Electrical engineers of the University of Princeton are working on a cheap solar-powered charging system that can be printed on plastic and transfers electricity wirelessly.

To make the solar cells the engineers used amorphous silicon (a-Si), a non-crystalline form of silicon. Crystalline silicon (c-Si) is much more efficient when it comes to converting sunlight into electricity but a-Si has other advantages. It can be processed at 75 degrees Celsius (167°F) against 300°C (572°C) for c-Si, allowing it to be cheaply printed on plastic sheets.

The electric circuit is made out of the same material as the solar cells. And again, a-Si has a lower electrical performance than c-Si. But the idea is to produce cheap electricity-generating plastic sheet which can be put up anywhere. On tents, jackets, windowsills but also in public spaces.

By making the charging system available at a large scale, the Princeton engineers aim to have wireless electricity everywhere.

Because amorphous silicon produces low-quality inductors the team designed an alternative electrical circuit to transfer electricity wirelessly. Energy is transferred by electromagnetic induction. An alternating current creates an oscillating magnetic or electric field in the charger. When the receiving device comes within range the field induces power.

Usually inductors are used to produce the alternating current. But because a-Si conductors do not perform good enough, the team produced an alternating current using only two solar panels, capacitors and n-type Thin-Film Transistors (TFTs). The solar panels are wired in reverse. The TFTs switch the current so it flows to the capacitors first from one solar cell and then the other thus turning the direct current produced by the cells to the desired alternating current.

Currently, the charging stations can transfer up to 120 microwatts of power to a receiving device. Performance can be increased by enlarging the photovoltaic surface area. Expanding the surface area from 5 by 5 centimeters to 10 by 10 delivers four times more power.

The engineers presented their findings at the 2012 IEEE Symposia on VLSI Technology and Circuits with the paper Integrated All-silicon Thin-film Power Electronics on Flexible Sheets For Ubiquitous Wireless Charging Stations based on Solar-energy Harvesting. Co-authored by Liechao Huang, Warren Rieutort-Louis, Yingzhe Hu, Josue SanzRobinson, Sigurd Wagner, James C. Sturm and Naveen Verma.

Source: Spectrum.ieee.org