Smart Window Technology Research Articles

Review of Angular-Selective Windows with Guest–Host Liquid Crystals for Static Window Applications

This review focuses on the development and advancements in angular-selective smart windows, with particular emphasis on static windows utilizing guest–host liquid crystal (GHLC) systems. Angular-selective windows are designed to adjust their transmittance based on the angle of incident light, offering enhanced energy efficiency and visual comfort in both architectural and automotive applications. By leveraging the anisotropic absorption properties of dichroic dyes, GHLC-based windows can selectively block oblique sunlight while preserving clear visibility from normal viewing angles. Various liquid crystal (LC) alignment configurations, including vertically aligned, homogeneously aligned, hybrid aligned, uniformly lying helix, and twisted aligned LC cells, have been investigated to optimize light control for different installation angles, such as for automotive windshields and building windows. These advancements have demonstrated significant improvements in energy conservation and occupant comfort by reducing cooling demands and regulating sunlight penetration. This review summarizes key findings from recent studies, addresses the limitations of current technologies, and outlines potential future directions for further advancements in smart window technology.

Assessing the Potential of Smart Windows for Energy Efficiency in Tropical Buildings: A Review of Current Research and Future Directions

Abstract Smart windows have energy-saving potential in buildings in tropical climates. Characterized by high solar radiation, humidity, and temperature, tropical climates demand innovative solutions for energy-efficient building design. Smart windows, which can regulate the transmission of light and heat through different thermochromic, photochromic, or electrochromic technologies, are promising to reduce energy consumption in such buildings. Several emerging window technologies, such as gasochromic, hydrochromic, polymer-dispersed liquid crystal, and suspended particle device technologies, also have promising energy-saving potential. However, their high initial costs, durability, and reliability of these technologies limit their applicability. Prospects for smart windows in buildings in tropical climates include advancements in materials science, cost reduction, and integration of smart window technology with other building systems, such as lighting and heating, ventilation, and air conditioning systems. The potential benefits of smart windows for energy-saving s in buildings in tropical climates are substantial, up to 37%. Thus, further research and development in this area would lead to significant advancements in sustainable building design for a better future.

Cholesteric Liquid Crystal Mirror-Based Smart Window Controlled with Ambient Temperature

In this paper, the authors demonstrate a small prototype of a smart window based on the thermo-optical properties of cholesterol liquid crystals. Due to its polymer-free design, the manufactured smart window is transparent and can reflect certain portions of visible or infrared light without requiring an external power source, and thus is easier to install and operate. The proposed smart window technology based on a cholesteric liquid crystal mirror will reduce energy consumption costs by reflecting excess sunlight and heat transfer, increasing comfort for residents of buildings and structures.

Compression-sensitive smart windows: inclined pores for dynamic transparency changes

Smart windows, capable of tailoring light transmission, can significantly reduce energy consumption in building services. While mechano-responsive windows activated by strains are promising candidates, they face long-lasting challenges in which the space for the light scatterer’s operation has to be enlarged along with the window size, undermining the practicality. Recent attempts to tackle this challenge inevitably generate side effects with compromised performance in light modulation. Here, we introduce a cuttlefish-inspired design to enable the closing and opening of pores within the 3D porous structure by through-thickness compression, offering opacity and transparency upon release and compression. By changing the activation mode from the conventional in-plane to through-thickness direction, the space requirement is intrinsically decoupled from the lateral size of the scatterer. Central to our design is the asymmetry of pore orientation in the 3D porous structure. These inclined pores against the normal direction increase the opaqueness upon release and improve light modulation sensitivity to compression, enabling transmittance regulation upon compression by an infinitesimal displacement of 50 μm. This work establishes a milestone for smart window technologies and will drive advancements in the development of opto-electric devices.

Highly Sensitive Two‐Dimensional Ruddlesden‐Popper Perovskites for Thermochromic Smart Windows

AbstractWith the growing demand for high‐performance smart windows, the discover of a class of thermochromic materials with reversible cycling and rapid response characteristics has become urgent. In this work, we have uncovered a two‐dimensional (2D) Ruddlesden‐Popper (RP) phase halide perovskite, (PMA)2MAPb2I7−xClx, where PMA=C6H5CH2NH3 and MA=CH3NH3, exhibiting exceptional reversible thermochromic properties. The 2D RP phase perovskite thin film features a low transition temperature (Tc=30 °C from the hydrated state to the hot state, along with a fast transition time of 40 s. Furthermore, the addition of 0.5 times excess MAI significantly enhances the visible light transmittance of the hydrated state. Characteristic hydration peaks in X‐ray diffraction patterns and O−H bond absorption peaks Fourier‐transform infrared spectra are observed in the thin film in its hydrated state, which disappear in the hot state, validating its reversible thermochromic properties. Additionally, a solar cell based on the thermochromic 2D RP phase thin film achieves a power conversion efficiency of 2.31 %, offering a promising solution for advanced smart window technologies.

A “Hand‐In‐Hand” Thermal‐Responsive Cyclodextrin Material Designed for Liquid Smart Windows

AbstractThermochromic smart windows have emerged as a focal point in energy research. In this study, a novel approach is introduced for developing thermoresponsive materials for smart windows, utilizing microscopic agglomeration of molecules to regulate incident light. The groundbreaking material, β‐CD‐Val‐IBAm4, is constructed by grafting four isobutylated valines onto natural β‐cyclodextrins. This material exhibits a unique “hand‐in‐hand” effect, resulting from the interaction of isobutylated valine side chains, which leads to molecular aggregation and reduced light transmission at elevated temperatures. The hydration–dehydration process is related to molecular aggregation rather than crosslinking, ensuring the structural integrity and uniformity of the smart windows. Molecular dynamics simulations and 2D NOESY confirm the thermal response mechanism of β‐CD‐Val‐IBAm4. When dissolve in a glycerol‐water binary solvent system, the material forms a thermochromic liquid with outstanding optical performance (ΔTsol of 1 mm liquid can reach 69.8%) and long‐term stability. This innovative approach opens new avenues for the advancement of next‐generation smart window technologies.

Recent Developments of Next-generation Smart Window using Metal Electrodeposition

Due to the energy and environmental crisis, indoor energy efficiency is of great importance. One of the solutions to cope with the problem is adopting a device with energy-saving functionality. From that perspective, smart window technology has received the foremost attention as of great potential, because it could modulate the solar energy stream such as light and heat. In addition to that, smart windows could offer visual aesthetic operation to our daily lives. Despite many technical attempts, no technology has been commercialized abruptly due to the restrictions or disadvantages. One of the rising options is a smart window based on reversible metal electrodeposition, as metal electrodeposits possess high contrast with color neutrality, and high coloration efficiency for privacy state. Since 2017, reversible metal electrodeposition-based smart window technology has been emerging, and advancing in terms of cycling, bistability, and color uniformity. This review aims to summarize the current technological trends and directions for the development of reversible metal electrodeposition-based smart window technologies.

Comprehensive analysis on building performance enhancement based on selective split-band modulated adaptive thermochromic windows

Thermochromic technology has exciting potential for building applications as it passively modulates its solar transmittance. Existing smart window technologies lead to unnecessary increases in lighting energy consumption during spectral modulation and limited application scenarios. Therefore, spectrally selective modulation is a key direction to improve the performance of smart windows. In this paper, a three-story office building is established in EnergyPlus, and how the characteristic parameters and spectral modulation properties affect the effectiveness of thermochromic windows are thoroughly investigated through actual spectra and hypothetical split-band modulation spectra. A comprehensive human-centered evaluation system is proposed, multiple light zones are divided to investigate in detail the light and thermal environment and comfort performance of office spaces with different orientations of near-window areas and verify the energy-saving effect. The impact of thermochromic adaptive windows on building part loads and equipment efficiency is discussed and the matching of this passive technology with renewable photovoltaics is briefly mentioned. Based on the two-way synergistic perspective of materials and buildings, the results inform the application of smart windows in specific scenarios, throwing light on more cross-cutting research.

Environmental Impact of Passive Smart Window Technologies

This paper draws attention to the environmental impact of passive smart windows, a novel high-performance glazing technologies that can change their solar transmittance to control the amount of solar gain, thus reducing cooling energy demand. Despite the large influence of building envelope technologies on overall embodied carbon in buildings, the environmental impact of passive smart windows has been inadequately addressed, with a dearth of numerical data on various impact categories beyond energy consumption and Global Warming Potential (GWP). While current literature focuses on the advantages of these technologies in terms of operational energy savings, other critical environmental considerations are currently missing. This paper aims to bridge the existing gap by introducing a novel framework for evaluating the broader environmental impact of passive smart windows through a multi-category LCA method. By analysing the life cycle of these technologies, including production, usage, and disposal, the research seeks to provide a holistic understanding of their contribution to sustainability. The framework is based on systematic literature review on current state-of-art approaches and Interviews with key stakeholders in the dynamic glazing value chain. Literature review and interview results are presented, and then the framework is demonstrated through a case study of a thermochromic technologies for an office building in the Netherlands. Preliminary results show the critical areas where improvements on the methods or on the performance of the technology are required for the achievement of holistically sustainable high-performing glazing.

Redox Potential Based Self-Powered Electrochromic Devices for Smart Windows.

Energy-efficient glass windows are pivotal in modern infrastructure striving toward the "Zero energy" concept. Electrochromic (EC) energy storage devices emerge as a promising alternative to conventional glass, yet their widespread commercialization is impeded by high costs and dependence on external power sources. Addressing this, redox potential-based self-powered electrochromic (RP-SPEC) devices are introduced leveraging established EC materials like tungsten oxide (WO3) and vanadium-doped nickel oxide (V-NiO) along with aluminum (Al) as an anode. These devices produce open circuit voltages (OCV) exceeding ±0.3V, enabling autonomous operation for multiple cycles. The WO3 film exhibits 1% transmission and 88% modulation in the colored state at 550nm with a mere 260nm thickness. The redox interactions facilitate coloring and bleaching cycles without external power, while photo-charging rejuvenates the system. Notably, the inherent voltages of the RP-SPEC device offer dual functionality, powering electronic devices for up to 81h. Large-area (≈28 cm2) device feasibility is demonstrated, paving the way for industrial adoption. The RP-SPEC device promises to revolutionize smart window technology by offering both energy efficiency and autonomous operation, thus advancing sustainable infrastructure.

Linking Occupant Behavior and Window Design through Post-Occupancy Evaluation: Enhancing Natural Ventilation and Indoor Air Quality

This study investigates how window design features, such as size, placement, and orientation, might impact occupants’ behavior related to natural ventilation in residential houses and how residents manage natural ventilation to affect indoor air quality (IAQ), comfort, and energy efficiency. By analyzing responses from a questionnaire distributed among 200 occupants, this article reveals that stuffy air, perceived outdoor pollutants, odors, and relative humidity, along with factors like inadequate ventilation, temperature fluctuations, and energy consumption concerns, emerge as primary issues affecting occupants’ comfort and well-being. This study proposes design recommendations for enhancing IAQ, including optimal window placement for cross-ventilation, window-to-wall ratio (WWR) considerations, and the integration of smart window technologies. This research recognizes that window design is not just a technical matter but involves understanding social and behavioral factors as well. By analyzing occupant responses, it aims to provide insights into the socio-technical parameters that should be considered in window design. The findings offer valuable strategies for architects, designers, and homeowners to optimize natural ventilation and underscore the importance of an occupant-centered approach in sustainable building design.

Empowering Zero‐Energy Buildings with Cutting‐Edge Self‐Powered Electrochromic Smart Window Technology

The development of energy‐efficient window technology is crucial for zero‐energy building approaches, as windows play a pivotal role in minimizing heat loss, optimizing natural light utilization, and contributing to overall energy conservation. In this study, a self‐powered electrochromic smart window is presented together and its application as a self‐charging battery is showcased. The self‐powered system is composed of a WOx electrochromic working electrode, an Al counter electrode, and an AlCl3 liquid electrolyte similar to a voltaic cell. This device generates an open circuit voltage of around 0.95 V. When two electrodes are connected, this voltage is applied as an external voltage and changes the color of the transparent WOx electrode into a blue state with approximately 62% transmittance modulation. However, if the two electrodes are connected via an light‐emitting diode (LED), the LED lights up for up to 4 h.

Refractive Index Modulation for Metal Electrodeposition-Based Active Smart Window Applications.

One of the remarkable choices for active smart window technology is adopting a metal active layer via reversible metal electrodeposition (RME). As the metal layer efficiently blocks the solar energy gain, even a hundred-nanometer-thick scale, RME-based smart window has great attention. Recent developments are mainly focused on the various cases of electrolyte components and composition meeting technological standards. As metal nanostructures formed through the RME process involve plasmonic phenomena, advanced analysis, including plasmonic optics, which is beyond Beer-Lambert's law, should be considered. However, as there is a lack of debates on the plasmonic optics applied to RME smart window technology, as research is mainly conducted through an exhaustive process. In this paper, in order to provide insight into the RME-based smart window development and alleviate the unclear part of plasmonic optics applied to the field, finite-difference time-domain electromagnetic simulations are conducted. In total, two extremely low-quality (Cr) and high-quality (Mg) plasmonic materials based on a nanoparticle array are considered as a metal medium. In addition, optical effects caused by the metal active layer, electrolyte, and nanoparticle embedment are investigated in detail. Overall simulations suggest that the effective refractive index is a decisive factor in the performance of RME-based smart windows.

Towards Room Temperature Thermochromic Coatings with controllable NIR-IR modulation for solar heat management & smart windows applications

Solar heat management & green air-conditioning are among the major technologies that could mitigate heat islands phenomenon while minimizing significantly the CO2 global foot-print within the building & automotive sectors. Chromogenic materials in general, and thermochromic smart coatings especially are promising candidates that consent a noteworthy dynamic solar radiation Infrared (NIR-IR) regulation and hence an efficient solar heat management especially with the expected increase of the global seasonal temperature. Within this contribution, two major challenging bottlenecks in vanadium oxide based smart coatings were addressed. It is validated for the first time that the NIR-IR modulation of the optical transmission (∆TTRANS = T(T〈TMIT) − T(T〉TMIT) of Vanadium oxide based smart coatings can be controlled & tuned. This upmost challenging bottle-neck controllability/tunability is confirmed via a genuine approach alongside to a simultaneous drastic reduction of the phase transition temperature TMIT from 68.8 °C to nearly room temperature. More precisely, a substantial thermochromism in multilayered V2O5/V/V2O5 stacks equivalent to that of standard pure VO2 thin films but with a far lower transition temperature, is reported. Such a multilayered V2O5/V/V2O5 thermochromic system exhibited a net control & tunability of the optical transmission modulation in the NIR-IR (∆TTRANS) via the nano-scaled thickness’ control of the intermediate Vanadium layer. In addition, the control of ∆TTRANS is accompanied by a tremendous diminution of the thermochromic transition temperature from the elevated bulk value of 68.8 °C to the range of 27.5–37.5 ºC. The observed remarkable and reversible thermochromism in such multilayered nano-scaled system of V2O5/V/V2O5 is likely to be ascribed to a noteworthy interfacial diffusion, and an indirect doping by alkaline ions diffusing from the borosilicate substrate. It is hoped that the current findings would contribute in advancing thermochromic smart window technology and their applications for solar heat management in glass windows in general, skyscraper especially & in the automotive industry. If so, this would open a path to a sustainable green air-conditioning with zero-energy input.

Thermochromic hydrogel-based energy efficient smart windows: fabrication, mechanisms, and advancements.

Thermochromic smart windows are regarded as highly cost-effective and easily implementable strategies with zero energy input among the smart window technologies. They possess the capability to spontaneously adjust between transparent and opaque states according to the ambient temperatures, which is essential for energy-efficient buildings. Recently, thermochromic smart windows based on hydrogels with various chromic mechanisms have emerged to meet the increasing demand for energy-saving smart windows. This review provides an overview of recent advancements in hydrogel-based thermochromic smart windows, focusing on fabrication strategies, chromic mechanisms, and improvements in responsiveness, stability and energy-saving performance. Key developments include dual-responsiveness, tunable critical transition temperatures, freezing resistance, and integrations with radiative cooling/power generation technologies. Finally, we also offer a perspective on the future development of thermochromic smart windows utilizing hydrogels. We hope that this review will enhance the understanding of the chromic mechanism of thermochromic hydrogels, and bring new insights and inspirations on the further design and development of thermochromic hydrogels and derived smart windows.

Transparent grating-based metamaterials for dynamic infrared radiative regulation smart windows.

Dynamic infrared radiation regulation has been widely explored for smart windows because of its vital importance for comfortable and energy-efficient buildings. However, it remains a great challenge to synchronously achieve high visible transmittance and pronounced infrared tunability. Here, we propose a dynamic infrared tunable metamaterial composed of indium tin oxide (ITO) gratings, an air insulator, and an ITO reflector. The ITO grating-based infrared radiation regulator exhibits a high emissivity tunability of 0.73 at 8-13 μm while maintaining a high visible transmittance of 0.65 and 0.72 before and after actuation, respectively. By adjusting the geometric parameters, the tunable bandwidth can be further extended to 3-30 μm and the ultra-broadband tunability reaches 0.62. The excellent infrared tunable performance arises from the insulator thickness-dependent effect of Fabry-Pérot and propagating surface plasmon resonance coupling and decoupling, which lead to perfect and low absorption, respectively. This work provides potential for the advancement of smart window technology and makes a significant contribution to sustainable buildings.

Energy-Efficient Liquid Crystal Smart Window with a Clear View

In this study, we enhance the angular-selective light absorption capabilities of guest–host liquid crystal (GHLC) cells by introducing a novel design featuring a uniform lying helix (ULH) structure. Previously GHLC cells, predominantly vertically aligned cells absorbed obliquely incident light but compromised x-direction visibility. In stark contrast, our ULH-based design allows incident light to seamlessly traverse transmittance in both z- and x-directions while efficiently obstructing oblique incident light in the y-direction. Our innovative ULH-based GHLC cell achieves an impressive optical performance. Specifically, it attains a substantial transmittance rate of 56.7% in the z-direction. Furthermore, in oblique views encompassing both the x- and y-directions, it maintains competitive transmittance rates of 44.2% and 29.5%, respectively. This strategic design not only ensures clear and unobstructed views for building occupants in the z- and x-directions but also contributes significantly to energy conservation by preventing oblique incident light from penetrating, thus reducing cooling requirements. Our ULH-based GHLC cell represents a breakthrough in smart window technology, offering an elegant solution to the challenge of balancing energy efficiency and occupant comfort in architectural settings. This advancement holds promising implications for sustainable building designs by enhancing indoor environmental quality while mitigating energy consumption for cooling, ultimately redefining the potential of smart windows in contemporary architecture.

A liquid crystal smart window for energy saving and harvesting

Energy conservation, particularly in buildings, has become a critical focus area. Among the various components of a building, windows play a significant role in the exchange of heat and light. Electrochromic systems, known for their ability to switch between transparent and opaque states with the aid of external energy, have served as the basis for conventional smart windows. This research introduces a novel approach to smart window technology by incorporating a luminescent solar concentrator (LSC) that incorporates thin photovoltaic cells at the window's edges, coupled with a thermally and optically adjustable liquid crystal cell. The device intelligently transitions between an energy-saving, opaque mode (day mode) and a transparent, idle mode (night mode), based on ambient temperature and incident light levels. Through this innovative design, the proposed smart window offers enhanced energy efficiency and harvesting capabilities, promising significant benefits for sustainable building practices.

High transmittance, fast response, and high contrast ratio smart window with lateral driving electrophoretic display

Smart windows can dynamically control the optical transmittance of natural light and efficiently reduce the energy consumption of buildings. Currently, the practical application of smart windows is limited by high power consumption, low transmittance, and complicated preparation process. Developing smart windows with fast response and high transmittance is challenging. Herein, we fabricated a smart window based on electrophoretic display (EPD) technology, which can switch between white and transparent states by changing the stacking states of the electrophoretic particles on electrodes. To further increase the transmittance of the smart window, the polyvinyl alcohol (PVA) hydrophilic layer was spin-coated on the electrode to reduce the adhesion of particles. Compared with other EPD smart window technologies, our device exhibits the fastest response time of up to 375 ms at 30 V, high transmittance of up to 78% at 632 nm, and a high contrast ratio of up to 89. In addition, to reduce the particle’s lateral diffusion and improve our device's stability, the microcup array was aligned on the interdigital electrode. The particles can be driven and stacked onto the side walls of the microcup to achieve a transparent state. This work demonstrated that the EPD device would provide a promising future for practical smart windows with high transmittance and fast response time.

84‐3: A Novel 2D Diffractive Liquid Crystal Smart Window using Single‐sided Engineered Electrode Structure

Smart window technology has been studied actively because it can simply control optical property of window by external power. Among various smart window technology, a phase grating liquid crystal (LC) device shows outstanding properties such as clear off state, high haze in on state, and no viewing angle dependence. Although it can have excellent optical performance, an LC phase grating device requires complicated pattern electrode structure on both top and bottom substrate. To make the device in cost effectiveness, it is required that simple and cost saving design of LC cell is essential.In this presentation, we will introduce our recent progress in LC phase grating smart window. Based on the optimal LC cell design, a trade‐off between performance and cost was minimized while maintaining the advantages of previously researched phase grating LC smart window.