Chapter 16. Electrochromic Smart Windows for Green Building Applications

This study comprehensively evaluates the energy-saving potential and visual comfort aspects of electrochromic (EC) smart windows in a hot-humid climate office building. Using an advanced building simulation tool, EC windows are compared to conventional low-E glazed windows, considering two control triggers: daylighting level and glare control. The primary objective is to determine energy savings achievable with EC windows while addressing visual comfort. Detailed analysis of the building’s energy performance and indoor environment is conducted. Results show significant energy savings of 23% with EC windows using daylighting control but limited visual comfort in some zones. Conversely, EC windows with glare control achieve 17% energy savings while maintaining visual comfort throughout the building. These findings highlight the potential of EC windows with glare control in saving energy and maintaining visual comfort in hot-humid office buildings. Further research is needed to optimize performance for different building types and climates. In conclusion, this study provides insights into energy-saving capabilities and visual comfort considerations with EC smart windows, emphasizing the importance of appropriate control triggers for maximizing energy savings and occupant comfort. Future investigations should explore EC window performance across diverse building typologies and climates to enhance the benefits of this innovative technology.

In recent years, with the increasing demand for building energy efficiency and comfort in daily life, electrochromic smart windows have attracted widespread attention. Electrochromic smart windows utilize electrochromic material (ECM) technology to change the color or transparency of the window under the action of an electric field, thereby regulating the light and heat entering the room and achieving energy-saving effects. A bistable electrochromic material is a material that can switch between two color states and maintain one of these states without an external electric field. Although ECMs with bistable properties can significantly reduce energy consumption, current research on their application in smart windows is limited. Therefore, three conjugated extended viologen polymers with thiophene derivative bridges, PDV-MMA, PBV-MMA, and PTV-MMA, were designed and synthesized, and electrochromic devices (ECDs) were prepared. The introduction of thiophene derivatives not only prolongs the effective conjugated chain length of viologen, but also extends the electrochromic (EC) response to the near-infrared (NIR) region. In addition, by inserting different thiophene derivatives to separate the two pyridinium moieties, viologen polymers emit strong fluorescence that can be quenched by applying a negative voltage. The electrochromic device based on PDV-MMA material has fast response time (1.8 s), low turn-on voltage (-0.9 V), high color contrast (73.7%), high coloring efficiency (550 cm2/C), and cycling stability (3300 cycles). In addition, ECD based on PDV-MMA exhibits bistable characteristics, with only a 10% transmittance decay within 221 min, displaying clear patterns even after 20 h of power outage, and significantly reducing indoor temperature (7.0 °C) in simulated indoor environments. These characteristics make the application of electrochromic smart windows highly attractive in energy-efficient buildings. The study also demonstrated an intelligent window system driven by a solar cell (SC), further validating its feasibility in practical applications.

Windows through heat exchange play a vital role in energy saving for realizing a net‐zero carbon emission society. Dual‐band electrochromic (EC) smart windows by dynamically regulate visible (VIS) and near‐infrared (NIR) light are necessary for improving both building energy efficiency and habitant comfort. However, the rational design of high‐performance electrochromic devices (ECDs) suffers sluggish EC response due to the slow ion transport. In this work, a locally dissociated Li+ concept is proposed to construct a solid‐state polymer electrolyte (SPE) with ultrafast Li+ transport. The succinonitrile (SN) is employed to loosen the Li+‐anion pair and the crystallographic C─O chain in the poly(ethylene glycol) methyl ether methacrylate (PEGMA) electrolyte by its strong solvation capability. The as‐prepared SPE shows a high ionic conductivity of 6.48 mS cm−1 at 30 °C and a high transmittance of >90%. The SPE‐based EC smart windows exhibited a fast switching speed (3.0/3.2 s for coloration/bleaching), a high coloration efficiency (CE) of 373.8 cm2 C−1, and a high optical modulation in the full solar spectrum (85%, 70%, 43% at 673, 1200, and 1600 nm, respectively). Finally, the SPE‐based EC smart windows shows three working modes with a temperature regulation range of 19.1 °C, exhibiting great potential in practical application.

A diamine containing carbazole (Cz) and triphenylamine (TPA) groups, N-(4-(dibenzocarbazol-7-yl) phenyl)-N-(4-aminophenyl) benzene-1,4-diamine (DC7TPA-NH2), is designed and used to form three polyamides (PAs): DC7TPA-SA, DC7TPA-CA, and DC7TPA-OA through linear condensation with three dicarboxylic acids. The photophysical properties including UV–visible absorption spectra and photoluminescence (PL) emission spectra of three PAs are measured. Among them, DC7TPA-CA exhibits an intense PL emission. Upon comparison of UV–visible absorption spectra and PL emission spectra of DC7TPA-CA in diverse solvents, a consistent red shift in the PL emission peak is observed with increasing solvent polarity. Most notably, electrochromic (EC) performance testing revealed that this series of PA films exhibit optical contrast exceeding 90% and have good cycling stability. Specifically, the DC7TPA-SA film shows the highest optical contrast, reaching an impressive 95%. Meanwhile, due to the presence of ether oxygen bonds stabilizing free radicals, DC7TPA-OA demonstrates the best cycling stability among the three PAs. To further investigate its potential for application in EC smart windows (ECSWs), DC7TPA-OA is fabricated into an EC device (ECD). Testing reveals a maximum (initial) optical transmittance change (ΔT) of 88% with only a 3% decay after 240 cycles. Compared to other ECSWs, device of DC7TPA-OA consistently ranks among the top in terms of ΔT performance across all-most categories, whether inorganic ECDs, organic ECDs, COF-based ECDs or composite ECDs, and also demonstrates excellent response times and coloring efficiency (CE). Furthermore, the three PAs exhibit distinct color-changing behaviors, and DC7TPA-CA and DC7TPA-OA maintain a colorless neutral state. This property meets diverse application requirements, especially for ECSWs. Additionally, DC7TPA-CA possesses a high PL quantum efficiency (φ) and sensitivity to trinitrophenol (TNP), making it a promising candidate for nitroexplosive detection.

Electrochromic (EC) technology can modulate visible-infrared (VIS-IR) optical properties under electrical stimulation, showing broad application prospects in smart windows and so on. This work demonstrates a colorful electrochromic smart window by employing vanadium oxide V2O5 with polaronic effects and ITO films with localized surface plasmon resonance (LSPR) to realize outstanding VIS-IR dual-band optical modulation and EC performance. It shows yellow and green reversible color switching, fast response (3.0 s for VIS bleaching and 0.32 s for IR emissivity rising), high optical modulation amplitude with ΔTmax = 53.2% at 850 nm and Δεmax = 0.57 at 4.8 μm, and a long lifetime of 104 reversible cycles. The new design methods of the excellent stability VIS-IR modulation in this smart window emphasizes the role of multiband modulation for the diversification of all the EC applications.

Since its discovery in 2011, the emerging family of two‐dimensional (2D) transition metal carbides, nitrides, and carbonitrides (denoted as MXenes) has shown tremendous promise in the field of energy storages, especially electrochemical capacitors (ECs). The intercalation of foreign species, including but not limited to proton, cations, organic ions, and solvents, is demonstrated as one of the dominant facts during the energy storage process of MXenes, through which interlayer spacing of MXenes can be reversibly tuned, followed with reversible redox reactions on the surface of MXenes. Such a mechanism provides MXenes extremely high capacitance up to 1500 F cm−3, in aqueous acidic electrolyte. In addition, because of the versatile terminations on their surface, the intercalation of ions into MXenes can simultaneously induce the reversible transformation of their electronic structure to trigger some other phenomenon, for example, electrochromic effect, which has great potential on electrochromic smart window—an extended application of ECs. Accordingly, regulating and facilitating the intercalation in MXenes is of great significance for MXene‐based ECs. In this review, we summarize the recent progress on the intercalation in MXenes towards ECs, discussing on the intercalated species, working mechanisms, and methods to promote the intercalation. Furthermore, we prospect the future research directions of intercalation of MXenes in ECs, such as the combination of simulation and experiment on finding the best intercalation species, precisely controlling the interlayer spacing and beyond, to boost the electrochemical performance of MXene toward practical applications and multi‐functional devices.

Smart windows represent a promising technology that enables the selective transmission of light and heat, and electrochromism technology is gaining interest in smart window applications. Electrochromic (EC) smart windows are the preferred choice for outdoor applications due to their ability to withstand high temperatures. Particularly, Tungsten Trioxide (WO3) is commonly used as an electrochromic layer in EC devices. Although Titanium Dioxide (TiO2) is a less expensive EC material compatible with optoelectronics applications (including solar cells), it has received little research attention. In the course of this study, the sol-gel spin coating method was utilised to deposit a thin film of TiO2 onto Indium Tin Oxide (ITO). This technique was chosen for its simplicity, affordability, and ease of coating thin films. The pre-heating temperature demonstrated a critical role in sol-gel fabrication, particularly in electrochromic applications. As the impact of the pre-heating temperature remains poorly understood, this study effectively investigated the effect of various pre-heating temperatures on the performance of TiO2-based electrochromic thin films. Moreover, this study effectively analysed the structural, optical, and EC properties of the TiO2 thin films pre-heated at different temperatures.

Electrochromic Smart Windows Ultra-large-scale, high-performance electrochromic smart windows are prepared using TAIG method, which exploits the instability of black phosphorus to achieve the control of the structural water and oxygen vacancy content in WO3 films. The simplicity of TAIG makes the commercialization of smart windows to be highly accessible. More details can be found in article number 2409790 by Maofei Tian, Rongzong Zheng and Chunyang Jia.

The ever-increasing aesthetically driven demand for fully glazed façades poses a design challenge; not least in controlling the cooling demand and occupant well-being of such buildings, especially in a central Mediterranean climate. This paper outlines the ever-important need to design for occupants and for designers to keep in mind, first and foremost, occupant well-being rather than aim solely to create energy-efficient buildings. The original objective of buildings was to provide shelter. Today however, the need for occupant comfort and its direct effect on productivity cannot be ignored. This need, therefore, ought to feature a central role in any building design. Studies show that occupant well-being is directly related to a range of environmental factors, particularly daylight distribution, glare and indoor air temperature. The use of external shading devices and more commonly, indoor blinds are often the adopted approaches to attempt to achieve indoor occupant comfort, often to the detriment of views. Adaptive facades seek to address the need to somehow strike a balance between occupant comfort and energy efficiency. These facades range from exterior and interior shading devices with varying control strategies, to the different forms of adaptive/switchable glazing technologies intended to control the visual light transmittance and solar radiation transmitted into a building’s interior. In the opinion of the authors, electrochromic glazing has a great potential in a cooling-dominated central Mediterranean climate, to achieve a compromise between occupant visual and thermal comfort whilst retaining unobstructed outdoor views at all times. Research shows that the potential benefits of electrochromic glazing have not yet been studied enough in real-life scenarios,and this paper further introduces the objectives for field study within two identical offices, having a South-South-East orientation, located in a central Mediterranean climate.

Flexible electrochromic and thermochromic hybrid smart window based on a highly durable ITO/graphene transparent electrode

Electrochromic (EC) smart windows offer a dynamic and energy-efficient solution for managing solar radiation in buildings by reversibly modulating light transmittance in response to an applied voltage. Among EC materials, amorphous tungsten oxide (WO3) is widely recognized due to its broad optical modulation and well-defined W6+ to W5+ redox transitions. However, its practical deployment is hindered by slow switching kinetics, limited coloration efficiency, and structural degradation during long-term cycling. In this study, rare-earth (RE = Neodymium (Nd), terbium (Tb), and gadolinium (Gd)) doping was employed to tailor the structural, electronic, and ionic transport properties of amorphous WO3 thin films synthesized via a peroxo-assisted electrodeposition method. Structural and spectroscopic analyses confirmed uniform dopant incorporation, elevated oxygen vacancy concentrations, and the preservation of the amorphous network. Among all compositions, the Tb-doped WO3 (WTb) film exhibited the most favorable EC performance, achieving an optical contrast of 78.34%, coloration efficiency of 138.82 cm2 C-1, fast switching times of 16 s and 11.4 s, and a lithium-ion diffusion coefficient of 9.249 × 10-11 cm2 s-1. A large-area EC device based on WTb demonstrated strong optical modulation, excellent reversibility, and long-term cycling durability. These findings establish RE doping as a powerful design strategy for engineering high-performance WO3-based EC materials for next-generation smart windows.

Electrochromic (EC) smart windows are able to decrease our energy footprint while enhancing indoor comfort and convenience. However, the limited durability of these windows, as well as their cost, result in hampered market introduction. Here, we investigate thin films of the most widely studied EC material, WO3. Specifically, we combine optical measurements (using spectrophotometry in conjunction with variable-angle spectroscopic ellipsometry) with time-of-flight secondary ion mass spectrometry and atomic force microscopy. Data were taken on films in their as-deposited state, after immersion in a Li-ion-conducting electrolyte, after severe degradation by harsh voltammetric cycling and after galvanostatic rejuvenation to regain the original EC performance. Unambiguous evidence was found for the trapping and detrapping of Li ions in the films, along with a thickness increase or decrease during degradation and rejuvenation, respectively. It was discovered that (i) the trapped ions exhibited a depth gradient; (ii) following the rejuvenation procedure, a small fraction of the Li ions remained trapped in the film and gave rise to a weak short-wavelength residual absorption; and (iii) the surface roughness of the film was larger in the degraded state than in its virgin and rejuvenated states. These data provide important insights into the degradation mechanisms of EC devices and into means of achieving improved durability.

Multi-band electrochromic (MBEC) smart windows, capable of decoupled management of solar radiation (visible and near-infrared transmittance) and thermal radiation (mid- and long-wave infrared emissivity), are crucial for zero-energy buildings and adaptive optical systems. However, they have not yet been developed due to the inherent material design challenges and kinetics-stability trade-offs. Inspired by lotus root vessels, we develop vertically aligned nano-helix tungsten oxide (NH-WO3) films with amorphous-crystalline heterophase. The multiscale microstructure provides numerous active sites, enhances local electric fields, and alleviates stress during repeated cycling. Consequently, NH-WO3 achieves unprecedented quad-band electrochromic performances with decoupled photothermal modulation across the visible to long-wave infrared spectra, featuring large optical contrast (ΔTVIS-NIR ~ 79%), significant temperature and emissivity variation (ΔT = 9.3 ℃, Δε = 0.47@5 μm), ultrafast switching (3.9/1.9 s@780 nm, 2.8/2.5 s@990 nm), and robust cyclability (10,000 cycles with only 9.1% optical-contrast loss). Furthermore, a prototype smart window based on the NH-WO3 film maintains a comfortable indoor temperature of 26 ℃ even under continuous AM 1.5G solar irradiation for 1 hour. This bio-inspired nano-helix design provides an efficient strategy for the development of advanced next-generation photothermal management smart windows towards practical energy-efficient architectures and adaptive optics.

Due to notable part of buildings in the global energy consumption, it is of significant importance to design various scenarios to optimize building energy consumption. Smart windows including electrochromic types are some recently developed ideas that may help through this way controlling the solar radiation. Here a residential building is studied to analyze the performance of electrochromic smart windows in compare with simple single glazing and also triple glazing windows in the cities of Tehran and Yazd, as two choices of hot and dry climate. Results indicate that in the city of Tehran, when comparing with simple single glazing windows, in the case of use of electrochromic windows, 20% and 10% saving is observable in gas and electricity consumption, respectively. These are 17.1% and 7.2% when implementing triple glazing windows. In the city of Yazd also this trend has been experienced with 22% and 12.6% saving in natural gas and electricity in the case of electrochromic smart windows, respectively and that of 17.5% and 8.1% for triple glazing windows, respectively. So implementing this smart window type has led to the building energy optimization by the mean of controlling the amount of solar radiation allowed in the building.

Review for "Crystal reconstructed Cubic Nickel Oxide with Energetic Reactive Interfaces for Exceptional Electrochromic Smart Window"