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| Fig. 1: Image of the Earth from Apollo 17. (Source: Wikimedia Commons |
The oceans account for over 70% of the earth's area (see Fig. 1). [1] They serve as a source of renewable energy. The energy of the oceans comes in various forms: thermal energy, kinetic energy from waves and tides, potential energy from changing tidal heights, chemical energy including fossil fuel stores, energy from osmotic gradients, as well as biological energy in the form of biomass. Ultimately, the Sun is the source of most of this ocean energy. Heat from the sun is the direct source of ocean thermal energy; transfer and distribution of thermal energy from the sun contributes to wind creation; waves result from wind motion over the oceans. Chemical energy in fossil fuels come from biological sources, and living things ultimately rely on the sun for energy. Tides are one source of ocean energy that is influenced more by two other bodies the earth itself, and the moon than it is by the Sun.
Ocean tides refer to the rise and fall (see Fig. 2 for an image of a beach at low tide) of the water levels on Earth due to the earths rotation with respect to the gravitational effects exerted on Earth's oceans by primarily the moon, as well as the Sun. [2,3] The earths rotation with respect to the sun and moons gravitational effects yields the tidal forces. Tidal force is inversely proportional to the distance between the center of masses cubed instead of squared as with gravitational force. [2] Because the earth-sun distance is so much larger than the moon-earth distance, the tidal force on earth due to the moon is greater than the tidal force due to the sun (by about 2.2 times). Though the tidal force exerted by the sun is less, it is still considerable. Thus, when the moon and sun are aligned, the tidal forces from both add up, creating larger resultant tidal forces and the maximum bulge of the oceans occurs. These tides are known as spring tides (see Fig. 3). [4]
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| Fig. 2: Image of a Beach at Low Tide. (Source: Wikimedia Commons) |
Tidal energy comes in two forms: tidal potential energy and tidal current energy. Tidal potential energy involves harnessing the potential energy stored in the oceans due to the height difference of high and low tides, whilst tidal current energy makes use of turbines to harness the kinetic energy due to the flow/ velocity of the oceans. Both types of energy can be and currently are harnessed for electricity generation.
Tidal currents flow in both horizontal and vertical directions. [1] The main requirement to be able to harness tidal current energy is a minimum water wave height of 1.5 m. [1] The instantaneous power generated by tidal currents is given by
where ρ is the density of sea water, C is the efficiency of the turbine, A is the cross-sectional area and v is the current flow velocity.
Compared to wind turbines, tidal current turbines also generate considerable amounts of power; though tidal currents usually have lower velocities than wind velocity, water is denser than air. Tidal current power potential per annum has been estimated as 61.3 TWh for China, 95 TWh for the UK and 230 TWh for Ireland. [4]
The estimate for Ireland breaks down like this: The
efficiency C of some commercially available turbines is about 40%. [5]
Rotor swept area A for available turbines is between 20 m2
and 320 m2, with a mid-range rotor swept area of 170
m2; rotor diameter ranges from 5m to 20m, with a mid-range
rotor diameter of 12.5m. [5] Given the length of Ireland's coastline as
approximately 1450 km, we can estimate that about 116,000 turbines could
be packed side-by- side along Ireland's coast. Sea water density is
1027 kg m
Estimated annual tidal energy off the shore of Ireland = 5.39 × 109 Watts × 24h day-1 × 365 days y-1 = 4.72 × 1013 Wh y-1 = 47.2 TWh y-1.
The estimated tidal power potential of 230 TWh per year for Ireland is about 5 times the estimate calculated above as 47.2TWh y-1.
Barrages are used to harness tidal potential energy. Barrages are dams that are controlled to allow water in and out at specific points in the tidal cycle. At high tide, water is allowed into the barrage through gates equipped with turbines. At low tide, water is allowed out of the barrage. The height difference for the high and low tides is known as the tidal range and the potential difference due to the tidal range results in the tidal potential energy harnessed as the water motion causes the turbines to move. The tidal potential energy harnessed is given by the equation below:
where ρ is the density of sea water, g is the acceleration due to gravity, A is the cross- sectional area and h is the tidal range.
Tidal potential energy can be harnessed with current systems if the tidal range is greater than 5m. The power potential of tidal current energy is estimated to be higher than for tidal potential energy. [4] The power potential per annum from ocean tidal power energy for the UK is estimated as 96 TWh. Considering both sources of tidal energy, the US's tidal electric energy generation is estimated to be 250 TWh per annum. [4] Tidal energy is estimated to cost 165,000/MW. [1]
The estimate for the tidal power for the La Rance Power Plant in France breaks down like this: The basin area A is 22 km2 = 2.2 × 107 m2. [7] The average tidal range h is 8.2 m. [7] Tidal potential energy = 1027 kg m-3 × 9.8 m sec-2 × 2.2 × 107 m2 × (8.4 m)2 / 2 = 7.81 × 1012 J. The total energy delivered per year is 2 × 7.81 × 1012 J day-1 × 365 days y-1 / (3600 J Wh-1) = 1.58 × 1012 Wh y-1 = 1.58 TWh y-1.
The net power output for the La Rance Power plant is estimated as 480 GWh per year, which is about a third of the estimated value 1.58 TWh y-1 calculated above. [7]
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| Fig. 3: Image of Spring Tide on Earth. (Source: Wikimedia Commons) |
Compared to non-renewable energy resources such as coal and crude oil, tidal energy is considered to have much less environmental impact. In harnessing most sources of energy, energy plants have to be set up and these usually take up space and modify the environment, affecting the living organisms that inhabit that space. This is a major concern for tidal energy plants. Most tidal energy plants take the form of barrages, dams, turbines within the ocean and these can and do change the physical properties of the aquatic environment and affect aquatic organisms. [8] Tidal barrages and dams affect the salinity of the water within and surrounding them, such that the zonation of different organisms can change. When established in estuaries, the change in salinity of the different areas of the estuary may lead to freshwater species expanding out of their previous zones. Turbines and dams also affect the water current velocity; with the water currents slowing down, usually mobile sediments may end up settling, leading to sediment accumulation. This would reduce water turbidity and allow for larger populations of invertebrate organisms since there will be an increase in stable substrate on which they can settle and grow. The noise and vibrations from the tidal plant are also likely to affect some aquatic organisms as well. [8]
It is important to note that the premise that tidal energy has lower environmental impact is questionable considering that harnessing tidal energy decreases tidal currents and could eventually lead to the cessation of tides. [3] This would have major environmental impacts, for example- what would happen to the intertidal habitat all across the world?
© Nana Ansuah Peterson. 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] N. Khan
[2] R. H. Stewart, "Introduction to Physical Oceanography," Texas A&M University, 2008.
[3] W. Chen, "Tidal Energy," Physics 240, Stanford University, Fall 2010.
[4] M. Melikoglu, "Current Status and Future of Ocean Energy Sources: A Global Review," Ocean Eng. 148, 563 (2018).
[5] A. Roberts et al., "Current Tidal Power Technologies and their Suitability for Applications in Coastal and Marine Areas," J. Ocean Eng. Mar. Energy 2, 227 (2016).
[6] F. O'Rourke, F. J. Boyle, and A. Reynolds, "Tidal Current Energy Resource Assessment in Ireland: Current Status and Future Update," Renew. Sustain. Energy Rev. 14, 3206 (2010).
[7] F. O'Rourke, F. Boyle and A. Reynolds, "Tidal Energy Update 2009," Appl. Energy 87, 398 (2010).
[8] A. Uihlein and D. Magagna, "Wave and Tidal Current Energy - A Review of the Current State of Research Beyond Technology," Renew. Sustain. Energy Rev. 58, 1070 (2016).
