The Endlessly Renewable Power of the Tides

The water flows high up the beach when the moon is directly overhead, as happens on the far side of planet. The rhythm is predictable with a spacing of approximately 12 hours and 25 minutes.

Half way between these points, the water level is at its lowest. There are subtle variations because of local factors. However, the power is always there.

This knowledge is not new. In approximately 150BC the Greek astronomer Seleucus of Seleucia, who came from Mesopotamia reasoned the earth rotated on its axis and revolved around the sun. He also thought the position of the moon determined the tides. 1,500 years later European astronomers Galileo Galilei and Isaac Newton reached the same conclusion. There is nothing new under the sun, and beneath the moon, it seems.

Tide Schematic: Source: CC 3.0
Tide Schematic: Source: CC 3.0

Early Efforts to Harness the Power in European Tides

Records of European tidal mills date back to the eighth century. The principle is simple. Build a dam with a sluice in a tidal estuary. Allow water to enter the dam at high tide so it fills. Close the sluice to trap the water. At low tide, release the water through a water mill and use the rotation to mill corn for flower.

Hydroelectric generators operate along the same lines in pumped storage systems. The difference is these use electricity to fill the storage dams, while water mills use the power of the moon and tides. It is quite possible Romans used designs similar to this simple system in use at Olhão, on the Mediterranean in the Algarve in Portugal.

Tidal mill at Olhão, Portugal: Marc Ryckaert: CC 3.0
Tidal mill at Olhão, Portugal: Marc Ryckaert: CC 3.0

Principles of Generating Electricity with Tides

Power stations make electricity by rotating a magnet within closed loops of a conductor wire. Until quite recently, the motive force came mainly from super-heated steam driving a series of turbine blades. In the 19th century, people began thinking of using falling water.

The first tidal power station entered commercial operation in the Rance River estuary in Brittany, France in 1966. Its 24 turbines have a total 240-megawatt capacity, although the average output is only 24% of that because of tidal changes. The site has the advantage of a wide average range between high and low tide. There has been progressive silting with some losses of local marine life.

Tidal Barrage on the Rance: Tswgb: Public Domain
Tidal Barrage on the Rance: Tswgb: Public Domain

The Rance Tidal Power Station recovered its construction costs in 20 years, and generates at a lower cost than nuclear. It is still fully functional after 50 years, which is something of a record in the power industry. A highway crosses the dam wall, while a lock admits ships up to 16,000 tonnes. A drawbridge allows larger vessels through.

Having worked at a nuclear power station with an exclusion zone around it and little interface with the surrounding community, I find it refreshing to know that cheap, safe renewable tidal power is within our grasp. What then is the future of global tidal power? Why do we not have more of it, and what are its limitations?

Koeberg Power Station: Idyllic Press: CC 3.0
Koeberg Power Station: Idyllic Press: CC 3.0

Let’s Talk About the Advantages of Tidal Power First

Tides are more predictable than either wind or solar. Like them, they are scalable and infinitely sustainable. However tidal is the least popular of the three because of cost, the limited presence of high tidal differences, and sufficiently powerful flow velocities. Rance has a tide differential of 8 meters, which is why it is there.

A nuclear power station delivers consistently around the clock. This is the commercial attraction, and the challenge tidal faces. This situation is changing with the arrival of more efficient tidal turbines. Thoughts of capturing current, not tidal power are growing. Currents flow in the same direction. Tides go in and out and up and down.

Axial Turbine (Concept): Anonymous: GNU Free Documentation
Axial Turbine (Concept): Anonymous: GNU Free Documentation

Axial Turbines are like giant windmills on the ocean floor. They draw energy from prevailing currents and come close to the consistency of a nuclear station. Higher density compared to air means more efficient energy conversion from water as opposed to wind. A few small commercial stations are operating, although take-up is slow.

Semi-Submerged Floating Turbines present an option where the seabed is too deep to exploit commercially. They have the added advantage of being portable, and are more accessible for maintenance purposes. A scaled-down prototype is floating in the Thames near London. This has positive implications for disaster management.

Tidal Turbine: Ocean Flow Energy: Free Use with Acknowledgement
Tidal Turbine: Ocean Flow Energy: Free Use with Acknowledgement

I increasingly wondered, as I explored this topic, why we are not harvesting more energy from the ocean. From own experience I know the reasons are technical opposition, and a lack of commercial and political will. I close with a photograph of Chernobyl, the nuclear power station that exploded.

Chernobyl exploded because of poor design, human stupidity, and arrogance. Until we learn to control these things – which we probably never will – I submit we should follow the power of the tides, the currents, the wind, and the sun that are the gifts of creation. Read this report on Chernobyl’s environmental impact. Then you decide. Will it be tidal and current, or potentially disastrous nuclear.

Chernobyl Unit 4: Calflier 001: CC 2.0
Chernobyl Unit 4: Calflier 001: CC 2.0

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3 thoughts on “The Endlessly Renewable Power of the Tides

    1. there are both 6 hour and 12 hour cycles. its roughly 6 hours from low to high and roughly 12 hours from high to high. Roughly because the timing rotates thru the day just as the ‘lunar noon’ and ‘lunar midnight’ do. Great article!

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