Coupled Modelling of a Non-Linear Wave Energy Converter and Hydraulic PTO

Hodge, Caitlin Worden (Industrial Doctoral Centre for Offshore Renewable Energy (IDCORE) & Zyba) | Bateman, Will (Industrial Doctoral Centre for Offshore Renewable Energy (IDCORE) & Zyba) | Yuan, Zhiming (University of Exeter) | Thies, Philipp R. (University of Strathclyde) | Bruce, Tom (University of Edinburgh)

OnePetro 

ABSTRACT

The mechanical motion of a wave energy converter (WEC) is converted by the power take-off (PTO) system into electricity, but these two systems are not independent, as they have been treated in WEC modelling. Treating them as such leads to inaccuracies in prediction of power output and reliability, and can erode confidence in numerical modelling tools. This paper presents a methodology for the two-way coupling of high fidelity modelling of WEC hydrodynamics with a more accurate representation of the PTO and investigates the impact of using simplified PTO models. Simplified models represent the full PTO with a single parameter which is in itself difficult to choose and may require a number of iterations. These different methods were used to assess the behaviour of the CCell WEC in a regular wave, with the calculations for mean power varying considerably in different wave conditions and the range of motion consistently under predicted by the simpler models. The coupled model increased the computational requirement for the simulation, however it provided the developer a better understanding of the impact on and utilisation of different hydraulic components.

INTRODUCTION

WECs are designed to convert the energy from waves into a mechanical motion which is then converted into electrical power through the power take-off (PTO) system. Oscillating wave surge converters, such as the CCell device, Fig. 1, have generally evolved as buoyant bottom-hinged flap WECs, which pitch back and forth from sea to shore under the influence of the horizontal motion of waves (Cameron, L, 2010). This pitching motion is transformed into useful energy usually through a hydraulic piston, which draws on the robustness and high power density that hydraulic circuits offer. Similar systems have also commonly been used in heaving buoys (Cargo, C, 2012).

Numerical modelling has become a valuable toolbox for WEC developers as it allows rapid modifications to a WEC design, without the additional manufacture and testing costs, or scaling issues. It can build up a picture of the Mean Annual Energy Production (MAEP) and the load estimates on the device which can aid design decisions and inform the required O&M procedures. However, numerical simulations can be slow to compute without adequate computer power and some simplifications must be introduced for efficiency, especially regarding the modelling of the power take-off system and/or the hydrodynamics.