Cool Earth Solar eyes rural power with balloons

Cool Earth Solar has one of those radical green-tech ideas that may actually make a real commercial impact.

In the next two weeks, the company plans to start testing a prototype solar plant built around rows of reflective balloons hung on poles. The solar balloons, which are eight feet in diameter, look something like a tube for sledding or laying around the pool, but each one can generate 1 kilowatt of electricity.

It's a design that combines cheap building materials, notably plastics, with expensive high-efficiency solar cells. Light goes through the side of the balloon facing the sun, is reflected on an aluminum coating on the bottom, and is concentrated onto solar cells in a "receiver."

The method can concentrate light between 300 and 400 times. To keep heat under control, the balloons have an automated water-cooling system.

The test installation for Cool Earth Solar, which was founded in 2006, will be small--on the order of a few dozen suspended balloons, according to a company representative. That's so that the company can make changes to the second version of its concentrating solar balloon.

Following that test system, which will generate about 100 or 200 kilowatts of electricity when done, the company intends to start building a 1.5-megawatt commercial solar power plant this winter, said CEO Rob Lamkin. One and half megawatts is enough power to supply about 400 or 500 U.S. homes. That installation, expected to be constructed in Tracy, Calif., will sell electricity to a utility.

Following that, Cool Earth Solar hopes to ramp up quickly, Lamkin said. It is planning another test facility, sized at 10 megawatts, for next summer, he said.

Unlike the rest
Cool Earth Solar's design is nothing like most concentrating photovoltaic (CPV) systems, which concentrate sunlight onto solar cells to make electricity.

Typically, concentrating solar devices use lenses and mirrors to direct and focus light to squeeze more electricity from high-end cells. The lenses, as well as the mounting and cooling systems, add significantly to the cost.

Cool Earth Solar's device, too, concentrates light onto expensive solar cells. But using materials like aluminum-coated plastic--the same sort of thing you'd find in a Power Bar wrapper--means the company can make electricity at $1 per watt, a few dollars per watt cheaper than other companies, Lamkin said.

He said that Cool Earth Solar will be able to sell electricity to utilities at the low end of what solar power plant providers project, which is in the range of 10 cents to 14 cents per kilowatt-hour.

Equally significant to cost is the flexibility that the balloons allow in terms of location.

Concentrating photovoltaic systems demand the right sun conditions, making them suitable only for certain areas like the southwest United States. That has set off a race among solar technology companies to find and get approvals for appropriate land. But Lamkin said that Cool Earth Solar is looking at a far broader set of possible locations where land is not as much in demand.

"It turns out, land is not an issue for us," he said. "What's exciting is because we are modular, we don't need to find super flat land in the lower Mojave desert. We're happy to be in farm land in rural California that no other solar companies are going after."

Cool Earth Solar raised $21 million in funding earlier this year. Lamkin said the company doesn't intend to raise more money in the coming year.