The buzz about alternative fuels and natural gas vehicles died down when oil went from $140 a barrel in 2008 to less than $40 a barrel in 2009. But with gas prices rising, and no end in sight to political turmoil in the Middle East, it may be time to take a look at how we could invest in alternative fuels. Here are four stocks that have caught my eye.

Clean Energy Fuels (Nasdaq: CLNE)
The transition to alternative fuels would be difficult without some infrastructure in place. That's where Clean Energy Fuels comes in, providing a growing network of natural gas fueling stations. Flying J has recently partnered with the company to begin providing a nationwide footprint of fueling stations for natural gas-powered trucks, and I expect more partnerships in 2011.

Fuel Systems Solutions (Nasdaq: FSYS)
Fueling stations would sit empty if not for the kind of technology Fuel Systems Solutions provides. It can turn gasoline-burning devices into natural gas-powered machines, serving a broad array of applications in the transportation, industrial and power-generation markets.

Rentech (AMEX: RTK)
The "Rentech Process" turns municipal waste, biomass, coal, and just about anything else into useful products like chemicals and fuel. Right now, the company is small, but it has proposed  projects in California and Mississippi, and its technology is very scalable. In a test case, someone had the wonderful job of driving a Rentech Diesel-powered Audi A3 on a 1,000-mile trip last October.

But Rentech isn't the only one making synthetic fuel from abundant sources of energy. Syntroleum (Nasdaq: SYNM) is also creating fuels from natural gas, fats, oils and greases. More than 400,000 gallons of the company's Syntroleum have been tested as synthetic diesel and jet fuel. Neither company is consistently profitable, but both hope to turn around  their finances as projects get built.

FuelCell Energy (Nasdaq: FCEL)
Last but not least, FuelCell Energy takes the natural gas or methane other companies produce and turns it into efficient electricity. Fuel cells can provide on-site power generation for hospitals, colleges and commercial buildings, lowering their reliance on the grid. In places that need energy to operate, natural gas fuel cells can be an attractive power source.

These companies all offer ways to invest in alternative fuels as oil prices rise. But be sure to do your due diligence when looking at these stocks. Profits can be elusive in this sector, and big oil companies are waiting to pounce the moment alternative fuels prove profitable.

Interested in reading more about one of these stocks? The best way is to add it to My Watchlist, and My Watchlist will find all of our Foolish analysis on this stock.

Fool contributor Travis Hoium does not have a position in any company mentioned. You can follow Travis on Twitter at @FlushDrawFool, check out his personal stock holdings or follow his CAPS picks at TMFFlushDraw.

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