Apple has been granted a patent for solar-powered portable devices by the US Patent and Trademark Office.

A report on the Patently Apple website suggests that when the application for the patent was first made it may have been filed under an engineer's name, rather than Apple's, as the company wanted to keep it as secret as possible.

The patent covers all types of portable device, so the technology could be under development for anything from the iPod to a MacBook. It is the company's fourth solar-power related patent.

Apple's latest patent involves the use of a voltage converter to recharge the battery of a portable device using solar power or other power sources. It also mentions a controller that could potentially manage the recharging process.

Recharging mobile computing devices when AC power is not available is a problem that has led to several solar solutions in recent years - for example, Solio's range of portable solar chargers and Intivation's solar powered phones.

Earlier this week it emerged that Apple was granted 563 patents in 2010, putting it 46th in the list for top patent winners last year. IBM topped the list with 5,896 patents.

This story, "Apple Granted Solar-power Patent" was originally published by Macworld U.K. .