Energy Efficient “Smart” Windows

According to the U.S. Department of Energy (DOE) over the next 20 years, energy consumption will increase:

• 45 percent for electricity
• 62 percent for natural gas
• 33 percent for oil

There are significant questions over the energy industry’s ability to meet this projected demand over the next 20 years. This implies that the price for all forms of energy will likely increase during this future time period. These consumption and price trends have been the catalyst for increased recent interest in developing low power consuming technologies. Among these technologies are “smart” windows, which allow consumers to control the amount light passing through a window by turning a knob or pressing a button. Smart windows apply one of a number of technologies:

• Suspended Particle Devices (SPD,)
• Polymer Dispersed Liquid Crystals (PDLC),
• Electrochromic, or
• Reflective Hydrides

These smart windows will block any extra heat, including any UV radiation. Since approximately 2 percent of all energy used in the U.S. dissipates through residential windows, this type of light control could save billions of dollars in heating and cooling costs and should be considered in any energy efficiency program.

Suspended Particle Devices

Windows serve a number of energy-related functions in homes and commercial buildings:

• Reduce the amount of energy needed to produce lighting
• Sunlight coming through the window provides heat

Because these functions are fairly basic there is little, if any, awareness of any cutting edge technology that might enhance the role that windows play in reducing energy costs. There is now a patented technology on the market, referred to as suspended particle devices (SPD), that enable a window to gradually change from clear to opaque with the flip of a switch. These SPD light control windows consist of the following:

• 2 panels of glass or plastic
• Conductive material to coat the panes of glass
• Millions of suspended particle devices placed between the panels of glass
• Liquid film to allow the suspended particles to float freely
• Control device which can be either automatic or manual

Coneptually, these windows either allow light to flow through or block light based on whether electricity comes into contact with the SPDs via the conductive coating or the electricity is taken away:

• When it comes into contact with electricity, the SPDs line up in a straight line, allowing light to flow through
• When electricity is not available, the SPDs return to a random pattern, thus blocking the light.
• A remote or automatic control device or process (i.e. rheostat or sensing device) moderates the amount of voltage to the conductive material.

Assuming that a consumer is interested in the benefits offered by this technology, one major objection is the issue of replacing the existing windows, which is certainly an expensive undertaking. This is not necessary as there are solutions that allow homeowners to upgrade their existing windows with SPD technology (reference: Methods for Retrofitting Windows with Switchable and Non-Switchable Window Enhancements; Research Frontiers patent No. 6,429,961), and studies have shown that 15 SPD “smart” windows require less power than that to operate a basic lamp.

Polymer Dispersed Liquid Crystals

Action similar to that of suspended particle devices (SPD) occurs with polymer dispersed liquid crystals (PDLC). Rather than dealing with the alignment and dispersion of suspended particle devices based on the presence or absence of an electrical charge, the medium here are liquid crystals. When the charge is present, the liquid crystals align parallel and let the light pass through; and when absent, they disperse randomly and block the light. Unlike with suspended particle devices, there are no intermediate settings: the window is either clear or translucent.

PDLC represents a developed technology, currently installed in offices where privacy is achieved without losing light. And, like SPD PDLC requires power to create transparency.

Electrochromic Windows

Electrochromic windows are able to be adjusted to varying levels of visibility. They darken as electricity increases in intensity and become transparent as electricity lessens. As the name implies, this type of window has electrochromic properties, described as being able to change color when exposed to an electrical current. In essence, electricity causes a chemical reaction or oxidation, which alters the way the window pane reflects and absorbs light. There are a number of different ways this occurs along with a broad array of materials and electrode systems. Like other “smart” windows, the basic construction involves the insertion of materials between 2 panes of glass. A typical electrochromic window uses the following:

• 2 glass or plastic panels,
• 2 layers of conducting oxide,
• Electrochromic layer,
• Ion conductor/electrolyte, and
• Ion storage

A source of power is connected to the two layers of conducting oxide and when energized it drives the ions from the ions storage layer through the conductor layer and into the electrochromic layer. This makes the window pane opaque. When the power is turned off, the ions leave the electrochromic layer and the window pane regains its transparency.

With this technology, electricity makes the initial change in opacity and maintaining a specific shade does not require a constant level of voltage. Rather, to change or reverse opacity only requires a specific amount of voltage, resulting in increased energy efficiency. One can run an entire house of electrochromic windows for the cost of powering a single light bulb.

Reflective Hybrids

Though technically classified as electrochromic, reflective hybrids are being developed to reflect rather than absorb light, offering the potential to be even more energy efficient than the more conventional electrochromic materials. They include thin films of a nickel-magnesium alloy that allow for the switching back and forth between transparent and reflective states, using either electricity or gas chromic technology for the switching.