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Smart glass or switchable glass (also smart windows or switchable windows in those applications) is a glass or glazing whose light transmission properties are altered when voltage, light or heat is applied. Generally, the glass changes from translucent to transparent, changing from blocking some (or all) wavelengths of light to letting light pass through.
When installed in the envelope of buildings, smart glass creates climate adaptive building shells, with the ability to save costs for heating, air-conditioning and lighting and avoid the cost of installing and maintaining motorized light screens or blinds or curtains. Blackout smart glass blocks 99.4% of ultraviolet light, reducing fabric fading; for SPD-type smart glass, this is achieved in conjunction with low emissivity coatings.
Critical aspects of smart glass include material costs, installation costs, electricity costs and durability, as well as functional features such as the speed of control, possibilities for dimming, and the degree of transparency.
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In suspended-particle devices (SPDs), a thin film laminate of rod-like nano-scale particles is suspended in a liquid and placed between two pieces of glass or plastic, or attached to one layer. When no voltage is applied, the suspended particles are randomly organized, thus blocking and absorbing light. When voltage is applied, the suspended particles align and let light pass. Varying the voltage of the film varies the orientation of the suspended particles, thereby regulating the tint of the glazing and the amount of light transmitted.
SPDs can be manually or automatically "tuned" to precisely control the amount of light, glare and heat passing through, reducing the need for air conditioning during the summer months and heating during winter. Smart glass can be controlled through a variety of mediums, such as automatic photosensors and motion detectors, smartphone applications, integration with intelligent building and vehicle systems, knobs or light switches.
Smart light-control technology increases users' control over their environment, provides for better user comfort and well-being and improves energy efficiency. The technology provides over 99% UV blockage and state switching in 1 to 3 seconds. In cars, the range of light transmission for the technology is 50-60 times darker than a typical sunroof to twice as clear as an ordinary sunroof. Published data by Mercedes-Benz shows that SPD technology can reduce cabin temperatures inside a vehicle by 18 °F (10 °C). Other advantages include reduction of carbon emissions and the elimination of a need for expensive window dressings.
SPD-Smart Glass was patented by the public company Research Frontiers.
Commercialization of SPD is accelerating in the automotive industry. SPD automotive side and rear windows and sunroofs offer many benefits to passengers in the vehicle. Because of their fast-switching and ability to be tuned, they reduce unwanted light and glare, which allows users to maintain their views of the outside while reducing glare on displays and video screens, or tint the windows for additional privacy. SPD automotive glass also minimizes heat build-up inside the vehicle because of their ability to block solar heat gain. Many SPD window systems automatically switch to their maximum heat-blocking state when the vehicle is not in use.
As of 2016 around thirty aircraft models had SPD windows.
Adaptability and control are especially important in the marine environment. SPD lets the user instantly and precisely control the amount of light, glare and heat passing through windows, skylights, portholes, partitions and doors.
Architectural SPD products – windows, skylights, doors and partitions – are available as laminated panels or insulated glass units for new construction, replacement and retrofit projects. These products offer a distinctive blend of energy efficiency, user comfort and security. Architectural products made with SPD technology:
Electrochromic devices change light transmission properties in response to voltage and thus allow control over the amount of light and heat passing through. In electrochromic windows, the electrochromic material changes its opacity: it changes between a transparent and a tinted state. A burst of electricity is required for changing its opacity, but once the change has been effected, no electricity is needed for maintaining the particular shade which has been reached.
First generation electrochromic technologies tend to have a yellow cast in their clear states and blue hues in their tinted states. Darkening occurs from the edges, moving inward, and is a slow process, ranging from many seconds to several minutes (20-30 minutes) depending on window size. Newer electrochromic technologies, also known as "smart-tinting glass," tackled the drawbacks of earlier versions by eliminating the yellow cast in the clear state and tinting to more neutral shades of gray, tinting evenly rather than from the outside in, and accelerating the tinting speeds to less than three minutes, regardless of the size of the glass. However, these newer electrochromic technologies have yet to pass ASTM-2141 for long term reliability and durability testing. This lack of third party independent ASTM certification is one of the limiting aspects of market acceptance in comparison to first generation electrochomric technologies that have successfully passed ASTM-2141 certification.
Electrochromic glass provides visibility even in the darkened state and thus preserves visible contact with the outside environment. It has been used in small-scale applications such as rearview mirrors. Electrochromic technology also finds use in indoor applications, for example, for protection of objects under the glass of museum display cases and picture frame glass from the damaging effects of the UV and visible wavelengths of artificial light. Electrochromic glass can be programmed to automatically tint according to the weather or the sun's position or user preferences. It can also be controlled via mobile applications and even via popular voice assistants.
Recent advances in electrochromic materials pertaining to transition-metal hydride electrochromics have led to the development of reflective hydrides, which become reflective rather than absorbing, and thus switch states between transparent and mirror-like.
Recent advancements in modified porous nano-crystalline films have enabled the creation of electrochromic display. The single substrate display structure consists of several stacked porous layers printed on top of each other on a substrate modified with a transparent conductor (such as ITO or PEDOT:PSS). Each printed layer has a specific set of functions. A working electrode consists of a positive porous semiconductor (say Titanium Dioxide, TiO
2) with adsorbed chromogens (different chromogens for different colors). These chromogens change color by reduction or oxidation. A passivator is used as the negative of the image to improve electrical performance. The insulator layer serves the purpose of increasing the contrast ratio and separating the working electrode electrically from the counter electrode. The counter electrode provides a high capacitance to counterbalances the charge inserted/extracted on the SEG electrode (and maintain overall device charge neutrality). Carbon is an example of charge reservoir film. A conducting carbon layer is typically used as the conductive back contact for the counter electrode. In the last printing step, the porous monolith structure is overprinted with a liquid or polymer-gel electrolyte, dried, and then may be incorporated into various encapsulation or enclosures, depending on the application requirements. Displays are very thin, typically 30 micrometer, or about 1/3 of a human hair. The device can be switched on by applying an electrical potential to the transparent conducting substrate relative to the conductive carbon layer. This causes a reduction of viologen molecules (coloration) to occur inside the working electrode. By reversing the applied potential or providing a discharge path, the device bleaches. A unique feature of the electrochromic monolith is the relatively low voltage (around 1 Volt) needed to color or bleach the viologens. This can be explained by the small over- potentials needed to drive the electrochemical reduction of the surface adsorbed viologens/chromogens.
In polymer-dispersed liquid-crystal devices (PDLCs), liquid crystals are dissolved or dispersed into a liquid polymer followed by solidification or curing of the polymer. During the change of the polymer from a liquid to solid, the liquid crystals become incompatible with the solid polymer and form droplets throughout the solid polymer. The curing conditions affect the size of the droplets that in turn affect the final operating properties of the "smart window". Typically, the liquid mix of polymer and liquid crystals is placed between two layers of glass or plastic that include a thin layer of a transparent, conductive material followed by curing of the polymer, thereby forming the basic sandwich structure of the smart window. This structure is in effect a capacitor.
Electrodes from a power supply are attached to the transparent electrodes. With no applied voltage, the liquid crystals are randomly arranged in the droplets, resulting in scattering of light as it passes through the smart window assembly. This results in the translucent, "milky white" appearance. When a voltage is applied to the electrodes, the electric field formed between the two transparent electrodes on the glass causes the liquid crystals to align, allowing light to pass through the droplets with very little scattering and resulting in a transparent state. The degree of transparency can be controlled by the applied voltage. This is possible because at lower voltages, only a few of the liquid crystals align completely in the electric field, so only a small portion of the light passes through while most of the light is scattered. As the voltage is increased, fewer liquid crystals remain out of alignment, resulting in less light being scattered. It is also possible to control the amount of light and heat passing through, when tints and special inner layers are used. It is also possible to create fire-rated and anti X-Ray versions for use in special applications. Most of the devices offered today operate in on or off states only, even though the technology to provide for variable levels of transparency is easily applied. This technology has been used in interior and exterior settings for privacy control (for example conference rooms, intensive-care areas, bathroom/shower doors) and as a temporary projection screen. It is commercially available in rolls as adhesive-backed smart film that can be applied to existing windows and trimmed to size in the field.
Micro-blinds—currently under development at the National Research Council (Canada)—control the amount of light passing through in response to applied voltage. Micro-blinds are composed of rolled thin metal blinds on glass. They are very small and thus practically invisible to the eye. The metal layer is deposited by magnetron sputtering and patterned by laser or lithography process. The glass substrate includes a thin layer of a transparent conductive oxide (TCO) layer. A thin insulator is deposited between the rolled metal layer and the TCO layer for electrical disconnection. With no applied voltage, the micro-blinds are rolled and let light pass through. When there is a potential difference between the rolled metal layer and the transparent conductive layer, the electric field formed between the two electrodes causes the rolled micro-blinds to stretch out and thus block light. The micro-blinds have several advantages, including switching speed (milliseconds), UV durability, customized appearance and transmission. Theoretically, the blinds are simple and cost-effective to fabricate. A video available on YouTube describes briefly the micro-blinds.
A thin coating of nanocrystals embedded in glass can provide selective control over both visible light and heat-producing near-infrared (NIR) light independently climates. The technology employs a small jolt of electricity to switch the material between NIR-transmitting and NIR-blocking states. Nanocrystals of indium tin oxide embedded in a glassy matrix of niobium oxide form a composite material. The voltage ranges over 2.5 volts. The same window can also be switched to a dark mode, blocking both light and heat, or to a bright, fully transparent mode. The effect relies on a synergistic interaction in the region where glassy matrix meets nanocrystal that increases the electrochromic effect. The atoms connect across the nanocrystal-glass interface, causing a structural rearrangement in the glass matrix. The interaction creates space inside the glass, allowing charge to move more readily.
A lower cost alternative to smart glass is Vistamatic Vision Panels, a privacy glass made up of three sheets of glass sealed as a single panel with evenly spaced, alternating lines to allow privacy or observation. Customized to the needs of the facility, these non-electric privacy glass panels are manually operated while still providing a frosted look similar to its electric counterpart.
A low cost alternative to high-tech intelligent windows is composed of two retroreflective panels mounted back-to-back with a narrow gap in between. When a liquid with the same refractive index as that of the panels is pumped into the cavity between them, the glass becomes transparent. When the liquid is pumped out, the glass turns retro reflective again. An example of this kind of window is the Norwegian brand Sunvalve.
This was invented by a professor at the University of Delaware, see US patent 8635817.
Another low-cost alternative to electronic smart glass is Smartershade. This glass consists of two panes of polarized glass with a patterned optical axis that allows it to transition smoothly between shades of gray to near complete blackout opacity. The advantage is a much higher light extinction (blackout) than EC or SPD glass at a much lower cost. The drawbacks are that it requires two panes, one of which must be able to move, and that at its most transparent it admits only 50% of incident light. This glass can also be produced as a clear to mirror, or smartmirror.
The expression smart glass can be interpreted in a wider sense to include also glazings that change light transmission properties in response to an environmental signal such as light or temperature.
These types of glazings cannot be controlled manually. In contrast, all electrically switched smart windows can be made to automatically adapt their light transmission properties in response to temperature or brightness by integration with a thermometer or photosensor, respectively.
The topic of smart windows in a further sense includes LED-embedded films which may be switched on at reduced light intensity. The process of laminating these LED-embedded films between glass will allow the production of transparent LED-embedded glasses. As most glass companies are not skilled in mounting LEDs onto metallized glass, the LEDs are located on a separate transparent conductive polymeric interlayer that may be laminated by any glass lamination unit.
Smart glass is produced by means of lamination of two or more glass or polycarbonate sheets.
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Smart glass using one of the aforementioned technologies has been seen in a number of high-profile applications. Large-scale installations were completed at the Guinness Storehouse in Dublin, where over 800,000 people per year can see smart glass being used in interactive displays and privacy windows. Smart glass was used to launch the Nissan Micra CC in London using a four-sided glass box made up of 150 switchable glass panels which switched in sequence to create a striking outdoor display. The main use for smart glass is in internal partitions, where many companies now enjoy the ability to switch screens and doors from clear to private.
Smart glass has found uses in the healthcare industry, where easily cleaned surfaces are essential and there are considerations of patient privacy. Smart glass products can replace traditional blind systems that are difficult to clean and can harbor dirt and bugs. Research has shown that patient comfort can help reduce recovery time.
One of the most popular smart glass applications is as projection screens.
Another example of use is the installation of PDLC-based smart glass, in The EDGE, a glass cube which protrudes out from the 88th-floor skydeck of the world's highest residential tower, Eureka Towers, located in Melbourne. The cube can hold 13 people. When it extends out of the building by 3 metres, the glass is made transparent, giving the cube's occupants views of Melbourne from a height of 275 metres. The same type of smart glass has also been proposed for use in hospital settings to controllably provide patients with privacy as needed.
PDLC technology was used in a display to unveil the Nissan GTR at the Canadian International Auto Show in Toronto.
Electro-chromatic glass was used on the 1988 Cadillac Voyage concepts body which adjusted the sun load on the car and can darken it.
In the media, the updated set for the Seven Network's Sunrise program features a Smart Glass background that uses liquid crystal switchable glass. The new set with Smart Glass allows the street scene to be visible at times, or replaced with either opaque or transparent blue colouring, masking the view.
Bloomberg Television currently features smart glass backgrounds in its studios in New York, Hong Kong and London.
The Boeing 787 Dreamliner features electrochromic windows which replace the pull down window shades on existing aircraft. NASA is looking into using electrochromics to manage the thermal environment experienced by the newly developed Orion and Altair space vehicles.
Smart glass has been used in some small-production cars. The Ferrari 575 M Superamerica had an electrochromic roof as standard, and the Maybach has a PDLC roof as option. Some Privacy Glass has been applied in the Maybach 62 car for privacy protection purposes.
A Hong Kong office uses 130 square meters of Privacy Glass, which is available in sizes up to 1,500 × 3,200 mm.
ICE 3 high speed trains use electrochromatic glass panels between the passenger compartment and the driver's cabin.
The city's restroom in Amsterdam's Museumplein square features smart glass for ease of determining the occupancy status of an empty stall when the door is shut, and then for privacy when occupied.
Bombardier Transportation has intelligent on-blur windows in the Bombardier Innovia APM 100 operating on Singapore's Bukit Panjang LRT Line, to prevent passengers from peering into apartments as the trains pass by and is planning to offer windows using smart glass technology in its Flexity 2 light rail vehicles.
The Bombardier INNOVIA APM100 (C801) trains are Singapore's first variant of LRT cars, which operates on the 14 station Bukit Panjang LRT Line operated by SMRT Light Rail Ltd. They were first developed by Adtranz as the CX-100, which was later acquired by Bombardier Transportation and renamed in 2001.
This electronically dimmable window technology provides unsurpassed thermal insulation: SPD-SmartGlass substantially rejects solar heat from entering through windows. When compared to conventional automotive glass, Mercedes-Benz reported that the use of SPD-SmartGlass significantly reduced the temperature inside the vehicle by up to 18 °F/10 °C. This increases passenger comfort and reduces air conditioning loads, thereby saving fuel and reducing CO2 emissions.
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