수치해석을 이용한 부유식 파력발전의 계류 시스템 특성에 관한 연구

Abstract

The aim of this study is to find and select a suitable mooring system to apply on to a floating wave energy system by studying the characteristics of its motion and response using a numerical analysis. The wave energy device converts the ocean wave motion into electrical energy. The waves influence the device's movement and in turn the device's motion influences the incoming waves. The behaviour of the device's motion is mainly affected by the type of mooring system that is attached to it. Therefore, as the mooring system has major influence on the output of the wave energy device, it is of high importance to study the response of the device with different mooring systems.

The results are summarized as follows:

1. In previous studies, the wave energy device was manufactured and deployed for sea tests. In order to study the internal flow in the Power Take Off (PTO) system, a commercial Computational Fluid Dynamic (CFD) code, ANSYS CFX ver.14, was used for analysis on a 1:1 scale model.

2. At a period of 4 seconds, the power output obtained was 407W. As the period was increased, the power obtained decreased. At a period of 6 seconds, the power output was only 235W. The highest efficiency obtained was 46.7% and at a period of 4.5 seconds, the efficiency was much lower than that obtained at 4 seconds. As the length of the device and period of the wave have a direct effect on each other. This specific device design is not recommended for deployment in wave climates where a significant number of waves have a period of 4.5 seconds.

3. A numerical case was given the same conditions as the sea tests with a single point mooring, at case 5(λ/L=4.0) the pitch response amplitude operator (RAO) was 2.401 and the heave RAO was 2.154. As the λ/L ratio increases, the results show that the RAO values also increase. At case 5 (λ/L=4.0), the largest pitch angle seen was 20.1°. When compared to actual device, the motion numerical case was seen to be similar.

4. In an actual sea, the currents and wind will affect the wave energy device. Therefore, there is a need for the device to have a multiple point mooring system to avoid unnecessary yawing on the surface of the ocean. It was expected that adding more mooring points the system, the RAO values would decrease. However, from the results of the multiple point mooring analysis, the RAO values obtained were similar to the single point mooring results. Therefore, it can be seen from these results of the numerical model that multiple point mooring do not interfere greatly with the motion of the wave energy device. In addition, when installing additional mooring lines or selecting the most suitable mooring system for wave energy devices, the results obtained in this study is needed for the selection process.