Tidal energy is powered by the natural rise and fall, or surge, of ocean tides and currents. While hydroelectricity refers to the use of dams to produce energy from water, tidal energy uses the ocean/sea's tides.
Tidal lagoon: a body of water partially enclosed by a natural or manmade barrier
How it Works
There are three main types of tidal energy.
Tidal Barrage: Tidal barrage acts in a similar fashion to a dam, installed across an inlet of an ocean bay or lagoon that forms a tidal basin. Sluice gates on the barrage control water levels and flow rates to allow the tidal basin to fill on the incoming high tides and to empty through an electricity turbine system on the outgoing ebb tide. A two-way tidal power system generates electricity from both the incoming and outgoing tides using the movements of the water.
Tidal Turbines: Tidal turbines act similarly to wind turbines. However, because water is much denser (800 times denser) and more powerful than air, tidal turbines have to be much sturdier and heavier than wind turbines. Tidal turbines still follow a similar principle to wind turbines, however, where the water turns the blades, which rotate a generator to generate electricity (for more detail, click here). Furthermore, tidal turbines are much more expensive than wind turbines because of their added weight, though this is offset with a great output than wind turbines.
Tidal Lagoon: A tidal lagoon generator uses the flow of water in and out of a tidal lagoon in order to turn turbines. The mechanics are much similar to a tidal barrage system, though they can be placed along coasts.
How it's Renewable Energy
Tidal energy does not require any additional energy to make the tides go, as the tides are a natural result of the moon's gravity, and it releases no greenhouse gas emissions.
Advantages and Disadvantages
Beyond being renewable, tidal energy requires much less space than other forms of renewable energy, such as wind, solar PV, and solar CSP. The advantage of this fact is that tidal energy can be employed by countries that lack land space while still taking up very little of the coastline. In addition, the tides are very predictable, much more so than wind, solar PV, or solar CSP, making it easier for engineers to design systems.
Unfortunately, tidal energy can have serious negative impacts on marine life. Some tidal systems emit electromagnetic emissions (EMF), which can be detrimental to marine life, migration patterns, and ocean patterns. In addition, tidal turbines can directly kill marine life, though there have been some preventative measures such as decreasing the speed for blades without decreasing output to allow fish to swim through them. Similarly, a tidal barrage can alter an entire ecosystem by changing water levels and blocking off certain species from their food sources.
Besides environmental impacts, tidal energy can be expensive. The tidal barrage system costs a lot to construct. Similarly, the tidal lagoon system, while having minimal environmental impacts, costs a lot with much less output than the rest.
Conclusion: Our Take
Tidal energy is still in its infancy. Lack of widespread deployment and limited research leave little information for a complete conclusion. Indeed, Yale argues that the complexity of tidal energy may make it impossible to ever get started. However, if it does, tidal energy can become a major player in the emerging renewable energy space. As its effects on the environment are mitigated, its output and prevalence can increase.
Renewable Energy: Hydroelectricity
Image: Levitan, Dave. Machines Designed by Pelamis Wave Power Operating Offshore at the Billia Croo Test Site of the European Marine Energy Centre (EMEC), Located in Scotland's Orkney Islands. A Central Challenge Faced by Wave Power Developers Has Proven to Be the Complexity of Harnessing Wave Power, Which Has Led to a Host of Designs. Yale Environment 360. Yale School of the Environment, April 28, 2014. https://e360.yale.edu/assets/site/_1500x1500_fit_center-center_80/Winter-testing_two-Pelamis-machines_16x9.jpg.