Next to solar energy, the largest source of energy on the planet is ocean energy—the motion of waves and tides. Both sources of energy are inexhaustible: Waves are caused by winds and the heating of water, which in turn are caused by the Sun; tides are caused by the gravitational attraction between the Earth, Sun, and Moon.
Essentially, because it derives from the Sun—as do wind and solar energy—marine energy is essentially concentrated solar energy. It's concentrated because water is dense, and so it absorbs and delivers a lot of energy.
Compare the amounts of energy delivered by sun, wind, and water per square meter by looking at Table 5.1. The density of the energy source has a huge effect on the power that can be generated from it. For example, because water is 850 times denser than air, tidal turbines can produce 40 times more power than windmills with similar gear, but with smaller equipment and lower costs per watt.
Of all forms of energy we use today, marine energy is our largest untapped energy source. Compared to other renewable technologies, it is still very much in its infancy, having received minimal attention and development capital.
Table 5.1. Energy Density of Renewable Energy Sources
Data sources: Wave energy: World Energy Council, "Survey of Energy Resources 2007,"