Potential renewable energy sources from seawater are classified as being either Tidal Energy or Wave Energy. Tidal Energy is derived from the rotation of the earth and the gravitational pull of the sun and the moon. With the moon’s pull on the earth being twice the force it exerts on the sun, the tides closely follow the gravitational pull on the moon as it spins around earth. These forces cause the ebb and flow of large amounts of water to rush into and out of ports and rivers around the world. The surge of water can be used to turn turbines and underwater propeller blades. However, the application of tidal power is limiting as it depends primarily on coastal availability, and marine current.

Wave energy is not as dependent on location: wave power can take advantage of movement at or near the surface of the sea. Wave energy can capture movement and convert it into electricity through the rising and lowering motion (vertical), the rocking back-and-forth cycles (horizontal), and the changes of underwater pressure from the seabed to the sea floor of the waves.

Many projects around the world are attempting to define and develop economical marine energy solutions. The difficulties inherent in harnessing this type of energy lie in the placement and anchoring the systems in fast-moving currents and in managing the harsh environmental conditions the corrosive salt water creates—these conditions exacerbate the heavier mechanical loads the water places on underwater turbines (compared to the load that air places on wind turbines).

Just as wind energy must utilize large structures, so must tidal and wave power. Due to the complex dynamics involved with tidal and wave power, their systems require more structural stability and sealed ruggedness than land-based solutions. In addition, these systems must be designed for minimal maintenance and optimized for size and efficiency. Some systems can be massive--for example, the base of a tidal turbine can weigh 200 tons.

Global Ocean Energy Capacity in 2005: 0.26GW

Global Ocean Energy Capacity in 2006: 0.3GW

There are a few sites in operation that leverage tidal energy. These include La Rance, France (240MW, built in 1967), Kislaya Guba, Russia (0.4MW, built in 1968), Annapolis, Canada (18 MW, built in 1984), and Jiangxia, China (3.9MW, built in 1985). Due to the cyclical nature and changes in direction every 12 hours and 25 minutes, tidal energy only nets an average efficiency of 25 to 27%. This is less efficient than wind, represents more substantial engineering challenges, and must overcome ecological objections of dangers to sea life.

A new industry development emerged in 2008 when SeaGen—a commercial tidal turbine with bi-directional blades—hit the market. The 1.2MW system is mounted on a 135ft tower and has two 600 kW blades separated by 95 ft. The installation of SeaGen cost $3.6 million (or $3,00 per kW of capacity).