New generation of energy-saving glass technology - The principle of electrochromic glass
Have you ever had a day like this, the sun doesn't know whether it is coming or going, prompting you to constantly open and close your blinds, like this Can you block reading your computer or stop your work from fading? Soon we will consign this particular problem to the history books, thanks to electrochromic glass ("smart" glass), which changes from bright to light (from transparent to opaque) and back again at the push of a button. Back to light. Relatively speaking, it's very simple, very convenient (no more prevarication!) and has huge environmental benefits. How exactly does it work? Let’s take a closer look!
Forget the curtains, forget the blinds! “Smart windows” made from electrochromic glass can change from clear to opaque and back to clear with the flick of a switch. Some are made of special glass ; some are plastic films added to ordinary glass.
What is electrochromic glass?
Glass is a magical material. Without it, our building interiors would be dark, gloomy, cold and damp. But it also has disadvantages. It comes in with light and heat even if you don't want it. The more heat that enters your building ("solar gain") on a blinding summer day, the more you have to use your air conditioner, which is a terrible waste of energy that costs you money and protects the environment. This is why most windows in homes and offices have curtains or blinds. If you work in interior design and remodeling, you might think furniture like this is beautiful and attractive—but in cold, practical, scientific terms, they're annoying. Let's be honest: Curtains and blinds are a technical flaw that's made up for the huge inherent flaw of glass: it's transparent even if you don't want it to be transparent (or translucent).
Since the beginning of the 20th century, people have been accustomed to the idea of buildings being increasingly automated. We have electric washing machines, dishwashers, vacuum cleaners and more. So, why not add electric windows to our home decoration ? Electric windows can automatically switch from cleaning to working. Smart windows (also known as smart glass , switchable windows and dynamic windows) utilize a process called electrochromism, in which when a voltage is applied across them. A typical smart window starts out blue and gradually becomes transparent (over a few minutes) when electricity is passed through it.
Picture: Electrochromic glass changes color under electronic control: Left: Here it is transparent and looks a lot like ordinary glass; Right: When a small voltage is applied, it becomes opaque (blue and dark). Photo provided by Warren Graetz U.S. Department of Energy/ National Renewable Energy Laboratory (Department of Energy/NREL).
How does electrochromic glass work?
There are quite a few different types of electrochromic glass: some just darken (like photochromic sunglasses, which darken in sunlight), some darken and become translucent, and others become opaque like a mirror. Each type is powered by a different technology, of which I will describe only one in detail here: The original technology, discovered by Dr. Satyen K. Deb in 1969, was based on lithium ions in transition metal oxides such as tungsten oxide ) movement in. [1] (As you may know, lithium is the best-known chemical element in rechargeable batteries lithium-ion batteries.)
A regular window is made of a vertical piece of glass, and a double-glazed window has two panes of glass in the middle. There is an air gap for insulation and sound attenuation (keeps heat and noise to one side or the other). More sophisticated windows (using low-E heat reflective glass ) are coated with a thin layer of metallic chemicals, so they keep your home warmer in the winter and cooler in the summer. Electrochromic windows work a bit like this, except they use a much more complex metal oxide coating and are deposited through a process similar to that used in manufacturing integrated circuits (silicon computer chips).
While we often talk about "electrochromic glass," windows like this can be made from glass or plastic (technically called the "base," or base material) through a process called sputtering, a process where one material is The precise method of adding a thin film to another material) process of applying multiple layers of thin film. On its inner surface (facing your home), the window has a double sandwich of five ultra-thin layers: a separator in the middle with two electrodes (thin electrical contacts) on either side of the separator. There are two transparent electrical contact layers on the side. The basic working principle involves lithium ions (positively charged lithium atoms - lacking electrons) migrating back and forth between two electrodes through a separator. Normally, when the window is transparent, the lithium ions are located in the innermost electrode (on the left side of the diagram, you can see it here), which is made of something like lithium cobalt oxide (LiCoO2). When a small voltage is applied to the electrode When on, the ions migrate through the separator to the outermost electrode (the one on the right in the picture). When they "soak" into this layer (which is made of something like polycrystalline tungsten oxide, WO3), they make it reflect light, effectively making it opaque. They are left there alone until the voltage reverses, causing them to move backwards so the window becomes transparent again. No electricity is required to maintain the clear or dark state of the electrochromic color-changing windows - just change them from one state to the other.
How the electrochromic window works: Applying a voltage to the outer contacts (conductors), lithium ions (shown here as blue circles) move from the innermost electrode to the outermost electrode (left to right in this image) . A window reflects more light and transmits less, causing it to appear opaque (dark). These layers are very thin coatings that are added to a heavy piece of glass or plastic called a substrate (not shown here for clarity).
Other technologies
That’s it for lithium-ion batteries, what other technologies are available? Here are some of them:
Instead of putting spacers between layers of electrodes, we can use electrochromic materials (a type of dye) that change color when an electric current is passed through it. This is similar to photochromic sunglasses, but under precise electrical control. The chemical dyes electrochemistry works with include violinist, which changes reversibly between clear and blue or green.
We can use nanocrystals (such as nanotechnology, which works at the atomic scale, which is about 1000 times smaller than what we call microscopic scales), in much the same way to let more or less light pass through smart windows.
Configuration
Different types of electrochromic windows have different configurations, but most have several different layers. In one popular design sold under the Halio brand, there are six separate surfaces. The electrochromic layer is sandwiched between two layers of PVB (polyvinyl butyral) polymer, flanked by thermally tempered glass. Then there's insulation, a low-e coating, and finally a layer of interior glass. Electrochromic devices can also be customized in various ways, with thicker outer layers for safety or weather resistance, different low-e coatings, more or less insulation, etc. Some can be controlled automatically via a smartphone app, or connected to a rooftop pyranometer (sun sensor) so your windows automatically darken when the sun is bright enough.
Adhesive electrochromic film
The smart windows we've seen so far have usually been installed as stand-alone units: you cover an entire window with purpose-made coated glass at great expense. You can also get smart window technology for slightly cheaper: Manufacturers like Sonte and Smart Tint make thin, self-adhesive, adhesive electrochromic films that you can apply to your existing windows, And turned on and off via a simple smartphone app.
Electrochromic films use technology similar to liquid crystal displays, which use liquid crystals, under precise electronic control, to change how much light can pass through.When the current is turned on, the crystals line up like open blinds, allowing light to flow directly through; when closed, the crystals orient themselves randomly, scattering light passing through from any direction, rendering the window opaque. The performance was impressive. According to Smart Tint, its film can transmit about 98 percent of light when the light is clear, and can switch to opaque in less than a second when the light drops to about a third; they It has been tested 3 million times switching back and forth.
How electrochromic films work: The film contains liquid crystals (blue). When the current is turned off, the crystals point in random directions, scattering incoming light and making the film opaque. When the current is turned on, the crystals align like open shutters, allowing nearly all light to pass through.
What are the advantages and disadvantages of electrochromic windows?
Advantages
Smart windows may sound like a gimmick, but they have a huge environmental benefit. In the dark, they reflect almost all (about 98%) of the light that hits them, so they can significantly reduce the need for air conditioning (both the huge cost of installing it and the daily running costs). (One manufacturer, View Glass, estimates that electrochromic glass can reduce cooling and lighting energy use by about 20 percent.) Because they are electrically operated, they can easily be equipped with smart home systems or sunlight sensors, whether or not there are any in the building. people. According to scientists at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL), windows like this could save 8% of a building's total energy use; they can switch from dark to light using only a small amount of electricity (100 windows use The energy is equivalent to the energy of a window incandescent lamp ) so the ahuge network is overall energy-saving. Other benefits of smart windows include privacy at the flip of a switch (no more fumbling with awkward, dusty curtains and blinds), convenience (auto-darkening windows prevent upholstery and pictures from fading), and increased security ( electric curtains is notoriously unreliable).
Hot stuff! This thermal (infrared) picture shows how hot it is when you park your car in direct sunlight: the colors represent the temperature, with red and yellow being the hottest and blue being the coldest. Electrochromic glass installed in cars may help solve this problem. When you park, you just flip a switch to dim the windows, and when you come back, your car will be nice and cool! Photo courtesy U.S. Department of Energy.
Disadvantages
It is well known that glass with printed electrodes and exotic metallic coatings will cost several times more to install than regular glass: a large smart window will typically cost about $500-$1,000 per square foot (approximately $500-$1,000). There are also questions about how durable the material is, with current glass deteriorating in performance after 10-20 years (much shorter than what most homeowners can expect from traditional glass). Another disadvantage of the current window is that it takes time to go from transparent to opaque and back to transparent. Some technologies can take several minutes (Halio quotes three minutes for its glass to go from clear to completely darkened), although stick-on electrochromic films are much faster, changing from clear to opaque in less than a second.
How will smart windows be improved in the future?
Another possibility is to combine electrochromic windows with solar cells so dark smart windows can absorb this energy and store it for later use, rather than uselessly reflecting sunlight away. It's easy to imagine windows capturing some of the solar energy that falls on them during the day and storing it in batteries that can power your home lights at night. Of course, a window can't be 100% transparent and also be 100% efficient with solar panels. . Incoming energy is either transferred through the glass or absorbed and stored, but not both. A window that doubled as a solar cell would likely involve compromises on both sides: It would be a relatively dark window, even though it was transparent, and much less efficient at capturing energy than a good solar cell.
One thing we are sure of is that we will see more electrochromic technology in the future!
