Venture capitalists are pouring more dollars into greentech companies each year, but only a handful have managed to make an exit – and a profit – via acquisitions or initial public offerings.

A report published Tuesday by PricewaterhouseCoopers and the National Venture Capital Association predicts that could change when the industry “comes of age” as early as next year.

From 2005 through 2007, only five companies were listed on Nasdaq, raising an average of $77 million each and reaching average post-IPO valuations of $245 million each, according to the report.

And a struggling economy has made it even more difficult to go public recently. For example, biodiesel producer Imperium Renewables in January delayed its IPO, citing unfavorable market conditions (see Imperium IPO Delay Underlines Feedstock Shortage, Analyst Says).

Mergers and acquisitions for venture-backed cleantech companies also have been "thin," the report said. It cited 21 such deals – averaging $31 million each -- between 2002 and 2007.

Tim Carey, the U.S. cleantech sector leader for PricewaterhouseCoopers, blamed the current state of the market for the lack of exits in cleantech, as well as in other industries. He said he hoped the exits would increase in 2009, when the markets are expected to improve.

Erik Straser, a partner at Mohr Davidow Ventures, had a similar prediction. In 2009, the climate for initial public offerings will start to heat up and large companies will begin hunting for acquisitions with which to diversify their holdings, he said in the report.

Still, exits could take some time to become very noticeable. “The market is still a couple years off before any significant M&A activity in venture-backed cleantech companies," Straser said in the report.

That doesn't mean interest in greentech has lost its luster for venture capitalists.

Over the last few years, the cleantech industry has grown from a niche to its own asset class, rising to 7.4 percent of total U.S. venture investment in 2007 from 1.9 percent in 2004, the report said. That makes cleantech the fourth-largest industry, just below life sciences, according to the report.

The report, which used data supplied by Thomson Reuters, said venture capitalists poured $2.2 billion into U.S. cleantech companies last year, up about 45 percent from the $1.5 billion invested in 2006.

Of course, PricewaterhouseCoopers and the National Venture Capital Association aren't the only ones keeping tabs on the cleantech industry. Different sources, which define and track the category differently, have reported widely varying quarterly and full-year investment totals (see Investment Keeps Growing, Greentech VC Hits $2.6B in U.S., Clean-Energy Fundings Reach $117.3B in 2007).

Four of the largest cleantech deals were in thesolarsector, including $101 million raised by thin-film developer HelioVolt Corp. and $76 million bagged by cell and panel maker Advent Solar.

Investment in wind-energy generation also grew, with companies blowing in $115 million in nine deals, up from $10 million in three deals in 2006.

The report also called the pollution and recycling sector a "promising growth area," saying it attracted 29 deals totaling $202.1 million last year. In 2006, the sector saw $137 million in 17 deals.

But not all the greentech sectors maintained bragging rights. Alternative fuels, which include biofuels and nuclear energy, dipped to $290 million in 2007 from $462 million in 2006.

Investors haven't been eager to pump money into the alternative fuels as biofuel producers struggle with tight margins and blame for damaging the environment and competing with the food supply (see Sugarcane Biodiesel Heads to Brazil and Pacific Ethanol Gets Much-Needed Cash).

Still, interest isn’t waning, Carey said. Although biofuel deal sizes were smaller, the alternative-fuel deals rose to 34 last year from 22 in tn 2006, he said.