Nonlinear 2D analysis of the efficiency of fixed Oscillating Water Column wave energy converters Yongyao Luo a , Jean-Roch Nader b , Paul Cooper c, * , Song-Ping Zhu b a State Key Laboratory of Hydroscience and Engineering & Department of Thermal Engineering, Tsinghua University, Beijing 100084, China b School of Mathematics and Applied Statistics, University of Wollongong, Wollongong, New South Wales 2522, Australia c Sustainable Buildings Research Centre (SBRC), University of Wollongong, Wollongong, New South Wales 2522, Australia article info Article history: Received 29 August 2012 Accepted 1 November 2013 Available online 8 December 2013 Keywords: Hydrodynamics Wave energy Nonlinear Numerical wave tank (NWT) Oscillating water column (OWC) Efficiency abstract This paper reports on the development of a two-dimensional, fully nonlinear Computational Fluid Dy- namics (CFD) model to analyse the efficiency of fixed Oscillating Water Column (OWC) Wave Energy Conversion (WEC) devices with linear power take off systems. The model was validated against previous experimental, analytical and numerical results of others. In particular, the simulation results show excellent agreement with the analytical results obtained by Sarmento and Falcão [1] for linear waves in a 2D channel and with previous experiments by others on the interaction between nonlinear waves and a fixed barge. Results are presented for linear waves on the influence of the seaward wall draft and thickness of the OWC device on the resonant frequency and the capture efficiency of the OWC. The key outcome of the present work is that for fully nonlinear waves a substantial decrease in the hydrodynamic capture efficiency of the OWC device was observed with increasing wave height, which represents a significant departure from the linear wave case. The optimal pneumatic damping coefficient for the OWC was also found to be dependent on the wave height. By analysing the magnitude of the first and higher order components of the incident nonlinear waves and the response of the OWC it was found that the first order capture efficiency decreases with increasing wave height, which in turn implies that the OWC hydrodynamic system is fully nonlinear and that the behaviour of an OWC in a nonlinear wave train cannot be accurately represented by the superposition of the linear response to a number of component linear waves. These results have significant implications for the design and operation of practical OWC systems. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Wave energy is generally considered to provide a clean source of renewable energy, with limited negative environmental impacts [2]. Of all the proposed and existing means of extracting energy from waves the Oscillating Water Column (OWC) device is arguably one of the most simple and elegant in design and principle of operation. Fundamentally, an OWC device consists of a partially submerged chamber, open below the free surface of the water, and within this structure an air pocket is trapped above the free surface. The oscillating movement of the free surface inside the pneumatic chamber, produced by the incident waves, forces the air to flow through a turbine which is coupled to an electrical generator [3]. Various analyses of the hydrodynamics of OWCs have been presented in the literature since the first analytical studies in the 1970s. Lighthill [4] developed a two-dimensional analysis of wave- energy extraction by means of a submerged duct, then Simon [5] and Miles [6] extended this to three-dimensional analyses of diffraction around a duct structure in an ocean of infinite depth in the early 1980s. Evans [7] used classical linear water wave theory to analyse OWCs and later Malmo and Reitan [8] conducted bench- mark numerical and experimental work. Sarmento and Falcão [1] developed an analytical two-dimensional linear model of the cap- ture efficiency of an OWC in a two-dimensional channel and calculated the optimal pneumatic damping required of an air tur- bine for maximum energy conversion efficiency. Air turbines with both linear and quadratic relationships between air flow and OWC chamber pressure were analysed. Brendmo et al. [9] later extended this work to consider viscous losses. Evans and Porter demon- strated an approach for optimizing a 2D device with respect to the OWC seaward wall draft [10] and Delauré and Lewis [11] used a boundary element method to study OWC performance against a * Corresponding author. E-mail addresses: luoyy@tsinghua.edu.cn (Y. Luo), jrn758@uowmail.edu.au (J.-R. Nader), pcooper@uow.edu.au (P. Cooper), spz@uow.edu.au (S.-P. Zhu). Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene 0960-1481/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2013.11.007 Renewable Energy 64 (2014) 255e265