Workers install equipment for an ocean thermal
energy conversion experiment in 1994 at Hawaii's Natural Energy
Laboratory. Credit: A. Resnick, Makai Ocean Engineering, Inc.
The ocean can produce two types of energy: thermal energy from the sun's heat, and mechanical energy from the tides and waves.
Oceans cover more than 70% of Earth's surface, making them the
world's largest solar collectors. The sun's heat warms the surface water
a lot more than the deep ocean water, and this temperature difference
creates thermal energy. Just a small portion of the heat trapped in the
ocean could power the world.
Ocean thermal energy is used for many applications, including
electricity generation. There are three types of electricity conversion
systems: closed-cycle, open-cycle, and hybrid. Closed-cycle systems use the ocean's warm surface water to vaporize a working fluid,
which has a low-boiling point, such as ammonia. The vapor expands and
turns a turbine. The turbine then activates a generator to produce
electricity. Open-cycle systems actually boil the seawater by operating
at low pressures. This produces steam that passes through a
turbine/generator. And hybrid systems combine both closed-cycle and
open-cycle systems.
Ocean mechanical energy is quite different from ocean thermal energy.
Even though the sun affects all ocean activity, tides are driven
primarily by the gravitational pull of the moon, and waves are driven
primarily by the winds. As a result, tides and waves are intermittent
sources of energy, while ocean thermal energy is fairly constant. Also,
unlike thermal energy, the electricity conversion of both tidal and wave
energy usually involves mechanical devices.
A barrage (dam) is typically used to convert tidal energy into
electricity by forcing the water through turbines, activating a
generator. For wave energy conversion, there are three basic systems: channel systems that funnel the waves into reservoirs; float systems that drive hydraulic pumps; and oscillating water column systems
that use the waves to compress air within a container. The mechanical
power created from these systems either directly activates a generator
or transfers to a working fluid, water, or air, which then drives a
turbine/generator.
