Sales could grow to a $167.2 billion by 2015, according to Clean Edge, whose forecasts on renewable energy have been met or exceeded by the market for the past four years.

If green sales grew to that size, it would still be tiny compared to the conventional energy market.

But green energy is the fastest growing energy sector, especially after oil prices hit a record of nearly $71 a barrel in August as a wave of hurricanes hit the Gulf of Mexico. Nearly a quarter of crude output in the United States, the world's top energy consumer, is still out after the storms.

The biggest green energy sector is biofuels - ethanol and diesel from renewable resources - particularly on rising numbers of flex-fuel vehicles, cars that run on both conventional and alternative cars.

"Flex-fuels was a term that was rarely uttered even a year ago," Joel Makower, a co-founder of Clean Edge told reporters on a conference call about the study. "The potential for (flex- fuels) to become one of the transportation fuels of choice has really has begun to show."

A few million flex-fuel cars are rolling on US roads today, but building a widespread biofuel distribution system for them is one obstacle the industry faces, the report said.

Other clean fuel obstacles include a lack of processed polysilcon, the main feedstock in the solar power industry and rising steel prices for the wind power sector.

But tightness in silicon supply could lead to innovation by companies making solar cells that use a fraction of the amount needed in conventional cells, the study said.

String ribbon solar technology made by Evergreen Solar and thin film solar made by private companies Miasole, Heliovolt and Nanosolar all use less silicon.

Shares in clean energy companies are doing well this year. The PowerShares clean energy portfolio was trading slightly down at $20.45 per share on Monday. But it's up nearly 25 percent on the year and near a 52-week high of $21.05 hit last month.