Ocean Energy

There are two broad types of ocean energy: mechanical energy from the tides and waves, and thermal energy from the sun's heat. Ocean energy is classified as tidal energy, wave energy and ocean thermal energy. Potential energy resources associated with major ocean currents, such as the East Australia Current or the Leeuwin Current, are not considered here.

Tidal energy

Tides result from the gravitational attraction of the Earth-Moon-Sun system acting on the Earth's oceans. Tides are long period waves that result in the cyclical rise and fall of the ocean's surface together with horizontal currents. The rotating tide waves result in different sea levels from one place on the continental shelf to the next at any one time, and this causes the water column to flow horizontally back and forth (tidal currents) over the shelf with the tidal oscillations in sea level.

Tidal energy is energy generated from tidal movements. Tides contain both potential energy, related to the vertical fluctuations in sea level, and kinetic energy, related to the horizontal motion of the water column. It can be harnessed using two main technologies:

• Tidal barrages (or lagoons) are based on the rise and fall of the tides ¿¿¿ these generally consist of a barrage that encloses a large tidal basin. Water enters the basin through sluice gates in the barrage and is released through low-head turbines to generate electricity.
• Tidal stream generators are based on tidal or marine currents ¿¿¿ these are free-standing structures built in channels, straits or on the shelf and are designed to harness the kinetic energy of the tide. They are essentially turbines that generate electricity from horizontally flowing tidal currents (analogous to wind turbines).

Wave energy

Waves (swell) are formed by the transfer of energy from atmospheric motion (wind) to the ocean surface. Wave height is determined by wind speed, the length of time the wind has been blowing, the fetch (distance over which the wind has been blowing), and the depth and topography of the sea floor. Large storms generate local storm waves and more distant regular waves (swell) that can travel long distances before reaching shore. Wave energy is generated by converting the energy of ocean waves (swells) into other forms of energy (currently only electricity). It can be harnessed using a variety of different technologies, several of which are currently being trialled to find the most efficient way to generate electricity from wave energy.

Ocean thermal energy

Oceans cover more than 70 per cent of the Earth's surface. The sun's heat results in a temperature difference between the surface water of the ocean and deep ocean water, and this temperature difference creates ocean thermal energy.

Ocean thermal energy conversion (OTEC) is a means of converting into useful energy the temperature difference between surface water and water at depth. OTEC plants may be used for a range of applications, including electricity generation. They may be landbased, floating or grazing.

World ocean energy market

There is only a small market at present for tidal, wave and ocean thermal energy. In 2009, commercial applications were limited to electricity generation based on tidal energy resources in France and Canada but significant investment in new tidal energy projects was taking place in the Republic of Korea. Feasibility assessments and RD&D investments in ocean energy technologies are taking place in several countries.

Australia's ocean energy resources

There has been limited progress in assessing Australia's ocean thermal energy resources, not least because of the greater prospectivity of other renewable energy resources (WEC 2007).

Tidal energy

Assessment of Australia's tidal energy resources is restricted to the tide kinetic energy present on Australia's continental shelf. Tidal currents off the shelf are minimal. Moreover, significant transmission losses would be expected for tidal energy converters located far from shore. The continental shelf for this assessment is defined as water depths less than 300m.

A variety of tide energy converters are presently available to generate electricity. Barrage-type systems require specific coastal geomorphic settings ¿¿¿ typically bays or estuaries ¿¿¿ as they are designed to harvest the potential energy of the tide, which depends on both the tide range and the surface area of the basin (i.e. the tidal prism). Because of their site-specific requirements and the complex response of the tide in very shallow water, it is not practical to undertake a detailed national scale assessment of the tidal potential energy.

Barrage-type tide energy systems generally require macro-tide ranges (greater than 4m), which are restricted to the broad northern shelf of Australia; from Port Hedland northwards to Darwin and the southern end of the Great Barrier Reef. Other types of tidal energy converters (tidal turbines) harness the kinetic component of tide energy. They are suitable for installation on the continental shelf, and while they do not necessarily require highly-specific coastal configurations they can be deployed in locations where local coastal configurations result in increased tidal flows.

The total tidal kinetic energy on the entire Australian continental shelf at any one time, on average, is about 2.4PJ. Since the tidal movement of shelf waters occupies the entire water column, the tide energy adjacent to each state at any one time reflects both the volume of shelf waters and the current speed of those waters. There are numerous other locations on shallower or narrower regions of shelf where the total tide kinetic energy is considerably less, but still more than enough for the purpose of electricity generation (e.g. Darwin, Torres Strait and Bass Strait).

The regions of shelf that have the largest kinetic energy densities are the North West Shelf and the southern shelf of the Great Barrier Reef, with large areas having densities of more than 100 Joules per cubic metre (J/m3). Darwin, Bass Strait and Torres Strait have localised areas with similar energy densities, despite more modest tide ranges. This is due to the convergence and acceleration of tidal streams on the shelf between the islands and mainland. The rate of delivery of tidal kinetic energy, or energy flux, is also referred to as tidal (kinetic) power. Tidal (kinetic) power is also greatest on the northern half of the Australian continental shelf, with many areas having more than 100 Watts per square metre (W/m2). The southern half of the Australian shelf (with the exception of Bass Strait) has relatively little tidal kinetic energy or power. The tidal kinetic energy delivered in a given time period, for example, in one year (total annual tidal kinetic energy), can be obtained by integrating the tidal (kinetic) power time series over one year.

The best resourced jurisdictions are Western Australia, Queensland and the Northern Territory. Western Australia has locations off its coast where the average tidal (kinetic) power in water depths less than or equal to 50m exceed 6.1 kW per square metre (KW/m2), delivering a total tidal kinetic energy of over 195GJ/m2 annually.

Wave energy

Previous studies of Australia's wave climate have focused mainly on the energetic south-western, southern and south-eastern margins of the continent, but there has been no previous publicly available comprehensive national assessment of Australia's wave energy resources. The wave energy resource assessment presented here is based on wave data hindcast by the Bureau of Meteorology at 6-hourly intervals over an eleven year period from 24 090 locations evenly distributed over Australia's entire continental shelf (Hasselmann et al. 1988).

Several types of wave energy converters are presently available to generate electricity. The choice of converter technology places limits on the locations from which wave energy can be harvested. For example, the Pelamis device is capable of generating electricity in water depths of 60 to 80 metres, whereas CETO is suited to shallower water depths (15 to 50 metres). Given these considerations, and the transmission losses expected if a wave energy converter is too far from shore, this resource assessment is restricted to the wave energy present on Australia's continental shelf. The shelf is defined here as water depths less than 300 metres. The northern Australian shelf (i.e. above latitude 23 degrees south) is characterised by relatively low wave energy densities of generally less than 2.5kJ/m2. The southern Australian shelf, on the other hand, is characterised by energy densities of more than 2.5kJ/m2, with large areas of the shelf experiencing twice this value (e.g. western and southern Tasmania). Much of the southern Australian coastline experiences significant wave heights (in excess of 1m) virtually all of the time. The total wave energy on the entire Australian continental shelf at any one time, on average, is about 3.47PJ. The wave energy adjacent to each jurisdiction at any one time reflects both the area of shelf waters and the energy density in those waters. For example, Victoria and Tasmania have, on average, about the same total wave energy as the Northern Territory; however, it is concentrated in a smaller shelf area.

The shelf waters off Victoria and Tasmania are suitable sites for harvesting wave energy, whereas the shelf waters off the Northern Territory are not suitable, at least with existing technology. Consideration must also be given, however, to the rate at which useful energy can be delivered. In the case of tidal and wave energy resources, the lack of control over the timing, rate or level of delivery can impact significantly on their potential as an electricity source.

The rate of delivery of wave energy, or energy flux, is also referred to as wave power. Wave power is also greatest on the southern half of the Australian shelf, with 25¿¿¿35kW/m being common on the outer shelf. Despite the fact that there is a considerable amount of energy on the northern half of the Australian shelf at any one time due to the large shelf area, the energy density and power or rate that the energy is delivered is small. For example, wave power off the Northern Territory shelf is typically less than 10kW/m and unsuitable for harvesting with current technologies.

The states with the best wave energy resource are Western Australia, South Australia, Victoria and Tasmania. Tasmania is particularly well endowed with wave energy resources. There are locations off its coast where the average wave power in water depths less than or equal to 50m reach almost 35kW/m, delivering a total wave energy of 1100GJ/m annually.