|Fig. 1: A schematic of three WEC types: an attenuator (top left), a point absorber (bottom left) and a terminator (right). |
|Fig. 2: A schematic of WECs operating with the pressure differential principle. |
Covering over 70% of the Earth's surface , the oceans represent an enormous source of renewable energy. [1,2] This recognition of the ocean as an energy resource has been proposed as early as the late 18th century, with the first patent for harnessing the ocean's wave energy proposed by Monsieur Girard in France in 1799. [1,3,4] Despite its promise, however, the ocean energy technology's development has staggered until the 1960s due to its high costs of construction, deployment and maintenance.  Only recently, when the interest in renewable energy was stimulated during the oil shortage crisis in 1973, was the ocean energy reconsidered as an alternative energy resource. [1-6] The research and development of the ocean energy technology started in the 1990s , and it is still in the early development stage. [1-6]
The ocean energy includes energy from waves, tides, currents, thermal gradients and salinity gradients. [1,2,7] The estimated global resource for these forms of energy is shown in Table 1. The wave energy represents the largest resource of ocean energy, and it is currently the most researched area.  Today, more than one thousand prototypes of wave energy converters (WECs) exist, and there are several classification schemes based on their characteristics. [4-6] In particular, the WECs can be categorized by converter types and energy conversion principles, based on a pressure differential, mechanical flexing and bobbing, overtopping, and hydraulic flapping. [5,7]
The WECs can be categorized into three types based on their size and direction of elongation: attenuators, point absorbers and terminators. [1,5,6] Fig. 1 shows a schematic of these converter types. The attenuators are elongated structures with dimensions larger than the wavelength of the waves, and are oriented parallel to the wave propagation direction. [1,5,6] Each attenuator consists of a chain of cylindrical components that are connected by hydraulic pump joints so that they can conform to the local shape of the oscillatory wave. [1,5,6] The point absorbers have dimensions much smaller than the wavelength of the waves. [1,5,6] Unlike the attenuators, their small structure allows them to absorb wave energy from all directions. [1,5,6] The terminators are similar to the attenuators with one main difference: they are oriented perpendicular to the wave propagation direction. [1,5,6]
|Table 1: An estimated global resource for various forms of ocean energy. [1,2]|
As shown in Fig. 2, the WECs that operate with variations in the air pressure inside a chamber can either be submerged, as in Archimedes effect converters, or semi-submerged, as in oscillating wave columns, under the sea water. [4-6] The Archimedes effect converters (ex. Archimedes Wave Swing) are air-filled chambers in the form of point absorbers with movable upper cylinders that are moored to the sea bed. [1,4-6] The variations in pressure exerted on the upper cylinder during the crests and troughs move the cylinder down and up, and this mechanical movement is turned into electricity by a linear electric generator. [1,4-6]
The oscillating wave columns are one of the first developments of WECs, with their floating version first developed by Yoshio Masuda in the 1960s and 1970s.  The oscillating wave columns (ex. Limpet) are air chambers with the bottom open to the sea water below the water free surface. [4-7] As the water level inside the chamber rises and falls due to the oscillatory movement of waves, the air inside is compressed and pushed out of the chamber or expanded and sucked into the chamber through a turbine. [4-7] The Wells turbine, a bidirectional turbine, is utilized so that both inflow and outflow of the air turn the turbine in the same direction. [4-7]
When floating structures conform to their local sea water altitudes, they move relative to each other in the case of attenuators, or with respect to the hinge point in the case of point absorbers.  The wave energy can be harnessed by turning these wave-induced mechanical movements into electricity. In attenuators (ex. Pelamis), hydraulic pumps at the joints between the cylindrical components are designed to resist mechanical flexing. [1,4,6,7] When the cylinders conform to the incoming wave shape as shown in Fig. 3, this mechanical flexing compresses these hydraulic pumps and pumps oil into a high-pressure tank, which then generates electricity via a hydraulic power take-off. [1,4,6,7] The floating point absorbers (ex. PowerBuoy) also operate with a similar mechanism, but in this case it is the bobbing of the floating structures with waves that pumps a fluid, usually oil or sea water, and generates electricity via a hydraulic power take-off or via a linear electric generator. [1,4,6]
|Fig. 3: A schematic of WECs operating with the mechanical flexing and bobbing principle. |
|Fig. 4: A schematic of WECs operating with the overtopping (right) and hydraulic flapping (left) principles. |
The wave energy can also be captured in the form of potential energy. As shown in Fig. 4, the WECs under this category are designed to capture sea water that enters a tapered channel into a reservoir raised above the sea level. [1,4-7] The accumulated sea water in the reservoir is then controllably released back to the ocean through a hydraulic turbine to generate electricity. [1,4-7] These WECs can either be stationed onshore (ex. Tapchan) or offshore (ex. Wave Dragon). [1,4-7]
When moored terminator structures oriented perpendicular to the wave propagation direction (ex. Aquamarine Power Oyster) are hit by waves, the wave energy is absorbed upon impact and deflects the structures. [1,4,5] As waves come in and out, this deflection generates a flapping motion as shown in Fig. 4. Similar to the mechanical flexing and bobbing principle, the flapping movement pumps a high-pressure fluid to generate electricity via a hydraulic power take-off. [1,4,5,7]
As the most research area, the wave energy technology is the most advanced among the various forms of ocean energy. [1,2] Many prototypes for harnessing wave energy have been proposed, and it is eventually crucial to develop cost-competitive WECs for commercialization. [1,2] As a relatively new technology, it is also important to establish its performance in harsh marine environments and test its environmental impacts through a full-scale demonstration. [1,2] Once these barriers are overcome, the ocean energy technology including wave energy may become a vital part of the renewable energy portfolio.
© Se Joon Lim. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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