Thermochromic Smart Windows Research Articles

Thermochromic performance enhancement by geometric structure optimization of VO2 nanoarrays for smart window application

The localized surface plasmon resonance (LSPR) exhibited by vanadium dioxide (VO2) has expanded its utility in thermochromic smart windows. Here, guided by the absorption characteristics of a single VO2 nanoparticle (NP) with a radius between 10 to 60 nm on a glass substrate, we numerically conducted an extensive analysis of the optical properties of VO2 nano-cylinder arrays. We achieved a significant enhancement in the performance of thermochromic smart windows by optimizing the geometric structure of the VO2 arrays. Such a performance enhancement relies on the on-off switch properties of LSPR in VO2 nano-cylinder arrays. Smaller (r≤ 60) ones with high aspect ratios (> 0.75) shows a higher absorption contribution in their extinction effect and a more energetic LSPR peak located in the near-infrared spectra. Those characteristics significantly improve the thermochromic performance of smart windows via enhancing the transmission contrast between the high-temperature rutile state and the low-temperature monoclinic state of VO2. At a transmittance luminous (Tlum) efficacy of 60.03%, an excellent 20.03% in solar spectrum modulation ability (ΔTsol) can be attained. Compared to flat films with an identical Tlum, ΔTsol of the periodically cylindrical-structured film is raised by 234%.

Freezing-resistant poly(N-isopropylacrylamide)-based hydrogel for thermochromic smart window with solar and thermal radiation regulation

Adaptive regulation of solar and thermal radiation by windows in diverse (hot and cold) climates is essential to reduce building energy consumption. However, conventional hydrogel-based thermochromic smart windows lack thermal radiation regulation, and have difficulty to combine high solar regulation with excellent freezing resistance. It is challenging to integrate the above performance into one hydrogel-based thermochromic window. Here, we firstly prepared poly(N-isopropylacrylamide-co-N, N-dimethylacrylamide)/ethylene glycol (PNDE) hydrogels with tunable and excellent freezing resistance (below −100 °C) by adding the anti-freezing agent ethylene glycol, and assembled PNDE hydrogels, polyvinylidene fluoride and polymethyl methacrylate-silver nanowires panels into a freezing-resistant smart window with solar and thermal radiation regulation (STR). PNDE hydrogels had an excellent thermochromic performance with luminous transmittance (Tlum) of 89.3 %, solar regulation performance (ΔTsol) of 80.7 % and tunable phase change temperature (τc, 22–44 °C). The assembled STR window showed high Tlum of 68.2 %, high ΔTsol of 62.6 %, suitable τc of ∼30 °C and freezing resistance to low temperature of −27 °C. Moreover, the different thermal emissivity (0.94 and 0.68) of the two sides of the STR window gave it the ability of radiative cooling in hot climates and warm-keeping in cold climates. Compared to the conventional thermochromic windows, the STR window promotes heat dissipation in hot conditions while reduces heat loss in cold conditions and is applicable to diverse climates, which is a promising energy-saving device for reducing building energy consumption.

Thermochromic smart window utilizing passive radiative cooling for Self-Adaptive thermoregulation

Being a major impediment, the development of advanced smart windows with exceptional thermoregulation in both the visible-near infrared (Vis-NIR) and the long-wave infrared remains a significant aspiration to scholars. Herein, an innovative strategy was implemented to achieve self-adaptive smart window that seamlessly integrates the thermochromic and radiative cooling technologies (TRSW), and holds great potential in effectively addressing the intricate thermal radiation requirements arising from unpredictable climatic conditions. The TRSW demonstrated an impressive capacity to regulate solar energy through automatic modulation of emissivity (ε) at both high (εLWIR-H = 0.68) and low (εLWIR-L = 0.35) temperatures, while effectively maintaining Vis-NIR regulation (20.12%). Moreover, the results of TRSW’s outdoor performance exhibited its efficiency in implementing dual solar energy transmission modes, effectively inhibiting the Vis-NIR transmission to mitigate solar overheating in high-temperature conditions, while diminishing heat dissipation and promoting radiative cooling in low-temperature situations. Compared to conventional cooling materials, the dynamic emissivity modulation of TRSW provided an effective approach to prevent overcooling at lower temperatures. Consequently, TRSW fostered an efficient synergy between the solar transmittance modulation (ΔTsol) and Δε, which led to heightened modulation of the entire spectrum and facilitated exceptional thermal management. This work paves an intriguing prospect of developing advanced smart windows with remarkable thermoregulation and promises a wide range of thermal regulation applications, such as windows, roofs, walls, and advanced textiles.

Employing Ion-Beam Sputter Deposited TiO2 Buffer Layers for VO2-Related Devices

Vanadium dioxide (VO2) is considered to be the most promising material for thermochromic smart windows due to its insulator-to-metal transition causing an abrupt change of transmittance near ambient temperature. TiO2 buffer layers are expected to have a beneficial impact on the growth and performance of the functional VO2 thin film, as they may induce crystallinity at lower growth temperatures and act as a barrier layer for ion diffusion from glass substrates. However, a systematic evaluation of TiO2 buffer layers of different phases and thicknesses to assess potentially competing impacts is still lacking. Here, we grow TiO2 buffer layers by ion-beam sputter deposition (IBSD) at different substrate temperatures to obtain amorphous TiO2 as well as crystalline TiO2 in the anatase and rutile phase, respectively. We investigate the buffer layers themselves and shed light on how these buffer layers influence the performance of the functional VO2 layer grown on top by combining experimental and theoretical means. We find that the VO2||TiO2 interface is the most crucial part of the multilayer structure with an urgent need for minimum roughness. The two types of crystalline TiO2 buffers yield similar results in terms of optical performance; however, we find that only rutile buffer layers significantly relieve the growth conditions needed.

Nanowire-based smart windows combining electro- and thermochromics for dynamic regulation of solar radiation

Smart window is an attractive option for efficient heat management to minimize energy consumption and improve indoor living comfort owing to their optical properties of adjusting sunlight. To effectively improve the sunlight modulation and heat management capability of smart windows, here, we propose a co-assembly strategy to fabricate the electrochromic and thermochromic smart windows with tunable components and ordered structures for the dynamic regulation of solar radiation. Firstly, to enhance both illumination and cooling efficiency in electrochromic windows, the aspect ratio and mixed type of Au nanorods are tuned to selectively absorb the near-infrared wavelength range of 760 to 1360 nm. Furthermore, when assembled with electrochromic W18O49 nanowires in the colored state, the Au nanorods exhibit a synergistic effect, resulting in a 90% reduction of near-infrared light and a corresponding 5 °C cooling effect under 1-sun irradiation. Secondly, to extend the fixed response temperature value to a wider range of 30–50 °C in thermochromic windows, the doping amount and mixed type of W-VO2 nanowires are carefully regulated. Last but not the least, the ordered assembly structure of the nanowires can greatly reduce the level of haze and enhance visibility in the windows.

基于CsPbIBr2的全无机半透明器件的光伏性能与热 致变色性能

The photovoltaic thermochromic smart window, which combines perovskite with thermochromic and photovoltaic properties, can not only control the solar transmittance at different temperatures, but also convert the absorbed light energy into usable electric energy. Herein, we designed a fluoride-doped tin oxide (FTO)/SnO2/CsPbIBr2/NiOx/indium tin oxide (ITO) semitransparent solar cell structure by using the optimized post-annealing process and obtained an average visible transmittance (AVT) of 33.2% and a power conversion efficiency of 3.89% under the optimal conditions. Meanwhile, based on the thermochromic properties of CsPbIBr2, it was found that the AVT of the device improved from 33.2% to 73.5% when CsPbIBr2 converted from the colored high-temperature phase to the transparent low-temperature phase. Besides, the transition from high-temperature phase to low-temperature phase is humidity-dominated. When the relative humidity rises from 50% to 90%, the phase transition time of perovskite decreases substantially from 1005 to 17 min. However, the excessively fast phase transition caused by high humidity will destroy the thin film morphology and crystal structure of perovskite, causing irreversible damage to CsPbIBr2 perovskite. This work is promising for the application of semitransparent photovoltaic and thermochromic smart window, which represents an attractive prospect in further reducing the energy consumption of buildings and automobiles.

Printable Thermochromic Hydrogel‐Based Smart Window for All‐Weather Building Temperature Regulation in Diverse Climates (Adv. Mater. 20/2023)

Hydrogel Smart Windows In article number 2211716, Xiancai Jiang, Jun Fu, and co-workers report a novel thermochromic hydrogel-based smart window for all-weather building-temperature regulation in diverse climates. The hydrogel smart windows spontaneously transform from transparent to opaque upon temperature changes. The outdoor solar radiation in summer and indoor thermal radiation in winter are both regulated by the smart window to modulate the building temperature and save energy.

Design of Antireflection and Enhanced Thermochromic Properties of TiO2/VO2 Thin Films

AbstractVO2‐based thin films exhibit great potential applications in thermochromic smart windows. However, it is still a challenge to synergistically achieve the high luminous transmittance (Tlum) and large solar modulation (ΔTsol). In this paper, antireflective (AR) TiO2/VO2 thin films are designed through optical simulations, and the thin films with optimized film thickness are prepared by magnetron sputtering. The overall performances of the thin films are significantly improved. Compared with single‐layer VO2, the Tlum value of the TiO2/VO2 thin film is significantly increased from 29.03% to 46.29% by 59.5%, and the ΔTsol value is increased up to 16.03%. The Tlum and ΔTsol values of the TiO2/VO2/TiO2 sandwiched thin films can be further improved up to 50.49% and 20.11%, respectively, owing to the light interference at the interfaces between the multilayers. The results provide an effective strategy for improving the performance of smart windows.

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.

Printable Thermochromic Hydrogel-Based Smart Window for All-Weather Building Temperature Regulation in Diverse Climates.

Thermochromic smart windows are widely developed to modulate building energy exchange to save building energy consumption. However, most smart windows have fixed working temperatures, moderate energy-saving efficiency, and are not suitable for diverse (cold and hot) climates. Here smart windows with strong temperature modulation over a broad range of hydrogels with adjustable transition temperatures for all-weather building temperature regulation in different climates are reported. Thermochromic poly(N-isopropylacrylamide-co-N, N-dimethylacrylamide) hydrogels, with lower critical transition temperatures ranging from 32.5 to 43.5°C, are developed for smart windows with solar modulation up to 88.84% and intrinsic transmittance up to 91.30% over full spectrum without energy input. Simulated indoor investigations are performed in different cities from 23 °N to 39 °N from winter to summer. The results indicate that smart windows have a strong solar modulation in summer to reduce indoor temperature up to 7.3°C and efficient heat conservation in winter to save energy up to 4.30Jm-3 , in comparison to glass windows. Smart windows with grid patterns and Chinese kirigami are fabricated by using 3D printing of the hydrogels to achieve both solar modulation and light incidence. The strategy offers an innovative path for thermochromic smart windows for low carbon economy.

Thermochromic Ni(II) Organometallics With High Optical Transparency and Low Phase-Transition Temperature for Energy-Saving Smart Windows.

Thermochromic smart windows with rational modulation in indoor temperature and brightness draw considerable interest in reducing building energy consumption, which remains a huge challenge to meet the comfortable responsive temperature and the wide transmittance modulation range from visible to near-infrared (NIR) light for their practical application. Herein, a novel thermochromic Ni(II) organometallic of [(C2 H5 )2 NH2 ]2 NiCl4 for smart windows is rationally designed and synthesized via an inexpensive mechanochemistry method, which processes a low phase-transition temperature of 46.3 °C for the reversible color evolution from transparent to blue with a tunable visible transmittance from 90.5% to 72.1%. Furthermore, cesium tungsten bronze (CWO) and antimony tin oxide (ATO) with excellent NIR absorption in 750-1500 and 1500-2600nm are introduced in the [(C2 H5 )2 NH2 ]2 NiCl4 -based smart windows, realizing a broadband sunlight modulation of a 27% visible light modulation and more than 90% of NIR shielding ability. Impressively, these smart windows demonstrate stable and reversible thermochromic cycles at room temperature. Compared with the conventional windows in the field tests, these smart windows can significantly reduce the indoor temperature by 16.1 °C, which is promising for next-generation energy-saving buildings.

Gradient Variation Oxygen-Content Vanadium-Oxygen Composite Films with Enhanced Crystallinity and Excellent Durability for Smart Windows.

Vanadium dioxide (VO2)-based smart windows show excellent promise for energy-saving and have been extensively researched. However, for the glass industry-compatible magnetron sputtering process, VO2 films are difficult to obtain and have homogeneous crystalline state, leaving them lacking the ideal solar modulation (ΔTsol) and sensitivity (narrow hysteresis loop). More importantly, the instability of VO2 hinders its commercialization. Multilayer structures have been repeatedly investigated to solve these problems. Unfortunately, the mediocre thermochromic properties as well as the complex and expensive manufacturing steps still hinder its commercialization. In this work, we prepared gradient variation oxygen-content vanadium-oxygen composite films (V2O3/VO2/V2O5, VOgv) with enhanced crystallinity and excellent durability by one-step continuous sputtering. According to optical measurements, the ΔTsol of the VOgv films was significantly increased by 145% (from 6.85 to 16.80%) compared to VO2 films, and the width of the hysteresis loop was reduced by 67% (from 19.34 to 6.36 °C), while the VOgv films exhibited a wider preparation window. The accelerated tests have shown that the film has an equivalent service life of approximately 20 years. We exploited the intrinsic similarity in properties of homologous compounds of vanadium oxide and simplified the preparation process, which is supposed to break the existing application bottlenecks and increase the commercializing possibility of VO2-based thermochromic smart windows.

Novel Photothermochromic Smart Window Based on PNIPAm‐glass‐MXene/PAM with High Shield, Fast Response, and Excellent Stability

In recent years, smart windows have emerged as an important method to achieve energy efficiency in buildings. The responses of thermochromic smart windows are very dependent on the photothermal absorption of nanoparticles. However, nanoparticles dispersed in thermochromic materials cause high thermal loads on the window's surface and become nonuniform and unstable with time. These problems seriously limit application of photothermochromic smart windows. Herein, a new photothermochromic smart window is designed using bicomponent poly (N‐isopropylacrylamide) PNIPAm and MXene/polyacrylamide (PAM), denoted as P‐g‐M smart window. Compared with other photothermochromic smart windows, the developed P‐g‐M smart window reduces the window's surface temperature, and there is no obvious visibility decrement due to the long‐term stable dispersion of MXene. The small particle size of PNIPAm microgels synthesized by the one‐step method shows high scattering efficiency, and the fabricated 1‐mm PNIPAm displays 91.2% visible light transmittance and 99.2% solar light shielding. While pursuing efficient light management, the P‐g‐M smart window responds 25 min faster than a 1 mm PNIPAm smart window. In addition, the device effectively improves the hydrogel thermal response stability. The indoor and outdoor energy‐saving demonstration shows that P‐g‐M smart windows can reduce indoor air temperature by 5–10 °C compared with normal windows.

High-transmittance pNIPAm gel smart windows with lower response temperature and stronger solar regulation

Improving the solar regulation and reducing the response temperature of thermochromic smart windows are pivotal importance for energy-saving buildings. However, such two strategies are challenging to be integrated into one window. Herein, a solution for simultaneously regulating the surface tension of the scattering center and the capturing-releasing capacity of pNIPAm for water is implemented, successfully carrying out the enhancement of solar regulation and the reduction of response temperature. The maximum solar shielding and regulation of the modified sandwich structured pNIPAm gel device can reach up to 88.45% and 71.81%, an increase of 52.29% and 73.71% respectively, compared to a pure pNIPAm gel device with the same thickness. Besides, the response temperature is reduced to around 28 ℃, making modified devices have the ability to respond promptly at the comfortable room temperature range boundary (22–28 ℃). Different from flowing, strengthless modified doped microgels, the high-transmittance modified pNIPAm gels maintain the stable patterning performance, and have great potential for further applications. Similar conclusions are obtained using different solvents to reduce the attraction between pNIPAm-water, and the surface tension of water. Therefore, this universal modification framework deserves attention for the future design of energy-saving windows.

Smart Window Structures Based on Highly Conductive, Transparent Metal Nanomeshes and Thermochromic Perovskite Films

AbstractSmart windows are energy‐efficient windows whose optical transparency can be switched between highly transparent and opaque states in response to incident solar illumination. Transparent and conductive metal nanomesh (NM) films are promising candidates for thermochromic smart windows due to their excellent thermal conductivity, high optical transparency at near infrared wavelengths, and outstanding stability. In this study, ZnO/Au/Al2O3 NM films with periodicities of 200 and 370 nm are reported. The ZnO/Au/Al2O3 NM film with a 370 nm periodicity exhibits a transmittance over 90% at 550 nm and sheet resistance lower than 20 Ω sq−1. Based on a standard figure of merit, this structure outperforms current state‐of‐the‐art NM films. Here, the integration of ZnO/Au/Al2O3 NM films into a thermochromic perovskite smart window is also demonstrated. The transparency of the smart window structure is manipulated by transient resistive heating to trigger the thermochromic transition to the opaque state, which can be then maintained solely by 1‐sun, AM 1.5 G illumination. This climate‐adaptive, low power‐activated, and fast‐switching smart window structure opens new pathways toward its practical application in the real world.

Review on Mechanoresponsive Smart Windows: Structures and Driving Modes.

The growing awareness about the global energy crisis and extreme weather from global warming drives the development of smart windows market. Compared to conventional electrochromic, photochromic, or thermochromic smart windows, mechanoresponsive smart windows present advantages of simple construction, low cost, and excellent stability. In this review, we summarize recent developments in mechanoresponsive smart windows with a focus on the structures and properties. We outline the categories and discuss the advantages and disadvantages. Especially, we also summarize six unconventional driving modes to generate mechanical strain, including pneumatic, optical, thermal, electric, magnetic, and humidity modes. Lastly, we provide practical recommendations in prospects for future development. This review aims to provide a useful reference for the design of novel mechanoresponsive smart windows and accelerate their practical applications.

Thermo-Responsive Poly(N-isopropylacrylamide)/Hydroxypropylmethyl Cellulose Hydrogel with High Luminous Transmittance and Solar Modulation for Smart Windows.

Thermochromic smart windows are considered to be promising energy-saving devices for reducing energy consumption in buildings. The ideal materials for thermochromic smart windows should have high transmittance, high solar modulation, low phase-transition temperature, and excellent high-temperature thermal stability, which are difficult to achieve simultaneously. This work reports a simple one-step low-temperature polymerization method to prepare a thermo-responsive poly(N-isopropylacrylamide)/hydroxypropylmethyl cellulose (PNIPAM/HPMC) hydrogel achieving the above performances simultaneously. The low-temperature polymerization environment endowed the hydrogel with a high luminous transmittance (Tlum) of 90.82%. HPMC as a functional material effectively enhanced the mechanical properties and thermal stability of the hydrogel. Meanwhile, the PNIPAM/HPMC hydrogel showed a low phase-transition temperature (∼32 °C) and high solar modulation (ΔTsol = 81.52%), which proved that it is an ideal material for thermochromic smart windows. Moreover, a PNIPAM/HPMC smart window exhibited high light transmittance (T380-760 = 86.27%), excellent light modulation (ΔT365 = 74.27%, ΔT380-760 = 86.17%, and ΔT940 = 63.93%), good indoor temperature regulation ability and stability, which indicated that it was an attractive candidate for application in reducing energy consumption in buildings. This work also provides an option and direction for modifying PNIPAM-based thermochromic smart windows.

Dual-function smart windows with dynamic and fast thermal response for building energy-saving/storage

Thermochromic smart windows are considered an important element for increasing building energy efficiency. However, the phase transition of conventional devices only depends on the surrounding temperature with solar irradiation, which limits their application under different conditions. Here, a thermoelectrochromic dual response device has been designed by combining thermochromic hydroxypropyl cellulose/potassium chloride (HPC/KCl) hydrogel electrolyte and optothermal electrochromic polyaniline (PANI) films. The thermoelectrochromic device (T-ECD) exhibits electrochromic switchable colour tones (light yellow to purple), visible luminous transmittance (15.70–75.53%), and high solar modulation efficiency (60.89%) when this electrochromic switching is combined with thermochromic phenomenon arising from the electrolyte. Further, since PANI is able to store charge reversibly during the electrochromic transition, this device may also be used as a photovoltaic energy storage. Under solar irradiation, the photothermal effect of PANI films can drive the fast phase transition of HPC/KCl hydrogel. The phase transition time of T-ECD is 30 s which is much faster than 3 min of conventional TCD. In addition, a field test further demonstrates that the house model equipped with the T-ECD can reduce the indoor temperature by 13.3 °C compared with that using a traditional glass window. Thus, it is a valuable device to be used in buildings for energy-saving/storage situations. • An innovative T-ECD was designed and shows a wide range of T lum (15.70–75.53%), high ΔT sol (60.89%), low LCST (30 °C). • The T-ECD exhibited photothermal property to drive the fast phase transition (30 s). • Simulating real building-based T-ECD is 13.3 °C lower than that of using a traditional glass window.

Broadband light and heat management with a self-regulating and user-controlled thermochromic smart window

Broadband light and heat management with a self-regulating and user-controlled thermochromic smart window

Thermochromic smart windows with broad-range customizable responsive temperature via the Hofmeister effect

This work proposes a novel responsive temperature customization stratagem for thermochromic smart windows by tuning the low critical solution temperature of thermally responsive materials via the Hofmeister effect.