Wave power generates electricity by harnessing energy from surface waves or pressure fluctuations below the surface.

Though the U.N.’s IPCC predicts ocean energy will be one of six renewable energy sources providing 77 percent of our power by 2050, it gets little attention relative to other renewables (i.e., wind, solar, biofuels). Thus, this series of blog posts on tidal power, ocean thermal energy conversion and wave power.

Wave power generates electricity by harnessing energy from surface waves or pressure fluctuations below the surface. Though the entire ocean is comprised of waves, there are only a few select spots in the world where the waves are conducive to power generation:

• Western coasts of Scotland
• Northern Canada
• Southern Africa
• Australia
• Northeastern and northwestern coasts of the U.S.

As outlined by the U.S. Department of Energy, wave power technology falls into one of two categories – offshore and onshore – the details of which are summarized below:

Offshore wave power, the systems for which are typically situated at least 131 feet below the surface using…

1) Tools that capture the bobbing motion of waves to power a pump that creates electricity.

2) Hoses connected to floats that ride the waves. As the hoses stretch and relax, they pressurize the water and turn a turbine.

Wave power may also be captured offshore via seagoing vessels that funnel waves through internal turbines.

Onshore wave power, the systems for which are located along shorelines where technology harnesses energy created by breaking waves using…

1) Oscillating water columns

These partially submerged concrete or steel structures have openings to the sea below the waterline where they enclose a column of air above a column of water:

As waves enter the air column, they cause the water column to rise and fall. This alternately compresses and depressurizes the air column. As the wave retreats, the air is drawn back through the turbine as a result of the reduced air pressure on the ocean side of the turbine.

2) Tapchan

This tapered channel system feeds into a reservoir constructed on cliffs above sea level:

The narrowing of the channel causes the waves to increase in height as they move toward the cliff face. The waves spill over the walls of the channel into the reservoir and the stored water is then fed through a turbine.

3) Pendulor device

A rectangular box is open to the sea at one end with a flap hinged over the opening:

A flap is hinged over the opening and the action of the waves causes the flap to swing back and forth. The motion powers a hydraulic pump and a generator.

Like the other two types of ocean energy, the development of wave power is dependent on sensitivity to its environmental impact – specifically the impact of wave power technology on scenic shorefronts and the flower patterns of sediment on ocean floors. Wave power is also challenged by start-up costs, but once constructed, the operation and maintenance of wave power technology is inexpensive relative to other forms of energy.

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For tidal power to be harnessed and turned into electricity, the difference in height of the highest and lowest tides must be at least 16 feet.

Much like ocean thermal energy conversion, the development of tidal power is still in its early stages despite  dating back a number of years. In fact, tidal power is a type of ocean energy that dates back to 787 AD when tide mills were used along the Spanish, French and British coasts.

The challenge?

For tidal power to be harnessed and turned into electricity, the difference in height of the highest and lowest tides must be at least 16 feet. And of all the coast areas in the world, only 40 sites meet this criterion. Sites in the Pacific Northwest and Atlantic Northeast are among them, but tidal power development in these areas is still limited to pilot projects, as construction is so cost-prohibitive.

There are three types of tidal power technologies, explained by the U.S. Department of Energy as follows:

Barrage or Dam Tidal Power

• Gates and turbines installed along dam
• When tides produce adequate difference in the level of the water on opposite sides of dam, gates are opened
• Water flows through turbines, turning an electric generator to produce electricity

Tidal Fence

• Tidal fences reach across channels between small islands or across straits between the mainland and an island
• Turnstiles spin via tidal currents
• Currents that run 5.6–9 miles per hour can generate as much energy as winds of much higher velocity

Tidal Turbines

• Arrayed underwater in rows, like wind farms
• Ideally located close to shore in water depths of 20–30 meters
• Function best where coastal currents run at between 4 and 5.5 mph, enabling a 15-meter  diameter tidal turbine to generate as much energy as a 60-meter diameter wind turbine

Of these three types of tidal power, tidal turbines are believed to be the least threatening to the migratory patterns of sea life – a top concern among scientists.

Though there are currently no operable tidal power plants in the U.S., as of this writing the nation’s first full-scale tidal power plant is in the final permitting stage for New York’s East River tidal turbine project. There are also three tidal power pilot projects underway in Washington, Maine and Alaska.

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In the simplest of terms, OTEC converts the warmth in our oceans into energy for electricity.

Ocean energy is one of six renewable energies the U.N.’s IPCC predicts will fuel 77 percent of the world’s power by 2050. Of the three types of ocean energy – 1) tidal power, 2) wind power and 3) ocean thermal energy conversion (OTEC) – OTEC gets the least amount of coverage, yet could prove the most promising of them all.

In the simplest of terms, OTEC converts the warmth in our oceans into energy for electricity. Beyond that, the U.S. Department of Energy identifies a number of other additional benefits associated with OTEC technology:

• Air conditioning – spent cold seawater can chill fresh water in a heat exchanger or flow directly into a cooling system.
• Chilled-soil agriculture – cold seawater can flow through underground pipes, chilling the surrounding soil.
• Fresh water production – in the open cycle process, the steam that leaves its salt behind in the low-pressure container is almost pure fresh water.
• Mining of trace elements – since OTEC already pumps water out of the oceans, it considerably reduces the cost of mining 57 trace elements from seawater, leaving only the issue of the extraction process.

Some have gone on to suggest that OTEC could even play a role in minimizing the impact of global warming.

As reported by Popular Science, the warming that causes seawater evaporation – and the subsequent production of salty water vapor that leads to desertification – could be mitigated by OTEC:

In the 1970s, engineers began using platform-based rigs to bring cold, deep water to the warm surface; the idea was that the temperature difference would drive a heat engine, generating energy.

Used on a large scale, OTEC could have the healthy side effect of lower the surrounding surface temperatures, and that would be a very good thing.

There are three types of OTEC, which the U.S. Department of Energy describes as follows:

Closed-Cycle OTEC

• Warm surface seawater is pumped through a heat exchanger where a low-boiling-point fluid, such as ammonia, is vaporized.
• The expanding vapor turns the turbo-generator.
• Cold deep-seawater – pumped through a second heat exchanger – condenses the vapor back into a liquid, which is then recycled through the system.

Open-Cycle OTEC

• Warm seawater is placed in a low-pressure container where it boils.
• The expanding steam drives a low-pressure turbine attached to an electrical generator.
• The steam, which has left its salt behind in the low-pressure container, is almost pure fresh water.
• It is condensed back into a liquid by exposure to cold temperatures from deep-ocean water.

Hybrid OTEC

Combining features of both closed-cycle and open-cycle systems:

• Warm seawater enters a vacuum chamber where it is flash-evaporated into steam, similar to the open-cycle evaporation process.
• The steam vaporizes a low-boiling-point fluid (in a closed-cycle loop) that drives a turbine to produce electricity.

The first OTEC plant was built in 1930 but the technology is still considered in its early stages.

What’s holding OTEC back?

Number one, the sites suitable for OTEC plants is limited to tropical coastal areas where the temperature difference between the warmest air on top and coldest air on the bottom is at least 20 degrees Celcius, or 36 degrees Fahrenheit. And number two, the large intake pipe necessary to bring cold water from the bottom of the ocean to the top is an excessively expensive endeavor.

Research continues at the National Energy Laboratory of Hawaii, one of the world’s leading facilities for OTEC development.

Meredith Simonds Ocean Energy air conditioning, chilled soil agriculture, fresh water, global warming, National Energy Laboratory of Hawaii, ocean energy, ocean thermal energy conversion, OTEC, Renewable Energy, tidal power, U.N., U.S. Department of Energy, Wind Power