Tidal Resources

What are Tidal Resources?

Tidal resources are the twice daily variation in sea level caused primarily by the gravitational effect of the Moon, and to a lesser extent the Sun, on the world's oceans. The Earth's rotation is also a factor in the production of tides. Tidal power is not a new concept and has been used since at least the 11^th Century in Britain and France for the milling of grains.

Tidal Physics

An understanding of the principles which give rise to tides is essential to explain tidal power. Whilst a thorough understanding of the interactions involved is quite complex the origin of tides can be explained in general terms by investigating the gravitational effects of the Moon and the Sun on the ocean and the effect of centrifugal forces.

Gravitational Effects and the Centrifugal Force

The interaction of the Moon and the Earth results in the Earth’s oceans bulging out towards the Moon. On the opposite side of the Earth from the Moon the gravitational effect is partly shielded by the Earth resulting in a slightly smaller interaction and the oceans on that side bulge out away from the Moon, due to centrifugal forces. This is known as the Lunar Tide. This is complicated by the gravitational interaction of the Sun which results in the same effect of the Earth’s oceans bulging towards and away from the Sun on adjacent and opposing sides of the Earth. This is known as the Solar Tide.

As the Sun and Moon are not in fixed positions in the celestial sphere, but change position with respect to each other, their influence on the tidal range (the difference between low and high tide) is also affected. For example, when the Moon and the Sun are in line with the Earth, the tidal range is the superposition of the range due to the Lunar and Solar Tides. This results in the maximum tidal range (Spring Tides). Alternatively when the Moon and the Sun are at right angles to each other, lower tidal differences are experienced (see Figure 1) resulting in Neap Tides.

Figure 1 Gravitational effect of the Sun and the Moon on tidal range
(Adapted from Boyle, 1996).

The range of a spring tide is commonly about twice that of a neap tide, whereas the longer period cycles impose smaller perturbations. In the open ocean, the maximum amplitude of the tides is about one metre. Tidal amplitudes are increased substantially towards the coast, particularly in estuaries. In some cases the tidal range can be further amplified by reflection of the tidal wave by the coastline or resonance. This is a special effect that occurs in long, trumpet-shaped estuaries, when the length of the estuary is close to one quarter of the tidal wave length. These effects combine to give a mean spring tidal range of over 11 m in the Severn Estuary in the UK. As a result of these various factors, the tidal range can vary substantially between different coastlines (World Energy Council, 2001).

The Need for Tidal Power in a Sustainable Energy System

Tidal power presents a very large potential for improving transportation, due to the development of traffic or rail bridges across estuaries and reduced greenhouse gas emissions by utilising tidal power in place of fossil fuels. The tides can provide base load power generation to displace fossil fuels and pollution technologies that directly harm the environment. There are reportedly problems with the larger barrage types of tidal systems, however there are other methods of generating power from the tides without these large constructions.

The Resource

The World Offshore Renewable Energy Report 2002-2007, released by the DTI in the UK stated that there was an estimated 3000GW of tidal energy available (BWEA, 2004). However due to the nature of the resource, the amount of energy obtainable from the energy in tides varies with location and time. Output changes as the tide ebbs and floods each day; it can also vary by a factor of about four over a spring-neap cycle. However this variability is highly predictable in both amount and timing due to the nature of the physics underlying the tides.

The Future of Tidal Energy

The future of tidal energy seems to becoming brighter with new developments with tidal generating technologies that have little environmental impact, have lower capital costs and therefore costs of generation. Tidal energy seems to becoming a valuable part of a sustainable energy future.

Further Information

RISE Resources - Information regarding available renewable energy resources.

RISE Technologies - An extensive collection of information regarding renewable energy technologies.

RISE Applications & System Design - Renewable energy application information and system designs.

RISE System Displays - Case studies and information on installed renewable energy systems & performance data.

Tidal Technologies - RISE Information Portal

Research Report by the Fujita Corp. Japan

Tidal Energy Fact Sheet - Australian Institute of Energy (PDF)

References

BWEA (British Wind Energy Association), 2004. “Marine Renewable Energy” (Online) http://www.bwea.com/marine/resource.html (Accessed 15 Februray 2007).

World Energy Council. 2001, “2001 Survey of Energy Resources – Tidal Energy” (Online) http://www.worldenergy.org/wec-geis/publications/reports/ser/tide/tide.asp (Accessed 15 February 2007).