Sustainable Infrastructure: The Guide to Green Engineering and Design by S. Bry Sarté mentions five types of hydrokinetic technologies: microhydroelectric, wave energy, vortex-induced vibration (VIV), tidal power and ocean thermal energy conversion (OTEC).  The Sherwood Institute focuses on the water-energy nexus, and although much of its work focuses on the balance of water and energy efficiency, I thought energy generation using water an appropriate topic to discuss.

In this first blog, I will summarize research on wave energy and tidal power.  In part two, I will discuss other popular types of water power.

Wave Energy:

Energy from the ocean can be generated using a variety of technologies that capture energy from surface waves or pressure fluctuations below the surface.  The devices are all installed at or near the water’s surface; however, they can be located nearshore, offshore or far offshore.

There are many devices being developed to harness this power. I’ve focused on four primary technologies: point absorbers, overtopping devices, attenuators and terminator devices.

• Point absorbers use the relative motion created by waves of a floating buoy to a fixed structure to power electromechanical or hydraulic energy converters.

• Overtopping devices consist of a reservoir and turbine system.  Incoming waves fill the reservoir, making the water height in the reservoir higher than that of the surrounding ocean.  When the water is released, the energy of the falling water drives a turbine or other conversion device to create electricity.

• Attenuators are floating structures that are situated in the same direction as wave travel.  The multi-segment device flexes as waves pass. This flexing drives hydraulic pumps and converters.

• Terminator devices are generally designed to be near shore.  They are oriented perpendicular to the wave direction.  One form of terminator device is the oscillating water column, which allows water to enter the bottom of a column filled with air.  Passing waves cause the water column to move up and down. The forced-out air powers a turbine to convert the energy to electricity.

The prospect of harnessing wave energy is very promising.  However, there are environmental considerations: visual appearance and operation noise above and below the surface, marine habitat changes, accidental release of hydraulic fluids, sharing sea space with recreational and commercial boaters, and possible reduction in wave height and site change while installing or decommissioning devices.  Many of these concerns can be addressed with careful site selection and monitoring; however, they are matters that must be addressed.

If you are interested in learning more about wave energy research, I suggest reading Technology White Paper on Wave Energy Potential on the U.S Outer Continental Shelf.

Ocean Tidal Power:

Tidal power harnesses energy from the changing tide.  In some areas, the gravitational pull of the sun and moon and the rotation of earth can cause up to a 40-foot water height differential between high and low tide.  Most of the technology for harnessing tidal power uses submerged turbines similar to wind turbines.  There are three well developed methods for capturing this energy: barrages, fences and turbines.

• Tidal barrages are dams across an inlet with gates that allow water to flow into the area and then empty during the ebb tide.  As the water flows in and out of this “tank”, the water flows through turbines which activate electric generators.

• Tidal fences are generally used in channels between two landmasses.  Vertical axis turbines are mounted on a fence.  As water flows past these turbines, the current causes the turbines to spin.  The turbine action, again, drives electric generators to produce electricity.

• Tidal turbines can be located anywhere where there is a strong tide.  The turbines are arrayed in rows, similar to wind turbines on a wind farm, only underwater.  The current of the ebbing and flowing tide cause the turbines to turn and produce electricity in the same fashion as the two previous technologies.

Water is about 800 times denser than air.  Therefore, turbines located underwater have enormous energy potential.  However, there are a lot of environmental concerns, including: sea-life migration, navigational and recreational disadvantages, and changing the ecosystem in barraged areas where silt buildup can occur.   Tidal turbines might prove to have the least environmental impact because they do not block the natural flow of water like the other two methods.

I believe wave and tidal energy hold enormous potential!  The ocean is so powerful, and both forms of renewable energy generation could produce an incredible amount of electricity.  However, there are cost and environmental concerns that still need to be addressed – and utilizing these technologies are only possible on a large scale and in very specific site locations.  These forms of power are still being intensely researched, so, hopefully, new technologies will minimize environmental impact and maximize the amount of electricity that can be generated so that more sites can be used to produce these types of power.

In my next post, I will continue the water power discussion by addressing micro-hydroelectric, ocean thermal energy conversion and vortex-induced vibration power.

Posted in BLOG, Energy, Water Resources | No Comments »