that enables reversible transition cycles atroom temperature is proposed. Under natural sunlight (>700 W m−2), it efficiently harvests 78% NIR light to trigger the thermochromism of perovskites, blocking the heat gain from both the visible and NIR light. Meanwhile, it also exhibits a low mid-infrared emissivity of <0.3, suppressing thermal radiation to the indoor environment. A field test demonstrates that this smart window can reduce the indoor temperature by 8 °C compared to a normal glass window at noon. The near-room-temperature color change, multispectral thermal management, outstanding energy-saving ability, and climate adaptability, and solution-based process of this window make it unique and promising for real applications.
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6. Study on the Halide Effect of MA4PbX6·2H2O Halide Hybrid Perovskites – from Thermochromic Properties to Practical Deployment for Smart Windows, Materials Today Physics, 100624.
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Y. Du, S. Liu, Z. Zhou, H.H. Lee, T.C. Ho, S.P. Feng, C.Y. Tso*
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https://doi.org/10.1016/j.mtphys.2022.100624
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Abstract: Thermochromic smart windows empowered by the unique material properties and reversible thermochromism of dihydrated methylammonium lead halide hybrid perovskites (MA4PbX6$2H2O; X: halide) have risen as novel yet promising candidates for thermochromic smart windows, in which the halide plays a crucial role to the functionality and performance. In this study, the effects of halide types and mixing ratios on the thermochromic properties and practical deployment of MA4PbX6$2H2O halide hybrid perovskites are comprehensively studied by extensive fabrication and characterization processes as well as fundamental and applicational investigation tools. The potential perovskite compositions for thermochromic smart window applications are identified with respect to the desired optical properties (i.e. luminous transmittance and solar modulation ability) and transitional properties (i.e. transition
temperature, hysteresis width and transition time). Moreover, thermal-regulating performance of the applicable candidate mixed halide hybrid perovskite MA4PbI5Br1$2H2O is demonstrated in a modelhouse field investigation. This work provides not only an inclusive database of the thermochromic properties of MA4PbX6$2H2O halide hybrid perovskites, but also an application manual about their deployment for thermochromic smart windows, facilitating the development of smart windows and sustainable buildings.
7. Bioinspired Flexible Thermochromic Transparent Hydrogel Wood with Advanced Optical Regulation Abilities and Robust Mechanical Properties for Smart Windows, Applied Energy 297 117207.
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S. Liu, C.Y. Tso*, Y.W. Du, L.C. Chao, H.H. Lee, T.C. Ho and M.K.H. Leung, 2021.
https://doi.org/10.1016/j.apenergy.2021.117207
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Abstract: The huge heat loss/gain through windows is the cause of great energy consumption in buildings. In addition, the traditional fabrication method for glass causes many environmental problems. Recently, transparent wood has emerged as a promising alternative to traditional glass because of its high transmittance, strong mechanical properties, excellent thermal insulation ability and sustainability. In this study, inspired by jellyfish, a thermochromic transparent hydrogel wood that can smartly regulate solar irradiation is proposed as a smart window material by impregnating Poly(N-isopropylacrylamide)-polyacrylamide hydrogel into delignified wood. The novel thermochromic transparent hydrogel wood inherits the excellent thermochromic properties of PNIPAM and strong mechanical properties of wood, showing advanced optical regulation ability (i.e. Tlum = 82.7% and 39.8% at the cold and hot states & ΔTsol = 38.1%), low transition temperature (i.e. Tc = 22.9 â—¦C), mechanically robust (i.e. σ = 11.6 MPa along the axial direction) and low thermal conductivity (i.e. K = 0.37 W mô€€€ 1 Kô€€€ 1 along the perpendicular direction of the wood growth). A field test conducted in October in Hong Kong shows that thermochromic transparent hydrogel wood can reduce the indoor air temperature by 4.3 â—¦C. Furthermore, a computational simulation for an office building proves that 2.6–10.2% energy could be saved by thermochromic transparent hydrogel wood in four different climate-zone cities. Besides, thanks to the flexibility, thermochromic transparent hydrogel wood can be easily fitted on existing windows, demonstrating the great potential for use in energy-efficient buildings.
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8. Self-Densified Optically Transparent VO2 Thermochromic Wood Film for Smart Windows, ACS Applied Materials & Interfaces, 13 22495-22504.
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S. Liu, C.Y. Tso*, H.H. Lee, K.M. Yu, S.P. Feng and B.L. Huang, 2021.
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https://doi.org/10.1021/acsami.1c03803
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Abstract: Optically transparent wood has emerged as a promising glazing material. Thanks to the high optical transmittance, strong mechanical properties, and excellent thermal insulation capability of transparent wood, it offers a potential alternative to glass for window applications. Recently, thermo-, electro-, and photochromic transparent woods that dynamically modulate light transmittance have been investigated to improve building energy efficiency. However, it remains challenging to widely replace windows with transparent wood because of its poor weather resistance. In this study, an environment-friendly thermochromic transparent wood film (TTWF) with thermal switching of transmittance is proposed and demonstrated. To achieve thermochromism, the bleached wood is impregnated with the vanadium dioxide (VO2)/polyvinyl alcohol composite. Due to the self-densification of cellulose microfibrils during the evaporation of solvents, the transparent wood is in the form of thin films, which can be attached on the inner face of a window to protect it from severe weather conditions, making the installation convenient and low-cost. Furthermore, the surface of VO2-TTWF is modified by octadecyltrichlorosilane to enhance the waterproof ability and achieve self-cleaning and antidust functions. The proposed VO2- TTWF shows great potential for application in energy-efficient buildings using sustainable materials with advanced optical properties (i.e., Tlum = 50.5%, ΔTsol = 3.4%, and haze = 70%) that are mechanically robust (i.e., σ = 130.6 MPa along the wood growth direction), have low-thermal conductivity (i.e., K = 0.29 W m−1 K−1 along the perpendicular direction to the wood fibers), and demonstrate hydrophobic self-cleaning and antidust functions (i.e., contact angle: 121.9°). An experiment, using a model house, showed that the VO2-TTWF attached on the inner face of the window could significantly reduce the indoor air temperature by 33.9°C compared with a bare glass panel, proving that VO2-TTWF has potential to be applied as a new-generation energy-efficient material for smart windows.
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9. Organic Hybrid Perovskite (MAPbI3-xClx) for Thermochromic Smart Window with Strong Optical Regulation Ability, Low Transition Temperature and Narrow Hysteresis Width, Advanced Functional Materials, 31 2010426 (Featured as Inside Front Cover). ​
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S. Liu, Y.W. Du, C.Y. Tso*, H.H. Lee, R. Cheng, S.P. Feng and K.M. Yu, 2021.
https://doi.org/10.1002/adfm.202010426.
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Abstract: Recently, organic hybrid halide perovskites have been found to show thermochromism with good optical performance, which can be applied in smart windows to reduce building energy consumption. However, these perovskites have shortcomings regarding their thermochromic performance, namely long transition time, high transition temperature, and large transition hysteresis width. In this study, a hydrated MAPbI3−xClx thermochromic perovskite smart window (H-MAPbI3−xClx TPSW) is proposed, which undergoes a reversible transition between a transparent state and a dark reddish-brown tinted state with a high solar modulation ability of 23.7%. Most importantly, the H-MAPbI3−xClx TPSW possesses a tunable low transition temperature of 29.4 to 51.4 °C, a controllable and narrow transition hysteresis width (7.7–13.2 °C) and a short transition time (1–4 min). Additionally, a mathematical model is developed to predict the transition temperature of the H-MAPbI3−xClx TPSW. A field test is also conducted, demonstrating that the H-MAPbI3−xClx TPSW fitted to a model house can reduce the indoor air temperature by 3.5 °C compared to using a quartz glass window. Overall, the H-MAPbI3−xClx TPSW can yield excellent optical properties, while simultaneously providing remarkable transition properties, making it potentially useful for a wide range of applications in energy-efficient buildings.
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10. Bioinspired TiO2 Nanocone Antireflection Layer for the Optical Performance Improvement of VO2 Thermochromic Smart Windows, Scientific Reports, 10(1) 1-14.
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S. Liu, C.Y. Tso* Y. Zhang, H.H. Lee, K.M. Yu, and C.Y. Chao, 2020.
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​https://doi.org/10.1038/s41598-020-68411-6
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Abstract: Vanadium dioxide (VO2) is a promising material for thermochromic glazing. However, VO2 thermochromic smart windows suffer from several problems that prevent commercialization: low luminous transmittance (Tlum) and low solar modulation ability (ΔTsol). The solution to these problems can be sought from nature where the evolution of various species has enabled them to survive. Investigations into the morphology of moths eyes has shown that their unique nanostructures provide an excellent antireflection optical layer that helps moths sharply capture the light in each wavelength from a wide angle. Inspired by this mechanism, a VO2 thermochromic smart window coated with a TiO2antireflection layer with a novel nano-cone structure, is presented in this study to achieve high Tlum and ΔTsol. Optimization for the key structure parameters is summarized based on the FDTD numerical simulations. The optimized structure exhibits a Tlum of 55.4% with ΔTsol of 11.3%, an improvement of about 39% and 72% respectively compared to the VO2 window without an antireflection layer.
Furthermore, wide-angle antireflection and polarization independence are also demonstrated by this nano-cone coating. This work provides an alternative method to enhance the optical performance of VO2 smart windows.
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11. Rapid Thermal Annealing assisted Facile Solution Method for Tungsten-doped Vanadium Dioxide Thin Films on Glass Substrate. Journal of Alloys and Compounds, 833 155053.
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M. Kong, K. Egbo, C.P. Liu, M.K. Hossain, C.Y. Tso*, C.Y. Chao and K.M. Yu, 2020.
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https://doi.org/10.1016/j.jallcom.2020.155053
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Abstract: Vanadium dioxide VO2 is an important thermochromic material with useful applications in smart energy
devices. This paper describes the synthesis of high-quality vanadium dioxide thermochromic thin films
directly on glass substrate without any barrier layer prepared using a simple solution method. We
demonstrate that 50 nm thick VO2 films display excellent visible transmittance (76.9% on glass, 69.4% on
fused silica) and a large NIR switching efficiency (33.8% on glass, 44.3% on fused silica) at 1600 nm. It is
found that the metal to semiconductor phase transition temperature Tc of undoped films can be reduced
from a theoretical value of 68 _C to 49.4 _C by rapid thermal annealing. WithWdoping a continuous drop
in the Tc is observed and for VO2 with 2% W doping, a Tc of 25.8 _C is achieved. These good optical
properties and the near room-temperature phase transition temperature suggest that solution processing
with the rapid thermal annealing process is a feasible method to obtain this promising material on glass substrate for practical application in thermochromic smart windows.
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12. Perovskite Thermochromic Smart Window: Advanced Optical Properties and Low Transition Temperature. Applied Energy, 254 113690.
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Y. Zhang, C.Y. Tso*, J.S. Iñigo, S. Liu, H. Miyazaki, C.Y. Chao and K.M. Yu, 2019.
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https://doi.org/10.1016/j.apenergy.2019.113690
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Abstract: Windows are one of the most inefficient components in buildings. Common thermochromic smart windows using VO2 can mitigate such energy loss. However, they suffer from several problems, namely, low solar modulation ability, high transition temperature (i.e. 68 °C) and low luminous transmittance. In this study, we propose a
perovskite thermochromic smart window towards achieving high solar modulation ability whilst maintaining a high luminous transmittance and a low transition temperature. Perovskite material shows a significant thermochromism in the visible and ultraviolet region. Since half of the photons lie in this spectral region, a high solar modulation can be achieved by perovskites. The material was optimized by varying the spin speed in the fabrication process as well as the mixing ratio between precursors. The optimized sample exhibits a solar modulation ability of 25.5% with luminous transmittance of 34.3% and higher than 85% in the hot (80 °C) and cold (25 °C) states, respectively, making this material suitable for practical device applications. The hysteresis loop, the transition temperature as well as transition time in relation to the relative humidity of a perovskite smartwindow during the heating and cooling process are investigated in this study. From field tests results, the perovskite smart window can help reduce the indoor air temperature by about 2.5 °C compared to a normal window. Overall, based on the results obtained in this study, the perovskite thermochromic smart window has potential to achieve excellent thermochromic properties, providing an alternative to alleviate the high energy consumed in buildings
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13. A Field Investigation of Passive Radiative Cooling under Hong Kong’s Climate. Renewable Energy, 106 52-61.
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C.Y. Tso*, K.C. Chan and C.Y. Chao, 2017.
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http://dx.doi.org/10.1016/j.renene.2017.01.018
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Abstract: This paper discusses the feasibility of cooling using radiation under Hong Kong’s hot and humid climate. Three different designs of a passive radiative cooler were studied in this work. The three designs include non-vacuum, and vacuum with seven potassium chloride (KCl) IR-Pass windows as well as one system with a single KCl IR-Pass window. The coolers were examined during daytime and night time operation as well as under different sky conditions, such as clear, cloudy and partly cloudy. Investigation was mainly based on the temperature difference between the radiative cooler and ambient air. The experimental results showed that the passive radiative cooler with seven KCl windows and the cooler design
without vacuum provided a satisfactory cooling effect at night (i.e. the ambient air temperature was reduced by about 6e7 _C), but the coolers could not produce a cooling effect during daytime under any of Hong Kong’s weather conditions. The same results were obtained for the passive radiative cooler with the single KCl window during daytime operation. However, the cooling capacity of the passive radiative cooler design without vacuum under a clear night sky achieved 38 W/m2.