Although still a fraction of the $34 billion global venture capital market, even conservative estimates show that investment in renewable energy companies nearly doubled last year, bringing the total to $1.2 billion and 140 rounds of financing, according to Dow Jones & Co.'s VentureOne.

Funding rounds also are getting larger. The median round size last year in a clean technology company was $7.5 million, eclipsing the $7 million average round for all venture-backed companies. That's because energy generation technologies, such as ethanol and biofuels, require expensive plants and equipment.

The largest funding round in recent years was raised by wind farm company Airtricity, which has used the capital to lease land, buy large General Electric Co. wind turbines and build pipelines to get the energy to ratepayers. Other big power generation plays include Nanosolar Inc., which not only is trying to create a new type of solar panel, but also is building a factory to manufacture the panels.

The renewable energy field is maturing quickly, with an increase in later-stage financings by venture capitalists, as well as a threefold jump in the funding raised by clean-energy firms from private equity firms. Last year, Series C fundings rose to $675 million, from $261 million in the previous year. Buyout shops poured $3.5 billion into alternative energy in 2006, up from $874 million in 2005, according to London-based research firm New Energy Finance Ltd.

Exits have included initial public offerings -- $10.3 billion worth in 2006, according to New Energy Finance, up from $4.3 billion in 2005 and $700 million in 2004.

The most active VCs in the renewable energy sector include Draper Fisher Jurvetson in Menlo Park, Calif., Element Venture Partners in Radnor, Pa., Khosla Ventures, also in Menlo Park, Nth Power in San Francisco and Rockport Capital Partners in Boston. When surveyed, VCs point to solar power as the technology most likely to make the greatest contribution to the world's primary energy supplies by 2020, reports New York's Jefferies & Co. Wind and hydroelectric power rank No. 2 and No. 3.

Following is a rundown of the alternative-energy companies that have drawn the most private investment since 2003.

Airtricity

Originally known as Eirtricity after its Irish roots, Dublin's Airtricity operates eight wind-generation farms, including a 124-megawatt facility located in Big Springs, Texas, that powers up to 124,000 homes. The eight-year-old firm plans to generate 6,000 megawatts of electricity using wind in the U.S. and Canada. Its other wind farms are in Europe, where it has 35,000 customers. Airtricity has raised $678 million from primary investors NTR plc, a private sector developer, financier and operator of public infrastructure in Ireland, and Ecofin Ltd., a London investment bank.

Cilion

The virgin investment for Richard Branson's Virgin Fuels, Goshen, Calif.-based Cilion Inc. plans to produce 440 million gallons of corn-based ethanol by 2008. It has raised $200 million from Virgin Fuels, Khosla Ventures and Western Milling, a grain milling company in Goshen. Rather than building a central plant to supply the whole country, Cilion is putting up multiple plants near population centers and livestock markets, including three plants in California and two in New York.

Altra

Altra Inc. of Los Angeles is betting on ethanol in California, Nebraska and Ohio, and it shares an investor with Cilion in Khosla Ventures. The company has raised $170 million since its founding in 2004. With one plant under construction and another purchased from Phoenix Bio-Industries LLC in Goshen, Altra plans to continue buying and building plants.

Bloom Energy

The Sunnyvale, Calif., company formerly known as Ion Energy is in stealth mode and declines to discuss its financing. According to VentureOne, Bloom Energy has raised $165 million. The company builds on-site power generation systems that use fuel cells. Developed at NASA, the technology can use a variety of different fuels to generate electricity and can store and use hydrogen when other power sources are unavailable, such as on a windless day or at night, according to the company.

Nanosolar

Bringing its total investment to $148 million, Palo Alto, Calif.-based Nanosolar raised an expansion round of $100 million in June 2006 to build a Bay Area factory for making thin-film solar panels. The panels, which use copper indium gallium diselenide, are expected to be thinner and more flexible than traditional ones made with silicon. They also may be more efficient. The goal is for the plant to manufacture panels that will generate 430 megawatts per year -- four times the current amount of solar energy produced in the U.S. Nanosolar is racing against several other thin-film solar competitors, including Lowell, Mass.-based Konarka Technologies Inc. and Miasole Inc. of Santa Clara, Calif.

Imperium Renewables

The Seattle biofuels company plans to build a 100 million gallon biodiesel plant in Grays Harbor, Wash., that will become operational in 2008. (Biodiesel is a type of biofuel that can be used in diesel engines.) Imperium Renewables Inc. relies on a proprietary technology that it says will result in production costs of 63 cents per gallon, as opposed to $1.50 per gallon for other biofuel producers. Imperium's strategy is to build plants where it expects strong demand, rather than near sources of feedstock that can be used to make biodiesel. It has raised $123 million in equity and has engaged France's Société Générale Corporate & Investment Banking to act as lead arranger of a $101 million credit facility for constructing plants in Hawaii, Argentina and an unnamed location.

Econcern

Europe has focused on sustainable energy development for about a decade longer than the U.S., and Econcern BV, a Dutch holding company that has raised $105 million, has been in the alternative energy business since 1984. The company has four subsidiaries -- Ecoventures, Ecostream, Evelop and Ecofys. Ecoventures commercializes sustainable energy technologies and spins out companies, while Evelop helps get projects off the ground through its experience in site selection, permit procedures and financing. Ecostream builds solar power facilities and plants that make photovoltaic cells, which convert sunlight into power and are used in solar panels. Ecofys focuses on consulting and research.

Renewable Energy Group

With three biodiesel plants in operation and four more under construction, Renewable Energy Group Inc. is one of the top U.S. producers of biodiesel. It can produce 100 million gallons of biodiesel, which amounts to roughly half of all the biodiesel produced in 2006. Renewable Energy Group plans to add another 150 million gallons of capacity by 2008 and 600 million gallons in total by 2009. As the largest U.S. marketer of biodiesel, the Ralston, Iowa, company, which has raised $100 million in funding, builds and manages its plants and produces its own feedstock and animal fats.

RealEnergy

Sometimes a company wants to go green even though there's no source of clean energy nearby. For such businesses, as well as those interested in clean backup power, Yountville, Calif.-based RealEnergy Inc. builds and manages on-site clean power plants that can use wind, methane or solar power. The company has constructed more than 40 of these plants, some of which are used in Merrill Lynch & Co.'s San Diego skyscraper, the Bechtel headquarters building in San Francisco and the Viceroy Hotel in Santa Monica, Calif. RealEnergy, which has raised $90 million, also wants to persuade U.S. regulators to allow it to inject clean energy into natural gas pipelines, which is already allowed in some European countries.

Biox

Biox Corp. of Oakville, Ontario, makes biodiesel from a variety of sources, including feedstock, vegetable oils, agricultural seed oils, recycled cooking oils and animal fats. The company operates a plant near Lake Ontario that produces 15 million gallons of biodiesel each year. Biox, which has raised $79 million in financing, claims it can save 50% in operational costs and 40% in capital costs in producing biodiesel.

Top 10 privately backed cleantech companies