Smart glasses

In the area of light control, there are three main technologies:Electro chromic, liquid crystal, suspended particle device(SPD)

LC smart glass – It is kind of laminated glass where two glass or plastic interlayers with polymer dispersed liquid crystal in between that include  a thin layer of a transparent, conductive material. This structure is in effect a capacitor.

Principle of working - 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. It is also possible to control the amount of light and heat passing through, when tints and special inner layers are used.

PDLCs operate on the principle of electrically controlled light scattering. In the scattering (opaque) state, the glass diffuses direct sunlight and eliminates 99% of the UV rays that fade carpet and furniture.

Application - 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.  Liquid crystal glazing is designed for internal applications, including partitions, display cases, bank screens. 

Disadvantages - As there is little change in performance properties and because it requires constant energy to maintain its clear state, this liquid crystal window provides no energy saving benefits.

Advantages -  The switch between the two states is nearly instantaneous.

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SPD smart glass - In suspended particle devices (SPDs), a thin film laminate of rod-like particles suspended in a fluid is placed between two glass or plastic layers, or attached to one layer OR two panes of glass separated by thin conductive film with suspended, light absorbing microscopic particles.

Principle of working - . When no voltage is applied, the suspended particles are arranged in random orientations and tend to absorb light, so that the glass panel looks dark (or opaque), blue or, in more recent developments, grey or black colour. When voltage is applied, the suspended particles align and let light pass.Advantage -  this type of smart window is capable of changing at the turn of a button. 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. Other advantages include reduction of buildings' carbon emissions.There are some advantages of this type of smart window over the other two types. Electrochromic glass responds slowly, has limited cycle lifetime, and has an “iris effect” where color change begins at the outer edges of the window and trickles its way toward the center. Liquid-crystal glass is either clear or opaque with no in-between states, and merely scatters light rather than blocking it, which limits it to certain interior privacy applications. 

Disadvantage-  one disadvantage is that electricity is required to keep the window transparent.

Electrochromic smart glass -  the electrochromic material changes its opacity: it changes between a colored, translucent state (usually blue) and a transparent state with the application of voltage. Principle of working -  Electrochromic windows consist of two glass panes with several layers sandwiched in between. It works by passing low-voltage electrical charges across a microscopically-thin coating on the glass surface, activating an electrochromic layer which changes color from clear to dark. The electric current can be activated manually or by sensors which react to light intensity. 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, TiO2) 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 a 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

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explained by the small over- potentials needed to drive the electrochemical reduction of the surface adsorbed viologens/chromogens. 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.

Advantage - One advantage of the electrochromic smart window is that it only requires electricity to change its opacity, but not to maintain a particular shade.

Disadvantage -  Electrochromic glass responds slowly, has limited cycle lifetime, and has an “iris effect” where color change begins at the outer edges of the window and trickles its way toward the center. 

Liquid Crystals: This familiar technology was commercialized for window use and later discontinued. Liquid crystal windows switch quickly from a transparent state to a diffuse white state. The primary function is to provide privacy and control glare as a substitute for conventional shading devices. In the diffuse state liquid crystals are primarily forward scattering so there is little control over solar heat gain.

Hydrides: These materials can be classified as electrochromics, but they are different in several ways from conventional oxide electrochromics. Originally deposited as a metal, they can be converted to a partially transparent hydride by injection of hydrogen from the gas or solid phase. Thus, they switch to a reflective state which has several potential advantages in terms of energy performance and durability

Photochromics: As the name implies, these materials darken under the direct action of sunlight. They are not considered as versatile as electrochromics becuase they cannot be manually controlled and because optimum energy performance requires consideration of temperature conditions as well as solar radiation. For example, a photochromic window may darken on a cold sunny day when more solar heat gain is desireable. They are used widely for automatically darkeningsunglasses.

Thermotropics - As photochromics respond primarily to light, thermotropics respond to heat. Again this is not as versatile a response as electrochromics. Daylight or view may have a higher priority for the occupant, at least temporarily, than reduction in solar gain.

Smart glass blocks infrared when heat is on - At most room temperatures the glass lets both visible and infrared light pass through. But above 29°C, a substance coating the glass undergoes

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a chemical change causing it to block infrared light. This will prevent room from overheating in bright sunshine or if temperatures outside start to soar.

Transition temperature

The glass is coated the chemical vanadium dioxide. This material transmits both visible and infrared wavelengths of light, and normally undergoes a change at about 70°C.

Above this transition temperature, the electrons in the material alter their arrangement. This turns it

from a semiconductor into a metal( ???????), and makes it block infrared light(?????).

Parkin and Manning lowered the transition temperature to 29°C by doping the material with the metal tungsten.

They also found a way to incorporate deposition of the coating into a conventional glass manufacturing process. This should make it relatively cheap to mass produce, they claim, with a commercial version of the glass ready within three years.

However, a number of issues still need to be overcome. Firstly, the substance is not permanently fixed to the glass. Also, the coating itself currently has a strong yellow tint.

But Manning believes it should be possible to overcome these issues. "You could add another substance, like titanium dioxide, to fix it to the glass," he told New Scientist. "And you could use a dye that would cancel out the yellow."

Tinted glass????/-----------------------------------------------------------------------------------------------------------------------------

http://www.cs.ubbcluj.ro/~moltean/switchable_glass_evolvable_hardware.htm

Suspended Particle Device refers to rod-like particles suspended in a fluid. With no applied voltage, the particles are randomly oriented and block light (dark state). When a voltage is applied, the particles align with the electric field and let light though (light state). By varying the applied voltage, we can continuously vary the amount of transmitted light.

Electrochromic glass becomes translucent when voltage is added and are transparent when voltage is taken away. Like suspended particle devices, electrochromic windows can be adjusted to allow varying levels of visibility.

Liquid crystal glasses become transparent when a voltage is added and have an opaque behavior when there is no electrical power applied. However, liquid crystal glass has only 2 states: opaque and transparent with no other degrees of visibility.

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The liquid suspension or film is then enclosed between two glass or plastic plates coated with a transparent conductive material. The mechanism behind SPD is similar to that of the dielectric in a parallel-plate capacitor which means that the atoms of the dielectric are polarized by the electric field

Electrochromic basically describes materials that can change color when energized by an electrical current. Electricity generates a chemical reaction in this material. This reaction (like any chemical reaction) changes the properties of the material. In this particular case, the reaction changes the way the material reflects and absorbs light. In some other electrochromic materials, the change is between different colors. In electrochromic windows, the material changes between colored (reflecting light of some color) and transparent (not reflecting any light).

In the design of electrochromic device, the chemical reaction at work is an oxidation reaction – a reaction in which molecules in a compound lose an electron. Ions in the sandwiched electrochromic layer are what allow it to change from translucent to transparent. It's these ions that allow it to absorb es and voltage drives the ions from the ion storage layer, through the ion conducting layer and into the electrochromic layer. This makes the glass opaque. By shutting off the voltage, the ions are driven out of the electrochromic layers and into the ion storage layer. When the ions leave the electrochromic layer, the window regains its transparency.

Advantages and weaknesses - An important advantage of Suspended-Particles Devices and Electrochromic Devices is their ability to continuously vary the amount of transmitted light based on continuous variations of the applied electrical power. This feature makes SPDs and ECDs very suitable for a large number of problems whose parameters are real-valued. By contrast, Liquid Crystal Devices are of ON-OFF type: they can have only 2 states.

A possible drawback is the speed of performing the changes. As a general remark, the speed is directly connected to the glass surface. A bigger surface requires more time to change its state than a smaller surface. However, for EHW tasks the required surface can be microscopic, which requires less than 1 microsecond to perform a complete cycle (in the case of SPDs).

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Because ECDs depend on ion injection and chemical reactions, the process is inherently slow. However, SPDs depend on a field effect, thus responding in very short time. As the technology advances we can expect to have smart glass which will respond faster to the applied electrical power.

According to [12] there are several characteristics that a material should have in order to become a possible candidate for EHW tasks:

the material should be configurable by applying some electrical power or any other source of energy (such as light),

the material should affect an incident signal (optical or electronic), the material should be able to be reset to its original state.

 Switchable glazing can change the light transmittance, transparency, or shading of windows in response to an environmental signal such as sunlight, temperature or an electrical control. 

BENEFITS -  "smart" windows can reduce a commercial building's energy use by 30 to 40 percent. In the summer months, electrochromic windows can block ultraviolet rays and radiant heat from direct sunlight from passing through windows and skylights to help lower cooling loads. They can also help slow the fading of interior furnishings by blocking out the sun's ultraviolet rays. Electrochromic windows offer the flexibility of control not available in photochromic or thermochromic windows (windows that turn opaque when exposed to light or warm temperatures). The cost of electrochromic glazing technologies, while currently high, continues to decline as the technology and manufacturing process matures. Potential uses for electrochromic technology include daylighting control, glare control, solar heat control, and fading protection in windows and skylights. By automatically controlling the amount of light and solar energy that can pass through the window, electrochromic windows can help save energy in residences.

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