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LOW-COST, TUNABLE SMART WINDOWS DEVELOPED WITH ‘ELECTROKINETIC PIXELS’

Low-cost, tunable smart windows developed with ‘electrokinetic pixels’

By Calum Williams on June 16, 2015 at 2:00 pm
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Researchers at the University of Cincinnati with industry partners, have created low-cost, smart window technology based on electro-kinetic pixels, which can dynamically adapt for brightness, color temperatures and opacity — something that neither blinds nor existing smart windows can do. This patent-pending research, supported by theNational Science Foundation, will hopefully lead to low-cost window tinting that can provide a wide range of optical functionality such as adjusting for color and brightness.

What is the problem?

Currently, most home and commercial windows use inexpensive, mechanical shades to provide privacy and to block light, heat or cold. These (as you may have guessed) are quite effective, which has slowed the adoption of relatively expensive and seemingly unnecessary electronically controlled window tinting. Previously these electronic windows could only mimic the clear-to-opaque performance of mechanical shades — therefore offering no additional performance or functionality benefit, but at a cost far greater than existing, and much simpler, age-old technology. For any real-world interest to gain hold, smart windows need to be both inexpensive and offer something that shades cannot.

What have the researchers actually created?

Mock up showing blue, yellow, black, clear, gray and all the intermediate shades possible.

The researchers have produced electrokinetic pixels for use as a fundamental smart window element. These are capable of dimming light transmission and fully altering the color temperature of transmitted light. They can be easily integrated into new windows or even easily applied to already existing windows, by means of a roll-on coating consisting of a honeycomb of electrodes (see bottom right image). The devices are based on electro-phoretic principles, which is the technology underlying e-paper and e-ink displays.

So far, the team has only created a proof-of-concept device, but envisage future functionality where “…[the pixels] could easily go milky for privacy so no one can see in, but still allow 90 percent (or more) of the available light in. Or, a setting change could dim the entering light or change the color of the light along a spectrum from cooler blue to warmer yellow…”

The image to the above right shows the different potential states the window tiles could operate in. This is a mock-up based on the results from the single device. The video below is an exciting presentation from the researchers on where they see this technology ending up.

How do the pixels work?

As stated, the device works off electrophoretic principles, the phenomena behind e-ink / e-paper displays, whereby opaque, charged particles (approx. 1 micron in size) are attracted to electrodes that posses an opposite charge. Thus the electric field created between the two electrodes causes the flow of these particles back and forth. The interaction of incoming light with each electro-phoretic pixel (two electrodes) depends on the position of the particles relative to these electrodes, and the light can be scattered (white state), or not (dark-state), or somewhere in-between (grey scale).

The figure below shows (left) the device construction schematic and (right) the operation. In reference to these, the device uses electrophoresis to attract charged color particles to the top transparent electrode to ‘spread’ their color, or to the bottom ‘compact’ electrode in order to hide them in the micro-pits, which are adjacent to the electrode. The device is filled with dual-colour, dual-particle colloidal dispersion inks. It is common in e-ink displays to use sub-pixels (i.e. multiple electrodes to do some fancy charged transport), and this is no different. They use three electrodes, and by altering the applied voltages between them all, many color states are created during operation, as shown in the right image.

: Top view and side-view diagrams of the device construction.

Haven’t I seen this before in e-ink?

You may even be reading this article on an e-ink device, so the obvious question may be: Is this even new? Well, the basic technology is similar to that in electronic display devices. The challenge for the the team from the university, and the two companies (Merck and HP), was how to apply common e-paper technology to larger structures such as windows, but inexpensively. For example, $30 per square foot is the industry standard for window manufacturing, so in order to become viable, the technology must attempt to meet this figure.

The researchers realized the potential in having a few selective, but compelling, operating modes (such as changing color temperature or privacy/shade) yet all in a form factor that can be inexpensive and simple to mass produce. E-ink devices are normally fabricated using photo-lithography, which is the same technology used to pattern microchips and is very expensive. But the design here can be integrated with HP’s roll-to-roll micro-patterning technique, a much cheaper and large scale alternative.

In summary, the work here provides a unique design for smart windows using existing e-ink technologies, but in an architecture that can be manufactured inexpensively, but still offer both electronic multi-color and transmission changing control. Rivals to this technology are liquid-crystal-based wallpapers and windows, and electrochromic materials (such as vanadium dioxide). It will be interesting to see which technology can provide the performance and low cost ratio needed to succeed. The specific details of the work can be found in their Applied Optics paper.