A civil and environmental engineering professor at Stanford has conducted a study on “clean energy” technologies. Professor Mark Z. Jacobsen concluded that wind is the cleanest of the technologies. Other winners, in order, are concentrated solar (the use of mirrors to heat a fluid), geothermal, tidal, solar photovoltaics (rooftop solar panels), wave, and hydroelectric.

The lesser forms of the clean energy technologies include biofuels, nuclear, and “clean coal,” which Jacobsen says do not live up to their “clean” hype.