2.2 Literature Survey
2.2.1 Smart Windows
In the United States of America (USA), air-conditioning and artificial lighting systems account for nearly 30% of the total energy demand [32]. Although traditional large-area window glazings provide visual comfort, warmth, indoor and outdoor visibility, they do not control the solar heat and light transmitted through them. Static solutions such as curtain blinds provide visual and thermal control to some extent, but they require manual interventions to adjust their position [33].
In the literature, a few passive windows with low-emissivity coatings have been demonstrated to block the solar heat and provide some energy-saving [34–36]. As shown in Fig. 2.4(a), Besteiro et al. theoretically showcased a design of passive win- dow to efficiently block infrared (IR) radiation by introducing random-sized metal- lic nanocrystals inside a transparent glass [37]. Their passive window with silver
Figure 2.4: A few recently reported nanoparticles and polymer based smart windows: (a) pictorial illustration of a plasmonic nanocrystals (NCs) based metafilm in a transparent glass.
Adapted with permission from: [37] Copyright c2018 American Chemical Society. (b) Schematic view of K0.3WO3/Ag2O films based smart window. Adapted with permission from: [38] Copy- right c 2019 Elsevier B.V. (c) Schematic of hydrogel particles based smart window showing transmittance modulation. Adapted with permission from: [39] Copyright c2020 Elsevier B.V.
(d) single-step dual stabilization based smart window showing light scattering state (left), trans- mission state (right), and corresponding schematic design. Adapted with permission from: [40]
Copyright c 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (e) Nanostructured grass-like glass showing transition between transparent (at 0 second) and haze mode (after 80 sec- ond) by putting water drops on the glass that evaporates in 80 seconds. Adapted with permission from: [41] Copyright c 2021, Optica Publishing Group.
2.2 Literature Survey
nanoshells produced excellent IR blocking, but the complex fabrication process limits its practical value for designing a low-cost window. For designing tunable windows, various techniques such as thermochromic, photochromic, and electrochromic effects may be adopted [46–49]. Since one does not have control over the outdoor weather conditions (e.g.heat and light), electrochromic effect driven by applied voltage became a natural choice for designing smart windows.
Over the past decade, electrochromic smart windows based on nanocomposites [38], hydrogels [39], suspended nanoparticles [50, 51], and liquid crystals [52] have been reported, a few of them are shown in Fig. 2.4. For example, Gao et al. experi- mentally demonstrated K0.3WO3/Ag2O nanocomposite based smart window [see Fig.
2.4(b)] [38]. For the first time, their design offered thermal insulation, near-IR shield- ing, and visible light induced self-cleaning properties. Weiet al.reported temperature- responsive Ag nanorods doped poly(N-isopropyl acrylamide) hydrogel for smart win- dows [see Fig. 2.4(c)] [39]. Their hydrogel showed enhanced solar modulation abilities with significant temperature reduction under direct sunlight. Yoonet al. reported liq- uid crystal-based smart windows [see Fig. 2.4(d)] [40]. They practically demonstrated fast switching at low voltage with good mechanical stability. Haghanifar et al. fab- ricated silica nanograss glass with ultrahigh-transmittance and ultrahigh-haze, both over 95% at 550 nm wavelength [see Fig. 2.4(e)] [41].
In industry, electrochromic, thermochromic, and photochromic smart windows have been developed (see Table 2.1) but voltage controlled electrochromic windows gained popularity due to superior control over outdoor climate conditions (e.g. heat and light) [12]. Recently, Sikdar et al. reported voltage-controlled self-assembly/disassembly of functionalized metallic nanoparticles at liquid-liquid [42] and liquid-solid electro- chemical interfaces [43–45] and realized voltage controlled mirror/window and mir- ror/absorber functionalities, respectively (see Table 2.1). They have demonstrated both
Table 2.1: Classification of smart glass technologies [12].
theoretically and experimentally that the optical properties of these electrochemical systems can be tunedin situwith application of very low potential, only±0.5 V. How- ever, use of liquids in their systems makes those challenging to be deployed in real- world applications.
Here we emphasize that most of these glasses are designed to block infrared radia- tion, which is suited only for warm climate conditions [53]. However, a typical ‘smart’
window is expected to control solar heat depending on the climate condition [54]. For example, low solar heat is desirable for a relatively hot climate to maintain ambient room temperature. Whereas, for cold climate conditions, a relatively high solar heat- ing is desirable. Therefore, a smart window that can regulate solar heat alongside visible
2.2 Literature Survey
Figure 2.5: Present day commercial smart window variants: (a) single-pane clear glass, (b) double-pane clear glass, (c) double-pane argon low-emissivity coating glass, and (d) triple-pane argon low-emissivity coating glass. [7, 8].
transmissioncould be a better value proposition that may find application over a wider geographical location.
The present-day smart windows are coming in single-, double-, and triple-pane glass variants, with or without low-emissivity coating, with empty spaces in between filled with inert argon gas, as shown in Fig. 2.5 [7, 8]. Among them, the single- pane glasses are considered the most suitable among those for designing thin and lightweight windows. Unfortunately, most of the single-pane glasses show poor IR blocking capability. For double- and triple-pane glasses, the outer glass that blocks long wavelength infrared (LWIR) radiations is called "hot pane". Similarly, the in- ner glass that controls the visible and near-IR radiations is called "cold pane". Even though the double- and triple-pane glasses are quite capable of blocking IR radiation, the multi-pane glass assembly significantly increases the overall cost and make the windows thick and bulky. Specifically, triple-pane glass suffers from low visible trans- mittance, making the outside-view tinted and unnatural for building occupants [7].
Moreover, integrating double- and triple-pane glasses into passenger vehicles is gen- erally impractical from the design aesthetic point of view.
Therefore, for most practical purposes, it is imperative to design a smart window us- ing a single-pane with a low-emissivity coating, which can still match the IR blocking capability of double- or triple-pane commercial glasses. To achieve such goals, elec- trochromic window glasses are considered the best-suited candidate.
2.2.1.1 Applications of Electrochromic Smart Glasses
Electrochromic smart glasses are remarkably convenient to use, environmentally friendly, and can dramatically reduce the need for air-conditioning. They can cut peak energy use for cooling and lighting by around 20% [55]. Since they are electrically operated, they can easily be controlled by a smart-home system or a sunlight sensor, or depending on whether there are people inside the building or not. They could provide improved security and privacy at the flick of a switch. However, they are expensive to install than ordinary glasses. For instance, a single large-area smart window typically comes in at around $500–1000 [55]. Another concern is the durability of the materials as compared to current commercial windows.