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Employees at Apple have filed a patent for integrating solar cells into portable devices by placing them underneath the layers of a touch-sensitive display, according to the filing.

Solar power could help make devices truly portable, freeing from the need for wires to connect them to a power supply.

When generating electricity from solar panels, the larger the panel the better -- but as the patent "Solar cells on portable devices" warns, after allowing space for buttons, screens and a way to hold the device, only a small area is left on most devices for solar cells.

One of the ways around that suggested in the patent is to stack a touch-sensitive layer, a display and solar panel on top of one another. That could make Apple's iPhone and iPod Touch good candidates for such a power supply, as the display occupies almost the entire face of those devices.

The use of solar powered charging in portable devices is starting to get more attention, for more immediate consumer use as well.

When Vodafone announced its plan in April to reduce its emissions of the greenhouse gase CO2 by 50 percent by 2020, it also announced plans for solar-powered phone chargers and universal phone chargers for Vodafone-branded handsets.

At the recent ITU Telecom Africa 2008 conference, Ugandan Minister for Communications and Information and Communication Technologies Ham-Mukasa Mulira talked about trials of solar-powered charging conducted there, which had showed promise.