수치해석을 이용한 OWC형 파력발전시스템의 통합성능해석

Abstract

Plenty of wave energy absorption devices have been invented, and several of them have been utilized in the electricity generation. The Oscillating Water Column (OWC) type has been widely employed in the application for the wave energy conversion for last 30 years. Due to the advantages of rather reliable converting technique and comparable production cost over other types of ocean energy, wave energy conversion system is considered to be feasible for the establishment of commercial power production.

The present paper deals with the numerical study on integrated OWC system for wave energy conversion. All the numerical study was based on Reynolds averaged Navier-Stokes(RANS) equations. An OWC wave energy converting system includes three energy converting stages: 1) The OWC inside a chamber forces air alternately into and out of the atmosphere through the duct. 2) A turbine with symmetric blades transforms the bi-directional air flow energy into a mechanical torque. 3) An electric generator linked to the turbine transforms the torque into electrical power. Since the hydro- and aero-dynamic performance analyses are of primary concern for the thesis, the third stage for electric generator is not considered in this study.

For the study of first energy converting stage, the numerical wave tank is established. The VOF model are adopted to calculate the wave generation and propagation in the numerical wave tank. The two-dimensional & three dimensional numerical wave tanks are validated with the analytic results and the predicted performance of oscillating water column is compared with the available experimental data. Various parameters of chamber geometry are investigated to demonstrate the effects of shape parameters on the wave field, water column oscillation in the chamber and wave energy conversion.

For the study of second energy converting stage, the numerical model for rotating machine is developed based on MRF technique. To optimize the impulse turbine, the effects of several shape parameters on operating performance are investigated such as number of blade, angle of guide vane, tip clearance, hub ratio, G/lr, sweep angle & staggered blade. Through the parametric study, the optimized impulse turbine is drawn out in diameter D=1.8m for 250kW capability. The fully transient calculation model is developed to investigate the unsteady characteristics of impulse turbine, especially self-starting performance under various incident air flow conditions.

For the study of integrated system of chamber and turbine, the orifice module is adopted. The experiments on relationship between the air flow velocity and the pressure difference between two side of the turbine and orifice are carried out. The corresponding simulation is performed to validate the capability of the numerical model on the prediction of pressure drop. The numerical wave tank is embedded with the orifice module to investigate the integrated OWC system and interaction of turbine effects.

The effects of wave directions on the performance of an OWC chamber have been investigated. Two test conditions which are with and without turbine effects was carried out in experimental study. The experiment was carried out in a 3-D wave basin. The wave elevation inside the chamber was measured at center point under various incident wave conditions and wave directions from 0o to 90o. A CFD study using a numerical wave tank was also conducted to compare the results with the experimental data and to reveal the detailed flows around the chamber.

In order to evaluate the operating performance of OWC facilities in real sea conditions, this paper proposed the integrated numerical techniques for OWC - turbine system to induce the influences of the other processing. The real sea conditions are considered as the combination of regular waves to estimate hydropower from incident waves. The estimating method for individual regular waves is based on the numerical simulation of the OWC chamber with effects of turbine effects.

The evaluation of the integrated OWC system by using look-up table to estimate the averaged power output of Jeju OWC plant is also presented. The look-up table contains three kinds of database: the orifice, OWC - orifice and turbine performance databases. The orifice database shows the relationship between the air flow rate and pressure drop for different orifice diameters. The OWC - orifice database shows the relationship between the incident wave condition and air flow rate generated inside the OWC chamber with various orifice devices. The turbine performance database presents the steady-state performance of the designed impulse turbine under various air flow rates and rotational speeds. The real-time power output of the OWC system can be predicted with the look-up table by iterations of each stage.

Finally the process of predicting the operating performance in the real sea conditions using look-up table is presented. The software with Graphic User’s Interface (GUI) contains all look-up table is developed using Visual C++ language, which can complete the iteration process automatically.