Design and Control of Hydraulic Power Take-Offs for Wave Energy Converters

  • Christopher Cargo

Student thesis: Doctoral ThesisPhD


Renewable marine energy has attracted considerable interest in recent years,especially in the UK due to its excellent location to take advantage of this sustainableenergy source. Dierent types of device have been developed over severaldecades to capture the energy of sea waves but they all need to be able to convertthis mechanical energy into electrical energy. The success of wave energy converters(WECs) depends on their eciency, reliability and their ability to react tothe variable wave conditions. Although a number of simulation studies have beenundertaken, these have used signicantly simplied models and any experimentaldata 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 theheaving 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 aconstant power output from a hydraulic motor coupled to a generator. Althoughthis design has been presented before, the ineciencies and dynamics of thecomponents have not been investigated in detail.The aim of this work is to create an understanding of the non-linear dynamicsof a hydraulic power take-o unit and how these aect the hydrodynamicbehaviour of the WEC. A further aim is to predict the eciency of the powertake-o unit and determine tuning and control methods which will improve thepower generation. In order to do this and test the device in dierent wave conditions,a full hydrodynamic and hydraulic model is developed using the Simulinkand SimHydraulics software package.The model is initially tested with regular waves to determine the behaviourof the power take-o unit and a method for adjusting the hydraulic motor displacementdepending on the frequency of the incoming wave is investigated. Theoptimal eective PTO damping to maximise power generation is found to bedependent on the signicant wave frequency and the values of PTO dampingare signicantly dierent to previous work using a linear power take-o modelwhich emphasises the importance of including the ineciencies of the hydrauliccomponents. The model is then analysed with irregular waves to predict the behaviourand power levels in realistic wave conditions. Power generation reducesin comparison to regular waves but a similar tuning method to maximise powergeneration still exists.A hydraulic motor speed control method is shown to increase power generationin irregular waves by maintaining the motor speed within an acceptable workingrange. Wave data from the Atlantic Ocean is then used to investigate the benetsof an adaptive tuning method which uses estimated wave parameters for a numberof dierent sea conditions. Results show only minimal gains from using activetuning methods over a passive method. However, results revealed signicantpower 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 thesimulation results. The power take-o unit is tested using a hardware-in-the-loopsystem in which the hydrodynamic behaviour of the WEC is predicted by a realtimesimulation model. The experimental results show good agreement to thesimulation with the PTO showing similar characteristics and tuning trends formaximising power generation.
Date of Award31 Dec 2013
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorAndrew Plummer (Supervisor) & Andrew Hillis (Supervisor)


  • wave energy
  • optimization
  • hydraulics
  • PTO

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