A Constant-Pressure Hydraulic PTO System for a Wave Energy Converter Based on a Hydraulic Transformer and Multi-Chamber Cylinder

This paper presents a constant-pressure hydraulic PTO system that can convert stored pressure energy into electrical energy at a stable speed through hydraulic motors and generators. A multi-chamber cylinder can be connected to the main power generation circuit by check valves, and the motor displacement can be controlled by a fuzzy controller to maintain the main power generation circuit under stable pressure. The hydraulic transformer can control the forces applied to the floater. The hydrodynamic parameters of the floater are calculated by AQWA, and the optimal PTO damping of the hydraulic system is analyzed as the target of transformer control. MATLAB/Simulink and AMESim are used to carry out the co-simulation. Three kinds of wave elevation time-series for the specific state are designed for the simulation. In the co-simulation, three approaches are carried out for the simulation including no control strategy, fuzzy control with a fixed transformer ratio, and fuzzy control with a variable transformer ratio. Under the fuzzy control with a fixed transformer ratio, the floater displacement and captured energy do not increase significantly, but the oil pressure fluctuation is very stable, which indicates that the fuzzy controller maintains the stability of the main power circuit. While under fuzzy control with a variable transformer ratio, the power generation is not larger than those under no control strategy or fuzzy control with a fixed transformer ratio, which proves that this hydraulic transformer concept is less efficient.

Prediction, Control and Energy Analyses of Hydraulic Transformer Based Heave Compensator

Heave compensators are widely used in offshore equipments to decouple the load motion from the wave induced ship motion. Hydraulic transformer based heave compensator (HTHC) can better compensate the ship’s heave in comparison with passive heave compensators (PHC) under changing sea conditions, meanwhile the accumulators’ volume is decreased significantly. Here in this paper, some further discussions about HTHC are given on the aspects of prediction, controller design and energy saving through simulation. Results turn out that heave prediction based on sliding mode observer is better than the Luenberger observer based one; chattering can be reduced through back-stepping method. The controller is robust when the load is perturbed by uncertain forces, and energy consumption of HTHC is about 56% of the one in an equal-sized hybrid active heave compensator (HAHC) meanwhile its total installed power can be reduced by 70%.

Four-port hydraulic transformer with inlet and outlet equal flow and its flow characteristics

Without changing the original valve control hydraulic system, a kind of hydraulic transformer, called four-port hydraulic transformer (FHT), is proposed to recover the energy loss caused by system throttling. The remarkable feature of FHT is that the flow rate at inlet and outlet ports are equal. This means that FHT can be connected into the load circuit of hydraulic system to recovery energy without refreshing flow rate. This paper investigates the flow characteristics of FHT, including instantaneous flow rates, average flow rates and flow pulsations in each port. The relationships between the structure of port plate and the flow characteristics are given. The variation rule of number of plungers connected to four ports is shown, and the relationship between the variation and the flow pulsation is revealed. The simulation results show that the flow rates and displacements of symmetrical ports are same, and the instantaneous flow rate of symmetrical ports has the same rule. The results also show the frequent changes of the number of plungers connected with each port lead to more flow jump points in instantaneous curves, and the jump point is the basic cause of its loud noise. The test shows that the flow rates of measurement data of the experiment are very close to the theoretical analysis, proved the theoretical analysis of flow characteristics for FHT are appropriate and reasonable, which has a certain reference for the development and energy-saving application for FHT.