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.

Fool contributor Anders Bylund holds no position in any of the companies discussed here. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. You can check out Anders' holdings and a concise bio if you like, and The Motley Fool is investors writing for investors.