Estimation of Cumulative Output Energy of Oscillating Water Column Wave Energy Converter Considering Power Take Off Damping

Abstract

Abstract The objective of this study is to investigate the influence of damping from the PTO device on the cumulative output of the oscillating water column wave energy converter under real sea conditions which encompasses wide range of wave period and height. In this presentation, a time domain dynamic simulation model of OWC motion is developed and validated with water tank test results. Then the model is used to calculate a wide range of wave period and height. Finally, annual cumulative air power output of OWC is calculated with different damping values. In the water tank experiment, a cylindrical oscillating water column with a diameter of 0.3 m and submerged depth of 0.2 m is tested. PTO damping was emulated by using several orifice plates. Since the orifice pressure is proportional to square of flow-rate of the orifice, In the simulation, a model was constructed to solve the dynamic equation of motion assuming water column as a rigid equivalent floating body. Validation showed the model captures the influence of PTO damping as observed in the water tank testing. Using simulation, output air power from real scale OWC was evaluated under wide range of inlet wave period and height. With the output power database from dynamic simulation and frequency of wave height and period at specific sites in Japan (Fukui and Kamaishi), annual cumulative output power is calculated. From the results, it was confirmed that a higher output can be obtained with higher energy in high waves by adopting a damping characteristic that increases the efficiency under a wide wave condition rather than a nozzle characteristic that achieves maximum efficiency in the resonance period. Furthermore, it became clear that the damping that increases the maximum efficiency does not necessarily increase the accumulated energy. When considering operation in real sea condition in the future, it is not always effective to select PTO damping that maximizes the output at specific wave height or period. And it is important to adopt a method that can estimate wide range of wave condition and evaluate the cumulative output power.