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Alternative Energy

Tides and Waves: Sources of Endless Energy

AMivehchi-199x300About the Guest Author:

Amin Mivehchi is in his 3rd year of the ocean engineering Ph.D. program at the University of Rhode Island. His background is in Naval engineering and advanced hydrodynamics. His research interests include bio-inspired engineering systems, ocean/offshore renewable energy systems, and studying fluid and structure interactions both experimentally and numerically.

 

 

The United States has more than 95,000 miles of coastline, and tidal energy might hold a great promise for clean energy generation. By recent estimates energy production from tidal resources could have a capacity of more than 350 MW of energy production which shows the great potential of this new source to fight global warming. According to EIA’s official website, average household power consumption in 2014 in US was about 10,000 KWh per year, therefore such a plant can power up more than 300000 houses in the United States.
 

Figure 1: Global Tidal energy resources (Image source: Atlantic Resources published on October 2014.) The estimates of global potential of tidal energy generation vary, but it is widely agreed that tidal stream energy capacity could exceed 120 GW globally. You can see the distribution of tidal energy around the world.

Figure 1: Global Tidal energy resources. (Image source: Atlantic Resources published on October 2014.) The estimates of global potential of tidal energy generation vary, but it is widely agreed that tidal stream energy capacity could exceed 120 GW globally. You can see the distribution of tidal energy around the world.


 

Why Tidal Energy?

The increasing interest and attention surrounding tidal energy can be attributed to several factors. First off, tidal energy can be harnessed wherever changing tides move a significant volume of water, including waterways near the coasts of many big U.S. cities, where electricity demand is high. A bonus is that tides, unlike winds or wind-driven waves can be forecast accurately, making tidal energy a renewable resource that is both reliable and predictable.

Basic technology for harnessing tidal energy is straightforward.  As tides flow back and forth, the current flow is transferred to an energy conversion device as mechanical energy, which the device converts to electricity. A variety of conversion devices are currently being used or are under development.
 

Georgia Tech interactive tidal energy map of United States  

Georgia Institute of Technology has developed an online database in cooperation with the U.S. Department of Energy that maps the energy available in the nation’s tidal streams. This interactive database allows users to zoom and pan over maps of color-coded information on water depth, mean current speed, and mean kinetic power density for tidal streams along America’s coastlines.

Figure 2: Southern New England Tidal Power Capacity map based on Georgia Tech study (Image source: http://www.tidalstreampower.gatech.edu/)

Figure 2: Southern New England Tidal Power Capacity map based on Georgia Tech study (Image source: http://www.tidalstreampower.gatech.edu/)

 

How Does Tidal Energy Work?

There are two general approaches to tidal energy extraction: a barrage approach and a hydrokinetic approach.

Figure 3: La Ranch tidal energy plants, France, - an example of the barrage approach (Image source: Wikipedia)

Figure 3: La Ranch tidal energy plants, France, – an example of the barrage approach (Image source: Wikipedia)

The barrage approach is based on the general hydropower principle of dams. This system requires a structure to enclose a large body of water. As tide height varies, water will enter into or discharge out of the enclosed area through conventional hydro-turbines. There are a number of active and prospective projects that use this principle worldwide, including a 250 MW project in France (La Rance tidal power plant) and South Korea (Sihwa Lake Tidal Power Station). Click here to watch a video about how the Sihwa Lake Tidal Power Station works. Understanding the environmental consequences of building such a structure requires a detailed study, so the feasibility and possibility of permitting such a project in U.S. is low.

On the other hand, the hydrokinetic approach might be a more environmentally practical solution, especially along U.S coasts.  Hydrokinetic devices are typically similar to wind turbines and use the same principal of energy extraction, but are modified to suit the harsh underwater environment. The most common devices in use are the three-bladed open turbine; however, other devices exist that use different principles, such as vertical-axis turbines or flapping hydrofoil.  Hydrokinetic devices will not need any impoundment or dam and will extract energy from moving water without and need to enclose a large body of water. This approach is produces considerably less energy due to smaller structural scale than the barrage approach, but it’s getting considerable support and development around the world because of its minimal environmental impacts.
 

Challenges to Harness Tidal Energy

There are some common challenges to harnessing energy from various tidal energy devices. Tidal power devices can have effects on marine life; the turbines can kill swimming sea life with their rotating blades and their noise may affect nearby fish populations, although precautions are made to ensure that as many marine animals as possible will not be affected.

Beside the environmental effect, the maintenance cost for these devices might be high due to salt water corrosion and growth of biological organisms. Also, as water is 800 times heavier than air, the amounts of forces on tidal renewable energy devices are much bigger than those on wind turbines.

The few studies to date are not sufficient to completely understand the environmental impact of tidal power, since the impacts depend greatly upon local geography of each specific site.
 

Wind Power is a Start, Tidal is the Future!

Although there are currently no tidal power plants in the United States, Georgia Tech’s database indicates favorable conditions for tidal power generation in both the Pacific Northwest and on the shores of Maine.  The tides along the northwest coast fluctuate dramatically, as much as 12 feet a day, showing huge volume displacement of water that potentially can be used in harnessing energy. This number could reach to more than 50 feet in Bay of Fundy in Maine. The coast of Alaska, British Colombia, and Washington, in particular, have exceptional energy producing potential. Besides large-scale power production, tidal streams may serve as a local and reliable energy source for remote and dispersed coastal communities and islands. This field will need more investment and research so it can contribute to the war against global warming.

This is a guest post by Amin Mivehchi, posted by Carrie McDonough. Feel free to leave any questions or comments for Amin below! 

 

 

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