Tidal currents are among the most predictable energy sources on Earth. Until recently, the only way to capture their power was to construct massive dams that impeded the flow of water, often in sensitive marine areas. The TidGen, developed by the Ocean Renewable Power Company (ORPC), sits at the bottom of a free-flowing deep river or bay instead. The company installed its first commercial unit in Maine's Cobscook Bay last summer, and it began delivering electricity to the U.S. grid shortly thereafter. A TidGen can produce up to 150 kW, or enough electricity to power 25 homes, but ORPC plans to add 5 megawatts of capacity within three to five years.