Thirty percent of the energy used by buildings in the United States is spent making up for heat loss or gain through windows. That adds up to about $40 billion in electricity costs each year. Windows that change color in response to changes in the weather can help save on electricity costs by absorbing sunlight in the winter and reflecting it in the summer. Such windows have existed for awhile, but they are expensive and not widely used. Now researchers are developing cheap printing methods for making these electrochromic systems, and hope to make electrochromic films that can be cut to fit existing windows.
Electrochromic windows sandwich materials that change color when a small electrical field is applied across them. This change is triggered by changes in light or temperature measured by sensors. “With electrochromic windows, everything happens dynamically–you don’t have to think about it,” says Anne Dillon, senior scientist at the National Renewable Energy Laboratory (NREL). “The problem is, they’re too expensive.”
This week at the Materials Research Society meeting in Boston, Dillon and research scientist Robert Tenent at NREL presented their new and potentially cheaper method for making electrochromic windows.
Typical electrochromic systems are made up of two electrodes separated by an electrolyte that shuttles ions between them. The electrode materials, usually oxidized metals, change color when an ion such as lithium moves into and out of them.
The NREL systems are based on electrodes made of nickel oxide and tungsten oxide and are the first electrochromic systems to be made by spraying down cheap precursors and then heating them. NREL has tested the systems using a liquid electrolyte, and is currently developing systems that rely on solid ion conductors. When a voltage is applied across the NREL system, lithium ions move out of the nickel oxide and into the electrolyte; on the other side, lithium ions move into the tungsten oxide. The movement of the ions causes the two electrodes to color.
Spraying the films is not only a cheaper alternative, says Tenent, it also provides some advantages in performance. The NREL team found that adding a small amount of lithium to the nickel-oxide ink solution before it’s printed made for a film that changes color much faster and within a wider range. In 29 seconds, as lithium leaves the nickel electrode and it darkens in color, the electrode goes from transmitting 80 percent of incident light to transmitting just 30 percent. Adding a small amount of lithium using conventional manufacturing techniques would be much more difficult, Tenent says.
There are other ways to make color-changing windows–by using materials that undergo a chemical change in response to light, for example. But these materials are prone to degradation. The NREL group is developing the metal-oxide electrodes in the hopes that these materials, which are robust and don’t degrade in response to light, will have long lifetimes.
So far, the NREL system has been tested on glass substrates. To make a truly affordable window coating, the group is working to make electrochromic films based on flexible, transparent plastics. The group is talking to DuPont, which makes plastics, about collaborating to fabricate electrochromic films sandwiched between one of the company’s heat-tolerant polymers. The nickel-oxide precursor must be heated to about 300 ºC in order to form the electrode material, a temperature many plastics can’t tolerate.