EGEE 101
Energy and the Environment

Tidal and Wave Power

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The water in the ocean is constantly moving due to waves (from the wind) and tides (mostly from the moon). This movement can be turned into energy in a number of different ways which we will discuss here. However, there are only a few locations where the electricity-generated is currently significant.

Tidal Power

The tides are associated with the gravitational pull from the moon (and to a lesser extent the sun). The moon rotates around the earth on a lunar cycle that is close to a calendar month. The waxing and the waning of the moon are associated with the sunlight being reflected from the surface of the moon (the moon does not generate light so the "Dark Side of the Moon" great Pink Floyd song is somewhat misleading). As the moon rotates around the earth, it exerts a gravitational pull on the oceans such that during a 24-hour cycle there will be at least one high and low tide. Hence, tidal energy is renewable. Unfortunately, the tides do not always coincide with the peak demand times but the production of electricity output is predictable.

The technology used for the conversion of tidal energy into electricity is similar to the technology used in hydroelectric power plants. There are various systems that are used to harness the potential energy supply by turning a turbine, to turn a generator, to electricity. The approaches for tidal power are either a barrage (similar to a dam),  to create a catchment zone, or to directly ??? from the flow. With a barrage, you close off the flow — allow the tide to come in when it is much higher than the water height in the catchment area: the flow is opened and water rushes in. Before the tide goes out the high tide water is captured, the flow stopped and the flow is started again when the tide is low so the water flows out of the catchment area. The process then repeats.

Watch

Click here for a transcript of the Tidal Power 101 video.

Tidal power. Tidal power is a form of hydro power that converts the energy from the natural rise and fall of the tides into electricity. Tides are caused by the combined effects of gravitational forces exerted by the moon, the sun, and the rotation of the Earth.

Tidal plants can only be installed along coastlines. Coastlines often experience two high tides and two low tides on a daily basis. The difference in water levels must be at least five meters high to produce electricity. Tidal electricity can be created from several technologies, the main ones being tidal barrages, tidal fences, and tidal turbines.

Tidal barrages are the most efficient tidal energy sources. A tidal barrage is a dam that utilizes the potential energy generated by the change in height between high and low tides. This energy turns a turbine or compresses air, which in turn creates electricity. Tidal fences are turbines that operate like giant turnstiles, whereas tidal turbines are similar to wind turbines only underwater. In both cases, electricity is generated when the mechanical energy of tidal currents turns turbines connected to a generator. The generator produces electricity.

Ocean currents generate relatively more energy than air currents because ocean water is 832 times more dense than air and therefore applies greater force on the turbines. Tidal power is easy to install and renewable, having no direct greenhouse gas emissions and a low environmental impact. Because the ocean's tidal patterns are well understood, tidal energy is a very predictable energy source, making it highly attractive for electrical grid management. This sets it apart from other renewables that can be more unpredictable.

However, adoption of tidal technologies has been slow, and so far the amount of power generated using tidal power plants is very small. This is due largely to the very specific site requirements necessary to produce tidal electricity. Additionally, tide cycles do not always match the daily consumption patterns of electricity and therefore do not provide sufficient capacity to satisfy demand. That's tidal power.

Read 

The U.S. Energy Information Administration Tidal Power webpage provides additional information on the three different tidal technologies: tidal barrages (currently in use), tidal turbines, and tidal fences are both emerging technologies.

Advantages of Tidal Power (barrage generated)

  • no air pollution
  • no fuel needed, no waste produced
  • relatively inexpensive to operate and maintain
  • can produce a great deal of energy
  • predictable energy output – tides are predictable and mostly constant
  • as much of the coastline is populated, there is often a site at which the product can be linked to the electric grid

Disadvantages of Tidal Power (barrage generated)

  • there is the possibility that some of the undesirable results of hydroelectric electric power generation can be carried over to tidal power. Consideration of fish movement, sediment flow, navigation, wetland areas, and other environmental issues must be made and environmental impact studies are required
  • tides do not always coincide with the peak demand times
  • must be able to withstand very rough weather
  • it is currently expensive to construct tidal power plants as they require high capital investments
  • environmental issues such as habitat change, particularly with tidal barrages

Wave Power

While tidal power is a proven technology, wave power is an emerging one. There are a few different ideas ranging from using wave power to move air to make electricity, underwater turbines similar to wind turbines (but much smaller), or devices that articulate.

Watch

Here is a 2:35 minute video about an emerging technology for using wave power at the coast. The wave is used to create a column of water, that works like a piston to compress air — moving it up though a Wells turbine on the upstroke, and moving the air in the other direction as the water column falls.
Click here for a transcript of the Harvesting the Waves video.

One concept for harnessing the wave energy is the oscillating water column principle. The take off for this technology is typically an air turbine. The most commonly used air turbine for this application is the Well turbine, which we will explain in this episode.

The water level in the chamber rises and falls with the rhythm of the waves and act as a piston. The air is forced forwards and backwards through the turbine and causes the rotation of the turbine. This generates mechanical energy that is converted into electricity by a generator.

The Wells turbine has a special feature. It always rotates in the same direction, regardless of which direction the airflow comes from. How is this possible? This is feasible because of the symmetrical shape of the rotor blades. As the air hits the rotor blade, most of the flow is deflected in one direction and pushes the blade in the opposite direction. Due to the symmetrical shape of the rotor blades, the same effect happens when the airflow comes from the other direction. Therefore, this Wells turbine always rotates in one direction, regardless of the air flow direction and guarantees the continuous rotation of the turbine.

The Wells turbine must be turned on initially. The airflow alone does not get it to rotate. This turbine is one of the simplest turbines for wave energy conversion. It has very few moving parts. None of them are in the water, need no gearboxes, is easy to maintain, and achieves an efficiency between 40% and 70%.

This turbine was tested in several research plants under real conditions. Limpert, the power plant on the Isle of Islay was the first commercial plant in operation between the year 2000 and 2018. It generated 500 kilowatts with two Wells turbines.

The Mutriku Breakwater Wave Plant in Bay of Biscay on the coast of the Basque country in Spain started in 2011. It has 16 Wells turbines and supplies 250 households with energy. The Wells turbine has the most operational experience and running hours of all air turbines for oscillating water column concept of harnessing the wave energy, and it makes a small contribution to the generation of sustainable energy.

Read

The U.S. Energy Information Administration's Wave Power webpage provides additional information on ways to capture the energy in the waves.

Advantages of Wave Power

  • Low cost
  • Predictable and reliable
  • Renewable
  • Produces no emissions

Disadvantages of Wave Power

  • High cost to build the initial equipment and machinery. Therefore, these devices must be placed in areas with the strongest waves
  • Might cause a negative effect on the marine system
  • Waves strengths can be variable (thus the electricity produced is variable