If you think investing in clean-energy technologies like solar photovoltaics, biofuels, and advanced batteries is the wave of the future, the First Trust NASDAQ Clean Edge U.S. Liquid Series Index Fund (NASDAQ:QCLN) might interest you. Launched in mid-February, it comprises 46 stocks of companies involved in clean energy, solar, and biofuels, and it tracks the NASDAQ Clean Edge U.S. Liquid Series Index. The index has caps to prevent high concentrations among larger alternative-energy stocks, which should help keep the fund more diversified.

Top sector exposure for QCLN is in Information Tech, at 61%, Industrials at 23%, and Materials at 11%. Individual stocks that make up the fund's largest holdings include MEMC Electronic Materials (NYSE:WFR), which is 10.5% of the fund. Linear Technology (NASDAQ:LLTC) is close to 9%, and ON Semiconductor (NASDAQ:ONNN) is just over 6%.

Clean and growing
According to Clean Edge research, biofuels, ethanol, and biodiesel will grow from $15.7 billion in 2005 to $52.5 billion by 2015. Wind power will expand from $11.8 billion in 2005 to $48.5 billion in 2015, while solar photovoltaics will grow from an $11.2 billion industry in 2005 to $51.1 billion by 2015. These are all annual growth rates of less than 20% and could be conservative. If the current expansion in alternative energy stays on track, it seems reasonable to expect some good returns in this area.

Clouds on the horizon
The solar industry has had difficulty lately getting enough silicon feedstock to keep pace with demand, which has put upward pressure on costs. Biofuels have experienced a run-up in feedstock prices and also face distribution channel restrictions. These fuels might be common in the Midwest, but here in Florida, I don't see any options other than the usual gasoline at the pump. Wind power faces a number of constraints, from people who don't want turbines in their backyard to environmentalists who worry about the impact on wildlife. (Apparently, turbines can act like a giant blender, which definitely isn't good for birds.) Another concern is that many clean-tech stocks are trading at or near their highs; how much further can they go?

Other clean options
QCLN is one of a small but growing lineup of clean-technology ETFs, including three funds from PowerShares: the PowerShares WilderHill Clean Energy Index (AMEX:PBW), the PowerShares WilderHill Progressive Energy Portfolio (AMEX:PUW) and the PowerShares Cleantech Portfolio (AMEX:PZD). These funds each carry a 0.60% expense ratio, while QCLN is slightly less at 0.50%. XShares Advisors LLC plans to collaborate with Chicago Climate Exchange to develop an ETF based on carbon-emission credits, so there may soon be another fund to consider.

Green investment bankers
Goldman Sachs, the world's premier investment bank, has designed its new-world headquarters to incorporate "green" building technologies, including water and energy conservation and recycled materials. Investment bankers usually focus on another kind of green, but this project is a clear sign that environmental consciousness is increasingly mainstream.

Oil and natural-gas prices have managed to stay at high levels and it seems certain that they will continue to be volatile. This high-cost energy environment is driving companies, governments, and consumers to seek alternatives and seems a perfect confluence of events for clean-tech, making this an attractive area to invest in.

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Fool contributor Zoe Van Schyndel lives in Miami and enjoys the sunshine and variety of the Magic City. She does not own any of the funds or securities mentioned in this article. The Motley Fool has a disclosure policy.