http://hbswk.hbs.edu/item/6499.html?wknews=101810

There are a couple of key differences that make the various clean-tech sectors and the broader green business world vastly different from the historically well-funded Internet start-up industry. In the Internet start-up world, VCs can reasonably assume that a company can go from launching a product to getting acquired—or even going public—within a few years. But clean-tech doesn't work that way.