Could a new ultra-thin coating that responds to heat and cold turn your dumb window into a smart one?

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The self-modifying coating, which is a thousand times thinner than a human hair, works by automatically letting in more heat when it’s cold and blocking the sun’s rays when it’s hot.

Smart windows have the ability to naturally regulate temperatures inside a building, leading to major environmental benefits and significant financial savings.  The breakthrough could help meet future energy needs and create temperature-responsive buildings.

Our technology will potentially cut the rising costs of air-conditioning and heating, as well as dramatically reduce the carbon footprint of buildings of all sizes.

Smart glass windows are about 70 per cent more energy efficient during summer and 45 per cent more efficient in the winter compared to standard dual-pane glass.

“Our coating doesn’t require energy and responds directly to changes in temperature.”

“This switch is similar to a dimmer and can be used to control the level of transparency on the window and therefore the intensity of lighting in a room,” Taha said. “This means users have total freedom to operate the smart windows on-demand.”

“The materials and technology are readily scalable to large area surfaces, with the underlying technology filed as a patent in Australia and the US,” she said.

Learn more: Clever coating opens door to smart windows


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Easy-to-manufacture switchable windows could improve energy efficiency in buildings and help keep cars cool

Researchers created a prototype smart glass that is retroreflective (left) and becomes clear (right) when a liquid with optical properties similar to the reflective structure is pumped into a chamber in front of the structure. Image Credit: Keith Goossen, University of Delaware

New type of smart windows use liquid to switch from clear to reflective

Researchers have demonstrated prototype windows that switch from reflective to clear with the simple addition of a liquid. The new switchable windows are easy to manufacture and could one day keep parked cars cool in the sun or make office buildings more energy efficient. The technology can also be used to make roof panels that keep houses cool in the summer and warm in the winter.

Although glass that uses an applied voltage to switch from clear to an opaque or tinted state is commercially available, its high cost— around $100 per square foot — has hindered widespread use.

“We expect our smart glass to cost one tenth of what current smart glass costs because our version can be manufactured with the same methods used to make many plastic parts and does not require complicated electro-optic technology for switching,” said Keith Goossen, who led the research team with Daniel Wolfe of the University of Delaware.

The new smart windows contain a plastic panel with a pattern of structures that is retroreflective. This means that rather than reflecting light in all directions like a mirror, it reflects light back in the direction it came from like a bicycle reflector.

In The Optical Society (OSA) journal Optics Express, the researchers demonstrate a prototype of the new smart glass consisting of a 3D printed plastic panel covered by a thin chamber. When the chamber is filled with the fluid methyl salicylate — which matches the optical properties of the plastic — the retroreflective structures become transparent.

“Although we had to develop new ways to process 3D printable plastics with good optical performance, develop inexpensive refractive index-matching fluids and come up with highly reflective optical structures, the innovation here is mostly in recognizing that such a simple concept could work,” said Goossen.

Keeping cars cool

One of the most promising applications for the new switchable glass may be in cars, where it could be used to change the windshield to a reflective state when the car is parked in the hot sun.

“You can’t use today’s commercially available switchable glass for this application because in the darkened state the windshield still absorbs sunlight and becomes hot,” said Goossen. “Because our glass is retroreflective in the non-transparent state, almost all the light is reflected, keeping the glass, and thus the car, from getting hot.”

The fact that the glass is retroreflective means that if it were used on the outside of skyscraper, for example, it would direct light up toward the sun rather than down to the street. This reduces the building’s contribution to city warming, which is a problem in many urban areas.

The researchers have also shown that the plastic retroreflective panels can be used as an inexpensive switchable roofing structure that cuts down on heating and cooling costs. In places that are warm and sunny year-round, white roofing materials have been shown to lower cooling costs by reflecting sunlight. However, in areas with cold winters, these white roofs prohibitively add to heating costs in the winter.

“Here in Delaware, you would like to have a white roof in the summer to keep the house cool and a dark roof in the winter to absorb sunlight and help lower heating costs,” said Goossen. “For smart roofing, our new technology offers a more effective type of cool roof because it is retroreflective while also allowing the roof to switch to dark in the winter.”

For roofing applications, a layer of material placed under the panels is used to absorb light when the panels are in their clear state. This helps keep the house warmer when outside temperatures are cold. Although the methyl salicylate used in the prototype could freeze in very cold climates (less than 16 degrees Fahrenheit), freeze-resistant fluids could be developed.

3D printing the prototype

To make the new switchable glass, the researchers started by using 3D printing to make plastic panels with repeating retroreflective structures of various sizes for testing. They used a commercially available clear 3D printable material and developed post-processing steps to ensure the plastic remained highly transparent after printing and exhibited very accurate corners, which were important to achieve retroreflection.

“Without 3D printing, we would have had to use a molding technology, which requires building a different mold for every different structure,” said Goossen. “With 3D printing, we could easily make whatever structure we wanted and then run experiments to see how it performed. For commercial production, we can use standard injection molding to inexpensively make the retroreflective panels.”

Once the researchers figured out the optimal size to use for the repeating structures, they performed optical testing to determine whether characteristics such as surface roughness or the material’s light absorption would cause unexpected optical problems. These optical tests showed that the structures worked exactly as indicated by optical simulations.

“Importantly, we also demonstrated that the device can undergo thousands of cycles from transparent to reflective without any degradation,” said Goossen. They did, however, find that some fluid stays on the structure instead of draining off. To solve this issue, the researchers are developing coatings that will help the fluid drain off the plastic without leaving any residue.

“To further demonstrate the technology’s usefulness as switchable glass, we are building an office door that incorporates the new smart glass as a switchable privacy panel,” said Goossen. “These types of panels are currently made with much more expensive technology. We hope that our approach can broaden this and other applications of smart glass.”

Learn more: New Type of Smart Windows Use Liquid to Switch from Clear to Reflective


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Smart windows get the ability to tint gradually

Figure: Dimming glass (20cm x 20cm) whose darkness can be varied from a completely darkened state (left) to a completely clear state (right)

NIMS, Waseda University and Tama Art University developed together smart glass capable of producing various shades on its surface.

NIMS, Waseda University and Tama Art University developed together smart glass capable of producing various shades on its surface. Unlike the conventional types, the newly developed tinting smart glass allows users to easily change the shaded area of a window. For example, a user would be able to change the shaded area of a window in accordance with the elevation of the sun. The technology may be applicable to various types of windows, including those of automobiles and buildings, enabling them to offer both shade and clear visibility simultaneously.

Electrochromic glass capable of blocking sunlight in response to electricity has been used in airplane windows in recent years. However, because these windows become shaded uniformly across their entire surface, once they are darkened, passengers are unable to enjoy any exterior views.

The joint research group recently developed a tinting smart glass that can produce gradational shade (Fig. 1) using electrochromic polymers (metallo-supramolecular polymers) studied by the group. The darkened part of the glass surface can be changed simply by applying a low voltage (3 V) for different durations. The desired shade can be maintained even after the voltage is reduced to turn off the current. In addition, the shading and transparency can be reversed by reversing the electric current, allowing users to easily change the darkened area of the glass surface as desired.
In the future, we plan to direct our research toward practical applications of this technology, including windows in vehicles (e.g., automobiles and airplanes) and in buildings.

This research was conducted jointly by a NIMS research group led by Masayoshi Higuchi (leader of the Electronic Functional Macromolecules Group, Research Center for Functional Materials); Research Organization for Nano & Life Innovation, Waseda University, and Faculty of Art and Design, Tama Art University. This study was carried out in conjunction with a research project entitled “Ultrafast, ultralow-power, ultralarge-area electrochromism” (Masayoshi Higuchi, principal investigator) in the research area “Innovative nano-electronics through interdisciplinary collaboration among material, device and system layers” (Takayasu Sakurai, research supervisor) funded by the JST Strategic Basic Research Programs (specifically the CREST program).

Learn more: Gradation-Tint Smart Window


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Self-powered smart windows can augment lighting, cooling and heating systems

Graduate student Nicholas Davy holds a sample of the special window glass. (Photos by David Kelly Crow)

Smart windows equipped with controllable glazing can augment lighting, cooling and heating systems by varying their tint, saving up to 40 percent in an average building’s energy costs.

These smart windows require power for operation, so they are relatively complicated to install in existing buildings. But by applying a new solar cell technology, researchers at Princeton University have developed a different type of smart window: a self-powered version that promises to be inexpensive and easy to apply to existing windows. This system features solar cells that selectively absorb near-ultraviolet (near-UV) light, so the new windows are completely self-powered.

“Sunlight is a mixture of electromagnetic radiation made up of near-UV rays, visible light, and infrared energy, or heat,” said Yueh-Lin (Lynn) Loo, director of the Andlinger Center for Energy and the Environment, and the Theodora D. ’78 and William H. Walton III ’74 Professor in Engineering. “We wanted the smart window to dynamically control the amount of natural light and heat that can come inside, saving on energy cost and making the space more comfortable.”

The smart window controls the transmission of visible light and infrared heat into the building, while the new type of solar cell uses near-UV light to power the system.

“This new technology is actually smart management of the entire spectrum of sunlight,” said Loo, who is a professor of chemical and biological engineering. Loo is one of the authors of a paper, published June 30, that describes this technology, which was developed in her lab.

Because near-UV light is invisible to the human eye, the researchers set out to harness it for the electrical energy needed to activate the tinting technology.

“Using near-UV light to power these windows means that the solar cells can be transparent and occupy the same footprint of the window without competing for the same spectral range or imposing aesthetic and design constraints,” Loo added. “Typical solar cells made of silicon are black because they absorb all visible light and some infrared heat – so those would be unsuitable for this application.”

Princeton engineers invented a window system that could simultaneously generate electricity and lower heating and cooling costs. The team, led by Professor Yueh-Lin (Lynn) Loo, center, includes graduate students Nicholas Davy, left, and Melda Sezen-Edmonds, right. Behind them is a cleanroom at the Andlinger Center for Energy and the Environment, where Loo is the director.

In the paper published in Nature Energy, the researchers described how they used organic semiconductors – contorted hexabenzocoronene (cHBC) derivatives – for constructing the solar cells. The researchers chose the material because its chemical structure could be modified to absorb a narrow range of wavelengths – in this case, near-UV light. To construct the solar cell, the semiconductor molecules are deposited as thin films on glass with the same production methods used by organic light-emitting diode manufacturers. When the solar cell is operational, sunlight excites the cHBC semiconductors to produce electricity.

At the same time, the researchers constructed a smart window consisting of electrochromic polymers, which control the tint, and can be operated solely using power produced by the solar cell. When near-UV light from the sun generates an electrical charge in the solar cell, the charge triggers a reaction in the electrochromic window, causing it to change from clear to dark blue. When darkened, the window can block more than 80 percent of light.

Nicholas Davy, a doctoral student in the chemical and biological engineering department and the paper’s lead author, said other researchers have already developed transparent solar cells, but those target infrared energy. However, infrared energy carries heat, so using it to generate electricity can conflict with a smart window’s function of controlling the flow of heat in or out of a building. Transparent near-UV solar cells, on the other hand, don’t generate as much power as the infrared version, but don’t impede the transmission of infrared radiation, so they complement the smart window’s task.

Davy said that the Princeton team’s aim is to create a flexible version of the solar-powered smart window system that can be applied to existing windows via lamination.

“Someone in their house or apartment could take these wireless smart window laminates – which could have a sticky backing that is peeled off – and install them on the interior of their windows,” said Davy. “Then you could control the sunlight passing into your home using an app on your phone, thereby instantly improving energy efficiency, comfort, and privacy.”

Joseph Berry, senior research scientist at the National Renewable Energy Laboratory, who studies solar cells but was not involved in the research, said the research project is interesting because the device scales well and targets a specific part of the solar spectrum.

“Integrating the solar cells into the smart windows makes them more attractive for retrofits and you don’t have to deal with wiring power,” said Berry. “And the voltage performance is quite good. The voltage they have been able to produce can drive electronic devices directly, which is technologically quite interesting.”

Davy and Loo have started a new company, called Andluca Technologies, based on the technology described in the paper, and are already exploring other applications for the transparent solar cells. They explained that the near-UV solar cell technology can also power internet-of-things sensors and other low-power consumer products.

“It does not generate enough power for a car, but it can provide auxiliary power for smaller devices, for example, a fan to cool the car while it’s parked in the hot sun,” Loo said.



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Admitting visible light, rejecting 90 per cent of the heat from sunlight

A*STAR researchers have developed a window coating that lets visible light through while blocking near-infrared radiation. via A*STAR Singapore Institute of Manufacturing and Technology

A*STAR researchers have developed a window coating that lets visible light through while blocking near-infrared radiation.
via A*STAR Singapore Institute of Manufacturing and Technology

A coating that blocks 90 per cent of the heat from sunlight could be used to develop smart windows

By fine-tuning the chemical composition of nanoparticles, A*STAR researchers have developed a coating that is promising for fabricating smart windows suitable for tropical countries. Such windows block almost all the infrared heat from sun rays, while admitting most of the visible light.

The transparency of glass to visible light makes it the most common way to let light into a building. But because glass is also transparent to near-infrared radiation — windows also let in heat, giving rise to the well-known greenhouse effect. While this heating is welcomed in colder climates, it means that air conditioning has to work harder to maintain a comfortable temperature in tropical climes.

Developing smart windows that allow most of the sun’s light in, while blocking near-infrared radiation, would cut energy costs and reduce carbon emissions.

“In tropical Singapore, where air conditioning is the largest component of a building’s energy requirements, even a small reduction in heat intake can translate into significant savings,” notes Hui Huang of the A*STAR Singapore Institute of Manufacturing and Technology.

Huang and his co-workers have developed such windows by coating glass with tin oxide nanoparticles doped with small amounts of the element antimony. By varying the nanoparticles’ antimony concentration, they could optimize their ability to absorb near-infrared radiation.

“Our infrared shielding coating, with 10-nanometer antimony-doped tin oxide nanoparticles, blocks more than 90 per cent of near-infrared radiation, while transmitting more than 80 per cent of visible light,” says Huang. “These figures are much better than those of coatings obtained using commercial antimony-doped tin oxide nanopowders. In particular, the infrared shielding performance of our small antimony-doped tin oxide nanocrystals is twice that of larger commercial antimony-doped tin oxide powders.”

The team produced the tiny nanoparticles using a synthesis technique known as the solvothermal method, in which precursors are heated under pressure in a special vessel, called an autoclave. The solvothermal method permits synthesis at relatively low temperatures. It also enables the nanoparticle size to be tightly controlled, which is important when trying to block some wavelengths of light while allowing others to pass through.

The work has already attracted the interest of industry. “A local glass company supporting this project is interested in licensing this smart window technology with infrared shielding,” says Huang. Potentially, the coating techniques could be applied on-site to existing windows, he adds.

Learn more: Admitting visible light, rejecting infrared heat



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