The last two months have seen an uptick in greentech companies filing for initial public offerings.

In July, STR Holdings, which supplies a protective material forsolarpanels, filed to raise up to $300 million, Chinese silicon and wafer manufacturer GCL Silicon Technology Holdings filed for an $862.5 million IPO and wind-project developer and operator First Wind Holdings Inc. filed to raise $450 million.

And so far this month, lithium-ion battery maker A123Systems filed to raise up to $175 million and Changing World Technologies Inc., which has developed technology to turn animal fat into biodiesel, submitted paperwork to raise up to $100 million.

Last week, German technology group Schott also told Reuters it plans to spin off its solar subsidiary, Schott Solar, in a €500 million ($740 million) IPO by the end of the year.

But the bevy of greentech IPO filings doesn't yet signify a trend, industry watchers say.

"The appetite for risk is drying up," said Robert Wilder, CEO of WilderShares, which manages several clean-energy indices, pointing to a slumping U.S. economy and the credit crunch.

Despite high fossil-fuel prices, there has been a drought of successful greentech IPOs since the beginning of the year, he said.

For example, solar-equipment manufacturer GT Solar received a cool reception from investors in July, when its shares dipped more than 11 percent during its first day on the Nasdaq (see GT Solar Goes Public).

The company's stock continued to get pummeled this month, when class-action lawsuits entered the scene.

In court documents, investors accused GT Solar of not fully disclosing the risk that it could lose out on future orders from one of its customers, LDK Solar (see GT Solar IPO Attracts Class-Action Suit). A day after GT Solar's IPO, LDK announced a contract to buy silicon ingot furnaces from competitor JYT Corp. (see GT Solar Sinks on LDK News).

Energy Recovery Inc. fared better. The company, which makes equipment that it claims can desalinate water more affordably than its competitors', offered shares at $8.50 each and saw them grow 16 percent to close at $9.83 per share in its first day of trading (see Green Light post).

But other greentech companies' public debuts didn't go so well.

In May, residential solar installer Real Goods Solar (NSDQ: RSOL) offered its shares at $10 each, then saw them fall 12 percent to close at $8.80 per share.

Some industry watchers called the dip a case of bad luck, as Real Goods had the misfortune to go public the same day Akeena, the largest public solar installer in the United States, missed its earnings (see Real Goods IPO Suffers From Akeena Solar Fallout).

"Clearly the markets have punished IPOs," Wilder said.

But even with the market's looming sense of uncertainty, there are reasons why some greentech companies are starting their push to go public now, said Tim Carey, who heads PricewaterhouseCoopers'  U.S. cleantech sector.

Filing paperwork with the SEC doesn't mean the companies will actually go public soon. The road to an IPO can take four to six months, he said. "They are just trying to get the process started so they are in the position to complete an IPO when the markets improve."

Another reason, according to Carey, is that greentech industries can be capital-intensive. Some companies are looking to support pivotal growth through public fund raising, even if that means they have to do it during an unfavorable time.

The solar industry, for example, is trying to figure out what to do if tax credits for renewable-energy projects in the United States are not renewed before  expiring by the end of this year.

Solar companies also are facing declining solar incentives in lucrative markets such as Spain, Carey said (see Senate Rejects Renewable Tax Credits Bill, Spain Could Reduce Solar Subsidies by 35% and Spanish Solar Group: Don't Change Feed-In Tariffs).

Also, solar companies could face profit declines if analysts prove correct about a predicted oversupply of solar panels.

As more silicon-manufacturing plants come on line, some industry watchers have forecast that the supply of solar equipment could exceed demand, leading to lower prices - and profits - and increased competition (see Incentives, Tech to Spark Debate at Intersolar: Pricing, Oversupply of Silicon to Be Worse Than Expected and Solar Sector Heading for a Shakeout).

Several solar companies are increasing their production capacity, which could help them survive a potential shakeout, and that means lining up their capital now.

Carey said he expects more greentech IPOs in 2009.

Others, including venture capitalists surveyed by auditing firm KPMG, expect slow IPOs until 2010.