Making Smart Windows that Are Also Cheap

"Tunable" windows would let people adjust light and heat levels, but so far it's been hard to make them affordable.

Windows that absorb or reflect light and heat at the flick of a switch could help cut heating and cooling bills. A company called Soladigm has developed methods for making these "electrochromic" windows cheaply, making them more viable for homes and office buildings.

Existing electrochromic window designs cost around $100 per square foot. Soladigm has not disclosed how much its windows will cost, but some experts say the method could reduce the cost to around $20 per square foot.

The Milpitas, CA-based company uses a thin-film deposition process that creates conducting layers between two panes of glass for controlling the amount of sunlight and heat that can pass through. A homeowner or office dweller could control how much light or heat a window lets in or absorbs and reflects.

The company's windows contain two transparent conducting oxide films sandwiching an ion storage layer, an electrolyte, and an electrochromic layer--all between two layers of glass. Applying a low voltage to the conductive oxide kicks the ions out of the storage layer and across the electrolyte to meet with the electrochromic layer. The collision prompts the electrochromic material to absorb or reflect light. It also causes the material to darken, giving the window a tinted look. Reversing the voltage sends the ions back to its storage layer, causing the window to lighten in color and let more light in.

"We did a case study in five cities, and the average savings in commercial buildings are about 25 percent of the heating, ventilation, and air-conditioning energy use annually," says Rao Mulpuri, CEO of Soladigm.

The trick to making electrochromic windows cheaply is the right materials and latest manufacturing method, says Mulpuri. Today's thin-film deposition equipment--the same used to make flat panel display and thin-film solar panels--is much better than that used a few decades ago, when the electrochromic window concept emerged.

Soladigm will use a tungsten oxide-based electrochromic layer for its first windows. Tungsten oxide can endure repeated cycling between ion-rich and ion-free stages-which makes it durable, says Delia Milliron, a Lawrence Berkeley National Laboratory (LBNL) researcher in electrochromic materials. However, using tungsten oxide can heat up a window until it's too hot to touch; it also doesn't block infrared light very well, meaning it lets plenty of heat through. 

Source:  Technology Review