Philips Research, Holst Centre and imec Illustrate Feasibility of Autonomous Smart Windows

At the 17th International Display Workshops (IDW ‘10) in Fukuoka, Japan in December 2010, a demonstration was presented that showed how an e-Skin panel, developed by Philips Research, could be powered by energy harvested from a mobile phone.

Thanks to an RF energy harvester developed by Holst Centre and imec, the e-Skin device could be switched from black to transparent (and vice versa) without the need of a battery. The combined effort illustrates the feasibility of applications such as energy-autonomous smart windows.

RF energy is a hot research topic. With the increasing number of small, ‘smart’ devices that we use to make our lives more comfortable, questions are being raised on how to efficiently supply them with electrical power. Harvesting energy from ambient RF waves is a prime candidate for wirelessly powering small devices and eliminating the need of batteries. RF energy can be harvested from energy already present in the environment, for example from wireless networks. Also, just as wireless internet routers transmit data, one can consider the possibility of RF energy sources that transmit power.

Philips e-Skin is a low power display targeted at ‘green’ applications such as smart windows or building surfaces that change color to reflect or absorb sunlight. In a joint effort, Philips Research, imec and Holst Centre have illustrated the low-power capabilities of the e-Skin. For the demonstration, the researchers used a mobile phone as RF energy source, showing that switching the e-Skin could be realized even with this small amount of power.

The demonstration comprises a 75mm by 75mm e-Skin panel connected to an energy harvesting antenna. A mobile phone is placed 10-15cm from the antenna. Simply making a call to the phone provides enough ambient RF energy for the antenna to directly generate the 6V needed to power the e-Skin panel, and would actually be sufficient to switch over 250 of such e-Skin panels.

The antenna used in the demonstration needed to be quite large as it did not include a voltage boost converter and also because it was tuned to the 900MHz operating frequency of the mobile phone. Smaller antennas can be used for higher frequencies such as the 2.45GHz frequency used by Bluetooth devices, or by incorporating a boost converter to raise the output voltage.

In other experiments, imec and Holst Centre have achieved charging voltages of 4V, the minimum needed to charge a mobile phone, at distances of over 60cm and has reached distances of up to 2m for lower voltages, using a RF energy harvester measuring only 5.5cm x 4.0cm x 0.80 cm. The harvester includes the receiving antenna, rectifying circuit, voltage boost converter and rechargeable battery.

Holst Centre and imec are also working on large antennas with an estimated operating range of up to 10m. It is investigating the possibility of manufacturing these large area antennas on flexible foils so that they can be concealed in walls, ceilings and windows. In addition to developing the antennas, the Micro-power generation and storage program is also developing the rectifiers and the DC:DC boost converters as part of a fully optimized energy harvesting solution.

“This will be a breakthrough in the development of office and home automation and assisted living applications,” Huib Visser, senior researcher on the Micro-power generation and storage program, stated. “Today, applications are limited by the lifetime of batteries or cabling. With wireless energy harvesting, no batteries or cables are needed. It is also an ideal solution for addressing the growing amount of electronics in the car, where the cabling alone can account for anywhere between 20 and 50kg of a car’s weight.”