Sea energy

The sea is a great potential source of renewable energy. This marine energy mainly takes the form of waves, tides and currents, and the temperature difference between the surface and the sea bed.

Wave power

The wind creates waves which can be used for harnessing energy. A number of different "wave converters" have been developed, some floating and some anchored to the coast or seabed, at various stages of development and offering varying levels of efficiency.

Investigation is making good headway and sea energy is becoming a more and more significant feature throughout Europe, because it allows energy to be generated in a sustainable and environmentally-sound way.

Oscillating water column

This is one of the most technologically mature systems for harnessing wave energy. It consists of an air chamber attached to the coast, the bottom of which is submerged in the water.

• One of the advantages of these facilities is that they can be integrated into civil engineering projects, such as breakwaters to shelter harbours.
  
• The waves enter the air chamber, forcing the water level up and compressing the air inside. The air is expelled through the top opening, driving a turbine connected to a generator, which produces power.
  
• When the wave goes out again, a vacuum is created in the chamber and air is sucked in, again driving the turbine.
  

The Pelamis

The Pelamis is a semi-submerged floating structure, made up of cylindrical steel sections, linked by hinged joints.

• The underwater anchorages connect the different Pelamis devices together and feed the power back to land over an underwater cable.
   
• The relative movement between the parts activates hydraulic rams, which pump high pressure oil to drive the hydraulic motors. The motors are connected to a generator which produces electricity.
   
• Pelamis Power Conversion Module Internal view
  
 

Sea Buoys

Several kinds of buoys can be used for harnessing wave energy.

The example shown here uses submerged buoys for converting energy.

The wave motion moves the cylinder up and down and this mechanical movement drives the generator, which produces electricity.

Tidal Energy

Tidal energy is the energy obtained by harnessing the movement of large masses of water between high and low tides.

Until recently it was thought that the best places to harness this type of energy was in estuaries and bays, where a large flow of water entered through a narrow channel. However, because of the environmental problems of building a dam to hold back the water, this type of facility has been ruled out.

There are two requirements for this type of facility:

• An estuary or bay with sufficient capacity
• A tidal range of around of 8 metres

Estuary
When the tide rises along the estuary the sea water passes through the dyke, turning the turbines to produce electricity. At high tide, the hatches are closed and the water is held in. When low water is reached, the sluices are opened and the water flows back into the sea through the turbine, again producing electricity.

Tide Energy

One variant of tidal energy uses the current of water which forms at high tide and low tide.

Turbine
As it flows through the estuary, the water passes through a turbine without being held back. This makes it unnecessary to build a dam with all the environmental impact an infrastructure of this type would have.
The turbines operate in a similar way to wind turbines, but because water is denser than air, the size of blade required is considerably less.

Ocean Thermal Energy Conversion

Sunlight shining on the earth represents a source of energy, which is mostly stored in the seas in the form of heat. The slight difference in temperature between the surface of the sea, which has been heated by the sun, and the colder sea depths, represents another source of energy which can be transformed into electricity. This is called ocean thermal energy.

An ocean thermal energy power station is a thermal machine in which the surface water acts as a heat source and the water extracted from the depths acts as a coolant.

It operates in a similar way to conventional thermal power stations where a liquid is evaporated and then passed through a turbine. In this case, the liquid is either the sea water itself, used directly, or a second fluid with a low boiling point, such as ammonia, which circulates in a closed circuit heated by the sea water.