The IPO window may be slammed firmly shut for conventional firms as investors seek to come to terms with a slowing economy and tightening credit market, but that has not stopped clean tech firms bucking the trend with two major alternative energy IPOs announced in the past week alone.

US-based wind farm developer First Wind is the latest to announce its intention to float, filing plans with US regulators for a $450m IPO. The company, which plans to trade on the NASDAQ under the gloriously appropriate ticker symbol "WINDY", said it planned to use the funds to service debt and meet capital expenditures for next year.

The news came in the same week as Specialized Technology Resources Holdings, a US manufacturer of solar cell components, filed plans to raise $300m through an IPO. The company, which makes the encapsulants that serve as protective sheets for solar cells, plans to trade on the New York Stock Exchange under the ticker symbol "PVS".

The two announcements appear to be part of a wider trend that has seen a number of clean tech companies defy the sluggish IPO market in recent months and go public.

GT Solar International fetched $500m through its IPO late last month, while solar installer Real Goods Solar floated earlier this year, raising around $55m. In addition, Germany-based SMA Solar Technology raised over €100m through its June IPO, while wind energy specialist Noble Environmental Power has filed plans to raise $375m through its upcoming IPO.

Richard White of investment research firm Library House said that there were encouraging signs that the clean tech sector could continue to defy the otherwise sluggish IPO market.

"The market conditions for IPOs aren't great and overall venture capital investment and IPOs both dropped during the first half of the year," he said. " But while the economic outlook is uncertain, investors realise that the other factors that make clean tech an attractive proposition, such as rising energy process and underlying regulatory issues, are still there."

He added that the outlook for further IPOs from clean tech firms remained positive. "What we're seeing is that the first wave of wind and solar firms in particular are starting to mature," he said. "We've also seen early indicators that while IPOs and venture capital investment across all sectors was slow in the first half of the year it has picked up again in the last month."

Yoni Alemu of investment firm Leaf Clean Energy agreed that the clean tech sector was well positioned to continue to defy the sluggish IPO market. "The market has been very receptive to the stories clean tech firms are telling and we've also seen great progress in terms of technology and business models in recent years," he said. "Investors see huge growth potential with these companies and feel that a lot of these businesses are making a lot of sense."