Scientists at the Nanyang Technological University (NTU) in Singapore have developed a smart window that is able to tint itself blue, partially blocking incoming light, without the need for an external power source. The device also functions as a small transparent battery that recharges on its own simply by interacting with the oxygen in its surroundings.

Indoor cooling can make up a sizeable percentage of a building’s power consumption. To help with the problem, we’ve seen a number of "smart windows" pop up that act as high-tech shutters to shield incoming light during those hot sunny days.

The prototypes we’ve seen so far  –  whether designed for use in buildings or public transport, controllable with a light switch or wirelessly through a smartphone  –  generally rely on an external power source to function, which is both a hindrance and an ongoing additional cost. And though they are better looking and make for techier gadgets than your standard blinds or curtains, in the end, even the self-powered models are fairly limited in their functionality.

Professor Sun Xiaowei and colleagues at NTU have developed a self-powered smart window that is bi-functional. It can not only tint itself blue at the flip of a switch, but also act as a transparent electrochemical battery that can slowly charge itself by absorbing oxygen from it surroundings.

The window consists of two glass panes connected by electrical cables, with an oxygen-carrying liquid electrolyte in between them. The panes are wrapped inside a transparent conductive coating which is attached to an aluminum foil on one side and coated with an oxygen-sensitive blue dye on the other.

When the circuit between the two glass panes is opened, the dye reacts with the oxygen in the electrolyte and spontaneously turns blue, blocking up to 50 percent of light passing through the window. Conversely, when the circuit is closed the process is reversed and the dye becomes transparent again in a matter of seconds. "[The window] charges up and turns blue when there is oxygen present in the electrolyte  –  in other words, it breathes," says Sun.

The scientists also used a small section of their device to power a red LED, showing that their window can find use as a transparent, self-rechargeable electrochemical battery for low-power electronics. This means that in principle this technology could turn out as an interesting, more versatile alternative than transparent solar cells, a technology which is currently also in its infancy.

Sun and team are now focusing on further improving the performance of their device and are looking into commercializing the technology, which could be attractive for hi-tech green buildings or even to promote energy savings in the household.

A paper describing the advance appears on the journal Nature Communications.

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