Switchable Windows Research Articles

High-contrast electrochromic coatings and devices for health assurance, visual comfort and energy-saving in buildings

High-contrast electrochromic coatings and devices reduce the ultraviolet-radiation from sunlight, ensure human health and visual comfort, save the electricity consumption of the heater in winter, and save electricity consumption of air conditioners in summer. Anodically coloring polymer coatings (P(CDBP), P(CDBP-co-BTPH), P(CDBP-co-TFU), P(CDBP-co-CDT) and P(CDBP-co-CDTK)) based on 4,4′-bis(carbazol-9-yl)-2,2′-dimethylbiphenyl (CDBP) and four different dithiophene derivatives (2,2′-bithiophene (BTPH), 2-(2-thienyl)furan (TFU), 4H-cyclopenta[2,1-b:3,4-b’]dithiophene (CDT), and cyclopentadithiophene ketone (CDTK)) are polymerized electrochemically on ITO electrode. The P(CDBP-co-BTPH) electrochromic coating has high transmittance change (ΔT = 71.1 %) and coloration efficiency (160.6 cm2 C−1). P(CDBP-co-BTPH) film reveals yellow-gray and dark blue in neutral and oxidation states, respectively. Electrochromic energy saving devices (ECDs) are assembled using P(CDBP), P(CDBP-co-BTPH), P(CDBP-co-TFU), P(CDBP-co-CDT) and P(CDBP-co-CDTK) films as the anodic layers and poly(3,4-(2,2-dimethylpropylenedioxy)thiophene) (PPD-M2) film as the cathodic layer. P(CDBP-co-BTPH)/PPD-M2 ECD shows a high transmittance change (ΔT = 30.3 %) and a rapid response time (≤0.3 s). Moreover, P(CDBP-co-BTPH)/PPD-M2 ECD also has sufficient optical memory and redox cycling stability. These findings provide us new insights for the useful design of polymer coatings in rapid switching electrochromic energy-saving devices and switchable architectural windows.

New graphene oxide doped polymer dispersed liquid crystal nanocomposites targeted to eco-friendly and energy-efficient smart windows

Graphene oxide-polymer dispersed liquid crystal (GO-PDLC) composites are proposed as a solution for energy-efficient switchable windows. These composites were prepared by incorporating varying concentrations of graphene oxide (GO) into a mixture of nematic liquid crystal (E7) and a UV–Vis curable resin using the polymerization-induced phase separation technique. The results indicate that the dispersion of GO at an optimum concentration of 0.005 wt% significantly affects the morphology, phase transition temperatures, and electro-optic properties of polymer dispersed liquid crystal (PDLC). In particular, the GO-PDLC composites exhibit a transition from an opaque to a transparent state at a remarkably low voltage (∼11.5 V) compared to pure PDLC cell. Dielectric analysis reveals that adding GO increases the permittivity, conductivity, and dielectric strength of PDLC composites. Moreover, the Cole-Cole plot indicates a non-Debye type relaxation in a higher frequency region. Furthermore, a blue shift in emission wavelength and enhanced photoluminescence intensity suggest alterations in electronic transitions within the composites. This comprehensive study provides valuable insights into optimizing the properties of PDLCs through the dispersion of GO, thereby offering promising prospects for their utilization in various optoelectronic applications.

Smart Switchable Window with Bistability and Tunability of Transmission

AbstractWindows are used everywhere, from buildings to vehicles. Smart switchable windows are highly sought‐after in order to reduce energy consumption. An ideal smart window should have two functions. First, it should be able to control privacy. Second, it should be able to control radiant energy flow. Here, a smart window is reported that possesses the two functions. The smart window is based on a dichroic dye‐doped cholesteric liquid crystal. It exhibits two stable states in the absence of applied voltage. One of them is the planar state that is optically absorbing without haze and can control radiant energy flow. The other state is the focal conic state that is optically scattering and can control privacy. Furthermore, when voltage is applied, the liquid crystal is switched to a heliconical state, where the transmittance can be tuned continuously. Thanks to its superior performance, this smart window has a big potential for applications in architectural and vehicle windows.

Amine Gas-Induced Reversible Optical Bleaching of Bismuth-Based Lead-Free Perovskite Thin Films.

Reversible optical property changes in lead-free perovskites have recently received great interest due to their potential applications in smart windows, sensors, data encryption, and various on-demand devices. However, it is challenging to achieve remarkable color changes in their thin films. Here, methylamine gas (CH3 NH2 , MA0 ) induced switchable optical bleaching of bismuth (Bi)-based perovskite films is demonstrated for the first time. By exposure to an MA0 atmosphere, the color of Cs2 AgBiBr6 (CABB) films changes from yellow to transparent, and the color of Cs3 Bi2 I9 (CBI) films changes from dark red to transparent. More interestingly, the underlying reason is found to be the interactions between MA0 and Bi3+ with the formation of an amorphous liquefied transparent intermediate phase, which is different from that of lead-based perovskite systems. Moreover, the generality of this approach is demonstrated with other amine gases, including ethylamine (C2 H5 NH2 , EA0 ) and butylamine (CH3 (CH2 )3 NH2 , BA0 ), and another compound, Cs3 Sb2 I9 , by observing a similar reversible optical bleaching phenomenon. The potential for the application of CABB and CBI films in switchable smart windows is investigated. This study provides valuable insights into the interactions between amine gases and lead-free perovskites, opening up new possibilities for high-efficiency optoelectronic and stimuli-responsive applications of these emerging Bi-based materials.

Enhanced thermo-electro-optical and dielectric properties of carbon nanoparticle-doped polymer dispersed liquid crystal based switchable windows

The effect of temperature as well as voltage on switchable smart windows was studied. These smart windows are composites of a polymer and a liquid crystal (LC) and collectively known as polymer dispersed liquid crystal (PDLC) films. In order to enhance the efficiency of the smart windows/PDLC films, the LC material was doped with carbon nano particles (CNP) in various concentrations ranging from 0 to 0.1%. To examine the efficacy and the temperature dependent performance of the CNP doped PDLC (CPDLC) film, firstly morphological and thermo-morphological studies were performed using polarizing optical microscopy (POM). POM in combination with temperature changes allows to record texture changes and the nematic-isotropic transition of the composite films for applied voltage. The complete electro-optical (EO) study comprising of voltage and temperature dependent transmittance, threshold voltage (Vth), saturation voltage (Vsat), contrast ratio (CR) and response time measurement was performed through an assembled EO set-up. To infer intermolecular cooperative processes between polymer and the dipoles of the LC, the relaxation mechanism of the composite films for various temperatures was studied by employing Debye and Cole-Cole models in dielectric spectroscopy. Parameters like dielectric strength and dissipation factor were determined to allow optimization of the CPDLC films.

Challenges and Optimization of Building-Integrated Photovoltaics (BIPV) Windows: A Review

PV windows are seen as potential candidates for conventional windows. Improving the comprehensive performance of PV windows in terms of electrical, optical, and heat transfer has received increasing attention. This paper reviews the development of BIPV façade technologies and summarizes the related experimental and simulation studies. Based on the results of the literature research, the average comprehensive energy-saving rate of BIPV façades can reach 37.18%. Furthermore, limitations and optimization directions of photovoltaic integrated shading devices (PVSDs), photovoltaic double-skin façades, and photovoltaic windows are presented. To improve the energy-saving potential of windows as non-energy efficiency elements of buildings, smart PV windows are proposed to be the key to breakthrough comprehensive performance. However, not all switchable windows concepts can be applied to PV windows. Typical studies on smart windows and PV windows are sorted out to summarize the challenges and optimization of smart PV window technical solutions. Considering the technological innovations in smart PV windows, two requirements of energy-saving materials and building envelopes are put forward. The advances in materials and the building envelope are complementary, which will promote the sophistication and promotion of solar building technology.

An evaluation of the demand response potential of integrated dynamic window and HVAC systems

Demand response (DR) increases the flexibility and reliability of the electricity grid as use of intermittent renewable energy sources increases. HVAC and envelope DR measures present the largest aggregate energy and peak demand savings potential of all commercial building end uses because their net demand savings occur during critical peak demand periods. Controllable envelope measures include switchable electrochromic windows, operable window attachments such as outdoor louvers, roller shades, and awnings, as well as other innovative facade technologies that can modulate both solar heat gain and daylight admission over a broad solar-optical range. This study evaluated the technical potential of DR-enabled dynamic windows to reduce critical peak demand for a prototypical medium office building situated in all 16 U.S. climates. Model predictive control (MPC) algorithms were designed to minimize electricity cost in daylit perimeter office zones through control of an electrochromic window with and without HVAC thermostat setpoint control. Conventional and time-of-use rates were used to shape the degree of DR. Median annual peak demand savings with window and thermostat control across all climate zones were 24.3 kW (4.4 W/m2) per building or 15.9 W/m2 for non-north perimeter zones. Resource adequacy at the whole building level was estimated to be 13.1 to 43.4 $/kW per year over the 30-year life of the installation. Co-benefits were increased energy efficiency, and reduced electricity cost and emissions. Visual and thermal comfort requirements were met at all times. Dynamic facades controlled by MPC have substantial technical potential for DR across all U.S. climates and warrant serious consideration for inclusion in DR portfolios.

Haze-Suppressed Wideband Light Shutter from Near Ultraviolet to Infrared Rays Utilizing Electrophoretic Carbon Black Particles.

Carbon black (CB) particles that can absorb from near ultraviolet to infrared rays are well dispersed into an isotropic dielectric liquid and their optical properties can be kept even under exposure to sunlight over a long time. The shutter which controls the position of CB particles by electrophoretic force can be applied to switchable light shutters for windows in buildings and automobiles for the purpose of energy savings. Here, a wideband light shutter with three terminal electrodes is proposed, exhibiting excellent dark (transmittance ≈1.4%) and transparent state (transmittance >60%). The device operates at a low field intensity of about 1Vµm-1 to obtain transparent state and its wide temperature range operation from -50 to 120°C is confirmed while conventional liquid crystal-based shutter cannot perform such levels due to a limited temperature range in the nematic phase. In addition, haze is minimized in a transparent state by adopting an insulation layer over electrodes and a solution is found to keep a transparent state without applying power. It is believed that the proposed broadband shutter with fast response time could open a new chapter in switchable windows owing to its low power consumption and working in a wide temperature range.

Energy assessment of advanced and switchable windows for less energy-hungry buildings in the UK

Globally, greenhouse gas emissions from the operational phase of buildings are significantly contributing towards climate change. Global and national efforts, through the Sustainable Development Goals and the UK's 2050 targets, aim to reduce these emissions with net zero energy buildings (NZEBs). A building's glazing plays a significant role in overall building energy consumption due to their traditionally ‘leaky’ nature. This study utilises experimental data from test cells and the International Glazing Database to evaluate the performance of advanced and smart/switchable windows on an existing low energy building (LEB) situated in north Wales, UK, as a step towards making the modelled building a NZEB. A number of glazing constructions were considered in this work; advanced window – vacuum, aerogel, vacuum-aerogel and smart window – PDLC, PDLC-aerogel and PDLC-vacuum, in their fixed and switching states. Results revealed that PDLC-vacuum offered the greatest reduction in building energy, yielding a theoretical U-value of 0.810–0.831 W/m2K and a G-value of 0.257–0.455. Despite its successes, it was notably susceptible to window orientation and window-to-wall ratio. Vacuum and aerogel glazing both offered similar energy savings, with the latter prone to overheating, stressing cooling loads. These advanced windows offered differing daylighting potential with vacuum able to meet 78% of useful daylight illuminance compared to aerogel's 60%. Given the prioritisation trilemma between heating, lighting and cooling needs of a building, PDLC-vacuum presents the best step towards a NZEB. As such, further efforts should concentrate on the development of a PDLC-vacuum window, maintaining smart window functionality and achieving low U-value for cold climates.

Novel warm/cool-tone switchable VO2-based smart window composite films with excellent optical performance

Vanadium dioxide (VO2) is a key material for thermochromic smart windows that can respond to environmental temperature to regulate near-infrared transmittance automatically. To date, VO2-based smart windows have made great progress, but practical applications still remain restricted by two tough challenges, i.e., low solar regulation ability and unpopular color. In view of the above two challenges, in this paper, novel warm/cool-tone switchable VO2-based composite films were designed and prepared. The novel composite films demonstrate outstanding optical properties: ΔTsol = 18.92% and Tlum,l = 48.70%. The high solar modulation efficiency is attributed to the modulation of the thermochromic microcapsules in visible light waveband and VO2 in near-infrared region. Meanwhile changes in color of the composite films at different temperatures were also studied in order to develop high-performance warm/cool-tone switchable thermochromic smart windows. These promising results will benefit to promote the popularization and application of smart windows.

Numerical Investigation of the Effect of Symmetry on Evaporation Triggered Elastocapillary Top-Gathering of High Aspect Ratio Micropillars

High-aspect-ratio (HAR) micropillar arrays offer a wide range of applications in micro-contact printing, switchable transparent optical windows, superhydrophobic surfaces, mechanical sensors, and actuators, due to their properties such as large surface area and excellent mechanical compliance. However, owing to their high aspect ratio, these microstructures are prone to lateral deflection by elastocapillary forces in liquid environments, which is known as top-gathering, limiting their manufacturing processes and applications. Here, the impact of symmetry on evaporation triggered top-gathering of micropillars was studied numerically. The initiation of the micropillar deflection due to capillary forces under varying force distributions was simulated using a COMSOL Multiphysics simulation package. The simulation was carried out for the configurations of two, four, and an array of micropillars. For the four micropillar configuration, a new equation was suggested for calculating the micropillar deflection due to elastocapillary forces, using force distributions around the micropillars. The suggested equation was verified by comparison with the experimental observations. The effect of droplet evaporation on deflection/top-gathering of micropillars was also investigated. It was found that initiation of deflection is due to asymmetry at the rim of the droplet, generating domino-like deflection of the other micropillars. This study provides a new equation/criterion for estimating deflection of the micropillars, suggesting array designs that are resistant to such deflections when interacting with liquids.

Can we make color switchable photovoltaic windows?

The development of smart windows could enhance the functionality of the large glass facades found in modern buildings around the globe. While these facades offer occupants views and natural light, the poor insulating qualities of glass cut against the desire for more efficient use of energy resources. In this perspective article, we explore recent developments for next-generation smart window technologies that can offer improved energy management through dynamic color switching, reducing heating and cooling loads, while also generating electricity through the photovoltaic effect. Approaches with chromogenic organic dyes and halide perovskite semiconductors have been developed for switchable photovoltaic windows, but each of these comes with unique challenges. These approaches are briefly discussed and evaluated with an eye to their future prospects. We hope that this perspective will spur other researchers as they think about the various materials and chemical design challenges associated with color switchable photovoltaic windows. Perhaps these initial demonstrations and research ideas can then become marketable products that efficiently use space to improve occupant comfort and reduce the energy demand of the built environment.

Optical Tunability and Characterization of Mg-Al, Mg-Ti, and Mg-Ni Alloy Hydrides for Dynamic Color Switching Devices.

Mg shows great potential as a metal hydride for switchable optical response and hydrogen detection due to its ability to stably incorporate significant amounts of hydrogen into its lattice. However, this thermodynamic stability makes hydrogen removal difficult. By alloying Mg with secondary elements, the hydrogenation kinetics can be increased. Here, we report the dynamic optical, loading, and stress properties of three Mg alloy systems (Mg-Al, Mg-Ti, and Mg-Ni) and present several novel phenomena and three distinct device designs that can be achieved with them. We find that these materials all have large deviations in refractive index when exposed to H2 gas, with a wide range of potential properties in the hydride state. The magnitude and sign of the optical property change for each of the alloys are similar, but the differences have dramatic effects on device design. We show that Mg-Ti alloys perform well as both switchable windows and broadband switchable light absorbers, where Mg0.87Ti0.13 and Mg0.85Ti0.15 can achieve a 40% transmission change as a switchable window and a 55% absorption change as a switchable solar absorber. We also show how different alloys can be used for dynamically tunable color filters, where both the reflected and transmitted colors depend on the hydrogenation state. We demonstrate how small changes in the alloy composition (e.g., with Mg-Ni) can lead to dramatically different color responses upon hydrogenation (red-shifting vs blue-shifting of the resonance). Our results establish the potential for these Mg alloys in a variety of applications relating to hydrogen storage, detection, and optical devices, which are necessary for a future hydrogen economy.

Liquid crystal-polymer composites switchable windows for radiant energy flow and privacy control

Global warming is becoming a more and more severe crisis for humans. One way to resolve the concern is to reduce energy consumption. Smart switchable windows for office and residential buildings and vehicles can help reduce energy consumption. An ideal smart window should be able to control radiant energy flow and privacy. We investigated the capability of switchable windows based on liquid crystal/polymer composites, such as polymer dispersed liquid crystal (PDLC), polymer stabilized liquid crystal (PSLC), and polymer stabilized cholesteric texture (PSCT), to control the privacy and radiant energy flow. Through a systematic study, we identified methods to improve their capabilities. We demonstrated that PDLC and PSCT windows of sufficient thick film thickness can control both privacy and energy flow.

Fast dual-mode switchable smart window based on the dual-frequency liquid crystal electrohydrodynamic instability

In the past decade, liquid crystals for use in switchable smart windows have become a research hotspot. Dual-frequency liquid crystals (DFLCs), in particular, are now widely studied and applied in fast-response liquid crystal devices. In this work, we determine that a DFLC scatters light when a voltage frequency close to its crossover frequency is applied. The principle behind this phenomenon is then investigated. The experimental results show that the scattering of light is a dynamic process that is caused by ion perturbation. Based on the results of this investigation, a double-layer liquid crystal (LC) shutter is prepared. It is shown that the fabricated device can be quickly switched between transparent, dark, and scattering states solely by changing the voltage frequency; this ability also has a useful application in connection to privacy-protecting smart windows. The DFLC is combined with dynamic scattering to accelerate the response time of the scattering.

Polymer-Dispersed Liquid Crystal (PDLC) smart switchable windows for less-energy hungry buildings and visual comfort in hot desert climate

Smart switchable glazing windows have become an alternative energy-saving solution for achieving visual comfort and reducing energy consumption. In Saudi Arabia, significant amounts of energy are lost through external envelope façades due to the integration of conventional glazing systems. Thus, in this work, experimental characterisation and numerical analysis were carried out on a scale model integrated with a Polymer-Dispersed Liquid Crystal (PDLC) smart window in a hot desert climate. In the first part of this paper, an outdoor scale model was used to evaluate the thermos-optical properties of a PDLC smart window. The experiments were conducted in the spring season in actual conditions, while the state of the window (transparent and opaque) was adjusted. The data were validated and employed to develop a numerical model for energy consumption and visual comfort analysis. A comprehensive parametric analysis of the window-to-wall ratio (WWR) in cardinal orientation was carried out using the EnergyPlus9.4 and Climate Studio packages. The results indicated that the PDLC glazing in the system was able to achieve the Leadership in Energy and Environmental Design (LEED) LEED4.1 standard partially due to the increased Annual Sunlight Exposure ASE>1000lx 250h, even with low transmittance and WWR in both states. The binary behaviour of the PDLC glazing was able to reduce the emission of CO2 from 32.5–89 kg CO2/m2 using the WWR fully for the transparent and opaque states, respectively. Overall, this study contributed to the knowledge on the integration of smart glazing in buildings with low energy consumption and indoor daylighting in a hot desert climate.

Assessment of the overall energy performance of an SPD smart window in a hot desert climate

In this study, the overall energy consumption and visual comfort of a switchable suspended particle device (SPD) smart window were investigated as part of the glazing integration of an adaptive building designed to consume less energy in a hot desert climate. A typical floor of a commercial office building in Riyadh was chosen for the energy and visual comfort simulation, which was based on Energy-Plus and Diva-for-Rhino software for cardinal orientation. A comprehensive simulation analysis of various state of SPD glazing, namely opaque (OFF), transparent (ON) and automated controlled (based on solar radiance: from 100w/m2 – 900w/m2), was conducted and the results were compared against traditional single glazing and double-glazing low emissivity (DG low-e) coated windows for reference. The simulation results indicated that switchable SPD smart windows (in the OFF and automated states) achieved a promising reduction of net energy by up to 58% against DG low-e, apart from at the northern orientation. Conversely, the opaque (OFF state) had a counterproductive impact on lighting energy consumption and visual comfort. Acceptable daylight autonomy (DA300lux) and Useful daylight illuminance (UDI100lux–2000lux) was observed for the SPD smart window in the ON and automated controlled states; moreover, it offered a significant reduction in daylight glare probability (DGP). Thus, controlled switchable SPD glazing can be a good alternative to standard glazing in a hot desert climate in terms of reducing energy use and providing visual comfort.

Direct Plasma‐Enhanced‐Chemical‐Vapor‐Deposition Syntheses of Vertically Oriented Graphene Films on Functional Insulating Substrates for Wide‐Range Applications

AbstractGraphene has inspired wide‐range research interests and practical applications due to its intriguing materials properties and versatile application prospects. Specifically, vertically oriented graphene (VG) constructed by vertically aligned nanosheets emerges as a unique material type, usually achieved through a plasma‐enhanced chemical vapor deposition (PECVD) route. The VG films possess abundant exposed active edges, large surface areas, relatively high electrical conductivity, and excellent thermal property in both in‐plane and out‐of‐plane directions. Direct synthesizing of such VG films on various functional substrates enables their transfer‐free integrations and direct applications in thermal management, sensing, energy storage and conversion, etc. In this review article, it is planned to showcase the recent advances in direct PECVD growth of VG films on some novel functional insulating substrates (e.g., glass, functional ceramics), mainly in terms of improving the electrical conductivity, enhancing the interfacial adhesion, and graphene growth orientation modulation. Meanwhile, the applications of the PECVD‐derived products in various new fields will also be discussed, such as photothermal conversion, switchable windows, light attenuation in photography, transparent heating, and thermal management for high‐power devices. Finally, current challenges and future opportunities will also be presented regarding controllable syntheses of high‐quality VG films and practical application explorations in different fields.

Building energy analysis using EC and PDLC based smart switchable window in Oman

Worldwide energy consumption and CO2 emissions increase yearly and the building industry contributes the most out of all the other sectors. The residential building sector of the building industry provides the largest portion of energy consumption. Reducing energy gains through elements of the building envelope such as windows is a way to combat increasing energy consumption. Smart switchable glazing can contribute to energy savings by adjusting its properties in response to user settings or an external stimulus. This study explored the potential energy savings of electrochromic (EC) and polymer disperse liquid crystal (PDLC) switchable glazing against common static window glazing for a residential building in Oman. The results showed that switchable windows displayed better optical properties than static windows, with electrochromic windows under daylight illuminance control having the highest total energy savings at 23.56% reduction compared to a single-glaze window. In a PDLC window configuration, using silver coated glass as the inner pane in the double glazing reduces energy consumption even further.

PDLC with controllable microstructure using wavelength-selective two-stage polymerization

A wavelength-selective, two-stage polymerization of acrylate-thiol for the formation of polymer dispersed liquid crystal (PDLC) films is developed. Via this method, the PDLC with microsphere polymer morphology was successfully prepared, which greatly improved the contrast ratio (CR) and adhesion without causing other performance degradation. The change of morphology from the porous to the microsphere structure could be controlled by adjusting the reaction time of the Michael addition reaction. Furthermore, the size and uniformity of the microsphere structure could be controlled by tuning the content of the free radical initiator. This work is expected to provide an effective method to control the microstructure of acrylate-thiol polymers in solution polymerization, and can possibly allow the fabrication of PDLCs with high CR and adhesion, which is of great significance in large, flexible displays, switchable windows, and light valves.