When I say "cleantech," which publicly traded powerhouses pop into your mind?

Perhaps you immediately think of General Electric (NYSE:GE) and its wind power preeminence, or Google (NASDAQ:GOOG) and its gigawatt-scale solar ambitions. There's also smart grid-savvy IBM (NYSE:IBM), not to mention DuPont (NYSE:DD) and its cellulosic ethanol enthusiasm.

Oil refiner Valero Energy (NYSE:VLO) isn't likely to make anyone's cleantech short list, but the firm has quietly begun to flex its muscle when it comes to funding next-generation fuels.

The big splash recently was Valero's recent plunge into ethanol, picking up various VeraSun Energy assets for dimes on the dollar. Less widely covered are the firm's recent investments in more cutting-edge cleantech efforts.

Back in November, Valero was identified as one of the key investors in a Series A funding round for Solix Biofuels, a firm working to commercialize algae-derived fuels. This is an increasingly crowded field, with Chevron (NYSE:CVX)-allied Solazyme and Bill Gates-backed Sapphire Energy representing just two of Solix's most promising competitors.

In true Easter fashion, it appears that Valero is trying not to put all its eggs in one basket. On Friday, the refiner made an investment in Terrabon, a firm that's attempting to turn municipal solid waste and other sludge into green gasoline. This approach has definite appeal, as it's possible that, unlike a designer microorganism, such feedstocks could come free of charge. Better yet, Terrabon could conceivably get paid to take the sludge off other people's hands, in a business model reminiscent of waste-to-energy provider Covanta Energy (NYSE:CVA).

As with renewables-reluctant Royal Dutch Shell, Valero is investing in the thing it really knows well: liquid fuels. While detractors might criticize hydrocarbon players for shying away from areas like solar, I think it makes sense to stay out of a sandbox better suited to semiconductor players.

My belief (hope?) is that cleantech investing is beginning to see a firmer division along industry lines, with tighter budgets keeping companies within their investing "circle of competence." This should in turn result in more intelligent allocation of capital among very high-risk ventures, such as the pair being partly funded by Valero.

Google is a Motley Fool Rule Breakers recommendation. See what other stocks we've tapped as future cleantech stars with a free 30-day trial.

Fool contributor Toby Shute doesn't have a position in any company mentioned. Check out his CAPS profile, or follow his articles using Twitter or RSS. The Motley Fool has a sensible disclosure policy.