It's thin film and solar technology, but not the way it's usually done.

Prism Solar and Cool Earth Solar are trying to exploit the properties and economics of plastic by producing solar concentrators out of the same stuff that wraps Pop Tarts. Cool Earth makes Mylar balloons coated with a thin layer of aluminum that reflects light onto a silicon solar cell. The balloon achieves a 400x concentration of sunlight.

Prism, meanwhile, inserts a plastic film into a solar panel itself: The company's power-generating modules consist of bands of solar cells separated by bands of holographic film. The film traps and reflects sunlight light onto the cells.

Why plastic film? It's cheap and made in high volumes. 500 billion pounds of plastic get made a year, according to Rob Lamkin, CEO of Cool Earth. Five billion pounds of that total goes to coated plastic wraps. By building concentrators out of film, the somewhat sketchy economics of concentrators-a sector that seems to be rapidly losing interest among VCs and analysts-begins to look a little better.

"We have plenty of land [to build solar farms] in the U.S. Land isn't the problem," Lamkin said. "We have plenty of Sun. Financing isn't really the problem. Government isn't the problem. It is a materials problem."

The company's balloons-which measure eight feet across – cost $2 each. If you wound up the five billion pounds of thin film produced a year onto a ten foot-wide spool, the plastic wrap would go from the earth to the moon 28 times, he added.

Prim's holographic film likewise allows Prism to reduce the amount of silicon required by 72 percent, and hence drop the cost, without denting efficiency. (Prism's film contains some additional intellectual property, but the basic roll-to-roll processes and underlying materials are similar.)

This year, Prism can make limited numbers of solar cells for $1.92 a watt that are 11.2 percent efficient. That's about twice the price of First Solar, the mass manufacturing king. By 2015, however, Prism hopes to be at 75 cents a watt and 18.7 percent efficiency, says director of business development Stephen Filler.

"By 2012, we want to be at grid parity," Filler said.

Prism is currently raising money to build a 60-gigawatt solar facility as well as a 1-gigawatt holographic film facility. It hopes to sell the film to other solar manufacturers.

Lamkin also noted that plastic film has structural advantages. For one thing, it's light, which makes shipping easy. It also forms into the perfect shape for Cool Earth's purposes. Mirrored concentrators work best when curved. Manufacturing curved mirrors out of more solid materials is difficult. To do that with a balloon, all you have to do is blow it up with air.

"It is one of the strongest structures known to man given the weight of the materials," he added.

The amount of aluminum required also remains somewhat low. The amount of aluminum required for a soda can could coat the reflective surface of 750 of Cool Earth's eight foot-diameter balloons.

Will it take off? Hard to say. Prism has been seeking this money for two years (see Holograms Not Just for Sci-Fi Anymore). Cool Earth, meanwhile, always faces a barrage of questions: How do you control leakage? (An air hose dynamically re-inflates.) What's the energy balance with the air hose? (The generator only adds a bit of energy to the system). What do you do about BB gun attacks or balloons that float away (They only cost $2 – get a new one.)

Photovoltaic concentrators still are dimly viewed. Travis Bradford of the Prometheus Institute predicts that concentrators may only constitute two percent of the solar market by 2020 (see Concentrating Solar to Reach 18GW by 2020).

"The technology is so different and unique that it will take a while," said Lamkin, who says Cool Earth will have to build its own prototype power plants first. "They don't understand that it's a materials issue yet."

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