Tidal forces

Tidal forces are caused by the interaction of the gravitational fields of astronomical objects, and tides are caused by physical deformations of astronomical bodies subjected to gravitational fields. The cyclic relationships between the orbits of the moon, Earth, and sun cause periodicity in the cycles of tidal movements on Earth. Tidal power is generated by utilizing the kinetic energy contained within moving tidal waters. This power generates electrical energy for human use.

The Physics of Tides

Every astronomical object, including stars, planets, and satellites, generates a gravitational field, which is defined by the way in which the object’s mass deforms the surrounding space. Larger objects have more mass and therefore generate a larger gravitational field.

When two astronomical objects interact, they exert force on each other through their gravitational fields, a force sometimes described as a gravitational pull or attraction. This force grows stronger the closer the objects move toward each other. The earth orbits the sun because of the gravitational pull exerted by the sun; the earth and the moon orbit each other because of their mutual gravitational attraction. Together this constitutes the earth-moon-sun system, which is responsible for many physical phenomena that affect Earth’s climate, weather, and physical cycles.

As the moon orbits the earth, the structure of both the earth and the moon deform slightly in response to the gravitational forces they exert on each other. This causes both the earth and the moon to elongate slightly as they pull toward each other. The pull of the moon is strongest on the side of the earth facing the moon. The moon’s pull causes the sides of the earth at right angles to the moon to depress slightly. This occurs because the gravitational pull of the moon is unequally distributed over the spherical surface of the earth.

The side of the earth facing away from the moon receives the least gravitational pull and, therefore, also undergoes the least amount of deformation. This differential deformation causes the phenomena known as tidal forces, which are most clearly demonstrated in the cyclic movement of ocean currents.

Earth’s hydrosphere, which includes oceans, seas, lakes, and rivers, is more responsive to tidal forces because of its fluid state. The waters closest to the moon are pulled toward the moon as the earth deforms, while the waters on the side of the earth farthest from the moon experience less pull; therefore, more of those waters are “left behind.” This causes a high tide on both the side of the earth closest to the moon and on the side farthest from the moon. The sides of the earth located approximately at right angles to the moon’s position experience intermediate pull from the moon and therefore exhibit low tides as the waters are either pulled toward the moon or toward the side opposite the moon.

Tidal Variations

As the earth spins on its axis, the position of the moon changes relative to the earth, creating two high tides and two low tides at each location per day. However, the tides are also affected by the topography of the earth, as landmasses affect the overall direction and force of tidal flow. Therefore the bathymetry, or overall measure of water depth, depends on both the tidal cycle and the particularities of the lithosphere.

Many locations experience the standard semidiurnal cycle of two high and two low tides per day, while in other areas, landmasses blocking ocean currents can lead to a cycle—the diurnal—of only one high and one low tide per day.

The timing of the tide coincides with the rotation of the earth and the position of the earth and the moon as they orbit around each other. The lunar cycle is the time it takes the moon and the earth to return to the same position with regard to one another. For instance, if the moon is directly above St. Louis, Missouri, it will take one lunar cycle for the rotation of the earth and the orbit of the moon to realign to bring the moon to the same point above St. Louis.

The lunar cycle reflects the relationship between the 24-hour rotation of the earth combined with the 27.3-day orbit of the moon around the earth (called the moon’s sidereal period). Because the earth rotates on its axis in the same direction that the moon travels around the earth, the relationship between the two lags by about 30 minutes each day. The combination of these factors creates a cycle of 29.5 days for the earth and the moon to return to the same position, sometimes called a lunation. The cumulative effect of this relationship is that the timing of the tides also changes on a daily cycle as the moon-earth relationship progresses through each lunation. High and low tides arrive approximately fifty minutes later each day.

While the moon exerts the most important influence over the tides, the position of the sun also has a powerful effect on tidal strength. The gravitational pull of the moon has a far more immediate effect on the earth because the moon is much closer. On average, the moon is approximately 384,403 kilometers (239 miles) from the earth, whereas the sun is approximately 150 million km (93 million mi) from the earth. However, because of the sun’s enormous mass, its gravitational field extends beyond the edges of the solar system, throughout the entire heliosphere. Therefore, a full understanding of tidal forces must take into account the relative positions of all three bodies.

Twice each lunar cycle, the moon, the earth, and the sun fall into a line with respect to one another. This state of alignment, called syzygy, causes the gravitational pull of the sun and the moon to become complementary or opposing, depending on whether they are on the same side or the opposite side of the earth. In both cases, the earth experiences higher high tides and lower low tides, called the spring tides, because the differential pull of gravity exerted on the earth is stronger. When the moon and sun are at right angles to each other with respect to the earth, their respective gravitational fields interfere with each other, leading to periods with less variation between high and low tides, called the neap tides.

Spring and neap tides are often associated with the phases of the moon, which is a measurement of which part of the moon is visible from Earth. As the moon completes its 29.5-day lunation, the portion of the moon that is visible from Earth changes as the angle between the earth and the sun alters the portion of the moon that is hit by sunlight and therefore visible to an observer on the earth.

Spring tides coincide with the full moon, which occurs when the sun, moon, and earth align so that the sun and moon are on opposite sides of the earth; spring tides also coincide with the new moon, which occurs when the sun and moon align on the same side of the planet. Neap tides are associated with the quarter moon and three-quarter moon phases, when the moon and sun are at right angles to one another.

Harnessing

Tidal energy is a type of alternative, renewable energy gained by harnessing the kinetic energy of tidal movement to create electricity. Kinetic energy is the energy possessed by an object by virtue of its movement through space. This energy can be used to perform such physical work as powering generators that transform motion into electrical signals.

Although not widely used, tidal energy offers a virtually limitless source of energy and does not produce atmospheric pollutants such as the carbon dioxide that results from burning fossil fuels. Because the movement of the tides follows a predictable cycle, generation of tidal energy constitutes a more predictable system of energy generation than many other forms of renewable energy, including solar and wind power.

One method of harnessing tidal energy is to create a tidal barrage, which is a dam built across an estuary or bay that experiences sufficient tidal variation. A tidal barrage works in a similar way to a hydroelectric dam, except that tidal barrages depend partially on tidal changes in water levels to push water into and out of the main reservoir of the system. In addition, tidal barrages tend to be heavier than hydroelectric dams because they must support not only the weight of the overlying water but also the force of tidal currents. Tidal range must be in excess of 5 meters (16 feet) per day for the barrage to be effective.

As water flows towards the barrage, it is channeled through gates called sluices and flows into the basin or estuary on the other side of the barrage, creating a body of water called the hydrostatic head. When the tide recedes, gates at the bottom of the barrage open, allowing the water to rush through the lower passage (driven by gravitational energy). The water channeled through these gates flows over a series of turbine engines, which are attached to an electric generator.

While tidal barrages produce little pollution, they require significant destruction of ecological habitat and may have long-term detrimental effects on ocean life. Because the installation of the barrage changes water levels on both sides, the lives of many animals may be altered. In addition, the barrage greatly restricts the movement of fish, crustaceans, and other oceanic organisms to and from the bay or estuary.

Another method of harnessing power from tidal energy is the tidal fence, which is similar to a tidal barrage but generally smaller in scope. Tidal fences are usually placed in areas where features of the surrounding landmasses cause a relatively rapid flow of tidal waters. Tidal fences also use turbines to generate energy from tidal movement. Tidal fences are less expensive, are easier to install, and cause less environmental damage overall, but they still affect the movement of sea creatures and cause localized environmental destruction.

Another system used to capture energy from tidal streams are tidal turbines, which are individual turbines mounted on the sea floor. Tidal turbines function similarly to air turbines used to capture energy from wind power. However, tidal turbines must be constructed more sturdily because water is more than eight hundred times denser than air and exerts a far greater pressure on the turbine structure. Tidal turbines cost less to construct than either tidal fences or tidal barrages and have less potential to cause environmental damage. In addition, tidal turbines can be installed under the water in such a way that they do not impede ocean travel or shipping. Tidal turbines may be the most promising method of harnessing tidal energy, but the technology is still in its infancy and few tidal turbine systems have been installed.

Energy generated by tidal kinetic energy can be stored as electric energy, and this energy can be channeled later to provide electricity for human use. One of the main challenges facing tidal energy and other types of renewable energy is how to store excess energy until it is needed. Current research programs are trying to refine lithium-ion battery technology to create methods for storing vast amounts of energy for later use. If this technology is successful it may be possible to generate most of the world’s electrical energy needs from renewable sources, thereby reducing reliance on fossil fuels and reducing overall environmental pollution.

Principal Terms

bathymetry: the measure of water depth at various points on Earth; also, the study of components affecting water depth

gravitational field: the area surrounding an astronomical body affected by the deformations in space and time caused by the mass of that body

heliosphere: area of outer space affected by the sun

hydrosphere: all the waters on the earth’s surface, including seas, oceans, lakes, rivers, and the water vapor in the lower atmosphere

lithosphere: outer portion of the earth consisting of the earth’s crust and part of the upper mantle

lunar cycle: the time it takes for the moon and the earth to return to any single position relative to each other as dependent on the combination of the rotation of the earth and the orbit of the moon; approximated to 29.5 rotations of the earth on its axis

syzygy: alignment of three astronomical bodies with regard to a certain direction of reference, such as when the sun, moon, and Earth align twice each lunar cycle

tidal force: secondary effect of the gravitational forces between two objects that causes elongation of the objects along the axis of the line connecting their respective center points

tidal power: utilized kinetic energy created by moving tidal currents; generates electrical energy

tidal turbine:turbine housed within a tidal system and used to generate electrical energy from the kinetic energy of tidal movement

Bibliography

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"Tides and Water Levels." National Oceanic and Atmospheric Administration, oceanservice.noaa.gov/education/tutorial‗tides/tides02‗cause.html. Accessed 26 July 2024.

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