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| Fig. 1: Tidal power plant in the Eastern Scheldt storm surge barrier in the Netherlands. (Source: Wikimedia Commons) |
Tidal energy is a form of renewable energy which extracts from the ocean's tides the energy generated by the coupled Earth-Moon system. [1] People have historically been aware of the tide's potential utility as far back as Roman times, with the first tidal power plant going online in 1966 and generating power since. [1] To a greater extent than other forms of renewable energy such as solar or wind, tidal power is predictable due to the predictability of tides. [2] Additionally, tidal power has great potential for energy generation. [2] Globally, tidal power resources are estimated to be 3 terawatts, with the technically harvistable resources estimated to be 1 terawatt. [3]
Tidal power is generated through using the movement of water to spin a turbine, thus generating electricity. It differs from hydro-electric power, another form of power generation using moving water. Hydro-electric power requires the damming of rivers to create reservoirs from which the controlled release of water can be used to spin the turbines. With tidal power, instead the natural movement of water in the ocean is the source of energy tapped into for spinning turbines. [2] There are multiple ways in which this natural movement of water in the ocean can be tapped into.
Tidal barrage plants are the more mature form of tidal power plant when compared to the newer tidal current designs, with the oldest operational tidal power plant being within this category. [1] Tidal barrage power plants extract the potential energy provided by the difference in elevation between the ebb and flood tides. The plants can be located wherever ocean water flows into areas compounded by land, such as bays. [3] The plants can be powered by either of both water flowing into and out of the compounded region. [3] The turbines used in this form of tidal power are similar to those used in hydro-electric dams. [3] In 2012, the total amount of power estimated to have been extracted from tidal barrage power plants globally is around 514 megawatts. [3] An example of a tidal barrage power plant can be seen in Fig. 1.
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| Fig. 2: Diagram of a tidal current turbine. (Source: Wikimedia Commons) |
Tidal current power generation is relatively newer, and extracts the kinetic energy from moving currents through the use of turbines deployed within the current's path. [3] The turbines used to generate electricity in this system operate the same as wind turbine, with the movement of water within the ocean replacing the movement of air in the atmosphere, yet they differ in that the tidal turbines have smaller blades, and spin slower compared to wind turbines due to the higher density of water. [3] The turbines can be oriented either horizontally or vertically, being parallel or perpendicular to the flow of water respectively. [3] According to a survey conducted on existing tidal current projects in 2013, 76% of tidal current turbines are oriented horizontally, with only 12% being oriented vertically. [3] In 2012, the total amount of power estimated to have been generated from tidal current globally is around 6 megawatts. [3] A diagram of a seabed turbine can be seen in Fig. 2.
There are a few barriers holding back tidal energy: cost, environmental impact, and lack of development. The lack of development could be credited with causing the other two barriers. Currently, tidal power plants come with a large upfront construction cost, and the reliability, and feasibility of the technology makes it hard to market against other, more developed forms of renewable energy. [2,3] Tidal range power plants in France and South Korea had construction costs of $340/kW and $117/kW respectively. [3] Beyond up-front costs, a proposed tidal power plant off the island of Alderny, for example, would have an levelized cost of energy (LCOE) of €0.09/kWh, or around $109/MWh. [2] Environmental impact, similarly, could be solved with time and further development, however, currently the impact is not fully understood. Tidal barrage plants can cut off access to estuaries effecting marine life, however some types of barrages mitigate this impact. [3] It is believed that tidal current power plants have less of an impact when compared to tidal barrage plants, with some studies showing little impact caused by the physical presence of seabed turbines, with fish and other ocean life able to avoid the new obstacles. [3,4] However, key environmental questions still remain unanswered, such as the effect the acoustic output of the turbines may have on sea life. [4]
© Jake Wilson. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. 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.
[1] S. P. Neill et al., "Tidal Range Energy Resource and Optimization - Past Perspectives and Future Challenges," Renew. Energy 127, 763 (2018).
[2] E. Segura, R. Morales and J. A. Somolinos, "Cost Assessment Methodology and Economic Viability of Tidal Energy Projects," Energies 10, 1806 (2017).
[3] "Tidal Energy: Technology Brief," International Renewable Energy Agency, June 2014.
[4] A. Copping et al., "Environmental Effects of Marine Energy Development around the World: Annex IV Final Report," Pacific Northwest National Laboratory, PNNL-22176, January 2013.
