A resource from the ocean
Traditional sources of energy such as oil, gas, and coal are non-renewable. They also create pollution by releasing huge quantities of carbon dioxide and other pollutants into the atmosphere. In contrast, waves are a renewable source of energy that doesn't cause pollution. The energy from waves alone could supply the world's electricity needs. The total power of waves breaking on the world's coastlines is estimated at 2 to 3 million megawatts. In some locations, the wave energy density can average 65 megawatts per mile of coastline. The problem is how to harness wave energy efficiently and with minimal environmental, social, and economic impacts.
Ocean waves are caused by the wind as it blows across the open expanse of water, the gravitational pull from the sun and moon, and changes in atmospheric pressure, earthquakes etc. Waves created by the wind are the most common waves and the waves relevant for most wave energy technology. Wave energy conversion takes advantage of the ocean waves caused primarily by the interaction of winds with the ocean surface. Wave energy is an irregular oscillating low-frequency energy source. They are a powerful source of energy, but are difficult to harness and convert into electricity in large quantities. The energy needs to be converted to a 60 Hertz frequency before it can be added to the electric utility grid.
Wave energy generation is a developing technology. Although many wave energy devices have been invented only a small number have been tested and evaluated and very few of these have been tested in ocean waves testing is usually undertaken in a wave tank.
There are three approaches to capturing wave energy: floats or pitching devices, oscillating water columns, and wave surge or focusing devices, and energy collection devices can be placed either on the shoreline, near the shoreline, or offshore. Shoreline devices have the advantage of relatively easier maintenance and installation and do not require deep water moorings and long underwater electrical cables. The wave energy is less on the shoreline but this can be partly compensated by the concentration of wave energy that occurs naturally at some locations by refraction and/or defraction.
Nearshore devices are situated in 10-25 metres of water near the shore. The most common device for this situation is the oscillating water column. Offshore devices are situated in deep water, with typical depths of more than 40 metres. The incidence of wave power at deep ocean sites is three to eight times the wave power at adjacent coastal sites. A range of devices are being trialled for offshore use.
The wave energy converting device placed on the sea bed may be completely submerged, it may extend above the sea surface, or it may be a converter system placed on an offshore platform. The visual impact of a wave energy conversion facility depends on the type of device and its distance from shore. Onshore or nearshore devices could change the visual landscape from one of natural scenery to industrial.
In general terms wave energy generation has the following advantages and disadvantages.
- the energy is free no fuel is needed and no waste is produced
- not expensive to operate and maintain
- can produce a significant amount of energy.
- depends on the waves variable energy supply
- needs a suitable site, where waves are consistently strong
- some designs are noisy
- mist be able to withstand very rough weather
- costly to develop
- visual impact if above water or on shore
- can disturb or disrupt marine life including changes in the distribution and types of marine life near the shore
- poses a possible threat to navigation from collisions due to the low profile of the wave energy devices above the water, making them undetectable either by direct sighting or by radar
- may interfere with mooring and anchorage lines with commercial and sport-fishing
- may degrade scenic ocean front views from wave energy devices located near or on the shore, and from onshore overhead electric transmission lines.
Despite inventors actively making systems to capture power from the waves, for the last two centuries, there is still not a wide application of wave energy devices as power generators. The availability of devices to fit different applications is not the problem the technology is definitely there. The reality is that the only long term problem is making the technology work at a cost of power which a consumer is willing to pay. The system will work itself out. The price of fossil fuel generation will become more and more expensive and wave generated power will fall in price.
One of the biggest difficulties is in introducing a new, fledgling technology into a commercial market dominated by subsidised low cost fossil fuel and nuclear generation.