Edison International's Southern California Edison utility has announced plans to build the largest photovoltaic solar system in the United States at 250 megawatts, enough for 162,000 homes.

With many of the earth's sunniest spots falling in the middle of the ocean or desert, the balloons, designed by a team from the Technion Institute of Technology, could be used to harness the sun's energy in those remote areas.

However, the application may turn out to have strictly niche appeal given the vast area available in remote locations to park solar panels on rooftops in cities, and on cheap scrub land.

"The idea is to take advantage of the height dimension. When you do that, you save a lot of land resources and can get to places otherwise hard to reach," said Pini Gurfil, the concept's developer.

The helium-filled balloons, covered with thin solar panels, hover as high as a few hundred metres in the air, and are connected via a wire cable to an inverter, which converts the electricity into a form households can use.

It will be about a year before the system is ready, Gurfil said. But initial research, both computerised and using a crude prototype, showed a balloon with a three metre (10 ft) diameter could provide about one kilowatt of energy, the same as 25 square metres (269 square feet) of traditional solar panels.

That's about enough energy for an average person to operate a washing machine and drier. While 25 square metres of traditional solar panels may cost about $10,000, the target cost of the balloon is less than $4,000, with most of the savings coming from the minimal structural support needed, Gurfil said.

"The balloons have no carbon footprint or negative impact on the environment," Gurfil said. "Helium is a naturally occurring gas and environment-friendly. The system saves land from being occupied as well as resources like glass and metal used in ground-based solar energy systems."

However, solar power installers believe the idea may have limited application, given available space on roofs and scrub land, and that the biggest single cost is not land but the solar panel, where much research is focused.

"There's 'free land' on the roofs of buildings, there's space enough for gigawatts of electricity even in cloudy Britain," said Jeremy Leggett, chairman of UK-based solar installation company Solar Century.

"There's plenty of low-value land that can be used for solar farms in the (Mediterranean) sun belt."

LEAVES OF A FLOWER

John Loughhead, executive director at the UK Energy Research Centre, said there was no reason the solar balloon system could not work, but it would be practical only in a few specific circumstances.

"I can see that there could be advantages if the ground is already being used for another purpose ... or if there were no land and the balloon was tethered to a ship at sea," Loughhead said.

The balloons, made from durable material used in meteorological balloons, are filled with helium and insulated on the inside by silicon to reduce leaks. They can stay afloat for up to a year before needing to be reinflated, Gurfil said.

They are lined with solar panels, about 0.2 mm (0.008 inches) thick, and a three-metre balloon weighs about 2.5 kg (5.5 lb).

While land-based solar panels, usually pointing only in one direction, are affected by the sun's position in the sky and can be obscured by taller structures, the balloons' circular shape ensures they always receive direct sunlight, Gurfil said.

"Mostly it's meant to be an auxiliary system, not a main power system," Gurfil said.