Canadian Maple Leaf Turning Color, Algonquin Park, Ontario, Canada. Photograph by Dennis Edwards

clean energy noun Clean energy is any energy that causes little or no harm to the environment. Wind energy, solar energy (in all its forms--photovoltaic, geothermal, solar thermal, etc.), hydrogen and fuel cells, wave and tidal energy, and biomass are all examples of clean energy. In this portal, clean energy encompasses "green" and "renewable" energy, but also includes entities such as building systems and enhanced conventional power.