That's the conclusion of New Energy Finance in a new analysis they released today. They studied 129 clean energy investments in Europe by 37 investors and found 15 IPOs, 10 trade sales, 21 up rounds, 19 down rounds or write-downs, and 10 liquidations. All told, they estimated a 54.9% gross annualized return across the portfolio of 129 companies. The study period covered 1998 to the present, and included an estimate of 1.2x valuation on unrealized gains on funds invested. It's a very positive study, certainly, and helps further illustrate why investors are so keen on this sector right now. It's also a very useful analysis -- but it's important to note a lot of caveats involved. The methodology is hard to figure out from the press release alone, but it's clear there's a lot of potential for some data bias (while it's impressive that 37 investors participated, it looks like the study only covered about half of relevant investments; and as well, it appears the study relies somewhat upon self-reported data from the investors). I'd also like to see more information about the way that "unrealized holdings, calculated on an industry-standard, conservative basis, are valued at 1.2 times the total funds invested." It's unclear, but in light of the statements that 35 exits have returned 1.4x the funds invested, and the total estimated gains are 2.6x (1.4x + 1.2x), it seems like there's an implication that the ~75% of funds invested that have NOT led to exits may be assumed to have seen some pretty nice paper gains. We may ask the good folks at NEF to write up their methodology for benefit of readers at some point... Before we all pat ourselves on the back about the healthy returns in this sector, in any case, consider the following:

• Neal has some cautionary words about solar...
• These researchers have some cautionary words about cellulosic ethanol...
• Here's a cautionary tale for anyone who's planning on generating a lot of value out of carbon offsets, and planning on today's carbon market pricing in their financial modeling...
• And here are some cautionary words about VCs stretching their investment approach to do quasi project finance (something we've discussed on this site a bit)

One funding announcement so far this week: Pentadyne has closed on a $14mm round of financing, led by Loudwater Investment Partners. GTM's Rachel Barron reports that the capital will go toward sales channel and product development. Jonathan Shieber at Dow Jones pointed out in CTI that the PR described it as a "recapitalization," and wondered about the company's previously-announced AIM IPO plans. It's unclear from the PR whether existing investors participated in the latest financing, which brought total investments in the company to about $60mm. Other news and notes: Cleantech is heating up in Australia, too... And finally, it's not cleantech, really, but how about $20mm to put a robot on the moon?

Why is cleantech venture capital suddenly the hottest thing since Disco? Because the exits are starting to happen. While M&A remains the dominant exit path for VC exits in this and other sectors, the Cleantech Group's latest figures, there were 17 cleantech IPOs in the second quarter, totaling $1.7 billion in proceeds. That's nearly double the number of IPOs in the first quarter... The Cleantech Group's numbers also showed that North American cleantech venture investments totaled $843mm in the quarter -- about the same as in Q2 2006. The amazing growth in cleantech VC investments appears to have slowed recently. Is it a pause before a second wave? Or is the recent hype covering up an underlying flatline in activity? Or is it all just a collection of lumpy figures that don't really tell a coherent story because a couple of deals here and there dominate the totals? Time will tell. Although August was brutally busy for many cleantech VCs, and September's starting out even busier (especially for yours truly, so apologies for the sporadic postings), so anecdotally at least, things are heating up once again... Red Herring also had a nice piece looking at the numbers and discussing how VCs are starting to look more into some of the less-fished corners of the cleantech pond. Deals and noteworthy items from the past few days:

• Virent Energy Systems, which is developing a catalytic process for the production of gasoline from sugars, announced a $21mm Series B, co-led by Stark Investments (a Wisconsin hedge fund) and Venture Investors of Madison, WI. Existing investor Cargill Ventures also participated. The fact that the co-leads of this round were local investors is an interesting development.

• SynapSense, which offers technology for monitoring and optimization of energy use in data centers, announced a $10mm Series B, led by Emerald Technology Ventures. Existing investors American River Ventures, Nth Power and DFJ Frontier also participated in the round.

• Solar thermal-based electricity generation startup Ausra announced a "more than" $40mm round of financing. Khosla Ventures and Kleiner Perkins provided the financing, and Vinod Khosla and Ray Lane have taken director seats.

• PV installer SolarCity raised a $21mm round of financing, led by DFJ. JP Morgan and company Chairman Elon Musk also participated in the round.

• UK-based Lysanda has raised GBP 1mm to develop what they're calling "Eco-Log", which you may be relieved to learn is an on-board vehicle emissions monitoring technology. Half of the capital was a venture capital investment by the Sustainable Technology Fund, managed by E-Synergy Ltd. GBP 300k was angel financing.

• Israel-based Galten, a Jatropha biodiesel startup, has raised $10mm. UK-based Capital Partners led the round.

• Cleantech investors in the news: Greylock is officially confirming that they're going to be investing in cleantech... And Globespan has closed on their fifth fund, with $380mm.

Other news and notes: Tech Confidential had a couple of very good posts (here and here) covering the AlwaysOn GoingGreen conference, which featured a good group of cleantech VC presenters -- reinforcing the need for sectoral expertise when making early-stage bets in the space... Here's a great update on all the cleantechy goodness going on down in Austin... "No clue" is the clear winner in the "who will win the race for commercialization of algal biodiesel" contest, according to this fun poll from Inside Greentech... Finally, although this came out last month, it's worth reading today.

Don't miss the upcoming Greentech Media webinar on solar financing services:

Wed. the 19th, noon eastern.

First off, thanks to everyone for all the kind words regarding the move to Greentech Media. So far so good!

• Expansion Capital Partners is announcing today [9/11 update: here's a link to the PR in pdf format] that they have closed their Clean Technology Fund II at more than $100mm. To all my former colleagues at Expansion Capital, many congrats!!

• Tioga Energy has added an additional $4mm to their Series A financing, bringing the total amount of the round up to $14mm. Nth Power provided the bulk of the second closing, according to PE Hub. We mentioned the first closing back in June.

Picked a bad week to fall behind on deal updates, it would appear...

• CPV developer SolFocus announced $52mm in new funding this week, and they may not be done pulling together what could be a $70mm round when completed. According to PE Week Wire, $27.4mm of the new financing is targeted at launching a new Spanish subsidiary, and $24.6mm is Series B financing into the parent company. Insider NEA led the round again, with participation by Moser Baer India, Metasystem Group, NGEN Partners, Yellowstone Capital, David Gelbaum, and other investors. SolFocus also recently acquired a Spanish tracker company, Inspira.

• Also according to PEWW, secretive solar startup Sierra Nevada Solar has raised $4.5mm in financing. AlarmClock reports that the financing was provided by EarthBright, a San Francisco-based cleantech investment group funded and led by Jonathan Bonanno, chairman of the Cleantech Investment Committee of the Keiretsu Forum, as well as angel investor Ryan Scott.

• Plextronics, which is developing organic PV cells, OLEDs and RFID technology, has raised $20.6mm in Series B financing -- Matt Marshall has details. Strategic investor Solvay Group led the round, which included participation by Firelake Capital, Birchmere Ventures, Draper Triangle Ventures, and Newlin Investment Company. The round brings the total investment raised by the company to $37mm over the last five years.

• Jonathan Shieber in today's Clean Technology Investor has an interesting column about the birth of Fisker Automotive Inc., a new plug-in hybrid vehicle startup with plans to introduce a four-door car in 2009. Shieber reports that the company, which has been founded by car designer Henrik Fisker and Quantum Fuel Systems CEO Alan Niedzwiecke (they met during the course of a used car transaction), is looking for outside capital -- and they estimate they'll need $100mm over several rounds to make a go of it. Interesting benchmark for EV and related startups... Of course, the obvious question is: What was an auto designer doing buying a used Land Rover, shouldn't he be driving around in some fancy prototype?

• Also according to Shieber (this time in VWire), BC-based General Fusion has raised $1.2mm in seed financing to pursue the commercialization of the company's small fusion reactor technology. We briefly mentioned fusion in passing last week, alluding to the fact that General Fusion isn't alone in pursuing the commercialization of "hot" fusion approaches. If one of the entrepreneurial teams driving toward this goal could achieve "net positive" energy from any of these approaches, it would potentially be a big game-changer... There is more info in this interesting article about Canadian clean tech ("the sexiest area of the North American venture-capital world"), including hints that the company may be looking to raise additional capital.

• UK-based Solarcentury has raised a GBP13.5mm round of financing, co-led by zouk ventures and Good Energies. Vantania Holdings Ltd and Foursome Investments, along with existing investors VantagePoint Venture Partners and Scottish and Southern Energy, also participated.

• PEWW also broke the news that fuel cell backup power system ClearEdge Power has raised $6.47mm of an anticipated $19.47mm Series C. Applied Ventures is mentioned as an investor.

• Tonga Capital Corp, d/b/a Momentum Biofuels, has raised a $4.3mm PIPE.

• Climate Change Capital, a private equity firm based in the UK, has closed on a $200mm Climate Change Capital Private Equity Fund, which will invest in expansion-stage startups all the way up to buyouts.

• CTI also reported this week that Terrabon, a cellulosic ethanol technology developer, has raised $1.25mm in seed financing, bringing their total investment from "individual investors" up to $4.5mm.

• PEWW reported last week that PV startup Solexant (with technology to "harvest energy from the entire solar spectrum") has raised a $4.3mm Series A, with backing from Trident Capital, Firelake Capital, and X/Seed Capital.

• Wind power software company 3Tier has raised a $2mm second round from Good Energies.

... And even more updates to follow soon...