Cleantech VC poster boy, Ira Ehrenpreis of investment firm Technology Partners, has "never been more bullish on cleantech," but according to an article in Dow Jones by Yuliya Chernova, when asked for examples of successful companies in the sector, he had trouble citing a single one during the REFF-Wall Street event last month. In my experience, Ehrenpreis is rarely at a loss for words.

(Ehrenpreis has been a staunch champion of the greentech transformation over the last decade, and Chernova's reporting in the WSJ tends to be critical of the sector. Ehrenpreis might be smarting from his investment in now bankrupt Abound Solar.)

We've contacted Ehrenpreis to give him another chance to respond but have yet to hear back.

Josh Green, a partner at Mohr Davidow Ventures and incoming chairman of the NVCA, was more able to use his words to cite a cleantech success story -- naming Tesla and Solazyme as contenders when questioned by GTM at a Stanford University event last week. Green's examples were two recently public firms; Tesla is the wildly ambitious electric vehicle upstart while Solazyme is developing algae-based chemicals and fuels. And while Green confined his responses to publicly-traded firms, I'd suggest that MDV portfolio firm Recurrent Energy, purchased by Japan's Sharp Solar in a move towards the downstream solar market, was a positive if not wildly successful example of a greentech VC exit.

Kleiner Perkins cleantech partner Trae Vassallo (the more quotable of the Vassallo VCs) was quick to name Opower, Nest, RecycleBank, and Enlighted as KP cleantech portfolio firms with a fighting chance at success while speaking at the same Stanford event. She called herself "an IT person at heart" with an eye towards "using IT to change the way we interact."

"Now that we have everyone online, let's get everything online," said Vassallo.

She called Opower a "social benchmarking and customer engagement firm for utilities." If customers are the most important thing for a startup, then Opower's record of winning more than 75 utility customers speaks for itself.

She cited Nest and its Apple iPhone-like thermostat as a "really exciting initiative to drive energy efficiency in the home" and an opportunity for "disruption at the edges." She called Enlighted "the Nest concept for commercial buildings" and saw RecycleBank as a way to incentivize and reward and "get consumers aligned" with the goal of recycling.

Larry Kelly, an entrepreneur and angel investor in the Band of Angels, has made a number of investments in cleantech recently through the Band, including water companies, lighting firms, and transportation firms. Kelly cited MBA Polymers as a success story with the potential to go public next year after having raised "seven easy rounds" of capital.

While we're at it, allow me to name a few standout greentech firms. How about SolarCity with its 1,900 employees and its impact on the residential and commercial solar industry? Or Enphase and its microinverter -- although the stock of the recently-public company is sluggish, the company's revenue growth is impressive and its margins are headed in the right direction. What about Kurion, a VC-funded startup playing a huge role in the Fukushima cleanup -- and moving to its next phase in handling the waste materials? Or Silver Spring Networks, a leader in networking the smart grid with 65 partners, 11 million smart meters deployed and 11 million more under contract? How about Lemnis Lighting or Switch Lighting in the LED lighting sector?

MDV and KP and every other investor have a long list of success-challenged portfolio firms -- such is the nature of venture capital.

MDV's Green said that when it comes to cleantech, "We are now firmly in the valley of despair" of the Gartner hype cycle, but added that "there are investment opportunities in all of these phases." He noted that the category has existed for only a few years and it can take nine years to liquidity in a sector where "the markets are tantalizing but access is difficult."

He suggested, "Everything's doing just fine. We will continue to have our failures; it's inherent in venture capital."

What firms are on your list of greentech success stories?