Wave Resources

What Causes Waves? | The Need to Incorporate Tidal Energy into a Sustainable Energy System | The Resource | The Future for Tidal Energy | Further Information | References

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What Causes Waves?

Ocean waves are produced by a variety of forces including meteorological forces (such as wind & air pressure), astronomical forces (gravitational effects of the Moon and Sun), and geological forces (sub sea earthquakes can produce tsunamis). This portal file will focus on the meteorological forces that act upon Earth's oceans.

The wind actually transfers some of its energy into the water. The water is able to "gain" energy from the wind because of the friction between the wind and the water. We can all test this by blowing over a cup of water and observing the "ripples" or waves that result. Over oceans and lakes, the waves that are generated as a result of the wind are called "ocean surface waves" (NOAA, 2002).

Initially, light winds generate small ripples called capillary waves on the water surface. If the wind increases, the added roughness created by the capillary waves increases the rate of energy transfer and waves begin to form on the ocean surface. In the region where wind is blowing across the ocean surface and waves are being generated, the sea surface is characterised by steep waves with many different lengths moving randomly in the direction of the wind. This condition is referred to as "sea". How large the waves get is dependent on 3 factors:

1. The strength of the wind
2. The amount of time the wind blows
3. The distance (called fetch) over which the wind blows in a straight line across the ocean

The stronger the wind and the longer it blows across the fetch, the larger the sea will become. A fully developed "sea" occurs when the waves reach the maximum size possible for a specific wind, duration and fetch (New Jersey Marine Science Consortium, 2005).

Figure 2 shows a model for the 20^th of April 2006 of the global wave heights (indicated by colour) and the magnitude and direction that the waves are traveling (indicated by the white arrow and its length).

 

The Need to incorporate Wave Resources into the Energy Supply System

In the context of sustainable development, environmental impacts, proper waste management, increased safety and security, intergenerational issues, and international equality are being introduced into energy policy decision-making. With the concern over anthropogenic climate change (i.e. man-made climate change), there is a growing awareness that we must utilise energy resources that are sustainable. Wave power is one such sustainable resource.

 

The Resource

The World Energy Council has estimated the global wave energy potential to be 2,000 GW (New Scientist, 2003). The global wave power resource is in excess of 2 TW, with potential for generation of more than 2000 TWh annually (World Energy Council, 1993. Thorpe, 1998).

Wave energy (or wave power) is the name given to the energy contained in the waves. The amount of energy in a single wave is considerable. The potential energy of a set of waves is proportional to wave height squared times wave period (the time between wave crests). Longer period waves have relatively longer wavelengths and move faster. The potential energy is equal to the kinetic energy (that can be expended). Wave power is expressed in kilowatts per metre at a location, such as a shoreline. Excluding waves created by major storms, the largest waves are about 15 metres high and have a period of about 15 seconds. Such waves carry about 1700 kilowatts of potential power across each meter of wavefront. A good wave power location will have an average flux much less than this: perhaps about 50 kW/m (Wikipedia, 2007).

For a typical swell of 10-second period, the power flux rate is 40 kW/m if the amplitude is 1 m (gentle waves) and 1000 kW/m if the amplitude is 5 m (large waves). In the latter situation (high waves of a single frequency), the theoretical power available is 1000 MW per kM of the coastline. This is comparable to an ordinary coal-fired power station. However in practice only a fraction of this energy can be extracted, because wave intensity has a large variability hourly, daily and seasonally.

 

The Future of Wave Power

Wave power has the potential to yield much more energy than tidal power. The energy potential of waves is vast, and wave power can be exploited in many locations. Countries with large coastlines and strong prevailing winds could produce five percent or more of their electricity from wave power (Wikipedia, 2007).

 

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.

 

Wave Technologies - RISE Information Portal

Energy Resources – Wave Power

World Energy Council

 

 

References

New Jersey Marine Science Consortium, 2005. “All About Waves” (Online) http://www.njmsc.org/Sea_Grant/Extension/All_About_Waves_Brochure.pdf (Accessed 16 February 2007).

New Scientist issue 2413, 2003. “Power from the Waves” (Online) http://www.newscientist.com/article/mg17924134.900.html (Accessed 16 February 2007).

NOAA, 2002. “What causes ocean surface waves?” (Online) http://seaboard.ndbc.noaa.gov/educate/pacwave.shtml (Accessed 16 February 2007).

Thorpe, T W.  “An Overview of Wave Energy Technologies”, A report produced for the Office of Science and Technology, AEA Technology Report Number AEAT-3615.

Wikipedia, 2007. “Wave Power” (Online) http://en.wikipedia.org/wiki/Wave_power (Accessed 16 Februaryl 2007).

World Energy Council, 1993.  “Renewable Energy Resources: Opportunities and Constraints 1990-2020” (Online) http://www.worldenergy.org/wec-geis/publications/reports/ser/wave/wave.asp (Accessed 16 February 2007).