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The New Generation of Energy saving Glass Technology - Principles of Electrochromic Glass

2022-07-26
1508 次

What is electrochromic glass?


Smart glass utilizes a technology called electrochromism, where when applied voltage passes through them, a typical smart window begins to appear blue, and gradually becomes transparent within a few minutes as current passes through.



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Image: Electrochromic glass changes color under electronic control:

Left: This is transparent and looks very like ordinary glass;                      Right: Applying a small voltage will make it opaque (blue and dark).


How does electrochromic glass work?


There are quite a few different types of electrochromic glass: some just darken (such as photochromic sunglasses, which darken in sunlight), some darken and become semi transparent, while others become opaque like mirrors. Each type is powered by different technologies, and I will only describe one in detail here: the initial technology, discovered by Dr. Satyen K. Deb in 1969, based on the movement of lithium ions in transition metal oxides such as tungsten oxide.


A regular window is made of a vertical piece of glass, while a double glazed window has two pieces of glass with an air gap in the middle for insulation and noise reduction (keeping heat and noise on one side or the other). More complex windows (using low radiation heat reflective glass) are coated with a thin layer of metallic chemicals, so they can keep your home warm in winter and cool in summer. The working principle of electrochromic windows is a bit like this, except that they use much more complex metal oxide coatings and are integrated circuits (silicon computer chips) deposited through processes similar to those used in manufacturing.


Although we often talk about "electrochromic glass," windows like this can be made of glass or plastic (technically referred to as "substrate," or substrate material) and coated with multiple layers of film through a process called sputtering (an accurate method of adding a thin film of one material to another). On its inner surface (facing your home), the window has a double-layer interlayer composed of five ultra-thin layers: there is a partition in the middle, with two electrodes (thin electrical contacts) on both sides of the partition, and two transparent electrical contact layers on both sides of the electrodes. The basic working principle involves the migration of lithium ions (positively charged lithium atoms lacking electrons) back and forth between two electrodes through a separator. Under normal circumstances, when the window is transparent and lithium ions are located at the innermost electrode (on the left side of the figure, as you can see here), it is composed of lithium cobalt oxide (LiCoO2). When a small voltage is applied to the electrode, the ions migrate through the separator to the outermost electrode (on the right side of the figure). When they "soak" into this layer (made of something similar to polycrystalline tungsten oxide, WO3), they make it reflect light, effectively making it opaque. They stayed there alone until the voltage reversed, causing them to move backwards, making the window transparent again. No electricity is needed to maintain the clear or dark state of electrochromic windows - just change them from one state to another.


How the electrochromic window works: Applying voltage to the external contact (conductor), lithium ions (shown as blue circles here) move from the innermost electrode to the outermost electrode (from left to right in this figure). Windows reflect more light and transmit less, making them appear opaque (dark). These layers are very thin coatings added to a heavy piece of glass or plastic called the substrate (not shown here for clarity).



What are the advantages and disadvantages of electrochromic windows?


Advantage: Smart windows may sound like a gimmick, but they have a huge environmental benefit. In the dark, they almost reflect off all (about 98%) of the light shining on them, so they can significantly reduce the demand for air conditioning (including the huge cost of installing air conditioning and daily operating costs). A manufacturer estimates that electrochromic glass can reduce cooling and lighting energy consumption by about 20%. Because they are electrically operated, they can be easily controlled by smart home systems or sun sensors, regardless of whether there are people in the building. According to scientists at the National Renewable Energy Laboratory (NREL) of the US Department of Energy, windows like this can save 8% of total building energy consumption; They can transition from darkness to light with only a small amount of electricity. Other benefits of smart windows include privacy when pressing the switch (no longer using clumsy, dusty curtains and blinds), convenience (automatically dimming windows can prevent interior decoration and image fading), and improved safety.


Disadvantages: As is well known, the installation cost of glass with electrodes and unique metal coatings printed on it will be several times that of ordinary glass: a large smart window typically costs around $500- $1000 per square foot. There is also a question about how durable this material is. Another drawback of the current window is that it takes time to transition from transparent to opaque and then to transparent. Some technologies may take a few minutes.


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