How Does Tidal Power Work?
Tidal power is a form of hydropower that uses the movement of water to turn a mechanical turbine that, in turn, generates electricity. The interesting thing about tidal power is that the source of the energy is gravity, just like it is in more standard forms of hydropower. How that gravitational energy is harnessed, however, is a different matter. Understanding tidal power begins with an understanding of the tides themselves.
How the Tide Works
We have all heard of high tide and low tide. All over the earth, every body of water rises and falls two times throughout the day. For the most part, this rise and fall is only noticeable for reasonably large bodies of water, so we will focus on the oceans.
There are two major gravitational influences on our planet. One is the sun, which keeps us in orbit and prevents us from flying off into space. The other is the moon, which orbits around the earth much as the earth orbits around the sun. Though the sun’s gravity is much stronger, its distance from the earth means it has much less effect on our plant than the moon, which is very, very close.
The sun is about 149,600,000 kilometers from earth whereas the moon is only 384,400 kilometers away. Because gravity varies with the square of the distance, it drops off very quickly as one moves away from the source. For this reason, the effect of the sun on the tides is less substantial than the effect of the moon. In fact, the force of gravity from the sun has about 49% of the effect that the force of gravity from the moon has. Thus, we will focus only on how the tides respond to the moon.
As the moon orbits the earth, it pulls on the planet. Because water, which makes up roughly three fourths of our planet’s surface, is more distensible (moveable) than the land, it moves more under the effect of the moon’s gravity. Thus, the water 'follows' the moon as it orbits our plant. The diagram below shows this.
You will note that the water is not only high on the side facing the moon, but also on the side facing AWAY from the moon. This is why there are two high tides during the day even though the moon is only overhead once. The question you might ask is why water should be high on the side of the earth opposite the moon if gravity is pulling strongest on the side near the moon.
The answer has to do with gravity again. Gravity is pulling on all of the earth, not just the water. The water closest to the moon is pulled the most followed by the earth itself and then the water on the far side of the planet. The water on the far side of the planet is experiencing the weakest gravitational force so it is not being pulled nearly as hard. In other words, that water “lags” behind the water nearest the moon and the earth, which is in between the two. Thus, it is not pulled as hard against the surface of the planet. The diagram below shows arrows that represent the force of gravity in each location.
The water never has a chance to 'catch up' and flow to the moon side of the earth because the moon is constantly moving and the earth is constantly spinning. The result is two high tides every day spaced about 12 hours and 25 minutes apart (half a lunar day). The high tide that occurs when the moon is overhead is HIGHER than the high tide that occurs when the moon is on the opposite side of the planet.
Mentioning the Sun
The tides are very complex and while the location of the moon is the major driving force for the tides, it is not the only force. The sun also plays a role and while we won’t go into detail, it is worth mentioning that when the gravitational fields of the sun and moon align, tides are at their highest. These are called Spring tides and they contrast with Neap tides, which occur when the gravitational fields of the sun and moon are at right angles to one another and thus opposed.
Spring tides occur every full and new moon, while neap tides occur with every quarter and three-quarters moon. Thus, spring tides occur roughly every two weeks, interspersed with neap tides, which occur every two weeks as well. This results from the fact that full and new moons only happen when the moon is in alignment with the earth and the sun whereas quarter moons occur when the three celestial bodies create right angles to one another. To throw a wrench into all of this there is a phenomenon called a Proxigean Spring Tide, which occurs when the moon is closer to the earth (the orbit of the moon around the earth is not a perfect circle) roughly every 1.5 years.
So, gravity is responsible for the tides. As we will see, this means that gravity is ultimately the source of power generated using the tides.
How Energy is Obtained from the Tides
There are several ways to generate power from changes in ocean water height due to tides. The first is to simply let the water turn a generator as it moves in and out with high and low tide. This is often called tidal stream generation. The equipment used looks much like the wind turbines we use to capture wind energy on the surface.
The other way to capture energy from the tides is to trap water at high tide and use the energy of it falling under the force of the earth’s gravity to turn a turbine. This is much more similar to how traditional hydropower works. There are two systems that operate under this premise. The first is known as tidal barrage. Basically, a dam is built that traps water at high tide and releases it at low tide. During release, the water turns a turbine.
The second form of tidal power in this category is dynamic tidal power, which is much more complicated in theory. Basically, the water is trapped by a different kind of barrier that makes use of the way water moves along the shore rather than into the shore. This system has yet to be tried in practice.
In all cases, one thing to keep in mind is that tides vary from place to place. In some locations, due to the shape of the land and structure of the ocean floor, tides very drastically from high to low tide while in other locations the difference is very slight. In some places the tides differ by tens of meters while in others the difference is measured in centimeters. Clearly, locations with drastic tidal differences will be more amenable to the generation of electricity.
At its core, tidal power is simply gravitational hydropower and not all that different from building a damn. In either case, the movement of water is used to push a turbine, which in turn generates electricity. In both cases, the energy for moving water comes from gravity, though the source of the gravity differs. The biggest problems facing tidal power, which are less relevant to standard hydropower, are cost, corrosion due to saltwater and the ecological impact of building massive facilities in sensitive costal areas.