Active-passive dual-control smart window with thermochromic synergistic fluidic glass for building energy efficiency

Zhou P., Wang H., Li F., Dai Y., Huang C.

• A comprehensive window opening model of residential buildings is developed. • The main factors affecting the window opening behavior change in different seasons. • The window operation model is validated by indoor CO 2 concentration. This study monitors the open/closed states of the windows in 16 residential apartments in representative cities of the hot summer and cold winter zone of China, along with outdoor air temperature, outdoor relative humidity and outdoor PM2.5 concentration in 2017, to investigate the window opening/closing behavior. Three window opening models are developed. All the models are validated by modeling a typical residential apartment to compare the predicted indoor CO 2 concentration with monitored data. It is found that the window opening probability is negatively correlated to both outdoor PM2.5 concentration and outdoor relative humidity, and the probability of window opening reaches the peak value when the outdoor air temperature is 23.7 °C. In terms of season, the shortest window opening duration occurs in winter due to the cold outdoor air temperature and high outdoor PM2.5 concentration. Finally, by comparing the correlation between the measured date and simulation results of indoor CO 2 concentration, this study provides an appropriate window opening model for reference.

Wang S., Jiang T., Meng Y., Yang R., Tan G., Long Y.

A passive turnoff Passive radiative cooling technology uses the infrared atmospheric window to allow outer space to be a cold sink for heat. However, this effect is one that is only helpful for energy savings in the warmer months. Wang et al . and Tang et al . used the metal-insulator transition in tungsten-doped vanadium dioxide to create window glass and a rooftop coating that circumvents this problem by turning off the radiative cooling at lower temperatures. Because the transition is simply temperature dependent, this effect also happens passively. Model simulations suggest that these materials would lead to energy savings year-round across most of the climate zones in the United States. —BG

Wang S., Zhou Y., Jiang T., Yang R., Tan G., Long Y.

Radiative cooling (RC) is a technique that spontaneously radiates long-wave infrared (LWIR) to the cold outer space, which provides cooling power to buildings, however only preferred in hot seasons. RC has been widely researched on walls and roofs but rarely on windows while windows are the least energy-efficient part of buildings. Moreover, in the current smart window design, consideration of tunable RC is missing. For the first time, we proposed an ideal smart window with a switchable front side LWIR emissivity ( ε Front ) and solar modulation ability (Δ T sol ). Such window needs not only to have high luminous transmission ( T lum ) and Δ T sol , the two major conventional performance indexes but also possess a switchable ε Front to cater for the variable seasonal thermal performance requirements and energy-saving demands. We further fabricated a tunable emissivity thermochromic (TET) smart window with large Δ T sol (51%) and T lum (72%) and switchable ε Front (0.95–0.1) to prove the efficacy of the proposed ideal window. Compared with current smart window technology solely regulating solar transmission and other RC materials with fixed emissivity, the TET smart window panel gives a wide tunability in the selective solar spectrum for dynamic climate conditions in energy-saving buildings, achieving largely enhanced energy saving performance globally • Smart window to regulate both solar transmittance and radiative cooling. • Switchable emissivity to prompt cooling in summer and warm keeping in winter. • High luminous transmittance, good solar regulation. • Better energy efficiency than commercial low-E glass across climate zones.

Xu G., Xia H., Chen P., She W., Zhang H., Ma J., Ruan Q., Zhang W., Sun Z.

Conventional buildings consume about 40% of global energy, smart window technologies have been developed for sunlight modulation and energy management. Most current smart windows change from transparent to opaque as the temperature rises, which is detrimental to indoor lighting at daytime or privacy protection at night. In this work, a versatile thermochromic hydrogel system by introducing sodium dodecyl sulfate (SDS) micelles into a crosslinked copolymer of hydrophilic acrylamide and hydrophobic stearyl methacrylate (C18) is developed. The liquid precursor solution can be encapsulated within two glass panels and in situ gelated to prepared smart windows, which showed excellent solar modulation ability (Tlum = 99.05%, DTsolar = 33.42%), dual responsiveness (thermal and pH) and tunable phase transition temperature (20–50 °C). Moreover, this design selectively blocks infrared light, while allowing ultraviolet and visible light through at daytime, which is beneficial for indoor illumination and heat insulation. When temperature drops at night, C18 units aggregate within SDS micelles to increase their dimensions, causing enhanced light blocking properties (opaque) to protect the customers’ privacy. The as-prepared hydrogel-based smart windows present a facile strategy to meet the stringent requirements of high transparency, excellent solar modulation ability, easy to fabricate and mechanical flexibility, holding great promise for the next-generation energy-saving buildings.

Sun Y., Wilson R., Liu H., Wu Y.

Smart window designs have emerged as a means of providing dynamic regulation of solar energy and daylight, enhancing indoor comfort, and achieving building energy conservation. We evaluated a novel window design that integrated a thermotropic (TT) material and Transparent Insulation Material (TIM) and present the investigation in this paper. The Parallel Slat TIM (PS-TIM) structure contained within the window unit provides extra thermal resistance and helps to redirect daylight. The TT material, which is applied to the slats, provides automatic daylight and solar adjustment. Firstly, the TT PS-TIM window system has been characterised thermally and optically. Then, a comprehensive approach including both building energy and daylight simulation packages was used to predict building performance. The effects of geometry (i.e. slat spacing and slat tilt angle) and thermotropic features (i.e. transition temperature and optical properties) on building performance were investigated. The simulation results show that use of TT PS-TIM window system with carefully selected features can simultaneously improve building energy efficiency (up to 22% saving when compared with a conventional double-glazed (DG) window) and attain homogenous daylight distribution with an average Useful Daylight Illuminance, UDI 500–2000 lux , of 52.2%. It was also found that both the geometric configurations and thermotropic features of a TT PS-TIM have significant influence on energy and daylight performance. TT PS-TIM with horizontally placed slats performs better than the unit with tilted slats, in terms of balance between energy efficiency and daylight availability. This research provides design guidance and material development suggestions for integration of this novel window system in buildings. • TT PS-TIM smart window provides building energy conservation and daylight autonomy. • The impacts of geometric configurations and thermotropic features of TT PS-TIM are studied. • Up to 22% building energy saving can be achieved under the selected London climate. • Daylight quality is significantly improved through applying the TT PS-TIM window. • A comprehensive and rigorous method is developed to evaluate novel complex system.

Zhang R., Xiang B., Shen Y., Xia L., Xu L., Guan Q., Tang S.

The smart window is highly transparent to allow solar transmittance at low temperatures, while turns opaque automatically to cut off solar energy gain when exposed in sunlight.

Zhang Q., Huang A., Ai X., Liao J., Song Q., Reith H., Cao X., Fang Y., Schierning G., Nielsch K., Bai S., Chen L.

Integrating transparent solar-harvesting systems into windows can provide renewable on-site energy supply without altering building aesthetics or imposing further design constraints. Transparent photovoltaics have shown great potential, but the increased transparency comes at the expense of reduced power-conversion efficiency. Here, a new technology that overcomes this limitation by combining solar-thermal-electric conversion with a material's wavelength-selective absorption is presented. A wavelength-selective film consisting of Cs0.33WO3 and resin facilitates high visible-light transmittance (up to 88%) and outstanding ultraviolet and infrared absorbance, thereby converting absorbed light into heat without sacrificing transparency. A prototype that couples the film with thermoelectric power generation produces an extraordinary output voltage of ≈4 V within an area of 0.01 m2 exposed to sunshine. Further optimization design and experimental verification demonstrate high conversion efficiency comparable to state-of-the-art transparent photovoltaics, enriching the library of on-site energy-saving and transparent power generation.

Yang J., Lim T., Jeong S., Ju S.

A smart window, which can easily adjust light transmittance, can provide barrier functions, such as improvement in energy efficiency, glare prevention, and privacy protection. However, a smart window that can selectively provide real-time information and display various colorful characters and images at a desired location has not been developed. In this study, a novel smart window capable of real-time information conversion is developed by advancing the light transmittance control of the existing smart windows. A transparent and flexible window display is fabricated by synthesizing poly(N-isopropylacrylamide) (pNIPAM)-N,N-methylenebisacrylamide-crosslinked hydrogels (NBcH) and near-infrared (NIR) absorption-heating films sandwiched between two plastic substrates. When the NIR laser irradiates the window display panel surface, the temperature rises rapidly, as the NIR absorption-heating film absorbs the NIR wavelength. The generated heat is transferred to pNIPAM in contact with the NIR absorption-heating film, and an image forms in real time. In addition, if the NIR laser and projector simultaneously irradiate the window display panel surface, various colorful images can be displayed. The smart window for real-time information provision proposed in this study acts like a glass curtain that can selectively make a desired location transparent or opaque by controlling the transmittance of light and acts as a display that can present various colorful characters and images in real time. Therefore, it is expected to be highly convenient for users.

Ren J., Zhou X., An J., Yan D., Shi X., Jin X., Zheng S.

Research on the window operating behavior of offices is of great significance for reducing building energy consumption and improving indoor comfort. The open-plan office is a common office form that involves a large number of people and a complex staff composition. The window operating behaviors in open-plan offices are also random and various. This study took three open-plan offices with different situations (area, office type, staff composition, etc.) as an example, which provides a new perspective on how people behave differently when opening or closing windows. The window operating behaviors in two typical seasons (summer and transition seasons) were recorded and analyzed. The occupants’ schedules and influencing factors of window operating behavior were investigated by questionnaire surveys. In addition, the indoor environmental parameters, occupancy situation, and on-off statuses of windows and air conditioning were acquired through field measurements. Furthermore, the differences in window operating behaviors in the three open-plan offices were compared from the perspectives of influencing factors, duration of the window on-off statuses, and cause of window control actions, among others. In addition, Spearman Correlation Coefficient was used to analyze the ranks of the candidate motivations for window operating behaviors. The preliminary results show that influenced by the personnel composition, type of air conditioner and adjustable degree of windows, the window operating behaviors of different office buildings have larger discrepancies than that in the same building. However, there were some common characteristics in the window regulation behaviors of the three open-plan offices: they were generally influenced by the coupling of environmental factors, schedule factors, and equipment factors. This study reveals that when expand the research object from a single building to multiple buildings, more difficulties and challenges would be involved into behavior research.

Tian J., Peng H., Du X., Wang H., Cheng X., Du Z.

Energy-efficient smart windows can control solar energy transmission, which is one of the most promising ways to reduce building energy consumption. However, the current thermochromic smart windows based on Poly(N-isopropylacrylamide) (PNIPAm) are still far from satisfactory in regulating the infrared region of sunlight. To expand the modulation ability of infrared region, an inorganic nanoparticle filled hybrid hydrogel that blocks out most part of sunlight as the temperature rises has been developed by up-conversion nanoparticles dispersed in hydrogel matrix effectively converts infrared light into visible light, thus greatly enhancing the infrared transmittance modulation (ΔTIR) and the solar modulating ability (ΔTsol) of the hydrogel. The 4% (wt.) UCNPs@SiO2–NH2 nanoparticles doped in PNIPAm exhibits the best solar modulation ability (ΔTIR = 78.3%, ΔTsol = 79.76% and Tlum, 20°C = 82.79%). In addition, this thermochromic hybrid material, which integrates inorganic components with organic components, is also durable and reversible. By sealing the hydrogel with glasses in a sandwich structure, a smart window that can regulates broadband solar light is obtained.

Zhang Q., Jiang Y., Chen L., Chen W., Li J., Cai Y., Ma C., Xu W., Lu Y., Jia X., Bao Z.

Smart windows have shown powerful potential in modulating sunlight and energy management. However, there has not been a single system that can simultaneously possess all desired properties, that is, high compliance, wide tunability, and autonomous regulation. Here, a novel system based on polyhedral oligomeric silsesquioxane crosslinked poloxamer is demonstrated. The as-synthesized material simultaneously possesses a number of the desired properties for smart windows. In addition, with the incorporation of gold nanorods, the as-fabricated smart window can be operated in an autonomous fashion with its transmission being modulated automatically upon environmental temperature shifts. Finally, the high toughness and high stretchability (10 000% strain) of the material also pave a way for future applications in wearable displays or flexible window screens.

Li Z., Lei H., Kan A., Xie H., Yu W.

Graphene is one of the most attractive materials due to its unique features, including high aspect ratio, excellent mechanical, thermal, and optical features. Especially, graphene and its derivatives exhibit the significant photothermal effect and are among the prominent candidates for the utilization of solar energy. This article reviews the progress on photothermal applications of graphene and its derivatives. Graphene and its derivatives are expected to be good candidates for a host of applications, such as solar collector, solar-driven water evaporation, photothermal catalysis, photothermal therapy, and photothermal antibacterial. The commercial potential, challenges and research trends of graphene-based photothermal materials are also discussed. • Photothermal applications of graphene and its derivatives were summarized. • Main advantages of graphene were presented. • Applications as solar collector and solar-driven water evaporation were analyzed. • Applications as photothermal catalysis, therapy and antibacterial were discussed.

Tang L., Wang L., Yang X., Feng Y., Li Y., Feng W.

Development of polymer-based smart materials, which can autonomously alter their physical and/or chemical properties when exposed to external stimuli, is a thriving research frontier in contemporary advanced functional materials science. Poly(N-isopropylacrylamide) (PNIPAM)-based smart hydrogels are known to exhibit distinct thermo-responsive properties near a lower critical solution temperature (LCST), which have found diverse promising applications such as smart coating, drug delivery, tissue regeneration, and artificial muscles. In this review, we provide an up-to-date account on the recent developments in advanced functional PNIPAM-based smart hydrogels and their emerging technological applications in the fields of smart actuators, photonic crystals, smart windows and novel biomedical applications. The fundamental design and synthetic strategies of PNIPAM-based smart hydrogels are discussed. Their unique properties, underlying mechanisms and potential applications in different fields are highlighted. Finally, this review provides a brief conclusion and enumerates the challenges and opportunities in this rising area of research and development involving these intriguing polymer-based advanced smart systems rooted in chemistry and materials science. It is expected that this review would provide significant insights for the development of reconfigurable and programmable advanced smart materials with numerous possibilities, prompting the rapid advancement of this highly interdisciplinary area, which encompasses materials science, polymer science, synthetic chemistry, device engineering, physics, biology, nanoscience and nanotechnology.

Zhou Y., Wang S., Peng J., Tan Y., Li C., Boey F.Y., Long Y.

Summary Buildings account for 40% of global energy consumption, while windows are the least energy-efficient part of buildings. Conventional smart windows only regulate solar transmission. For the first time, we developed high thermal energy storage thermo-responsive smart window (HTEST smart window) by trapping the hydrogel-derived liquid within glasses. The excellent thermo-responsive optical property (90% of luminous transmittance and 68.1% solar modulation) together with outstanding specific heat capacity of liquid gives the HTEST smart window excellent energy conservation performance. Simulations suggested that HTEST window can cut off 44.6% heating, ventilation, and air-conditioning (HVAC) energy consumption compared with the normal glass in Singapore. In outdoor demonstrations, the HTEST smart window showed promising energy-saving performance in summer daytime. Compared with conventional energy-saving glasses, which need expensive equipment, the thermo-responsive liquid-trapped structure offers a disruptive strategy of easy fabrication, good uniformity, and scalability, together with soundproof functionality that opens an avenue for energy-saving buildings and greenhouses.

Liu X., Mishra D.D., Wang X., Peng H., Hu C.

This review systematically discusses the whole process of solar-driven interfacial desalination and the critical issues involved from the perspective of energy flow.

You Y., Yang X., Xu Z., Cui C., Shi F., Hong X.

Dong W., Ma M.

Hassan M., Alaa S., Sherief F.

Chen Y., Li Y., Zhang X., Jiang H.

Zhang C., Li D., Wang L., Zhang S., Shen C., Gao R.

Lu V.L., Chen X., Chen J., Jiao K.

You Y., Yang X., Deng X., Shi F., Wang C., Hong X.

Wu Y., Li X., Li Y., Wang H., Zhang Y., Lu X.

In recent years, thermochromic smart windows have played an important role in enhancing energy efficiency and contributing to carbon neutrality in building energy consumption. However, thermal-shielding smart windows with high visible light modulation and structural colors are rarely reported. This study focuses on the development of thermochromic smart windows utilizing poly (N-isopropylacrylamide) (PNIPAm)-based nanogels, which exhibit dynamic solar modulation and high visible light transmission without additional energy input. Unlike traditional electrochromic materials such as tungsten trioxide, these smart windows achieve structural color states through the self-assembly of nanogels, eliminating the need for inorganic materials. The fabricated hydrogel contains thermo-responsive poly(N-isopropylacrylamide-random-acrylic acid-random-N-tert-butyl acrylamide) P(NIPAm-r-AA-r-TBA) (PNTA) nanogels with tunable phase transition temperatures (Tp) that align with ambient conditions. These smart windows demonstrate excellent stability over 100 heating-cooling cycles and significant temperature regulation, achieving an indoor temperature modulation of 10 °C and energy savings of 14.24 KJ m−3 compared with normal windows. The production process is simple and scalable, making it feasible for industrial applications. Furthermore, these smart windows offer additional functionalities such as information encryption, adding value to their practical application.

Kong X., Han Y., Zhang X., Zhao X., Yuan J.

Hong X., Yang X., You Y., Shi F., Yang F., Cui C., Gong Y., Qian D.

Ming Y., Wu Y.

Window, as a part of building envelope, can always provide outside visual for indoor occupants and effectively impact the indoor daylight and thermal environment and occupants’ health. Over the past decades, the demand for optimal indoor environments has been steadily increasing. In response to this need, several technologies integrated within conventional double-glazed (DG) systems for solar energy and daylight regulation were developed and investigated. This paper aims to comprehensively review the daylight, solar control of existing advanced passive technologies of DG windows for further building energy consumption reduction, focussing on Transparent Insulation Materials (TIM) and adaptive passive thermotropic (TT) materials. From the review of current studies, it can be concluded that, the traditional smart thermotropic DG window can effectively control glazing spectral response to prevent indoor occupants from over-illuminated and daylight glare, however, occupants’ view communication with the external environment was totally blocked when the TT material was in its translucent state. Meanwhile, TIM window systems can improve the indoor daylight and thermal comfort, but they also have some limitations, for example, they have fixed visible light and solar optical properties. As a result, a DG window integrated with TIM may allow less solar heat and visible light to be transmitted into a building space, which may lead to a lower cooling energy demand in summer but a higher heating energy consumption in winter and also a higher electric lighting requirement. To effectively avoid the disadvantages brought by windows integrated with single TT or TIM system, TT-TIM-coupled window system is recommended to further improve the passive daylight and solar control technologies in the future.

Li Y., Xu M., Wang D., Liu Z., Mao W., Sun J., Shen W., Lee H.K., Tang S.

Wang L., Li D., Wang Z., Ma A., Lang Y., Jin Y., Fang J.

Liu W., Tian Y., Yong W., Xiong T., Fu G.

AbstractOrganic–inorganic composite photochromic coatings receive significant attention in energy utilization and conservation due to their easy application and rapid photoresponse. However, their high sensitivity to moisture limits practical applications. This study fabricates organic–inorganic composite functional coatings using a simple one‐pot method, producing materials with excellent photoresponsive, water resistance, and thermal insulation. Unmodified phosphotungstic acid serves as the inorganic photochromic filler, while methacrylate derivatives act as the polymer matrix, resulting in coatings with high initial transparency (over 85%). After 5 min of UV exposure, the samples shift to nearly non‐transmittable states in the visible range and maintain color depth through 14 coloring‐erasing cycles. Unlike polyvinylpyrrolidone substrates, which absorb moisture and soften quickly, this coating has a static water contact angle of up to 91.7°, showing excellent water resistance. Additionally, the composite material provides effective heat insulation in simulated chamber experiments, lowering the temperature by 15 °C compared to untreated chambers. In summary, this composite coating, with its excellent thermal insulation, rapid light responsiveness, stable cycling performance, and outstanding waterproof capabilities, proves highly suitable for applications such as smart windows, information storage, and building energy efficiency.

Xie G., Li Y., Luo J., Wu C., Cao M., Zhu D., Wu X., Xie H., Yu W.