Study on the Efficiency of a Wave Energy Converter Using Anti-rolling Tank

自由液面附近之細長浮體受波浪作用時會在六個自由度(縱移、橫移、起伏、橫搖、縱搖跟平擺)上產生運動，其中橫搖運動因其興波減衰力較小而經常對細長浮體造成較顯著的橫搖運動。對於遊輪會影響旅客舒適度，對於貨船則過大的運動可能損害貨物，因此自1874年開始漸漸有減搖水槽構想之出現與實體之實現。 減搖水槽其主要原理為當水槽受到搖晃時，內部裝載之流體會往位能較低之位置流動，並對乘載之水槽產生力矩，此力矩對於波浪所產生之力矩存在相位差，因而能削減橫搖運動。本研究室在2011年以此橫搖水槽為構想，初步探討橫搖水槽作為波能轉換器之可行性[1]。本研究以此為基礎，主要討論水槽內水流之動能，並分別探討主動式強制橫搖與受波浪作用而被動橫搖的水槽內產生水流動能之能量轉換效能。 本研究之模擬是運用計算流體力學軟體FLUENT進行二維橫搖水槽之動態模擬分析。以動態網格進行波浪作用於浮體之運動模式驗證，並以強制運動模擬對附加質量與減衰係數求解驗證，利用文獻[2]與文獻[3]進行比對驗證，確定FLUENT計算浮體受波浪作用的受力和運動，以及流體動力係數計算分析之可靠性。接著模擬橫搖水槽在主動強制簡諧橫搖運動下的物理現象，以文獻[4]檢驗內部水流運動對水槽本身產生之力矩與其相對於強制橫搖運動之相位差，確定計算內部水流對水槽本身作用力與力矩之準確性，進而推算主動橫搖水槽的能量轉換效率。最後將水槽放置於設定的波浪環境內，以六自由度(SDOF)使用者定義函數模擬水槽在波浪中之運動，並分析水槽內部水流動情形，並推估被動橫搖水槽從波能吸收轉換之效率。A floating body subjected to wave forces will perform motion in six degrees of freedom, i.e. surge, sway, heave, roll, pitch and yaw. The roll motion is the most critical one because it is lightly damped and therefore prone to dynamic magnification. Excessive motions interfere with the comfort of passengers and in some situations this may cause internal damage to the contents of the containers. Dates back to 1874, Froude installed water chambers in upper part of a ship for the first time to achieving stabilization against roll. Anti-rolling tanks contain water that follows the ship’s rolling and acquires a phase lag with respect to the ship movement, the moment generated partially counteracts the sea wave moment. Following the study in 2011 [1], we focus on internal water flow kinetic energy generation and the efficiency of energy conversion from a tank rolled by active excitation and rolled in passive way by water wave . In this study, we applied ANSYS FLUENT, a commercial CFD software package, to analyze a two dimensional anti-rolling tank, verified floating body motions in wave conditions, calculated added mass and damping coefficient, and compared these results with those published in the references [2][3]. And then we verified the calculated moment and phase difference due to internal water flow of anti-rolling tank under harmonic roll motions by comparing with the results published in the reference [4]. Finally, we used SDOF UDF to simulate the internal water flow of two kinds of anti-rolling tank excited by waves of various conditions, and investigated the efficiency of energy conversion of these anti-rolling tanks