Cleantech power-plant builder FuelCell Energy (Nasdaq: FCEL) may be on the rebound, but it's nowhere near a safe bet yet.

I told you three months ago to keep your hands off this stock until the company got closer to profitable operations. Looking at a stock chart since that day, you might conclude that I was wrong, because the trend has been positive. But yesterday's trading once again revealed the high risk involved in this investment. That's why I still wouldn't advise buying FuelCell shares yet.

For another reminder, consider that the share price is down 60% so far in 2010, and 80% over five years. This stock has destroyed a lot of value over the years -- and it might not be done.

In the just-reported fourth quarter, $19.7 million of revenue led to a net loss of $12.9 million. Ouch. The product-to-cost ratio FuelCell likes to report is coming down a bit, but still remains at 1.21. That's a nicer way of saying that gross margins are negative.

If your heart is set on green energy investments, you might sleep better at night with a profitable company like FuelTech (Nasdaq: FTEK), or perhaps Satcon Technology (Nasdaq: SATC) with its larger revenue base. Also, don't forget that giants General Electric (NYSE: GE) and United Technologies (NYSE: UTX) also develop fuel cells for commercial applications. They could play the green-tech part in your portfolio with much lower risk than the relatively unproven business model of FuelCell.

All that said, you could certainly buy FuelCell shares in the hopes of a massive rebound from years of negative returns. Just remember that you'd be gambling with cold, hard cash -- and gambling is not investing.