|Fig. 1: Onehunga Lagoon, Auckland New Zealand. (Source: Wikimedia Commons)|
Tidal energy is growing in popularity as a utilized energy source. Tidal energy harnesses the power of nature in the way wind turbines do, using movement to create electricity. Caused by the "gravitational pull of the sun and moon, leading sea levels to rise and fall reliably," tidal energy relies on Earth's most abundant resource: water.  Water is significantly denser than air, almost 1000 times denser, giving it the potential to produce much more energy per single turbine. 
As great as tidal energy sounds, there are a number of problems that have kept the total number of tidal plants in the world low. The potential for negative environmental impact depends on where the plant is built, but almost every tidal plant disturbs ecosystems. Dredging and construction alone bother the sea floor, and the existence of turbines can affect sea life.  It is possible to limit environmental impact by selectively choosing the location of a plant, as well as exploring the different ways one can harness tidal power. Currently there are three ways to access tidal energy: streams, barrages, and lagoons. Each way comes with positives and negatives. But tidal lagoons are the most sensible option because of their feasibility and limited environmental damage potential.  Fig. 1 shows a picture of the Onehunga Lagoon in Auckland New Zealand.
Tidal barrages have the greatest potential energy output. Functioning similarly to a dam, a tidal barrage is built across a river estuary, allowing water to flow through tunnels in the dam as the tide goes in and out. This flow of water powers turbines, which then create electricity.  Barrages allow the operator to control the flow of water, and also utilize the tunneling technique to maximize power output. Both of these features make a barrage an attractive energy option. But these small dams have a large environmental impact. The construction of a barrage completely changes the landscape of the sea floor, creates a harmful change in water level, dangerously decreases salinity, and stops migration of fish.  For these reasons, barrages are not a very suitable option for tidal energy.
Tidal streams utilize much smaller bodies of water than barrages and do not require the construction of a dam. When placing turbines at sections of a stream with high flow rates, one can harness powerful tidal energy. Streams have extremely fast currents and varying water levels, giving them a high capacity to produce electricity. Another important characteristic of a stream is that the horizontal speeds of the current exist at an almost identical rate throughout the entire depth of the water.  This allows for flexibility when placing a turbine, since flow rate will be the same at any depth of the stream.
Tidal streams, however, are still not the best option for tidal energy. While the nature of a stream allows for some placement flexibility, streams are often high-traffic bodies of water for both sea life and ships.  Any stream large enough to produce sufficient energy would also contain a lot of sea life. Placing turbines in these areas can kill sea creatures and disrupt migration patterns. Utilizing tidal streams is less harmful than building barrages, but the energy output is not as powerful and the number of viable sites is limited.
|Table 1: Output and efficiency projections for lagoon vs. barrage tidal energy sources.  (Source: JP Cannistraro)|
A lagoon is a body of water partly enclosed by any barrier.  By placing tidal generators at the entrance of a lagoon, one can harness the energy provided by the change in depth. Lagoons operate similarly to barrages but present fewer environmental complications. Lagoons can easily be constructed out of natural materials and are built along the coastline. Barriers would keep out sea creatures too large to swim into the lagoon, while the smaller animals would be able to enter and exit easily.  Since the water levels in lagoons are constantly changing, the turbines would be constantly generating electricity. The biggest drawback of a lagoon is its relatively limited potential to produce energy based off of a relatively slow flow rate.
A closer look at the data, however, suggests that this analysis is too simplistic. A study done by Friends of the Earth (FOE), an organization dedicated to limiting mankind's impact on the environment, explains that every metric ton of material used in the creation of a tidal lagoon would produce three times as much energy as a metric ton of coal.  FOE also projects a number of other factors that lean heavily in the favor of lagoons as being the best possible tidal energy source. Table 1 summarizes the main results of their study, which explored the potential impact of a building a barrage versus multiple lagoons in the Severn Estuary.
In contrast to turbines placed in tidal streams, lagoons will not impede shipping. When compared to both barrages and streams, lagoons have a significantly smaller environmental impact. As Table 1 shows, lagoons generate 24/18 or 1.33 times as much power as barrages. Even though barrages have a much higher capacity to generate electricity, they do not operate at their full capacity nearly as much as lagoons could. This is made clear by the 35 percent capacity factor difference between the two. Lagoons eliminate more emissions at cheaper cost.
As interest grows in tidal energy as a renewable energy source, more information about its feasibility will be discovered. When looking at specific types of tidal energy, it appears that energy harnessed from lagoons is the most feasible and ethical option. The affect on sea life is minimal, the technical feasibility exceeds that of streams, and the projected output exceeds that of barrages.
© J. P. Cannistraro. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 B. Handwerk, "Five Striking Concepts for Harnessing the Sea's Power", National Geographic, 21 Feb 14.
 M. Kadiri et al., "A Review of the Potential Water Quality Impacts of Tidal Renewable Energy Systems," Renew. Sustain. Energy Rev. 16, 329 (2012).
 N. Crompton, "A Severn Barrage or Tidal Lagoons?", Friends of the Earth, January 2004.
 R. E. Thompson, Oceanography of the British Columbia Coast," Canadian Department of Fisheries and Oceans, 1981, p. 45.