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Wave energy converter strings for electricity generation and coastal protection

Alexandre, Armando Emanuel Mocho fernandes e

[Thesis]. Manchester, UK: The University of Manchester; 2013.

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Abstract

Generation of electricity from ocean waves has seen increasing research and commercialinterest in recent years. The development of projects of several hundred megawattsrated capacity is now being considered. There is a clear need for improved understandingof the environmental impact of large-scale wave energy extraction, particularly innearshore regions where sediment transport and cliff erosion may be affected. Thisthesis investigates the change in nearshore wave conditions and sediment transportdue to energy extraction by long strings of wave energy devices.The influence of wave energy converter (WEC) arrays has been studied using transmissioncoefficients implemented within a spectral wave model. It is shown that thebreaking wave height nearshore is larger (5%) if transmission is defined as frequencydependent. This is due to the energy dissipation processes associated with differentwave frequencies. Linear wave theory is employed to determine frequency dependenttransmission and reflection coefficients across a line of wave energy devices based onthe amplitude of scattered and radiated waves. This approach is compared with experimentalmeasurements of the wave field in the vicinity of an array of five heaving floats.The transmitted wave amplitude is predicted with reasonable accuracy but additionalnumerical damping is required to predict the measured float response amplitude. Thiscomparison indicates that linear analysis is an acceptable approach for predicting floatresponse and wave field in the vicinity of the array for a certain range of conditions.Linear wave analysis is subsequently applied to investigate the variation of transmissioncoefficients with distance inshore of a long array of heaving WECs undergoingfree response and with damping specified to optimise power extraction. A methodis presented for identifying representative transmission and reflection coefficients suchthat change in wave energy is equal to energy extraction by the devices. These coefficientsare employed to quantify the change in nearshore conditions due to deploymentof a long line of wave devices at a site near the East Anglian coastline. Wave conditionsare modelled at 12 points along the shoreline over a 140 year period and significantwave height reductions up to 30% were obtained. Importantly, changes in nearshorewave direction are also observed. Analysis using the sediment transport model SCAPE(Soft Cliff and Platform Erosion model) indicates that the introduction of the arrayreduces both the sediment transport rate and cliff recession rate by an average of 50%.