Ocean Energy

Worldwide, enormous potential exists for ocean-generated electricity.  Capturing the energy in tides and waves can make power.  Additionally, Ocean Thermal Energy Conversion (OTEC) produces electricity from the heat stored in the ocean's depths.  

There are currently no tidal or wave power plants in the US.  However, the geographical features of both the Pacific Northwest and Atlantic Northeast coasts offer good conditions for making tidal and wave power.  OTEC works best in water conditions that exist between the Tropic of Capricorn and the Tropic of Cancer.  In the US, Hawaii and the seas south of Florida fall within this area.

Tidal power technologies activate generators with:

• Barrages or dams: force seawater through submersed turbines
• Tidal fences: rotate like a giant turnstile on the ocean currents
• Tidal turbines: spin like a wind turbine, ideally located close to shore in water 65-100 feet deep

Wave power technologies:

Offshore Systems are generally placed in water deeper than 125 feet and capture energy with:

• Edinburgh (Salter's) Duck - powers a pump from the up-and-down motion of the waves
• Floating hoses - pressurize water that turn a turbine
• Floating platforms - funnel waves through internal turbines and back to sea

Onshore Systems are built on shorelines and capture energy with:

• Oscillating water columns - enclose a column of air above a column of water
• Tapchans - increase wave height by moving water through a narrowing channel
• Pendulors - power a pendulum device that runs a hydraulic pump and generator

OTEC technologies create electricity with:

Closed-Cycle systems:

• Pump warm surface seawater through a heat exchanger to vaporize a low-boiling-point fluid (such as ammonia) that then turns a generator
• Cold deep-seawater pumped through a second exchanger condenses the vapor back to liquid

Open-Cycle systems:

• Boil warm surface seawater to create steam that turns a low-pressure turbine
• Steam is turned back to liquid by exposure to cold temperatures from deep-ocean water

Hybrid systems: 

• Combine elements of open-and closed-cycle OTEC systems
• Warm seawater is flash-evaporated into steam (open-cycle)
• Steam vaporizes a low-boiling-point fluid (closed-cycle) that turns a turbine

A by-product of open or hybrid-cycle OTEC plants is the production of fresh water from seawater, known as desalinization.

Challenges:
Currently, most ocean power technologies are not economically competitive with conventional fossil fuel power.  They tend to have low operating costs but high construction costs with a long payback period.  Careful site selection is also extremely important to keep the environmental impacts of ocean power technologies to a minimum.  Protecting shorefronts, keeping sea life migration patterns and habitat intact, and preventing alterations in ocean temperature or sedimentation processes must all be considered.  In addition, water and geographic conditions must be right for the technologies to work.

Additional Resources

• Marine Renewable Energy Center (MREC)
• Massachusetts Technology Collaborative (Ocean Energy (Wave, Tidal, Ocean Thermal)

• US Department of Energy (Exploring Ways to Use Ocean Energy)

*Thanks to the DOE webpage Exploring Ways to Use Ocean Energy for portions of this text.