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.