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Wednesday, November 14th, 2018
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Home » Energy » Wave Tidal Energy
Wave Tidal Energy
Ocean Energy

The ocean can produce two types of energy: thermal energy from the sun's heat, and mechanical energy from the tides and waves. 

Ocean Thermal Energy

Oceans cover more than 70% of Earth's surface, making them the world's largest solar collectors. The sun's heat warms the surface water a lot more than the deep ocean water, and this temperature difference creates thermal energy. Just a small portion of the heat trapped in the ocean could power the world. 

Ocean Thermal Energy Conversion (OTEC) is a means of converting into useful energy the temperature difference between the surface water in tropical and sub-tropical seas and cold water at a depth of about 1 000 metres, which emanates from the polar regions. A temperature difference of 20ºC is adequate for OTEC.

Workers install equipment for an ocean thermal energy conversion experiment in 1994 at Hawaii's Natural Energy Laboratory. Credit: A. Resnick, Makai Ocean Engineering, Inc.
Ocean thermal energy is used for many applications, including electricity generation. There are three types of electricity conversion systems: closed-cycle, open-cycle, and hybrid. Closed-cycle systems use the ocean's warm surface water to vaporize a working fluid, which has a low-boiling point, such as ammonia. The vapor expands and turns a turbine. The turbine then activates a generator to produce electricity. Open-cycle systems actually boil the seawater by operating at low pressures. This produces steam that passes through a turbine/generator. And hybrid systems combine both closed-cycle and open-cycle systems.
Unlike most Renewable energy technologies, OTEC has the advantage of providing base-load power,  available at a constant rate throughout the 24 hours, and varying very little with the seasons.  Currently there are no plans to utilise this technology in Australia.

Ocean Mechanical Energy (Tidal/ Wave Energy)  

Ocean mechanical energy is quite different from ocean thermal energy. Even though the sun affects all ocean activity, tides are driven primarily by the gravitational pull of the moon, and waves are driven primarily by the winds. As a result, tides and waves are intermittent sources of energy, while ocean thermal energy is fairly constant. Also, unlike thermal energy, the electricity conversion of both tidal and wave energy usually involves mechanical devices.
A barrage (dam) is typically used to convert tidal energy into electricity by forcing the water through turbines, activating a generator. For wave energy conversion, there are three basic systems: channel systems that funnel the waves into reservoirs; float systems that drive hydraulic pumps; and oscillating water column systems that use the waves to compress air within a container. The mechanical power created from these systems either directly activates a generator or transfers to a working fluid, water, or air, which then drives a turbine/generator.
In other models, electricity is generated by allowing water to flow from one side of the barrier to the other, through low-head turbines. Various configurations have been proposed, utilising single or multiple basin layouts
The high capital costs of tidal barrage systems are likely to restrict their development in the near future. However, with interest in entrainment schemes higher than in the past, it is increasingly likely that new barrage and lagoon developments will emerge in due course, especially where they can be combined with new transport infrastructure

Tidal
A tidal power station is part of a dam or barrage, built across a narrow bay or river mouth. As the tide flows in and out, it creates uneven water levels on opposite sides of the barrage. Water flows from the high side to the low side through turbines to generate electricity. It is also possible to use oceanic power generation to desalinate seawater and produce drinking water.
The tides – cyclic variations in the level of the seas and oceans – give rise to water currents which constitute a potential source of power. There are two basic approaches to tidal energy exploitation: one exploits the cyclic rise and fall of the sea level through entrainment, whilst the other harnesses local tidal currents.


Wave
Surface waves and pressure variations below the ocean’s surface can generate intermittent power. Floating buoys, platforms, or submerged devices placed in deep water, generate electricity using the bobbing motion of the ocean’s waves. Power comes from the water’s movement, i.e. either the changes in height of the tides or the ocean’s current.

The global wave power resource in water depths of over 100 m has been estimated as between 1 and 10 TW, while the economically exploitable resource ranges from 140-750 TWh/yr for current designs when fully mature, and could be as high as 2 000 TWh/yr if the potential improvements to existing devices are achieved. 


Greenhouse gas savings
Ocean power is a zero-emission electricity source. One megawatt hour of hydro-derived electricity avoids approximately one tonne of CO2.
Oceanic power provides a realistic solution to reducing the high costs of distribution and grid connection that other power sources face.


Issues in Australia (Current and Potential)
With its vast coastline, Australia’s near shore wave energy resources could create around four times the nation’s current national power needs. The Southern Ocean, in particular, is one of the world’s largest and most consistent wave energy resources. Regions such as Port MacDonnell in South Australia, Portland, Warrnambool and Phillip Island in Victoria, Albany and Geraldton in Western Australia and parts of the Tasmanian and NSW coastlines are optimal sites for wave energy plants.
The resource is so far almost completely undeveloped, but that is beginning to change. Currently two wave powered systems are working/planned having generating capacity of 0.6 megawatts. Another 915 megawatts of ocean power is being evaluated around Australia.

The future success of ocean power in Australia is dependent upon government policies to support the development and deployment of these emerging technologies. The sector requires a comprehensive policy framework for emerging technologies to take them from research to full scale demonstration.