Water is 800 times denser than air, and building a gen­erator able to withstand the tremendous force it generates has hampered the development of next-gen hydropower. If engineers can harness its energy, water holds great potential: about 1,420 terawatt-hours per year, or roughly a third of U.S. annual electricity usage.

 

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.