In the last chapter of our series on how electricity is produced we would like to focus on three clean, renewable, energy sources.

One of them is based on the heat that rises from inside the earth, another is possible through wave action and the third is as old as watermills.

Hydraulic energy

A study by NASA states that the origin of life may be found in the electricity generated naturally on the sea floor some 4 billion years ago. Water and movement are a source of life and, thus, also a source of energy.

Our ancestors knew this and used the currents in rivers to move large mills. The sophistication of these mills gave rise to hydropower plants. A dam blocks a river with a concrete wall, flooding the area around the plant and creating an artificial lake. The retained water harbours an enormous energy potential.

Water is one of the strongest and most powerful forces of nature. That torrent can be converted into kinetic energy (energy created by a body in movement). Using the force of gravity, the water travels downwards through a series of large pipes called penstocks. This makes the blades of the turbines spin quickly.

These devices power the plant’s electric generators by means of mechanical energy. A transformer increases the electric power and transmits it to the power grid, which then supplies power to your TV or washing machine.

What happens to all the water that goes through the turbines? Nothing, it is released and continues on its path along the river, eventually flowing into the sea.

Tidal energy

Another type of hydropower that is not so well known is tidal power. This system utilises the vertical movement of seawater that occurs due to the gravitational force of the moon and sun on the sea. The ebb and flow of the tides generates tidal power.

There are currently three types of tidal power systems:

• Tidal barrages: built at the mouths of rivers, they are quite similar to hydropower plants. They harness the potential energy that is generated by the difference in height between high tides and low tides. Although they produce large amounts of energy, building and maintaining these facilities is quite costly.
• Tidal turbines: water currents cause a series of axial turbines that are similar to windmills to spin, which then generates mechanical energy. This is the simplest and most economical method, with the lowest impact on nature. As no dam needs to be built, it does not alter the ecosystem in the sea.
• Dynamic tidal power: this method is merely theoretical, since it has never yet been tried. It would combine the two methods described above. To do this, dams would be built off the coast and further out at sea, creating a T-shaped structure that, on one side, would retain the force of the high tides and, on the other, the energy of the low tides.

“Tidal power comes from the movement of water caused by the high tide/low tide cycles.”

Geothermal energy

Moving out of the water and onto dry land, we’ll now analyse geothermal energy, a system that uses the heat stored inside the earth in the form of rocks and/or hot springs.

The thermal energy contained under our feet is tremendous. By simply digging some 10 metres underground, temperatures of approximately 17ºC throughout the year, due to the thermal inertia of the soil.

To harness this energy, geothermal heat pumps are used to add heat to the earth or remove heat from it, depending on whether the aim is to heat the air, cool it or to heat water.

One of the most precise techniques involves injecting liquid water under the earth’s surface to raise its temperature: the water is turned into steam and returns to the plant full of energy, ready to be transformed into electricity.

This energy can be used for different purposes depending on the characteristics of the source:

• Resources at high temperatures (more than 150ºC) are used to generate light.
• Below 100ºC they are used to supply electricity to heating/air conditioning systems.
• At very low temperatures (less than 30ºC) they are used directly for heating water.

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