An Integrated Piston-Variable Digital-Controlled Hydraulic Cylinder for Pendulum Wave Energy Converter

A pendulum wave energy converter (WEC) is one of the most important forms of wave energy harvesting devices. To obtain better performance under broader wave conditions, this article proposes an integrated piston-variable digital-controlled hydraulic cylinder (IPDC). A traditionally used single large cylinder is replaced by several pairs of subsidiary cylinders to compose a more compact and extendable integrated type. The working area of the IPDC can be regulated in real time using high-speed <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on–off</small> valves to change power take-off (PTO) damping and achieve maximum energy extraction in the pendulum WEC. In performance experiments, the average mechanical efficiency is around 95% for all area combinations, and the leakage is relatively lower. The results indicate that the IPDC has a lower extra energy loss compared with a conventional single hydraulic cylinder. Based on the theoretical analysis for maximum power extraction, AMESim–Simulink co-simulation and prototype experiments are conducted to verify the feasibility and effectiveness of the IPDC. The IPDC can significantly improve the total captured energy by 37.04% compared with the fixed area cylinder in the experiment. The results illustrate that the IPDC exhibits excellent harvesting performance by effectively changing the connected area to impose compatible PTO resistance under broader wave conditions.