Category Astrophysics

How tides are used to produce electricity?

The tides have been used to provide power for hundreds of years. In the 18th century, the coast of Europe was dotted with tidal mills, which let the incoming tide into a reservoir through open sluices. At high tide the sluices were closed and the only way the water could escape as the tide fell was by passing through and propelling a waterwheel, so providing turning power.

The same principle was used in a power station built in France in the 1960s. a dam was built across the estuary of the River Rance at St Malo in Brittany, with 24 machines that could be used as turbines in either direction.

As the tide comes in, it is allowed to build up against the dam until there is a difference of 5ft (1.5m) between one side and the other. Then it is allowed to pass through the turbines, driving them and generating electricity. When the tide begins to fall, the turbine blades are reversed, and the water generates electricity again.

The amount of electricity generated depends on the ‘head’ of water – the difference in the level of the water between one side of the dam and the other. The larger the head, the greater the amount of electricity that will be generated, because the water is under greater pressure and so turns the turbines with more force.

At high tide the sluices are shut and extra water is pumped from the sea into the estuary. The water level in the estuary is raised above high tide, so when the sea falls back to low tide the difference in levels has been accentuated.

Once all the water has been allowed to flow into the sea – driving the turbines as it does so – extra water is pumped out to make the level in the estuary artificially low.

When it is high tide again, the turbines are reversed, water flows back into the estuary, and the cycle starts once more. Of course, pumping consumes electricity, but the additional heads produce considerably more electricity than the pumps use.

The scheme at La Rance generates 240 megawatts at peak output – sufficient for a medium-sized city such as Rennes or Caen, but it has had few followers. The immense cost of building the dams and the lack of suitable sites have discouraged everybody except the Russians and Canadians.

The Bay of Fundy in Nova Scotia has the biggest tidal range in the world, with up to 59ft (18m) height difference between tides.

A successful pilot plant was opened across an inlet of the bay at Annapolis Royal in 1984. If the power of the tides across the whole bay could be harnessed it would produce ten times more energy than could be used locally. The surplus electricity could be used in New England and New York Experts believe that it is just a matter of time before the project goes ahead.

 

Picture Credit : Google

How are tides formed?

          The oceans and seas rise and fall at definite intervals. This rise and fall happens because of the gravitational force between the Earth and the Moon.

          We know that the Moon has a weak gravitational force. But it is strong enough to cause these periodic rises and falls known as tides.

          As our planet rotates, the Moon’s pull is exerted on different parts of the Earth. The Moon’s gravitational force exerts a pull on the oceans that generates tidal energy. It causes both the Earth and the oceans to bulge out in the direction of the Moon and away from it, and is known as tides. Because of the rotation of the Earth around the Sun, two tides occur each day- high tide and low tide.

Picture Credit : Google

 

 

Changing tides

 

 

Twice a day, the sea moves up and down the seashore. It is high tide when the beach is covered with sea. After high tide, the sea turns and goes back out again. When the sea is far out and the beach is uncovered, it is low tide.

 

 

 

At high tide the seashore is covered.

At high tide, the seashore is covered with water. The high tide line is the highest place the water reaches up the seashore. The tides are caused by the gravity of the Moon and Sun pulling the seawater towards them.

 

 

 

The seashore is uncovered at low tide.

At low tide, the seashore is uncovered and the sea is far out. On some seashore the difference between high and low tides is big. On others it is small. Low tide happens twice in about 25 hours.

 

 

 

 

Waves are made by the wind.

The waves that move across the sea and break on seashore are made by the wind. When the wind blows over the sea, it pushes and drags against the surface and forms waves. Strong storm winds make huge waves. On days when there is no wind blowing, the sea is calm and the waves are very small.

What causes tides in the sea?

          The giant water waves which rise and fall periodically in the sea are called tides. Sometimes these tides are so high that they appear like high walls moving on the surface of the sea. Do you know what causes these tides?

          The main cause behind the formation of these tides in the sea is the force of attraction between the earth and the moon. We know that all the bodies in the universe attract one another with a force known as gravitational force. This force also acts between the earth and the moon. The moon is pulled towards the earth and the earth towards the moon. This force of gravitation acting between the earth and the moon is so strong that it can easily break a steel rod with a diameter of 400 kms. This force affects the solid part of the earth slightly but it causes rise and fall in the sea water noticeably.

           At any point in the sea, the water level keeps on rising for six hours and subsequently for next six hours it keeps on falling. This rise and fall of the water level is known as tide. The water level rises after every 12 hours 25 minutes. This means that during one day and one night, i.e., 24 hours and 50 minutes, every place in the sea experiences tides two times. Tides are formed both in the seas facing the moon as well as those on the other side. Since the moon comes to the opposite side of the earth after 12 hours and 25 minutes, tides are again formed on the same two spots.

          The force of attraction between the sun and the earth also affects the formation of tides but to a very small extent. When the sun, earth and moon come in a straight line, the earth experiences the maximum force of attraction. And the tide caused on such occasions is very large. It is known as the ‘spring tide’. The sea experiences such tides on the full moon and the new moon days. When the sun and moon are in a perpendicular direction, the earth experiences the minimum force of gravitation. The tide formed on such occasions is very small and is called the ‘neap tide’. Such tides occur between the full moon and the new moon days. Tides can be as high as 15 metres.

          Tides are useful in many ways. They bring many precious items of sea, on to the shore. The tidal power of the sea is also being used to generate electricity.  

Why is it said that the wind and tides are the best way to produce electric power?

        The wind is a major source of renewable energy that is also useful to generate power. You must have seen windmills in real life, or on television. They are set up on farms for power generation.

        Although they look like fans, windmills work in just the opposite way. In fans, electricity is used to make wind, but in windmills, wind is used to make electricity.

        Let’s see how they work. When the wind blows, it turns the blades of the windmill. This in turn, spins a generator inside, and produces electricity.

         A single windmill can generate only a small amount of electricity. Hence, power companies build wind farms with a large number of wind turbines.

        In many countries including Denmark and Portugal, wind power contributes largely to power production.

        Yet another form of energy comes from ocean tides, which again, can be used to produce electricity. Just like wind turbines, there are tidal turbines too. They are turned by ocean currents. The spinning turbines are connected to devices that generate electricity.

         Here again, the process and products are safe, as there are no harmful emissions made.

 

How energy is obtained from high and low tides?

High and low tides come alternately, that is, one after the other. In high tides, the level of water rises and covers a part of the coast, and in low tides it flows away again. This energy of the tides is used to run turbines. Special water turbines are used in tidal power plants. Water – rising in case of high tides and receding in case of low tides – can flow through these turbines in both the directions. Electricity is generated in tidal power plants due to the rotary movement of the turbines. These power plants need a tidal hub – the difference in the height of water between high and low tides – of more than 5 m to be able to work economically. Such a power plant is in the French Bretagne, in St. Malo. Here, the tidal hub is at 12-15 m.