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General Electric seems to be covering its bases when it comes to solar power. The company, which four months ago bought a controlling stake in thin-film solar panel maker PrimeStar Solar, has added concentrating solar to its portfolio with a $2.5 million bet on Soliant Energy, based in Monrovia, Calif.

The two startups are about as different as they come. Whereas thin-film solar, such as PrimeStar makes, is very low efficiency but dirt cheap, Soliant makes panels that focus the sun’s rays on a single point, providing the equivalent of 500 suns’ energy to tiny, high efficiency panels. That approach, predictably enough, costs far more per panel than thin film, although they may be competitive on the cost per watt of electricity.

Each scheme has its advantages, but it’s unclear yet whether thin film, concentrating solar or traditional silicon-based solar photovoltaics will win a majority share of the solar market. Soliant, for its part, has been around longer than some concentrating photovoltaic (CPV) companies. We first covered it in 2006, when it was still called Practical Instruments and had just raised $8 million to help develop its panels.

Nowadays it has plenty of competition, from a slew of companies including GreenVolts, Sunrgi and SolFocus, the last of which recently edged close to $100 million in funding. Soliant’s not quite there yet, but its product is at least completed and has some notable differences to other CPV panels on the market. The company fits a number of lenses and mirrors into a nearly flat array, providing a lightweight setup.

However, like almost all CPV systems, Soliant requires a dual-axis tracking system to keep its panels aimed at the sun, so don’t expect to see the units on home rooftops. Instead, they can be installed on commercial roofs or on the ground. Next year, the company will open its first plant, making 40 megawatts worth of panels annually.

The $2.5 million that GE Energy Financial Services provided is part of a larger $21 million round the Convexa Capital, Rockport Capital Partners, Nth Power, Trinity Ventures and Rincon Venture Partners are also taking part in. Soliant is based in Monrovia, Calif.

Rockport Capital Partners, a cleantech-focused venture firm co-located in Boston, MA and Menlo Park, Calif., has closed its third fund with $450 million, almost twice the size of its last raise.

One of Rockport’s more notable deals of late, a partnership with Kleiner Perkins to start a North American subsidiary of the Norwegian electric car maker Think, points the way to Rockport’s shifting strategy: Mixing later-stage deals in with new startups, and helping existing companies to commercialize.

Overall, the firm is looking to invest in fewer companies with its new $450 million pot than the 40 startups it put money into with its $125 million first and $261 million second fund.

Although some are warning of overheating in the cleantech space, Rockport managing partner David Prend seems confident his firm will have no trouble doing well. “The prospects for exits look rosier all the time. When successes start materializing, you get a lot of me-toos, [but] I don’t think that’s necessarily bad,” he told me.

One positive side affect of ever more money pouring into cleantech is in the quality of the people it attracts, Prend says. “Historically, the challenge was management and entrepreneurs. If you look now, the quality is an order of magnitude higher.”

As to where the sector stands overall, Prend says it’s roughly comparable to IT some 35 years back, or biotech two decades ago, suggesting that plenty of work remains to make it mature. However, he does thinks that it will be permanently considered one of the most important areas for investment. “In 5 years if you interview anyone from Rockport, we’ll just be another venture fund. The fact that we do cleantech won’t be notable,” he said.

It looks like Tesla Motors will finally have some competition for headlines in the United States: Think, a Norwegian maker of electric cars, has announced a partnership with Kleiner Perkins and Rockport Capital Partners to export its vehicles stateside through Th!nk North America, a new subsidiary.

Out of the many small electric vehicle makers in existence, Think is one of the few that seems credible. The company has proved popular in its home country and taken investments from General Electric, DFJ Element, Kleiner Perkins and Rockport, and several European firms.

The Think City, the first model to come over, looks vaguely like a VW Beetle and has a range of 110 miles and top speed of 65 miles per hour. While those specs aren’t likely to inspire the masses to junk their SUVs and switch, its $25,000 price tag will prove attractive to plenty, including corporate fleets anxious to cut down on fuel expenses. More attractive will be the Think Ox, a model slated for production in 2010 with a higher top speed and range. (It’s the model pictured below.)

Demand has been unexpectedly high even for the few electric vehicles already sold in the US, so there’s no reason to think the market here isn’t ripe for new models. But Euro-style vehicles have never done that well in the States, and Think may get competition in the urban vehicle market from Smart cars, which have also started to appear this year and are far cheaper, starting at under $12,000.

Other consumers, who need to go longer distances or drive at highway speeds, will likely gravitate to larger hybrid models like the Prius, which the Think is attempting to undercut in price but not functionality.

For now, sales of the Think will be limited to trial and demonstration models, according to the company. When the cars began to be shipped to the US en masse, in 2009, they’ll probably show up first in the Bay Area; Think’s local offices are in Menlo Park, Calif.

Investments in clean-burning combustion engines are picking up, as VCs bet that batteries and exotic fuels like hydrogen won’t be the end of the story for powering automobiles.

The latest is Achates Power, a San Diego, Calif. startup working on a clean diesel. Although details are thin, the company’s website promises to create “new benchmarks in fuel economy and power density.”

Constructing a new type of engine is a risky bet, relying not only on the ability of a small team of engineers to quickly work out all the kinks, but also on a market that has changed little in decades to adopt a totally new type of equipment.

However, diesel engines have a number of advantages over spark-ignition engines, the kind typically used in American passenger vehicles. Those include a greater efficiency, more torque and lower emissions of some gases, including CO2.

A design that tackled the remaining problems — including higher weight and the toxic soot diesel engines tend to produce — would have a shot at capturing a vast market. Khosla Ventures also recently bet on a diesel motor maker called EcoMotors, which has many of the same aims as Achates.

The investment in Achates was reported by VentureWire (subscription required). Besides Sequoia Capital, investing firms included Rockport Capital Partners, Interwest Partners and Madrone Capital Partners.

With a fresh $32 million funding going to nanotechnoloy firm NanoGram, mainly for development of next-generation solar cells, it’s a good time to point out some up-and-coming technologies that work on very small scales to make photovoltaic cells more efficient.

NanoGram has already had several commercial successes, including inventions in both electronics and medicine. However, the company has of late turned its sights on boosting the efficiency of solar cells.

The company is working on ultra-thin crystalline silicon which it says will reduce the cost of silicon-based solar cells to below $1 per watt hour, a price point that is generally considered a breakthrough.

Its latest funding is notable because Nanogram had so far only taken $27 million in funding since its inception in 1996, growing to over $20 million in annual revenue. It plans to use the additional $32 million (investor details at bottom) in part toward a pilot plant for solar modules.

SunFlake A/S, a European company, makes the same claim of being able to manufacture a low-cost cell with about 30 percent efficiency, roughly double the efficiency of the average solar cell available today.

Headed by noted scientist Martin Aagesen, the company plans to make use of a type of nanowire discovered by Aagesen that he calls “nanoflakes.” Blessed with a perfect crystalline structure, nanoflakes are capable of absorbing nearly all light directed at them, according to the company.

By growing its nanowires into a low-grade silicon substrate, SunFlake will reduce the need for large amounts of high-quality polysilicon when making cells. However, it has yet to announce plans to commercially manufacture cells.

Another methods on the horizon is the use of metal oxide nanoparticals in cells. Dr. Jin Zhang of the University of California, Santa Cruz, plans to use a combination of nanoparticles and quantum dots (using nano-crystals, as SunFlake does) to make a highly efficient solar cell.

(Nanotechnology, by the way, refers the field of science that works at the atomic and molecular scale, roughly between 1 to 100 nanometers. Elements and compounds take on different characteristics when they are so tiny, and studying them is leading to new users and inventions, as we’re seeing here.)

A team led by Zhang and including other researchers from China and Mexico recently tested a prototype cell using a nanocomposite material of their own devising. The cell performed even better than the researchers expected.

“We’re manipulating the energy levels of the nanocomposite material so the electrons can work more efficiently for electricity generation,” Zhang told ScienceDaily. His research is currently supported by various governmental groups from the three countries involved.

One note when considering these up-and-coming technologies: It will probably be about five years before they hit the market in force. However, as new technologies become more common, existing cost balances between different solar technologies, like polysilicon and CIGS cells, will likely be upset.

Finally, returning to NanoGram’s funding, the company brought on new investors Global Cleantech Capital, Masdar Clean Tech Fund, Mitsui Ventures, Nagase & Company, Nanostart AG, TEL Venture Capital, and Yasuda Enterprise Development for the round. Existing investors ATA Ventures, Bay Partners, Harris & Harris, Institutional Venture Partners, Nth Power, Rockport Capital Partners, SBV Venture Partners, and Technology Partners also participated.

Norwegian electric car company Think Global AS, aiming to roll out its first electric vehicles this years, said it has raised $60 million in its second round of financing.

Its the latest electric car company to get significant funding, following Silicon Valley’s Tesla Motors, which is building a high-end, all-electric sports car.

The two companies are partnering, with Tesla providing batteries for the Think, a much smaller, more practical car.

Think has now raised $85 million this year. Investors include DFJ Element, RockPort Capital Partners, British Hazel Capital and CG Holding. They join existing investors Canica, Capricorn Investment Group and Wintergreen Advisers, among others (see entire statement here).

The cars will be priced at about $35,000. On top of that is an additional $100 to $150 monthly fee for the battery. To start with, the cars will be marketed in relatively small amounts in Europe

Ok, you’ve heard versions this headline before: “Investments in clean technologies doubled last year, compared to the year before.”

So far, definitions of clean technology have remained vague, and we’ve remained suspicious about the accuracy of industry data on investments in this area.

Today, Dow Jones Venture One has released a more precise definition of what “clean technology.” Earlier, the group also tightened the definition of “Web 2.0″ investments, which greatly improved upon previous efforts.

In 2006, venture investors pumped $1.28 billion into clean technology companies in China, Europe, Israel and the U.S., the group said today. That’s about double the $664.1 million invested in 2005, according to the research, which was compiled the data with help from Ernst & Young.

And here’s their definition:

Because of the significant level of attention being focused on cleantech, VentureOne’s research department adopted a strict methodology for categorizing potential companies in this new industry. They were defined as companies that directly enable the efficient use of natural resources and reduce the ecological impact of production. Areas of focus include energy, water, agriculture, transportation, and manufacturing where the technology creates less waste or toxicity. The impact of cleantech can be either to provide superior performance at lower costs or to limit the amount of resources needed while maintaining comparable productivity levels.

The most active global investors in cleantech in 2006 include Draper Fisher Jurvetson, DFJ Element, Khosla Ventures, Nth Power and Rockport Capital Partners.

See more info in the table below: