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Title: Design and control of hydraulic power take-offs for wave energy converters
Author: Cargo, Christopher
Awarding Body: University of Bath
Current Institution: University of Bath
Date of Award: 2013
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Renewable marine energy has attracted considerable interest in recent years, especially in the UK due to its excellent location to take advantage of this sustainable energy source. Dierent types of device have been developed over several decades to capture the energy of sea waves but they all need to be able to convert this mechanical energy into electrical energy. The success of wave energy converters (WECs) depends on their eciency, reliability and their ability to react to the variable wave conditions. Although a number of simulation studies have been undertaken, these have used signicantly simplied models and any experimental data is scarce. This work considers a heaving point absorber with a hydraulic power takeo unit. It employs a common hydraulic power take-o design, which uses the heaving motion of the buoy to drive an actuator that behaves like a linear pump. Energy storage is used to provide power smoothing in an attempt to give a constant power output from a hydraulic motor coupled to a generator. Although this design has been presented before, the ineciencies and dynamics of the components have not been investigated in detail. The aim of this work is to create an understanding of the non-linear dynamics of a hydraulic power take-o unit and how these aect the hydrodynamic behaviour of the WEC. A further aim is to predict the eciency of the power take-o unit and determine tuning and control methods which will improve the power generation. In order to do this and test the device in dierent wave conditions, a full hydrodynamic and hydraulic model is developed using the Simulink and SimHydraulics software package. The model is initially tested with regular waves to determine the behaviour of the power take-o unit and a method for adjusting the hydraulic motor displacement depending on the frequency of the incoming wave is investigated. The optimal eective PTO damping to maximise power generation is found to be dependent on the signicant wave frequency and the values of PTO damping are signicantly dierent to previous work using a linear power take-o model which emphasises the importance of including the ineciencies of the hydraulic components. The model is then analysed with irregular waves to predict the behaviour and power levels in realistic wave conditions. Power generation reduces in comparison to regular waves but a similar tuning method to maximise power generation still exists. A hydraulic motor speed control method is shown to increase power generation in irregular waves by maintaining the motor speed within an acceptable working range. Wave data from the Atlantic Ocean is then used to investigate the benets of an adaptive tuning method which uses estimated wave parameters for a number of dierent sea conditions. Results show only minimal gains from using active tuning methods over a passive method. However, results revealed signicant power losses in both calm and rough sea conditions with the PTO most ecient, at approximately 60%, in an average sea power. A scaled experimental power take-o unit is developed to help validate the simulation results. The power take-o unit is tested using a hardware-in-the-loop system in which the hydrodynamic behaviour of the WEC is predicted by a realtime simulation model. The experimental results show good agreement to the simulation with the PTO showing similar characteristics and tuning trends for maximising power generation.
Supervisor: Plummer, Andrew ; Hillis, Andrew Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: wave energy ; optimization ; hydraulics ; PTO