Effect of inclination on oscillation characteristics of an oscillating water column wave energy converter Mitsumasa Iino n , Takeaki Miyazaki, Hiroshi Segawa, Makoto Iida The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8904, Japan article info Article history: Received 1 November 2015 Accepted 6 March 2016 Keywords: Oscillating water column Wave energy Mechanical model Water tank test abstract Oscillating water column (OWC) wave energy converters are adopted in many marine applications such as buoys. This research investigates how inclination-induced changes in motion affect the oscillation characteristics of an OWC. As the collector of the OWC wave energy converter is inclined, the direction of motion of the water column is also inclined because it is constrained by collector walls. To investigate this idea, we developed a mechanical 1-degree-of-freedom oscillator model of an inclined OWC. The usefulness of the mechanical model was confirmed in a water tank test with OWCs inclined at 90° (vertical), 45°, and 18.4°. We then investigated the effect of inclination on the resonance period and the optimum power take-off damping of the OWC. The resonance period was mainly prolonged by the reduced restoring force as the motion direction changed. The optimum power take-off damping of an inclined OWC equaled the radiation damping, as observed in vertical OWCs. Finally, we varied the air chamber volume and observed much smaller effects than when varying the inclination. We conclude that changing the direction of motion affects the oscillation characteristics of OWCs mostly because of the reduction of gravity restoring force. & 2016 Published by Elsevier Ltd. 1. Introduction The oscillating water column (OWC) wave energy converter is among the most reliable wave energy conversion systems. Many full-scale prototype tests have been performed on floating offshore types (Ishii et al., 1982; Yukihisa et al., 2001) and shoreline fixed types (Hotta et al., 1986; Pecher et al., 2011). At Port Sakata (Suzuki et al., 2004), a shoreline fixed OWC wave energy converter has been continuously tested for 13 years without major accident. In these experiments, a water column oscillating in the vertical motion was constructed as a concrete or steel chamber. Recently, we conducted the Blow-Hole wave energy converter project in Japan (Miyazaki et al., 2014). In this project, we con- structed a cylindrical OWC collector inclined at 18.4° from the horizontal level at the Fukui test site. Unlike standard artificial collectors, our OWC collector is an inclined tunnel drilled into shore rock. As in traditional OWC devices, power take-off equip- ment with a turbine generator is attached at the top of the tunnel. The other side of the tunnel is a wave inlet. In this type of device, the inclination angle of the OWC is limited by the tunneling cost. Therefore, inclination effects should be properly considered at the preliminary design stage. Previously, the only inclined OWC wave energy converter subjected to full-scale testing was LIMPET in the United Kingdom. According to the LIMPET report, the inclination of the OWC increases the water surface area and hence the resonance period of the OWC. Moreover, the inlet loss is decreased because of the fluid dynamics. Elsewhere, Ram et al. (2010) experimentally compared the efficiencies of OWCs inclined at angles under lim- ited wave conditions. The DTI project in the UK demonstrated the superior performance of a floating buoy with an inclined OWC and a backward bent duct buoy OWC (Department of Trade and Industry Great Britain, 2004). However, previous studies appear to lack fundamental physical insight. To elaborate, inclining the OWC changes the direction of motion of the water surface. If the direction of motion is inclined from the vertical axis, the gravitational acceleration slows down, analogous to a box sliding or water falling down a slope. Moreover, if the water is allowed to freely oscillate in the water column, the slower acceleration increases the period of the oscillation, similar to a pendulum swinging on a slope. Thus, the inclination of the OWC might significantly affect the oscillation characteristics of the water column. This type of water motion (with an inclined direction) is pointed to in the literature of oscillating body type wave energy converters (Payne et al., 2008). Therefore, this phe- nomenon must significantly affect the converter. However, whe- ther inclination changes the oscillation or motion characteristics Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/oceaneng Ocean Engineering http://dx.doi.org/10.1016/j.oceaneng.2016.03.014 0029-8018/& 2016 Published by Elsevier Ltd. n Corresponding author. E-mail address: iino@ilab.eco.rcast.u-tokyo.ac.jp (M. Iino). Ocean Engineering 116 (2016) 226–235