Abstract:Considering the full dynamics of the different conversion stages from ocean waves to the electricity grid is essential to evaluate the realistic power flow in the drive train and design accurate model-based control formulations. The power take-off system for wave energy converters (WECs) is one of the essential parts of wave-to-wire (W2W) models, for which hydraulic transmissions are a robust solution and offer the flexibility to design specific drive-trains for specific energy absorption requirements of different WECs. The potential hydraulic drive train topologies can be classified into two main configuration groups (constant-pressure and variable-pressure configurations), each of which uses specific components and has a particular impact on the preceding and following stages of the drive train. The present paper describes the models for both configurations, including the main nonlinear dynamics, losses and constraints. Results from the mathematical model simulations are compared against experimental results obtained from two independent test rigs, which represent the two main configurations, and high-fidelity software. Special attention is paid to the impact of friction in the hydraulic cylinder and flow and torque losses in the hydraulic motor. Results demonstrate the effectiveness of the models in reproducing experimental results, capturing friction effects and showing similar losses.

Digital hydraulics is a key part of the continuing applicability of fluid power in the modern world. In order to realize the potential of digital hydraulic circuits, valves which are able to switch at high frequencies whilst retaining high flows are first required. This paper details the development of a valve which is capable of switching in 0.5ms whilst providing a flow rate of over 50L/min at a 10bar pressure drop. Unlike most of the other valves currently in development, position control is used as opposed to the more common bang-bang actuation. This has obvious benefits for valve robustness and offers the possibility of a hybrid control approach which utilizes both throttling and switching control. This paper will detail the design and empirical testing of the valve and benchmark it against published and commercial valves before proceeding to discuss the challenges present in developing the valve further.

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Oscillating surge wave energy converters (OSWECs) offer the possibility to convert ocean wave energy in the near shore region into electricity. One of the unique challenges they present is that the forces experienced in each direction differ. The curved geometry of the CCell device exaggerates this difference. Presented in this paper is a novel method of dealing with this difference which does not rely on high buoyancy within the paddle but, instead, variations in the control signals. This reduces the likelihood and severity of end stop collisions , resulting in improved survivability and reduced lifetime costs without increasing the volume and cost of the device. The algorithm presented may further be used to compensate for individual manufacturing tolerances or deterioration to OSWECs whilst in operation.

Piezo pumps provide an attractive alternative for driving small actuators (e.g. less than 100W) compared to traditional valve controlled cylinders powered by a central hydraulic supply. This provides the ability to distribute power electrically rather than hydraulically, which can bring both weight and efficiency savings. Currently the use of piezo pumps is severely limited by the maximum power and flows that can be provided. This paper documents the simulation of a new pump which makes use of disc type reed valves to rectify the flow generate by a single piezostack-driven piston. The proposed valves have the potential to overcome frequency limitations of more conventional poppet or ball type check valves. This enables the pump to operate at higher frequencies and thereby produce larger flows. Simulation results suggest that a pump capable of producing a no load flow in excess of 1L/min would be possible using an off-the-shelf piezo stack.