It's great to see the natural light streaming through a sunny window. But it can make you squint and sweat your forehead. The shades can be closed or the air conditioner turned on, but it leaves you with a darkened room and a rising electric bill.

Smart windows were invented 40 years ago to solve this problem. They can be tunable to suit your needs. These windows are made of special materials that block certain wavelengths of light. They can adapt to the weather and your individual comfort. Are you too hot? To block out light and keep the room cooler, tinting windows is an option. Too cold? Too cold? You want privacy? Glass can become cloudy with a flick of a switch

Smart window research has made the technology more accessible than ever before. The smart glass market is expected to grow to $7.5 billion in 2028. Smart windows are already found in cars, boats, and planes. They are now making their way into offices and terminals. Proponents believe that smart windows could be a key part of making homes more efficient, provided they can overcome some key challenges.

The U.S. buildings account for 39% of the energy consumed. Heating, ventilation, and air conditioning accounts for 35 percent. Smart windows are able to block heat or let it in. This can help reduce energy consumption.

Smart windows must be affordable for everyone in order to become mainstream and reduce energy consumption. Current smart windows can be 10 times more expensive than standard energy-efficient windows. Researchers are interested in making the technology cheaper and smarter.

Claes-Goran Grandqvist, materials physicist, believes that there will be no need for windows other than smart ones in the future.

Smart windows were invented long before televisions and phones were "smart". In the 1980s scientists from Chalmers University of Technology (Sweden) and Lawrence Berkeley National Laboratory (California) were looking for new ways to make energy-efficient building materials. Researchers came up with the idea of a responsive window that could dynamically alter its tint. Granqvist was part of the early research and used the term "smart windows” on a grant application. This name stuck, and in 1984 the first smart windows was born.

Original prototypes measured only centimeters in length and had glass that could be reversibly transformed from transparent to opaque. This was achieved by sandwiching glass, layers of materials, and transparent conductors that provided small electric jolts. The voltage changed the way the materials interacted to light, changing the wavelengths that were absorbed or reflected.

Researchers began to explore other substances that could manipulate light in response other prompts. These included heat, ultraviolet light, and magnetism. Researchers continue to explore new "smart" materials and a variety of other special "smart" materials.

Early prototypes were made from "electrochromic", which means that they can revert to their original state when exposed to voltage or electrical current. The five layers of electrochromic windows are typically five. These include two layers that act as electrodes, which is similar to the positive and negative poles in a battery, and an inner layer with ions. Positively charged ions can be driven into one layer by applying voltage, while electrons are drawn into the other. This creates a tint which blocks visible light and heat-packed infrared radiation. The tint will remain until another round voltage triggers a reverse react that extracts electrons and ions. This makes the window transparent again.

Granqvist of Uppsala University's Angstrom Laboratory says, "It's exactly like an electrical battery." It takes energy to charge the battery, but once it is charged you can keep it going for a while.

Smart windows are often made from electrochromic glass. It can be found in privacy screens and display panels as well as in aircraft windows, boat windows, and sunroofs. When they're not powered, some types of electrochromic glasses are almost opaque. The glass's responsive materials (e.g crystal droplets, suspended particles) are not arranged in a systematic way. This scatters light and makes it cloudy. If you zap the glass with an electric current, the crystals or droplets will line up and make it transparent.

These electrochromic windows, along with other electrochromic windows, allow building occupants to control tint levels. However, electricians are needed to correctly build the electrode arrays. This complicates design and installation. Prices are much higher than other smart windows designs. Granqvist states that the materials are inexpensive and can be made in scalable ways.

Electrochromic windows have much to learn. With the goal of increasing efficiency and longevity, researchers continue to explore new responsive materials. Scientists recently reported that a prototype containing tin dioxide and two other metals that block visible and infrared light, was able to withstand many more cycles than existing electrochromic windows.

Theother class of smart windows that respond to heat is called thermochromic. It doesn't require complicated wiring or power supply like electrochromic windows. Vanadium dioxide (VO2) is a thermochromic material that has been studied for a long time, according to Harlan Byker, a materials scientist who created Pleotint which makes dynamic window glass.

VO2 can shape-shift at higher temperatures (around 68 degrees Celsius or 154 degrees Fahrenheit), which increases its ability to reflect infrared sunlight. This allows visible light to continue streaming in, brightening and cooling the room. Byker states that researchers can make VO2 reflect at lower temperatures by adding special substances to it. However, this reduces the light's reflection, making it more difficult to move the product from the laboratory to the market.

The next generation of thermochromic windows could use materials that absorb light rather than reflect it. This creates a continuous tint as temperatures increase, similar to electrochromic Windows. A recently discovered substance uses interactions between metalions and other molecules to absorb visible and infrared light less when it's cold, allowing heat in. The materials are more sensitive to heat and absorb more light when it is hotter.

Each temperature-responsive material transitions at different temperatures and can look more or less tinted -- and has challenges, says materials scientist Long Yi of the Nanyang Technological University in Singapore. Hydrogel, a polymer known as hydrogel, is great at absorbing heat, but it has a tint that is too opaque to be seen through. Some, like VO2, can be transparent enough to preserve views but they aren't as efficient as heat-reflecting materials.

Yi states that there are no perfect materials. Yi says, "We can only find the right material for certain applications."

Engineering windows that trap heat and scatter light is one example. Yi's laboratory is working on thermochromic windows that store heat at the sunniest time of the day, and then release it as needed.

Hydroliquid, a mixture of water and gel, can be used to do this. The glass becomes transparent due to the expansion of the polymers in the hydrogel at temperatures below 30° Celsius (86° Fahrenheit). The chains become paper-like and form a tint by entwining with one another. Hydroliquid, due to its high heat resistance, absorbs heat and slowly releases it. According to Yi's simulation data, this can reduce heating, ventilation, and air conditioning energy requirements by 35 percent, compared with double-pane glass.

The window is a bi-directional one, according to her latest research. Yi's team developed a window that uses energy-efficient coatings, VO2 and other materials to block heat-packed infrared sunlight from entering buildings. Indoor heat radiates out, cooling the rooms. This was published in Science 2021. The winter version of this mechanism works in reverse. As temperatures drop, the materials allow light to enter the room and block indoor heat radiating out. Yi claims that this new technology could reduce energy consumption by up to 15% compared with existing commercially available energy-efficient glass coatings.

Smart windows are a great way to save energy, but as with other innovations, like electric cars, it can be a little bumpy.

Smart windows' high price tag is a major obstacle to expanding into larger spaces. This was noted in a review of smart windows materials published in the Annual Review of Chemical and Biomolecular Engineering. Because of the complexity of their powering systems and long installation times, electrochromic windows can be more costly than energy-efficient windows. Even though they are less expensive than regular windows, thermochromic windows still have a higher price tag. Even though smart windows offer greater energy savings, most consumers prefer to choose the less expensive option.

While scientists continue to improve the technology, smart windows' success or failure is largely dependent on economics. Smart windows must have lower prices in order to succeed, according to Carl Lampert (materials scientist and electrical engineer), a managing partner at Star Science, a window coatings consulting company. It could be a good idea to include windows in government rebate programs, such as the Energy Star symbol which recognizes energy-efficient products. Consumers could be better educated about emerging technologies, their performance, and energy savings.

Researchers are slowly making smart windows simpler, more affordable, and safer. However, consumers might have to wait: Yi estimates that it could take 10 years before new products enter the market.

These windows could become more common, passively adapting to customers' needs or responding to a press of a button, like a remote control for a TV. He envisions a future in which he could ask for a smart glass at Home Depot and not cause confusion among workers. Other researchers are also in agreement:

Byker states that dynamic windows are "unavoidable." It's just a lot birthing pains.