High-efficient Wave Energy Converter: Multi-Degree-of-Freedom with Hybrid Power Take-Off Systems

Wave energy is a promising source of renewable energy due to its high energy density, predictability, and wide availability. Wave Energy Converters (WECs) are devices designed to harness the kinetic and potential energy of ocean waves and convert it into usable forms of energy, typically electricity. They are a type of renewable energy technology that taps into the vast energy resources present in the world’s oceans. The basic idea behind WECs is to capture the movement of waves and convert it into mechanical motion, which is then transformed into electrical power through generators However, the efficiency and cost-effectiveness of WECs have posed challenges, limiting their maturation compared to wind and solar energy technologies. One avenue for improving WEC efficiency is the exploration of multi-degree-of-freedom (multi-DOF) systems with hybrid power take-off (PTO) mechanisms. To this note and in a new study published in the peer-reviewed Journal Energy Conversion and Management, where Associate Professor Xingxian Bao, Fumiao Li from China University of Petroleum, Huihui Sun from Qingdao Marine Science and Technology Center, and Professor Hongda Shi from Ocean University of China collaborated with Professor Gregorio Iglesias from University College Cork presented a novel multi-DOF wave energy converter (PUWEC) with hybrid PTO systems.

Unlike traditional single-DOF WECs that rely on capturing energy from either pitch or heave motion, the PUWEC introduces a groundbreaking approach to enhance wave energy conversion. The PUWEC’s design incorporates prismatic pairs in the hydraulic cylinder and universal pairs in the gear transmission system. These PTO systems are matched to the buoy’s motion in heave and pitch, respectively, resulting in improved power capture. The innovative concept revolves around the simultaneous capture of kinetic and potential energy of the floating body through multi-DOF motion.

To analyze the PUWEC’s performance, the authors developed a numerical simulation framework based on ANSYS-AQWA, a comprehensive software suite for marine dynamic analysis. This framework enables the dynamic analysis of floating bodies in various wave conditions, taking into account nonlinear hydrostatic and Froude-Krylov forces. The secondary development of ANSYS-AQWA allows for the incorporation of the PUWEC’s unique multi-DOF behavior and hybrid PTO systems. Validation of the numerical model was achieved through model experiments in a wave tank, affirming its predictive capabilities for the dynamic behavior of the PUWEC.

The research team conducted a series of parametric studies to explore the effects of P-PTO and U-PTO parameters on the captured power of single-DOF PWEC and UWEC under different wave conditions. The results yielded optimal damping coefficients for both P-PTO and U-PTO systems. Importantly, the PUWEC demonstrated a clear advantage in terms of captured power compared to single-DOF systems. The authors found that the pitch motion is the primary driver of energy capture in the PUWEC, contributing significantly to its improved efficiency.

The authors further investigated the trajectory analysis of the float’s center of gravity, shedding light on the intricate relationship between multi-DOF motion and power capture. The trajectory coefficient was introduced as a metric to evaluate the energy capture characteristics of the PUWEC. Their findings demonstrated that a higher trajectory coefficient corresponds to better wave energy absorption, providing insights into optimizing multi-DOF WEC design. Additionally, the working space analysis of the PUWEC indicated that the actual performance of the device was well-aligned with the design expectations, reinforcing its feasibility and potential for real-world application.

Xingxian Bao and colleagues’ comprehensive analysis of the proposed multi-DOF wave energy converter with hybrid PTO systems showcases a promising solution for enhancing wave energy conversion efficiency. According to first and corresponding author, Dr. Xingxian Bao, the PUWEC’s innovative design, coupled with the advanced numerical simulation framework, offers valuable insights into the complex dynamics of multi-DOF motion and its impact on power capture. The study findings emphasize the significance of optimizing PTO parameters and trajectory characteristics to achieve maximum wave energy absorption. Ultimately, the PUWEC holds the potential to drive advancements in wave energy technology and contribute to the global pursuit of sustainable energy sources.