Novel smart windows use liquid to switch from clear to reflective

20 Dec 2017

Affordable switchable windows could improve energy efficiency in buildings and help keep cars cool.

Researchers at the University of Delaware have developed prototype windows that switch from reflective to clear with the simple addition of a liquid. The developers say that the switchable windows are cheap and easy to manufacture and could one day keep parked cars cool in the sun or make office buildings more energy efficient.

The work has just been published in Optics Express.

Although glass that uses an applied voltage to switch from transparent to an opaque state is commercially available, its high cost — typically around $100 per square foot (0.1m²) — has limited its widespread use.

“We expect our smart glass to cost one tenth of today's typical smart glasses because our version can be manufactured with the same methods used to make many plastic parts and does not require complicated electro-optics,” said Prof Keith Goossen, who led the research team with Daniel Wolfe, who both work at the University of Delaware.

The 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 only in the direction whence it came.

Prototype

The 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.

The Delaware team has demonstrated 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.

One promising application for this glass may be in cars, where it could be used to switch the windshield to a reflective state when the car is parked in hot sun. “Because our glass is retroreflective in the non-transparent state, almost all the light is reflected, keeping the glass, and so the car, from getting hot,” explained Goossen.

Switchable windows and roofing

The fact that the glass is retroreflective means that if it were used on the outside of building, for example, it would direct light up toward the sun rather than down to the street. This reduces the building’s contribution to atmospheric warming, which can be 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.

Goossen added, “Here in Delaware, for example, residents 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. 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.”

3D printing

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, transparent, 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.

“To further demonstrate the technology’s usefulness as switchable glass, we are now building an office door incorporating our 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.”

Paper conclusion

The Optics Express paper concludes, “Optofluidic smart glass exhibits superior transmittance in the light transmitting state compared to other commercially available smart glasses, such as, electrochromic, polymer dispersed liquid crystal, and suspended particle devices. At normal incidence, in the light blocking state, optofluidic smart glass exhibits only slightly higher transmittance than EC, while blocking significantly more light than liquid crystal and suspended particle devices.

“The corner cube variable transmittance device is capable of modulating visible light transmittance from 8% to 85% at normal incidence. Solar weighted energy transmittance modulation ranges from 22% when the device is filled with air to 79% when the device is filled with index matching fluid. A cycling experiment showed that optofluidic smart glass performs well over time with only slight deviation in light transmittance.”

Video

The following video, produced by Prof Goossen, shows the smart glass technology in action: