J. Marine Sci. Appl. (2017) 16: 1- DOI: 10.1007/s11804-017-1405-y Optimization of Bottom-hinged Flap-type Wave Energy Converter for a Specific Wave Rose Hamed Behzad and Roozbeh Panahi * Department of Civil Engineering, Tarbiat Modares University, Tehran 14115-116, Iran Abstract: In this paper, we conducted a numerical analysis on the bottom-hinged flap-type Wave Energy Convertor (WEC). The basic model, implemented through the study using ANSYS-AQWA, has been validated by a three-dimensional physical model of a pitching vertical cylinder. Then, a systematic parametric assessment has been performed on stiffness, damping, and WEC direction against an incoming wave rose, resulting in an optimized flap-type WEC for a specific spot in the Persian Gulf. Here, stiffness is tuned to have a near-resonance condition considering the wave rose, while damping is modified to capture the highest energy for each device direction. Moreover, such sets of specifications have been checked at different directions to present the best combination of stiffness, damping, and device heading. It has been shown that for a real condition, including different wave heights, periods, and directions, it is very important to implement the methodology introduced here to guarantee device performance. Keywords: wave energy converter, bottom-hinged flap, power take-off system, directional analysis, optimization Article ID: 1671-9433(2017)02-0000-07 1 Introduction 1 Considering fossil fuel reserves reduction as well as global warming issues, attention has been paid to renewable energies although their technical and economic aspects still need to be discussed in depth. Recently, wave energy, with an average power density of 8 kW/m, compared to wind or solar sources, with a maximum production capacity of 500 W/m 2 , has received special attention (Flocard and Finningan, 2010) although its level of technological readiness needs to be justified. In recent years, numerous scientific and applied research have been performed in this field with the aim to design and optimize devices with an acceptable output. Such devices, in terms of their distance from the coast, are generally categorized into three main groups: shoreline, nearshore, and offshore. Most research had been historically focused on shoreline and offshore devices until Folly (2009) introduced the concept of exploitable wave energy resource. Despite the significant reduction of wave energy nearshore, Folly (2009) discussed that exploitable energy does not vary much if the effects of wave entrance into shallow waters is Received date: 20-Jul-2016 Accepted date: 02-Dec-2016 *Corresponding author Email: rpanahi@modares.ac.ir © Harbin Engineering University and Springer-Verlag Berlin Heidelberg 2017 taken into account (Folley and Whittaker, 2005). According to Xiros and Dhanak (2016), the horizontal speed of water particles sharply increases nearshore and strongly affects the efficiency of surging Wave Energy Converters (WECs). It has been proved in another research both theoretically and experimentally that absorbed energy by a surging device nearshore is directly related to the horizontal wave force rather than the total wave energy (Folley et al., 2007). In addition to the mentioned issues, these types of WECs are less expensive to install and maintain compared to offshore types. Additionally, in contrast to shoreline WECs, they do not suffer from visual and environmental pollutions while benefitting from higher energy levels due to wave breaking. Caska and Finnigan (2008) investigated the hydrodynamic performance of a cylinder hinged to the floor on average depths. Finnigan (2010) continued the study with physical models of the cylinder and investigated the effects of damping and diameter on power capture. In the meantime, Whittaker and Folly (2010) brought this idea into practice and created a commercial model of the surging WEC called Oyster. Zhao et al. (2013) of Zhejiang University optimized the density and damping of this device for short-wave sea states with wave periods lower than 6 s. Here it should be noted that that flap-type WEC has been the subject of many theoretical investigations (Renzi and Dias, 2013; Renzi and Dias, 2015a). They have used physical models to verify findings (Henry et al., 2015). Besides, it has been discussed with more accurate or recently developed numerical models (Wei et al., 2015; Wei et al., 2016; Wolgamot and Fitzgerald, 2015; Schmitt et al., 2016; Yeylaghi et al., 2016; Tomey-Bozo et al., 2016; Pezzutto, 2016). Also, there are lots of recent works discussing flap-type WEC in a farm (Renzi et al., 2014; Renzi and Dias, 2015b, Gunawardane et al., 2016), its Power Take-Off (PTO) mechanism (Bacelli et al., 2015); its combination with wind energy in turbines (Michailides et al., 2014), its extension over still water level (Kamkar et al., 2013) or its degrees of freedom (Kurniawan and Moan, 2012 ). This study presents a simple new methodology to investigate appropriate positioning of the apparatus and its optimized PTO system in the presence of wave rose. The study was performed in the Persian Gulf. The research shows that a slight change in the stiffness of the structure and thereby a change in its natural frequency has a drastic