How Tidal Power Works

In our quest to reduce our dependence on fossil fuels and develop renewable sources of energy, mankind is increasingly turning to renewable and alternative forms of power. We are all familiar with wind power and solar power, and with the power of water in the form of hydroelectric dams. All of these technologies harness the innate power of nature to produce electricity through clean, non-polluting methods that are not harmful to the environment. The focus of this article in on another form of hydropower that is not as widely used as wind, solar, or hydroelectric; one that is environmentally sound and shows much potential to help us meet our future energy demands: tidal power.

What Causes Tides

Our earth’s oceans are subject to two periods of high tide and two periods of low tide each and every day. The term “tides” refers to this succession of changes in water level as compared to the land. The tides are caused by the gravitational pull exerted upon the Earth by both the Moon and the Sun. The Moon, being our nearest celestial neighbor, has the strongest effect on the tides. The Sun has a smaller influence; due to its distance from the Earth, its gravitational pull is less than half that of the Moon. Owing to its mass, however, the Sun does have a noticeable impact.

Gravity from the Moon and the Sun constantly pull at the earth and try to pull everything closer. The Earth’s own gravity is able to hold onto nearly everything, with the exception of water. Due to its constantly moving, fluidic nature, the Moon is able to pull at the water. This causes a bulge in the water toward the Moon as the earth rotates, causing a high tide. This bulge is stationary relative to the Moon, so the Earth will rotate under it. The effect of this is that the sea level will drop at any given point as it rotates away from the Moon. One cycle of high to low tide takes approximately 12 hours.

Although the Moon is primarily responsible for tidal action, the gravity of the Sun can work to either enhance or hinder its pull. If the Sun and Moon are aligned, their combined gravity produces a higher than normal tide, called a spring tide. If they are not aligned, they work against each other and the resulting neap tide is lower and weaker than usual.

Tidal Barrages

Tidal barrages are essentially hydroelectric dams, except that it is usually much longer and situated at a tidal estuary. Such barrages are built to span river estuaries, the area where a river meets the sea. As the tides ebb and flow, water moves through spillways built into the barrage and spins a turbine that drives a generator, thus producing electricity. Alternately, the tidal action can force air through an intake pipe to turn the turbine. Since a tidal barrage spans the entire width of an estuary, locks have to be built in to allow shipping traffic to pass.

The main problem with tidal barrages is that they can only work when the tides are flowing. That means that they are only effective for 10-12 hours each day. The bright side is that tides are completely predictable, so other power stations can be brought online to make up for period when the barrage is inactive. Also of concern are the ecological effects of the tidal station on the river estuary.

Tidal Stream Generators

An emerging technology that comes without the drawbacks of tidal barrages is the development of tidal stream generators. Also called tidal energy converters (TEC), tidal stream generators operate in much the same way as a wind turbine, except that they are placed underwater. There are four different methods of harnessing tidal action using tidal stream generators.

Horizontal axis turbines utilize a configuration that looks just like a traditional windmill. As the tides flow, the horizontal-axis mounted turbine rotates and drives a generator. A vertically mounted TEC is placed inside of a tidal basin. Incoming water fills the basin at high tide and drains from it at low tide. Flow in either direction spins the turbine. Oscillating generators do not rotate but instead use airfoils and are pushed from side to side. Venturi effect generators amplify the effects of tidal flow and can be mounted horizontally or vertically.

Water is 832 times denser than air, so a TEC can generate a significantly higher amount of power for the same sized turbine. They are quite efficient and will work in a current as little as 2 knots. They can be anchored to the sea floor or suspended from buoys or barges.

Dynamic Tidal Energy

Created and patented by the Dutch-based firm H2iD, dynamic tidal energy (DTE) is the latest method of tidal power generation. Although it is as yet untested, it could provide high levels of nearly constant power. Dynamic tidal energy relies on the construction of long (30 to 60 km) T-shaped dams perpendicular to the shoreline. They run straight out to sea without enclosing an area like tidal barrages.

DTE takes advantage of the fact that in many parts of the world the tides run parallel to the coastline along continental shelves. This type of tide is common in the UK, Korea, and China that feature shallow coastlines with strong, oscillating currents. A DTE dam is long enough to affect the movement of the tide and create a water level differential of both sides of the dam. The water from the high side then flows through a series of turbines built into the dam, producing power.

It is estimated that a single DTE dam can produce up to 8 GW of electric power, or enough energy for 3.4 million Europeans per year at an annual consumption rate of 6,800 kW per year. If two such dams were built in a tandem configuration and properly spaced apart, approximately 200 km, they would complement each other and level the output of both. In other words, one dam would produce power while the other is idle. The result would be a nearly uninterrupted supply of electricity. Tidal range required for DTE to work is low, meaning there are many suitable sites where it could potentially be implemented.

We can expect to see tidal power become more prevalent in the future. Tidal power was once limited due to high cost and lack of suitable sites for its implementation. Recent advances in both methodology and turbine design have shown that there is much more potential in tidal power than was previously believed, and that it can be exploited at competitive prices. The predictability and renewable nature of tidal power make it a strong candidate for supplying much of our future energy needs.


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