At its essence, tidal power is a form of hydropower not that different from the hydropower that involves damming a river and using water under the influence of gravity to power a turbine. Tidal power actually turns out to have its roots in gravity as well. The difference is that the gravity comes from the sun and moon, which cause the tides to rise and fall. We then harness the power of the tides as they come in and go out under the influence of gravity from the moon and sun.
Tidal power is more reliable and predictable than either wind or solar. It also does not offer the aesthetic drawbacks of either of these two renewable technologies. That is not to say, however, that tidal power is not without its disadvantages. It is expensive, relatively untested, prone to corrosion, and has environmental drawbacks of its own. Like wind and solar, tidal power would also require changes to electricity delivery systems so that energy can be efficiently delivered inland from coastal areas.
History of Tidal Power
Of course, the oceans have been used for travel and fishing for thousands of years, but several hundred years ago, mankind began tapping the oceans for tidal energy to do mechanical work. The first known use of Tidal Power began in the middle ages with the advent of tidal mills. References date all the way back to 787 CE and there is a tidal mill still standing in Suffolk, England that was built in 1170 CE. These mills capture water at high tide and then release it through a sluice so that it can turn a water wheel to grind grain.
The tide was mostly left to milling until sometime in the late eighteenth and early nineteenth centuries when electricity became commonplace and mechanisms for generating it were sought. In 1920, Dexter Cooper made the first modern plan to use tidal power in Cobscook Bay (Maine, United States) to generate electricity. Since that time, tidal power use has been primarily theoretical with only a few commercial installations ever being constructed. Now, as our energy demands grow and our poisoning of the atmosphere reaches epic proportions, we are once again turning to the oceans in hopes of finding clean, abundant, renewable energy.
In 2012, the world used more than 19,090 terawatt-hours of electricity. If we include all energy consumption, the number jumps to nearly 150,000 terawatt-hours. Roughly 81% of that energy was derived from fossil fuels. The use of fossil fuel is problematic because supplies are dwindling and because of the pollution that these fuels create. As energy demands continue to grow (electricity use has nearly doubled since 1990 when it was roughly 12,000 Twh/year) the supply of oil not only shrinks, causing a rise in prices, but the level of pollution generated accelerates our already large environmental problems. An alternative is clearly needed.Solutions like solar and wind have been offered up and though they do provide renewable solutions, they suffer from availability and predictability drawbacks. Another alternative that has been known about for centuries, but has only recently become of interest in modern power generation is the ocean.
Power of the Ocean
There are five types of energy that can be harvested for the ocean as shown in the table below. All told, the potential to harvest upto 142,000 TWh/year of energy exists.
Marine Current Power
Marine Current Power (MCP) harnesses tidal power from the kinetic energy of currents like the Gulf Stream. The benefit of MCP is that currents are predicable and stable. The downside is that harnessing this form of tidal power means outstanding feats of engineering. Of course, the fact that 1/1,000th of the energy of Gulf Stream is equal to 21,000 times the energy in Niagara Falls makes many feel that the challenge is worth taking.
The biggest disadvantage to marine current power, however, is not the cost or difficulty, but rather the importance of these currents to ocean life. There is concern that interfering with the normal workings of the Gulf Stream could have devastating consequences on ocean food chains and thus on the entire planet.
The technology for harnessing MCP already exists in the form of vertical and horizontal axis turbines. The trick will be developing corrosion resistant, environmentally friendly, cost effective variations on traditional designs that can be used in deep ocean settings. Following that, the next challenge will be to transport the energy from the ocean to the locations where it is needed.
The basic premise of osmotic power is that difference in salinity can be used to drive the flow of water. In general, water flows from areas of low salinity to areas of high salinity. If a barrier exists between these two areas that only allows water to flow, then one side of the barrier can develop a head (water height above the other). This head can then be released under the force of gravity to spin a turbine.
Not only is the system effective, it is also inexpensive. However, the scale on which this technology would need to be implemented poses a risk to sensitive marine and freshwater wildlife that cannot tolerate changes in salinity.
Ocean Thermal Energy
This technology relies on differences between the temperature of water at the ocean’s surface and the temperature of water at greater depths. The movement of water in this scenario can be used to generate kinetic energy, which can then be harvested for power generation.
Ocean waves possess a great deal of kinetic energy, which can be harvested in a number of different ways. Ultimately, wave energy is generated by the wind and the tides, so it is actually a form of wind and gravitational energy being harvested.
Wave energy is difficult to harvest for several reasons including noise pollution and biophysical impacts on marine wildlife. Additionally, transporting the power also poses a problem.
The tides are predictable, constant, and occur in enough locations throughout the world to make them a potentially huge source of energy. Right now, there are several different mechanisms for harvesting tidal power that are discussed in greater detail on this site. Tidal power technology is the most advanced of ocean energy schemes and has even been implemented on commercial scales in France, Canada, the United States, China, and Korea. It is likely that the future will see greater reliance on tidal power, particular among countries trying to cut greenhouse gas emissions. This is not to say, however, that tidal energy is without problems. There are a number of ecological concerns that must be considered for any country interested in tidal power. The good news is that these concerns are being taken seriously by everyone looking to implement tidal power, so even though projects are proceeding, they do so with caution and careful diligence.