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Tidal Energy - Introduction

Tides are the result of gravitational forces that the sun and moon exert on the earth.

Tidal energy is generated by the relative motion of the earth, sun and the moon, which interact via gravitational forces. Periodic changes of water levels, and associated tidal currents, are due to the gravitational attraction by the sun and moon. The magnitude of the tide at a location is the result of the changing positions of the moon and sun relative to the earth, the effects of earth rotation, and the local shape of the sea floor and coastlines.

Because the earth's tides are caused by the tidal forces that are present due to gravitational interaction with the moon and sun, and the earth's rotation, tidal power is practically inexhaustible and classified as a renewable energy source.

Tidal Energy - How it works

A tidal energy generator uses this phenomenon to generate energy. The stronger the tide, either in water level height or tidal current velocities, the greater the potential for tidal energy generation.

Tidal Barrages – The concept of tidal barrage is quite old, and there are a few tidal barrages that have been in existence for decades. Tidal barrages make use of the potential energy in the difference in height between high and low tides. Tidal barrages are essentially dams built across a tidal estuary, and the technology used is similar to that used in hydroelectric plants. Owing to the large amount of construction required, tidal barrages suffer from very high infrastructure costs. Another bottleneck for tidal barrages is the shortage of viable sites worldwide; tidal barrages also pose serious environmental issues.

Tidal Stream Systems - Tidal stream systems make use of the kinetic energy of moving water to operate turbines. Unlike tidal barrages that require water to be stored, this method relies only on the moving water, and hence method is gaining in popularity because of the lower cost and lower ecological impact compared to barrages.

Tidal Lagoons - Offshore tidal lagoon power generation resolves the environmental and economic problems of the barrage system and puts tidal power generation back amongst the choices for commercial-scale renewable power generation. Rather than blocking an estuary with a barrage, offshore tidal power generators use an impoundment structure, making it completely self-contained and independent of the shoreline. It is similar to having a circular dam, built on the seabed. Tidal lagoons eliminate the environmental problems associated with blocking off and changing the shoreline. Likewise, the concept of a tidal lagoon is not a recent proposition. As of 2009, No tidal lagoon has ever been built anywhere in the world, and although the technologies used would themselves be classed as mature, the concept itself is currently unproven due to a number of remaining uncertainties over  design, construction methods and physical impacts. 

Tidal Energy Potential

A miniscule portion (less than 0.01%) of the world’s electricity requirements is currently being generated from tidal sources. Tidal constitutes about 0.02% of the total electricity generated from all renewable energy sources.

As of 2009, all electricity from tidal energy is obtained from tidal barrages. Tidal streams and tidal lagoons are yet to have any completed projects that generate electricity.

Top Ten Potential Locations for Tidal Energy

Source: WEC, 2001 

Data and Statistics 

Tidal Energy Status and Trends

Current Contribution of Tidal Energy

As of 2009, of the total electricity production from renewables, less than 0.03% is obatained from tidal sources.

Electricity generation from tidal energy could be much higher in future, with some estimates suggesting a potential generation in the range 1000 TWh to 3500 TWh. (For comparison, the total electricity generation worldwide was about 17000 TWh in 2008)

The United Kingdom is a leading country both in terms of activity and in terms of support to the sector. According to estimates, 20% of UK's total electricity requirement can be harnessed from ocean energy, comprising primarily tidal and wave energy sources. The Carbon Trust has also predicted that marine energy could contribute up to one sixth of the UK's '20% renewable energy by 2020' target. Scotland with its rich ocean energy resources plays an important role. In fact, 25% of Europe's tidal resources and 10% of Europe's wave energy resources are found in Scotland.

It has been estimated that if ocean energy technologies continue to be supported and achieve their predicted potential, approximately 3 gigawatts (GW) of installed capacity could be available in the EU by 2020.

Tidal Energy Projects & Companies

Existing Large Tidal Power Plants

Source: www.gcktechnology.com

Tidal Stream Resources 

Source: Black & Veitch-for Carbon Trust -2004-5

Tidal Barrage Projects and Proposals

Source: http://www.raeng.org.uk/policy/reports/pdf/energy_2100/David_Lindley.pdf

The problems and barriers mentioned below are relevant to all forms of tidal projects (barrages, streams and lagoons), unless specifically mentioned.

• High cost - The main detriment of tidal energy is the cost; tidal plants are expensive to build
• Effects on ecosystem – Presence of tidal plants can result in damages such as reduced flushing, winter icing and erosion, which can change the vegetation of the area and disrupt the balance.
• Regional limitations - Similar to other ocean energies, tidal energy has several prerequisites that make it available only in a small number of regions. For a tidal power plant to produce electricity effectively (about 85% efficiency), it requires a basin or a gulf that has a mean tidal amplitude (the differences between spring and neap tide) of 7 meters or above. It is also desirable to have semi-diurnal tides where there are two high and low tides every day. Thus, there are not too many suitable sites for tidal barrages.
• Time limitations - Tidal provides power for about ten hours each day, when the tide is actually moving in or out. So, it does not provide energy for electricity all through the day.
• Problems specifically related to tidal barrages
  • Tidal barrages may block outlets to open water. Although locks can be installed, this is often a slow and expensive process.
  • Barrages affect fish migration and other wildlife- many fish like salmon swim up to the barrages and are killed by the spinning turbines. Barrages may also destroy the habitat of the wildlife living near it
  • Barrages may affect the tidal level - the change in tidal level may affect navigation, recreation, and cause flooding of the shoreline

Tidal Energy – Reference

1)     Ocean Thermal Energy 

The main objective of ocean thermal energy or Ocean Thermal Energy Conversion (OTEC) is to turn the solar energy trapped by the ocean into useable energy. OTEC systems use the ocean's natural thermal gradient—the fact that the ocean's layers of water have different temperatures—to drive a power-producing cycle. As long as the temperature between the warm surface water and the cold deep water differs by about 20°C (36°F), an OTEC system can produce a significant amount of power.