Last year, a sewage-treatment plant in Washington, D.C., began converting excrement into electricity. The District of Columbia Water and Sewer Authority installed four digester tanks, each capable of holding about 4 million gallons of sewage, and populated them with microorganisms called methanogens. These creatures feed on the biosolids in sewage, and their digestion process creates methane gas that is used to drive turbines, which in turn power generators. This system now provides one-third of the plant's electricity.

A company called Lucid Energy installed four turbines inside pipes that carry drinking water from a reservoir to the residents of Portland, Oregon. The 42-inch turbines' spinning blades don't disrupt the flow of water. Their movement drives generators outside the pipes, which create electricity that is sent to the grid. Portland's pilot system will generate 1,100 megawatt-hours of electricity per year, enough to power 150 homes.

Subway trains rushing through the London Underground create heat with their brakes and lighting, and the mob of commuters adds body warmth. All that thermal energy is typically vented as waste heat, but soon it will be captured and used, as part of a pioneering project in the district of Islington. The hot air will go through a heat exchanger that will warm water, which will flow to boilers in apartment buildings, providing heat for more than 450 homes.