"There is a huge amount of energy available in high altitude winds," said coauthor Ken Caldeira. "These winds blow much more strongly and steadily than near-surface winds, but you need to go get up miles to get a big advantage. Ideally, you would like to be up near the jet streams, around 30,000 feet."

"We found the highest wind power densities over Japan and eastern China, the eastern coast of the United States, southern Australia, and north-eastern Africa," said lead author Archer. "The median values in these areas are greater than 10 kilowatts per square meter. This is unthinkable near the ground, where even the best locations have usually less than one kilowatt per square meter."

Several technologies have been proposed to harvest these high altitude winds, including tethered, kite-like turbines that would be floated to the altitude of the jet streams at an altitude of 20,000-50,000 feet and transmit up to 40 megawatts of electricity to the ground via the tether.

A new prototype charging system from the company is able to power itself on nothing more than ambient radiowaves - the weak TV, radio and mobile phone signals that permanently surround us. The power harvested is small but it is almost enough to power a mobile in standby mode indefinitely without ever needing to plug it into the mains, according to Markku Rouvala, one of the researchers who developed the device at the Nokia Research Centre in Cambridge, UK.

The difference with Nokia's prototype is that instead of harvesting tiny amounts of power (a few microwatts) from dedicated transmitters, Nokia claims it is able to scavenge relatively large amounts of power -- around a thousand times as much -- from signals coming from miles away. Individually the energy available in each of these signals is miniscule. But by harvesting radiowaves across a wide range of frequencies it all adds up, said Rouvala.

The trick here is to ensure that these circuits use less power than is being received, said Rouvala. So far they have been able to harvest up to 5 milliwatts. Their short-term goal is to get in excess of 20 milliwatts, enough power to keep a phone in standby mode indefinitely without having to recharge it. But this would not be enough to actually use the phone to make or receive a call, he says. So ultimately the hope is to be able to get as much as 50 milliwatts which would be sufficient to slowly recharge the battery.