Investigation of new type wave energy converter systems

Abstract

In consideration of the evolving global economy and increasing population, energy is increasingly occupying a more important place in our lives. As commonly understood, energy sources can be categorized into two groups: non-renewable and renewable. A renewable energy source is defined as an energy source that can exist the next day within the natural order of the Earth. The most significant features of renewable energy sources include reducing carbon dioxide emissions to help protect the environment, decreasing dependence on foreign energy sources due to being local resources, contributing to increased employment, and receiving widespread and strong support from the public. In other words, renewable energy sources possess characteristics of accessibility, availability, and acceptability. Among renewable energy sources, wave energy, despite being approximately 10 times more intense than solar energy per square meter and 5 to 10 times more intense than wind energy, has a very low utilization rate compared to others. Efforts to harness wave energy are rapidly increasing worldwide. It is a predictable fact that our country, surrounded by seas on three sides, has significant potential for wave energy, and the use of this energy type will reduce external dependence. Wave energy converters are used to convert wave energy in the seas into electrical energy. Among these converters, oscillating water column (OWC) type converters are the most efficient. The oscillating water column is a system consisting of two phases: water and air flow. Moving according to the periodic structure of the wave, the water converter compresses the air inside, allowing it to exit at a certain speed from the outlet. Therefore, the parameters affecting this system are the periodic movement of the wave, the dimensions of the converter, and the dimensions of the outlet. Until now, studies have been conducted on OWC energy converters, and prototypes have been developed by testing various models. However, due to the complexity of the energy conversion mechanism, commercial production has not achieved the desired efficiency from these studies. This thesis is composed of three separate articles, each written by designing new oscillating water column-type wave energy converters, calculating their efficiencies, and compiling the results into articles. In the first article, a new tank design is proposed for an oscillating water column (OWC) with an inclined wall to improve performance. Physical experiments were conducted for different orifice damping levels and sea conditions to calculate hydrodynamic performance. The results were compared with the results of a classic OWC design. According to the comparison, it was observed that the new chamber design had a significant impact on OWC efficiency. The proposed chamber design geometry increased OWC performance by up to 31% for wave parameters where the OWC operates effectively. Additionally, the results show that the optimal orifice damping level for the highest efficiency increase is dependent on the chamber design. The aerodynamic geometry of the chamber increased OWC efficiency by 54% maximum and 44% average even when there is internal slosihng in the tank. The second article focuses on optimizing the diameter of the cylindrical front wall entry of the OWC. Specifically, 240 physical experiments were conducted for five different diameter values, various orifice dampings, dimensionless wave frequencies (Kh), and wave heights. It was found that the cylindrical front wall entry geometry consistently improved OWC efficiency under all conditions. Maximum and average efficiency improvements were calculated as 45% and 25%, respectively. The simplicity of the modification at the front entrance makes this improvement even more interesting. A negative correlation was observed between the size of the diameter and Kh. Optimal sizes for the diameter were identified as 12 cm, 7 cm, and 5 cm for low, medium, and large Kh values, respectively. The diameter maximizing efficiency in a specific frequency band was determined based on wave height and orifice ratio. To validate the effectiveness of cylindrical geometry, experiments in free decay were conducted, demonstrating that modifying the front entrance mitigated or reduced flow separation caused by the sharp lower cut. Consequently, the utilization of a cylindrical front wall underlip geometry proved beneficial in reducing shear stresses on both sides of the front wall and enhancing the structural integrity of the OWC model. In the third article, hydrodynamic features evaluating the wave energy collection potential of a double-chambered OWC structure were examined through a series of experiments. The study used efficiency as a measure to quantitatively assess the performance of the OWC system by changing front wall designs, power orifice dampings, and wave conditions. The findings show that the double-chambered configuration consistently exhibited improved performance for the studied orifice ratios, opening heights, and incoming wave frequencies combinations. Increasing the opening height of the second chamber improved the performance of the OWC, as expected. Interestingly, the effect of applied orifice damping on efficiency was found to be relatively insensitive. The most significant improvements in efficiency values were observed for larger wave frequency values. For example, for dimensionless wave frequency (Kh) values of 1.43 and 1.68, efficiency increased from 0.47 to 0.72 and from 0.26 to 0.52, respectively, with the orifice ratio combination of τ1, 0.015 and τ2, 0.018. While the single-chambered OWC effectively operated in the resonance frequency range of 0.94-1.23, the double-chambered design expanded this frequency range, allowing effective wave energy collection over a wider spectrum of wave conditions. The double-chambered configuration offers significant advantages by increasing the effective frequency bandwidth for efficient energy conversion in wave energy conversion applications. Research findings have shown that this innovative design has the potential to significantly increase the efficiency and adaptability of wave energy converters. The results indicate that improvements and innovations in wave energy technologies will make it possible to use this energy source more efficiently and reliably. However, overcoming challenges at the application scale and economic barriers will require further work and investment. This thesis seeks to establish a comprehensive framework for both research and industrial applications within the wave energy field, highlighting the considerable potential of wave energy. In summary, the goal of this investigation is to enhance our understanding, make progress in wave energy technology, and play a role in shaping a sustainable energy future.