The CETO wave energy converter is shown schematically in Fig. 27.1. Submerged buoys are connected to pumps that are tethered to the seabed in an array. As a wave disturbance passes overhead the buoys are heaved upwards and exert tension on the tethers forcing the pistons inside the pumps to move upwards-expelling fluid at high pressure. The high-pressure fluid, usually water, is piped into a manifold where it moves to shore. The pressurised water may be used to drive a turbine directly for electricity production or for production of desalinated water, or a combination of both.
CETO thus distinguishes itself from other WEC's in that the output of the offshore plant is not electricity but rather pressurised fluid. Energy conversion from hydraulic to electric takes place onshore with standard off-the-shelf plant- Pelton, or similar high-head turbines coupled to electric generators.
CETO wave energy converters may be understood better when compared to a current snapshot of the competitors, as shown schematically in Fig. 27.2. Wave energy converters such as OPT's Powerbuoy1 and Oceanlinx2 that are on the sur-
1 OPT website: www.oceanpowertechnologies.com.
2 OCEANLINX website: www.oceanlinx.com.
Carnegie Wave Energy Limited, Level 1, 16 Ord Street West Perth, WA, Australia 6005 Perth e-mail: [email protected]
A. Schiller, G. B. Brassington (eds.), Operational Oceanography in the 21st Century, 721
DOI 10.1007/978-94-007-0332-2_27, © Springer Science+Business Media B.V. 2011
DECREASED EXPOSURE TO LAROE STORMS
DECREASED EXPOSURE TO LAROE STORMS
face of the water are exposed to breaking waves during large seas, for example, in storms, whereas the CETO device and the AWS buoy3 are fully submerged during normal operation so that they are significantly less prone to damage than floating devices. Pelamis is a surface going device but is designed to tolerate very large seas4. Another metric for comparison of WEC's is the simplicity of the power generation scheme. CETO, Oyster5 and the shore-based Limpet6 have the electricity generation equipment onshore rather than in the water and this leads to greater simplicity and should translate to better long-term reliability. Also, onshore electrical generation plant may be upgraded or modified without recourse to removal of the WEC's from the water.
Devices that are fully submerged in normal sea states, including CETO, are not able to access the full energy flux of the waves as can surface mounted devices such as OPT's Powerbuoy. This penalty in energy capture efficiency may be offset by reduced operating costs in the form of maintenance, as submerged devices are subject to a lower rate of occurrence of breaking waves and repetitive stress loading compared to surface devices. CETO will typically be deployed up to several hundred metres from shore so the pipeline lengths are typically greater than those of the other hydraulic wave energy converter, the OYSTER, which operates in the breaking wave zone much closer to shore. This means that the CETO balance of plant design must pay close attention to the optimisation of the energy losses and minimise the cost of piping. Fortunately this is made easier by the selection of very high operating fluid pressures of around 7 MPa which enables the hydraulic design of CETO to obtain acceptable piping losses in smaller diameter, and therefore cheaper, piping to shore.
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