What are “smart” windows?

Without glass, our buildings would be gloomy, chilly, and wet. Glass is an incredible material. However, it also has disadvantages. Even when you don’t want it to, it lets heat and light in. The more heat (“solar gain”) that enters your building on a sweltering summer day, the more you’ll need to turn on your air conditioning, which is an awful waste of energy that is bad for the environment and expensive for you. For this reason, the majority of windows in houses and workplaces have blinds or curtains installed. These kinds of furniture may seem tidy and appealing to those who are interested in interior design and renovation, but they are actually a nuisance from a scientific and practical standpoint. Since glass is transparent (or translucent) even when you don’t want it to be, let’s face it: curtains and blinds are a technical workaround for this major flaw.

Read More: electrochromic

People have become accustomed to the notion of more mechanized structures since the early 20th century. We have electric vacuum cleaners, dishwashers, and washing machines, among many more appliances. So why not install electronic windows in our houses that can go from clear to dark on their own? That is precisely what smart windows—also known as switchable windows, smart glass, and dynamic windows—do. The first ones were electrochromic, which means that when you apply an electrical voltage across them, they change color—or, more accurately, go from being nearly transparent to almost opaque. Other varieties of smart windows make use of different technology, more akin to laptop LCD panels. Smart windows typically have an initial blue tint that disappears when electricity flows through them. It takes many minutes for certain technologies to complete the procedure, whereas it just takes a few seconds for others.

How do traditional electrochromic glasses function?

Smart windows exist in a variety of forms: some only darken (similar to photochromic eyeglasses, which get darker in the sun), others darken and turn translucent, and still others turn opaque and mirror-like. Every variety is driven by a distinct technology, which I will discuss in depth below. This is the original electrochromic technology, which was identified in 1969 by Dr. Satyen K. Deb and is predicated on the mobility of lithium ions in transition metal oxides (like tungsten oxide). (You are probably most familiar with lithium as the chemical component of rechargeable lithium-ion batteries.)

Double-glazed windows feature two glass panes separated by an air gap to promote heat insulation and soundproofing (to keep the noise and heat on one side or the other). Ordinary windows are comprised of a single vertical pane of glass. More advanced windows are coated with a thin coating of metallic compounds (using low-e heat-reflective glass) to keep your house cool in the summer and warm in the winter. The operation of electrochromic windows is somewhat similar to this, with the exception that they employ far more advanced metal-oxide coatings that are applied using techniques like to those used in the production of integrated circuits, or silicon computer chips.

What is the composition?

Sputtering is a precise method of applying thin films of one material onto another. While we frequently refer to “electrochromic glass,” a window such as this can be formed of either glass or plastic (officially called the “substrate,” or base material) coated with numerous thin layers. This window has a double-sandwich of five ultra-thin layers on its inner surface (looking into your home): two electrodes (thin electrical contacts) on either side of the separator, two transparent electrical contact layers on either side of the electrodes, and a separator in the center.

How does it operate?

Lithium ions, or positively charged lithium atoms without electrons, are the fundamental building block of the primary operating principle, which is their migration back and forth across the separator between the two electrodes. The innermost electrode, which is constructed of something resembling lithium cobalt oxide (LiCoO2) and is located on the left in the diagram above, is where the lithium ions normally dwell when the window is clear. Ions go through the separator to the outermost electrode—the one on the right in this diagram—when a little voltage is given to the electrodes. They “soak” into that layer (which is composed of WO3, or polycrystalline tungsten oxide), reflecting light and rendering it nearly opaque. They stay there by themselves until the voltage is switched, which makes them migrate back and restores the transparency of the window. Electrochromic windows only require power to transition between their light and dark states; no energy is required to keep them in either condition.

What are the benefits and drawbacks of smart windows?

Benefits

Although they may seem like a gimmick, smart windows offer significant environmental advantages. By blocking nearly all (98%) of the sunlight that falls on them while they are darkened, they can significantly lessen the requirement for air conditioning, which saves on both the astronomical initial installation costs and ongoing operating expenses. (One producer, View Glass, claims electrochromic glass may reduce peak energy use for lighting and cooling by around 20%.)

Whether or whether there are humans inside the building, they can be readily controlled by a smart-home system or a sunshine sensor because they are electrically driven. Scientists at the National Renewable Energy Laboratory (NREL) of the US Department of Energy estimate that windows like these could save as much as one-eighth of the nation’s building energy use annually. This is because windows require very little electricity to turn from dark to light—100 windows use about the same amount of energy as one incandescent lamp—resulting in a significant net energy savings.

Additional advantages of smart windows include enhanced security (electrically operated curtains are notoriously unreliable), convenience (automatically darkening windows can save your upholstery and pictures from fading), and privacy at the touch of a switch (no more fumbling around with clumsy, dusty curtains and blinds).

Difficulties

Glass with electrodes and sophisticated metal coatings is obviously going to cost a lot more to install than regular glass; the average cost of a single big smart window is between $500 and $1000 (that is, $500 to $1000 per square meter or $50 to $100 per square foot). Concerns have also been raised over the materials’ durability, as evidenced by the fact that the performance of most modern windows deteriorates after just 10 to 20 years—a far shorter lifespan than most homeowners would anticipate from traditional glass. The time it takes for modern windows to transition from light to dark and back again is another flaw. Certain technologies require a few minutes to completely darken from clear (Halio mentions three minutes), whereas stick-on window films change from clear to opaque and back in less than a second.

How can smart windows get better in the future?

Combining solar cells and electrochromic windows might also be an option. This way, when sunlight is blocked out by darkened smart windows, the energy it absorbs can be stored for later use. Although it is obviously impossible for a window to be completely transparent and function as a 100 percent efficient solar panel at the same time, it is simple to imagine windows that collect some of the solar energy that shines on them during the day and store it in batteries so they can power lights inside your home at night. The energy entering the system is either absorbed and stored or transferred through the glass, but not both. A window that could also function as a solar cell would probably need compromise on both sides: even under clear conditions, it would be quite dark, and it would be far less effective at gathering energy than a truly excellent solar cell.