Tidal energy systems can extract either kinetic energy (energy caused by movement) from the moving water of rivers, tides and open ocean currents; or potential energy from the difference in height (or "head") between high and low tides. The first method - generating energy from tidal currents - is becoming more popular because people believe that it does not harm the environment as much as barrages or dams. Many coastal sites worldwide are being examined to see if they can be used to produce tidal (current) energy. 