Renewable Energy from the Ocean

“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. Oceans also produce mechanical energy from the tides and waves. Even though the sun affects all ocean activity, the gravitational pull of the moon primarily drives the tides, and the wind powers the ocean waves.” — Exploring Ways to Use Ocean Energy

Ocean Thermal Energy Conversion (OTEC):

Open-Cycle OTEC Plant
“On an average day, 23 million square miles of tropical seas absorb an amount of solar radiation equal in heat content to about 250 billion barrels of oil. If less than one-tenth of one percent of this stored solar energy could be converted into electric power, it would supply more than 20 times the total amount of electricity consumed in the United States on any given day.”

“OTEC systems use the ocean’s natural thermal gradient—the fact that the ocean’s layers of water have different temperatures—to drive a power-producing cycle. As long as the temperature between the warm surface water and the cold deep water differs by about 20°C (36°F), an OTEC system can produce a significant amount of power. The oceans are thus a vast renewable resource, with the potential to help us produce billions of watts of electric power…” — National Renewable Energy Laboratory (NREL)  Read more

Ocean's Thermal Gradient Map

Ocean Wave Power:

The total power of waves breaking on the world’s coastlines is estimated at 2 to 3 million megawatts. In favorable locations, wave energy density can average 65 megawatts per mile of coastline.
Ocean Wave Power
W a v e g e n —a world leader in wave energy and wave power
Wavemill Energy
Oregon coast could be wave energy hub
MagnetoHydroDynamics (MHD) Ocean Wave Energy Conversion
Wave Energy Potential Warrants Further Research And Development, Says EPRI

Ocean Tidal Power:
Tidal energy traditionally involves erecting a dam across the opening to a tidal basin. The dam includes a sluice that is opened to allow the tide to flow into the basin; the sluice is then closed, and as the sea level drops, traditional hydropower technologies can be used to generate electricity from the elevated water in the basin. Some researchers are also trying to extract energy directly from tidal flow streams.
Tidal Energy
Marine Current Turbines Ltd.

Power from Ocean Currents:
Blue Energy Canada Inc.
The Davis Hydro Turbine

Ocean Thermal Energy Conversion, Ocean Wave Power, Ocean Tidal Power, Energy from Ocean Currents, and…
Renewable Energy from Ocean Salinity:
Energy can be captured by exploiting the pressure difference at the boundary between freshwater and saltwater. This is called Osmotic Energy. The difference of potential between freshwater and saltwater is called the Salinity Gradient. The potential for osmotic energy exists where ever a stream or river enters the ocean.

Most people are familiar with reverse osmosis where freshwater is obtained from saltwater. Reverse osmosis consumes energy and produces freshwater from seawater. Osmosis consumes freshwater in the presence of seawater and produces energy (the freshwater becomes saltwater).

The principle of salinity gradient energy is the exploitation of the entropy of mixing freshwater with saltwater. The potential energy is large, corresponding to 2.6 MW m3/sec freshwater when mixed with seawater. This energy source is not easy to understand, as it is not directly sensed in nature in the form of heat, waterfalls, wind, waves, or radiation. Several methods have been proposed to extract this power. Among them are the difference in vapor pressure above freshwater and saline water and the difference in swelling between fresh and saline waters by organic polymers. However, the most promising method is the use of semipermeable membranes. The energy can then be extracted as pressurized brackish water by pressure retarded osmosis (PRO) or direct electrical current by reverse electrodialysis (RED).

With the RED method, ion selective membranes are used in alternate chambers with freshwater and seawater, where salt ions migrate by natural diffusion through the membranes and create a low voltage direct current. With the PRO method, another type of membrane, similar to reverse osmosis membranes used for sea water desalination, is used. These PRO method membranes are much more permeable to water than to salt. If fresh and saltwater are separated by such membranes, natural osmosis will force the freshwater through the membrane to the saltwater side where hydrostatic pressure up to 26 bars can be created. The two methods are quite different in their working principles, but it is the same potential energy that is exploited.

Salinity power is one of the largest sources of renewable energy that is still not exploited. The exploitable potential world-wide is estimated to be 2000 TWh annually. One of the reasons that this renewable source has not drawn more attention is that it is not readily evident to most people. Another reason is that considerable technological development is necessary to fully utilize this resource. Along with the the lack of efficient and suitable plant components, some pessimistic cost forecasts have been issued. The potential cost of energy from this source is higher than most traditional hydropower, but is comparable to other forms of renewable energy that are already produced in full-scale plants.

Salinity Energy
Author: Thor Thorsen, SINTEF, Norway

An Ocean Salinity Energy Example:
The Orange County Sanitation District, serving Orange County California, has a flow of 240,000,000 gallons of effluent going into the ocean each day, every day of the year. Figure the effluent has a salinity of .06 ppt, the ocean has a salinity of 35 ppt, and that the mixing ratio will be 5 gallons of seawater for each one gallon of effluent. Take the result to your local turbine manufacturer.

Bottom line- Think! Every time you flush the toilet- you could be generating electricity for your community for a nickel rebate! Which is more than enough to pay for the desalination of seawater for California’s thirsty golf courses and suburban lawns.

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