Energy-efficient Smart Windows Research Articles

Boron doped M-phase VO2 nanoparticles with low metal-insulator phase transition temperature for smart windows

Vanadium dioxide (VO2) is considered to be a promising candidate for energy-efficient smart windows because of its special reversible Metal-Insulator Transition (MIT) near the ambient temperature. However, its use is constrained by its high transition temperature (TC) relative to the room temperature. In this paper, VO2 doped by boron, could achieve an outstanding metal-insulator phase transition property with a low TC (28.1 °C) close to the room temperature. This enhancement strongly contributes to the studies of the VO2-based smart windows. A limit doping level of around 9.0 at% is observed for the boron-doped VO2. Moreover, the particle size is getting smaller and more uniform and the particle distribution becomes more equal and compact with the continued increase in the doping content. Such uniform grain size and grain boundary conditions suppress the extension of the hysteresis loop (ΔT decreases from 25 °C to 7 °C). In addition, the TC first declines with the increase in the boron content and it starts to increase after reaching its minima of 28.1 °C at 6.0 at% doping level. This feature is the consequence of the competition between the inhibition on the phase transition caused by the V5+ and the promotion on the phase transition caused by the heterogeneous defect-nucleation sites. VO2 doped with 6.0 at% boron exhibits a favorable thermochromic performance with ΔTsol of 12.5% and Tlum up to 54.3%, which is promising for the smart windows.

Flexible Electrochromic Materials

Electrochromic devices (ECDs) containing electroactive materials can reversibly change their optical absorbance and transmittance according to applied voltage, and this property is utilized in a variety of applications such as reflective displays, antiglare mirrors, smart windows and sensors. Electrochromic devices (EC), which dynamically change colour under applied potential, are widely studied for use in energy-efficient smart windows. To improve the viability of smart windows, many researchers are utilizing nanomaterials, which can provide electrochromic devices with improved coloration efficiencies, faster switching times, longer cycle lives, and potentially reduced costs. The flexible electrochromic films are promising film for applications in automobiles, buildings, etc. Solution processed tungsten trioxide (WO3)-based electrochromic devices (ECDs) are successfully demonstrated. The thin layers with nanostructured materials of WO3 inks were coated on flexible substrate at room temperature, solution-processed method. In contrast to conventional ECDs based on metal oxides, we controlled the substrate to perform flexible ECDs. The performances of flexible transparent conductive electrodes (TCEs) will be presented. The performances and reliability of with flexible TCE will be presented.

Preparation of TiO2@W-VO2 thermochromic thin film for the application of energy efficient smart windows and energy modeling studies of the produced glass

Buildings consume huge amounts of energy in order to maintain thermal comfort for users. Among different buildings components, windows are considered as one of the most inefficient parts; so studies on energy-efficient windows have recently become crucial due to the energy crisis in the world. Thermochromic (TC) glazing is a new generation of energy-efficient smart glazing that passively changes its transmittance rate in response to temperature. Vanadium dioxide (VO2) is the most widely used TC material in thermochromic windows. To increase the transparency of glazing, augment the thermochromic properties of VO2, and set the transition temperature near the comfort zone, many dopants have been investigated and many fabrication methods have been proposed.In this research, by using the sol–gel method, we doped tungsten ions (W6+) on a vanadium dioxide film. Titanium dioxide (TiO2) was also added to the solution to produce a nanoparticle structure with both thermochromic and photocatalytic properties. To quantify the energy saving achieved by the produced TC glazing compared to standard products, a simple model of a room in a residential building was created with EnergyPlus computer software. The simulation results showed that the fabricated TC glazing could bring about a significant reduction in energy demand of buildings compared to current approaches.

Bifunctional Template-Induced VO2@SiO2 Dual-Shelled Hollow Nanosphere-Based Coatings for Smart Windows.

Thermochromic vanadium dioxide (VO2) as one of the most promising candidates for smart windows has attracted widespread attention in recent years. Excellent optical performances (luminous transmittance, Tlum, and solar modulation efficiency, Δ Tsol) of VO2-based coatings are usually pursued as crucial issues. In the current work, we report an ingenious approach for the synthesis of VO2@SiO2 dual-shell hollow nanospheres (DSHNs) and the preparation of DSHNs thermochromic coatings. A sequential bifunctional template-induced mechanism for the formation of DSHNs was proposed. Because of the unique hollow-core and dual-shell structure, the as-prepared VO2@SiO2 DSHNs coatings exhibited appealing optical performances with enhanced luminous transmittance of 61.8% and solar modulation efficiency of 12.6%, compared with continuous and dense VO2 coatings. It has been proved that the improvement of visible transmittance could be ascribed to the effective reduction of refractive index (from 2.6 to 1.6 at 630 nm). In addition, its excellent thermochromic performance has been confirmed by the model cubes measurements, expressing a great potential as energy-efficient smart windows in high-rise buildings. The bifunctional template-induced synthetic strategy may inspire more facile, efficient and inexpensive processes for development of well-defined multishelled hollow nanostructures for varied applications.

Adaptive Thermochromic Windows from Active Plasmonic Elastomers

Summary Architectural windows that smartly regulate the indoor solar irradiation are promised to economize the building energy consumption. Here, we demonstrate a method for adaptive, broadband, and highly efficient solar modulation for energy-efficient smart windows through active plasmonics in reconfigurable structures. We develop a kirigami-inspired elastomer containing plasmonic vanadium dioxide (VO2) nanoparticles, in which the geometrical transition and the temperature-dependent localized surface plasmon resonance (LSPR) present dominant controls in ultraviolet-visible and near-infrared regions, respectively. The active LSPR control, via stretch-induced local dielectric changes is mitigated on reconfigurable metamaterials because of their unique strain distributions. This method demonstrated a desirable property in energy-efficient smart windows facilitating improved solar energy modulation (37.7%), surpassing the best-reported modulation in passive and transparent VO2 thermochromism systems. This first attempt to integrate the plasmonics and reconfigurable structures may inspire developments in smart windows, building energy economization, as well as fundamental studies of plasmonic controls in metastructures.

The influence of low-temperature surface induction on evacuation, pump-out hole sealing and thermal performance of composite edge-sealed vacuum insulated glazing

Hermeticity of vacuum edge-sealing materials are one of the paramount requirements, specifically, to the evolution of energy-efficient smart windows and solar thermal evacuated flat plate collectors. This study reports the design, construction and performance of high-vacuum glazing fabrication system and vacuum insulated glazing (VIG). Experimental and theoretical investigations for the development of vacuum edgeseal made of Sn-Pb-Zn-Sb-AlTiSiCu composite in the proportion ratio of 56:39:3:1:1 by % (CS-186) are presented. Experimental investigations of the seven constructed VIG samples, each of size 300 mm·300 mm·4 mm, showed that increasing the hot-plate surface temperatures improved the cavity vacuum pressure whilst expediting the pump-out hole sealing process but also increases temperature induced stresses. Successful pump-out hole sealing process of VIG attained at the hot-plate set point temperature of 50 °C and the approximate cavity pressure of 0.042 Pa was achieved. An experimentally and theoretically validated finite volume model (FVM) was utilised. The centre-of-pane and total thermal transmittance values are calculated to be 0.91 Wm−2K−1 and 1.05 Wm−2K−1, respectively for the VIG. FVM results predicted that by reducing the width of vacuum edge seal and emissivity of coatings the thermal performance of the VIG is improved.

Active Plasmonics in Kirigami Configurations Toward High-Performance Smart Windows

Architectural windows that smartly regulate the indoor solar irradiation are promised to economize the building energy consumption. Here we demonstrate a method for adaptive, broadband, and high-efficient solar modulation for energy-efficient smart windows through the active plasmonics in kirigami structures. We develop a kirigami-inspired elastomer containing plasmonic vanadium dioxide (VO2) nanoparticles, in which the geometrical transition and the temperature-dependent localized surface plasmon resonance (LSPR) present dominant controls in ultraviolet-visible and near-infrared regions, respectively. The active LSPR control via stretch-induced local dielectric changes is mitigated on kirigami metamaterials due to their unique strain distributions. This method was demonstrated a decent property in energy-efficient smart windows with a decent solar energy modulation (37.7%), surpassing the best reported passive and transparent VO2 thermochromism systems. This first attempt to integrate the plasmonics and kirigami may inspire developments in smart windows, building energy economization, as well as fundamental studies of plasmonic controls in metastructures.

Surface and interface engineering for VO2 coatings with excellent optical performance: From theory to practice

Known as “smart windows”, energy-efficient thermochromic coatings on architectural glasses have attracted great attention due to the modulation ability of solar irradiation in response to change of ambient temperature. As the most promising thermochromic inorganic substance, vanadium dioxide (VO2)-based smart windows have been widely investigated over the past few decades. However, pricey preparation process and poor optical performance (including low luminous transmittance as well as solar modulation efficiency) still limit its commercialization process. To date, various strategies have been proposed to tackle these issues. These studies have been mainly focused on antireflection, which are supposed to have enormous potential to make a further breakthrough in improving light transmission. Meanwhile, the localized plasmon surface resonance of the VO2 NPs has been gradually applied to boost solar regulation efficiency of smart windows. In this review, the basic principles of antireflection and thermochromism of coatings for smart windows are firstly discussed. Then, the state-of-the-art VO2-based thermochromic coatings fabrication strategies through aqueous process for improving the visible transparency and modify thermochromic performance, especially surface engineering techniques, are reviewed in detail. Afterwards, since the assessment of the thermochromic coatings’ control ability to thermal radiation is vital for their practical application, multiform energy-saving measurements are presented Finally, the current challenges and perspectives of VO2-based coatings in energy-efficient smart windows are discussed.

Triazole-based ionene exhibiting tunable structure and ionic conductivity obtained via cycloaddition reaction: A new polyelectrolyte for electrochromic devices

Main-chain poly(ionic liquid)s based on 1,2,3-triazole with pentaoxyethylene spacer was developed via copper-catalyzed alkyne-azide cycloaddition (CuAAC) of a novel azide/alkyne-terminal monomer and subsequently quaternized by alkyl halides followed by anion exchanges with several fluorinated salts. Introduction of the extended ethylene oxide fragments [–(CH2CH2O)5–] has made this new PIL with methyl substituent and bis(trifluoromethane)sulfonimide counteranion exhibit ionic conductivity of 1.16 × 10−4 S cm−1 at 30 °C. This new electrolyte efficiently switches an electrochromic device (ECD) with 18% optical contrast from its transparent state to a colored state in 4.75 s and while bleaching takes 11.8 s. The development of this polyelectrolyte with simple structure and excellent physical and thermal properties is promising for the design of new all-solid state energy efficient smart windows and devices and other electrochemical device applications.

Improved Electrochromic Characteristics of a Honeycomb-Structured Film Composed of NiO.

Color changes controlled by electronic energies have been studied for many years in order to fabricate energy-efficient smart windows. Reduction and oxidization of nickel oxide under the appropriate voltage can change the color of a window. For a superior nickel oxide (NiO) electrochromic device (ECD), it is important to control the chemical and physical characteristics of the surface. In this study, we applied polystyrene bead templates to nickel oxide films to fabricate a honeycomb-structured electrochromic (EC) layer. We synthesized uniform polystyrene beads using the chemical wet method and placed them on substrates to create honeycomb-structured NiO films. Then, the EC characteristics of the nickel oxide films with a honeycomb structure were evaluated with UV-Visible and cyclic voltammetry. FE-SEM and AFM were used to measure the morphologies of the nanostructures and the efficiencies of the redox reactions related to the specific surface area.

Transformation of crystalline structure and photoelectric properties in VO2/glass thin films by inserting TiO2 buffer layers

Vanadium dioxide (VO2) with reversible metal–insulator transition (MIT) is a promising energy-saving material for next-generation smart windows and infrared devices. However, the specific applications are largely limited by controllable preparation with different polymorphs on common glass substrates. Herein, VO2 thin films were prepared on transparent amorphous glass substrates by inserting TiO2 buffer layers using pulsed laser deposition (PLD) technique. The results showed that B-VO2 film would be formed on the amorphous glass substrate at 400 °C. While after inserting TiO2 buffer layer, pure M-VO2 films with clear phase transition properties were obtained. Based on the analysis of X-ray Diffraction (XRD) and Energy Dispersive Spectrometer (EDS) characterization, the crystalline transformation was attributed to the template effect of TiO2 layer and Ti ions diffusion. The characteristics of VO2/TiO2/glass thin films were closely associated with the thickness of TiO2 buffer layer. With 60 nm TiO2 buffer layer, the VO2/TiO2/glass film showed the sharpest resistance change with more than 2.5-order of magnitude across the MIT, and the T-vis value is as high as 53% with the ΔTsol up to 5.2%. Our current results demonstrated the importance of TiO2 buffer layer to the formation of M-VO2 film on amorphous glass substrate, which was very meaningful for the production of energy efficient smart windows in the future.

Nanohybridization of molybdenum oxide with tungsten molybdenum oxide nanowires for solution-processed fully reversible switching of energy storing smart windows

A multi-functional electrochromic material which modulates light over a wide spectral range and stores electrical energy is an intriguing substance, which would lead to a variety of novel applications, such as smart windows for energy-efficient buildings, solar power storage, nonemissive displays, electronic paper, optoelectronic switches, and variable-reflectance mirrors/surfaces. Developing such a versatile material entails merging the electrochromic phenomenon with electrical-chemical energy conversions in an electrochromic-battery platform. However, the challenges related to transition metal oxide electrochromic-batteries include poor energy storage, low optical contrast, and high fabrication cost. Here, we demonstrate the synthesis of aqueous nanocrystalline colloidal molybdenum oxide in a matrix of tungsten molybdenum oxide (MoO3-W0.71Mo0.29O3) for solution-processed fully reversible switching electrochromic-battery electrodes. The hybrid material exhibits both 50% optical contrast modulation over a wide optical bandwidth and an improved Columbic efficiency. An 8 × 8 cm2 energy storing smart (ESS) window is assembled to demonstrate a functional prototype that can power an LED for more than 10 min when charged under −2.5 V for 1 min. The enhanced electrochromic performance, high energy storage capacity, simple solution processability, versatility for large area fabrication, and low cost make this nanocrystalline MoO3-W.71Mo.29O3 composite a promising material with the potential to be deployed ubiquitously in energy-efficient smart window architectures.

Transparent and flexible Sb-doped SnO2 films with a nanoscale AgTi alloyed interlayer for heat generation and shielding applications.

Transparent and flexible Sb-doped SnO2 (ATO) films with a nanoscale AgTi alloyed interlayer were fabricated for use as plasma damage-free, indium-free, thermally stable electrodes for high performance heat generating films and shielding films in smart windows. The AgTi alloy-inserted ATO film on a PET substrate showed a low sheet resistance of 6.91 ohm per square and a high optical transmittance of 90.24% without thermal annealing or intentional substrate heating. Even after deformation using an outer bending radius of 4 mm, the ATO film with a AgTi interlayer showed a constant sheet resistance due to the mechanical robustness of the AgTi interlayer. Furthermore, the AgTi-inserted ATO film showed a constant resistance even after annealing at 500 °C, unlike the Ag-inserted ATO films. Furthermore, we demonstrated the feasibility of the AgTi-inserted ATO films as transparent heat generating films and shielding films for smart windows. The effective heat generation and shield performance of the ATO/Ag–Ti/ATO multilayer suggests that the multi-functional ATO/Ag–Ti/ATO films can be used to create energy-efficient smart windows for building energy management systems and automobiles.

Hydrothermal Synthesis of VO2 Polymorphs: Advantages, Challenges and Prospects for the Application of Energy Efficient Smart Windows.

Vanadium dioxide (VO2 ) is a widely studied inorganic phase change material, which has a reversible phase transition from semiconducting monoclinic to metallic rutile phase at a critical temperature of τc ≈ 68 °C. The abrupt decrease of infrared transmittance in the metallic phase makes VO2 a potential candidate for thermochromic energy efficient windows to cut down building energy consumption. However, there are three long-standing issues that hindered its application in energy efficient windows: high τc , low luminous transmittance (Tlum ), and undesirable solar modulation ability (ΔTsol ). Many approaches, including nano-thermochromism, porous films, biomimetic surface reconstruction, gridded structures, antireflective overcoatings, etc, have been proposed to tackle these issues. The first approach-nano-thermochromism-which is to integrate VO2 nanoparticles in a transparent matrix, outperforms the rest; while the thermochromic performance is determined by particle size, stoichiometry, and crystallinity. A hydrothermal method is the most common method to fabricate high-quality VO2 nanoparticles, and has its own advantages of large-scale synthesis and precise phase control of VO2 . This Review focuses on hydrothermal synthesis, physical properties of VO2 polymorphs, and their transformation to thermochromic VO2 (M), and discusses the advantages, challenges, and prospects of VO2 (M) in energy-efficient smart windows application.

(Invited) Durability of Electrochromic Films: Aging Kinetics and Rejuvenation

A major challenge for energy-efficient smart window technology is to ensure the durability of electrochromic (EC) devices over a service life of more than 20 years. In this paper, we report recent results from a fundamental study of the aging kinetics of EC tungsten oxide and nickel oxide thin films and describe electrochemical rejuvenation mechanisms that are able to restore the films to their initial state. The aging kinetics displays an approximate power-law decrease of the charge capacity as a function of cycle number. This decay of charge capacity can be understood in terms of models built on so-called dispersive chemical kinetics. Tungsten oxide and nickel oxide EC films can be rejuvenated by applying a high electrochemical potential or a small constant current. Trapped ions in the bulk or at the surface of the films can be released by these procedures.

(Invited) Durability of Electrochromic Films: Ageing Kinetics and Rejuvenation

A major challenge for energy-efficient smart window technology is to ensure the durability of electrochromic (EC) devices over a service life of more than 20 years. Hence, their degradation under operating conditions must be better understood and preferably avoided. In this paper we review our recent results from a fundamental study of the ageing kinetics of EC tungsten oxide and nickel oxide thin films, as well as electrochemical rejuvenation mechanisms that are able to restore the films to their initial state. The ageing of tungsten oxide films upon cycling in the range 2-4 V vs. Li/Li+ displays a power-law decrease of the charge capacity as a function of cycle number. In the case of severe ageing at potentials below 2.0 V there is evidence for a superposition of two processes. In the case of nickel oxide the ageing process seems to be more complex and it can be described by either a power-law or a stretched exponential function. The ageing of nickel oxide was also observed to decrease and even stop after a few thousand cycles just prior to final breakdown of the film. The decay of charge capacity can be understood in terms of models built on so-called dispersive chemical kinetics. The term “rejuvenation” describes electrochemical processes that may restore aged films to full EC functionality. During severe ageing of tungsten oxide coatings, Li ions are trapped in the film, and subsequently these ions can be released by application of a high electrochemical potential for a few hours. The trapping/detrapping of Li has been verified by Elastic Recoil Detection Analysis (ERDA) measurements. Very recently we have found that a similar process can be applied to EC nickel oxide films, although using a lower potential than for the case of tungsten oxide. For nickel oxide, the rejuvenation mechanism is not yet clear, but ERDA measurements are in progress and are expected to give detailed information. An increased understanding of ageing and rejuvenation will facilitate large-scale practical application of electrochromic materials. Our phenomenological models of ageing may make it possible to predict the service life of EC devices using empirical relationships. An understanding of rejuvenation processes will be of major importance in order to prevent or minimize performance decrease under extended cycling. The next step will be to investigate whether rejuvenation treatments can be applied successfully to full EC devices.

A roll-to-roll process for multi-responsive soft-matter composite films containing CsxWO3 nanorods for energy-efficient smart window applications.

This work provides a roll-to-roll processed flexible multi-responsive smart film containing tungsten bronze nanorods and a phase-separated liquid crystal-polymer composite, which can reversibly control the passage of visible light in response to temperature, an electric field and near infrared light, and also screen the heat rays from 800 nm to 2500 nm.

Optimization of VO2 nanowire polymer composite thermochromic films by optical simulation

Thermochromic films with high efficiency, transparency, and flexibility are highly desirable for energy-efficient smart window films. Vanadium oxide (VO2)-nanoparticle-embedded flexible polymer composite films are the most promising thermochromic films because of the sharp phase transition of insulating to metallic phases of VO2 at 68 °C with visible transparency and a large change in transmittance at near-infrared wavelengths before and after the metal–insulator phase transition. This paper describes the simulation of high-efficiency thermochromic polymer composite films embedded with VO2 nanoparticles of various sizes to investigate the optimum VO2 nanowire size and length.

Electrochromic Devices Fabricated with Electrolyte Impregnated on Porous Substrate

Electrochromic devices, which dynamically change colour under applied potential, are widely studied for use in energy-efficient smart windows. To improve the viability of smart windows, many researchers are utilizing nanomaterials, which can provide electrochromic devices with improved coloration efficiencies, faster switching times, longer cycle lives, and potentially reduced costs. The flexible electrochromic films are promising film for applications in automobiles, buildings, etc. Here, we synthesized nanostructured tungsten trioxide (WO3) particles for electrochromic materials. The thin layers with nanostructured materials of WO3 inks were coated on ITO glass at room temperature, solution-processed method. For comparing the effects of electrolyte, liquid electrolytes were used with/without a porous substrate. Porous substrates can be helpful in leakage issues of liquid electrolytes. The porosity and pore size of porous substrate were parameters for characterization of electrolyte. The coloration efficiencies and cycle performance will be discussed with characteristics of porous substrate.

Electrochromic Devices with Ionic Liquid Based Electrolyte By Room Temperature Process

Electrochromic devices (EC), which dynamically change colour under applied potential, are widely studied for use in energy-efficient smart windows. To improve the viability of smart windows, many researchers are utilizing nanomaterials, which can provide electrochromic devices with improved coloration efficiencies, faster switching times, longer cycle lives, and potentially reduced costs. The flexible electrochromic films are promising film for applications in automobiles, buildings, etc. Here, we synthesized nanostructured tungsten trioxide (WO3) particles for electrochromic materials. The thin layers with nanostructured materials of WO3 inks were coated at room temperature, solution-processed method. For comparing the effects of electrolyte, electrochromic devices were fabricated with various kind of electrolyte, which can be optimized to obtain higher performances. Two types of polymer based electrolytes were fabricated and characterized. The operation voltages were controlled with electrolyte types and compositions. Ionic liquid based electrolyte showed higher ionic conductivity than liquid type or polymer type electrolyte. Also EC with the liquid electrolyte exhibited 40% of coloring/bleaching efficiencies and EC with the polymer gel electrolyte 60%. All solid EC could be fabricated with the polymer gel electrolyte, which exhibited the higher coloring / bleaching efficiencies than that with the liquid. The effects of composition and process variables will be discussed.