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Are We Headed Toward a Green Bubble?

Billions of dollars are pouring into clean-tech companies--with little results. Is this an intelligent investment or the beginning of another bubble?

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This story appears in the April 2010 issue of Entrepreneur. Subscribe »

Solar Power

How it works: Photovoltaic cells, in modules () or thin film, convert sunlight to electricity. Solar thermal power uses sunlight to heat water, which turns to steam, which runs turbines.

Top companies: First Solar, Nanosolar, SunPower, United Solar Ovonic

Pros: Sunlight is free; state and federal subsidies available to users; works on individual roofs.

Cons: Sun only shines half the time, or less; systems are expensive--payback in reduced energy cost can take more than seven years; requires large amounts of land; dependent on silicon prices, which fluctuate.

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