Electrochromic Windows Basics

What Are Electrochromic Windows?

Electrochromic windows are windows that can be darkened or lightened electronically. A small voltage applied to the windows will cause them to darken; reversing the voltage causes them to lighten. This capability allows for the automatic control of the amount of light and heat that passes through the windows, thereby presenting an opportunity for the windows to be used as energy-saving devices.

How Do Electrochromic Windows Work?

Electrochromic windows consist of up to seven layers of materials. The essential function of the device results from the transport of hydrogen or lithium ions from an ion storage layer and through an ion conducting layer, injecting them into an electrochromic layer.

The electrochromic layer is typically tungsten oxide (WO₃). The presence of the ions in the electrochromic layer changes its optical properties, causing it to absorb visible light. The large-scale result is that the window darkens. The central three layers are sandwiched between two layers of a transparent conducting oxide material. To protect the five layers of materials, they are further sandwiched between two layers of glass. All of the layers, of course, are transparent to visible light.

To darken (or "color") the windows, a voltage is applied across the two transparent conducting oxide layers. This voltage drives the ions from the ion storage layer, through the ion conducting layer and into the electrochromic layer.

To reverse the process, the voltage is reversed, driving the ions in the opposite direction, out of the electrochromic layer, through the ion conducting layer, and into the ion storage layer. As the ions migrate out of the electrochromic layer, it lightens (or "bleaches"), and the window becomes transparent again.

The challenges in fabricating electrochromic windows lie in achieving low costs, high durability, and practical sizes. The largest samples fabricated by participants in DOE's Electrochromic Initiative thus far measure 14 by 16 inches, which is suitable for a window divided into small panes.

Benefits of Electrochromic Windows

Electrochromic windows can control the amount of daylight and solar heat gain through the windows of buildings and vehicles. The ability to control these parameters using an electronic circuit suggests a variety of applications.

For instance, a small photovoltaic cell could be used to sense the amount of sunlight, darkening the window when the sun is brightest. This would be an appropriate application in a hot climate where solar heating is not desired. An alternate approach would be to only lighten the window when direct sunlight is available, but darken it for privacy at other times. This approach would be useful in areas where solar heating is desired. Either approach could feature an override switch for the convenience of the inhabitants.

In a building with ample solar heating, one could even conceive of using electrochromic windows as a thermostatic control, darkening or lightening the windows as needed to provide more or less solar heating to the living space. In this case, the electrochromic window would be controlled by an electronic thermostat.

Electrochromic windows also could be used to help keep cars cool. An electrochromic sunroof could darken in the direct sunlight but lighten at other times, providing function while keeping the car cool. Electrochromic rear or side windows in a car could conceivably darken while the car is parked, keeping the car cool, and then lighten again once the car is started.

Advanced electrochromic technologies such as photo-electrochromic windows would darken in sunlight without the use of an external control circuit. Such windows would be ideal for applications such as car sunroofs. Because of their ability to control daylighting and solar gain, electrochromic windows have the potential of reducing the annual U.S. energy consumption by several quadrillion (10¹⁵) Btus, or quads. The United States currently consumes a total of about 94 quads of energy per year.