ocean energy
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CT: What is ocean energy?
Ocean energy is a term used to describe all forms of renewable energy derived from the sea.
There are two broad types of ocean energy: mechanical energy from the tides and waves, and thermal energy from the sun’s heat.
Ocean energy is classified as:

  • wave energy: generated by converting the energy of ocean waves (swells) into other forms of energy (currently only electricity). There are many different technologies that are being developed and trialled to convert the energy in waves into electricity.
  • tidal energy: generated from tidal movements. Tides contain both potential energy, related to the vertical fluctuations in sea level, and kinetic energy, related to the horizontal motion of the water. It can be harnessed using technologies using energy from the rise and fall of the tides or by technologies using energy from tidal or marine currents).
  • ocean thermal energy: generated by converting the temperature difference between surface water and water at depth into useful energy. Ocean thermal energy conversion (OTEC) plants may have a range of applications for Australia, including electricity generation. They may be land-based, floating or grazing.

S: http://arena.gov.au/about-renewable-energy/ocean-energy/ (last access: 8 February 2015)

N: 1. ocean (n): late 13c., from Old French occean “ocean” (12c., Modern French océan), from Latin oceanus, from Greek okeanos, the great river or sea surrounding the disk of the Earth (as opposed to the Mediterranean), of unknown origin. Personified as Oceanus, son of Uranus and Gaia and husband of Tethys. In early times, when the only known land masses were Eurasia and Africa, the ocean was an endless river that flowed around them. Until c.1650, commonly ocean sea, translating Latin mare oceanum. Application to individual bodies of water began 14c.; there are usually reckoned to be five of them, but this is arbitrary; also occasionally applied to smaller subdivisions, such as German Ocean “North Sea.”
energy (n): 1590s, “force of expression,” from Middle French énergie (16c.), from Late Latin energia, from Greek energeia “activity, action, operation,” from energos “active, working,” from en “at” + ergon “work, that which is wrought; business; action”.
Used by Aristotle with a sense of “actuality, reality, existence” (opposed to “potential”) but this was misunderstood in Late Latin and afterward as “force of expression,” as the power which calls up realistic mental pictures. Broader meaning of “power” in English is first recorded 1660s. Scientific use is from 1807. Energy crisis first attested 1970.
2. The energy that may be harnessed by exploiting an aspect of the physical or chemical characteristics of oceans, namely, tidal movement, wave motion, thermal gradients, salinity gradients, ocean currents.
3. Ocean energy: 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.
4. 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.
5. 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.

S: 1. OED – http://www.etymonline.com/index.php?allowed_in_frame=0&search=ocean&searchmode=none; http://www.etymonline.com/index.php?allowed_in_frame=0&search=energy&searchmode=none (last access: 8 February 2015). 2. GDT. 3, 4 & 5. http://www.renewableenergyworld.com/rea/tech/ocean-energy (last access: 8 February 2015).

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CR: ocean thermal energy conversion, ocean wave energy, tidal energy.