Rico Hjerm Hansen

Design of a Magnetic Lead Screw for Wave Energy Conversion

This paper deals with the development of a magnetic lead screw (MLS) for wave energy conversion. Initially, a brief state of the art regarding linear permanent-magnet generators and MLSs is given, leading to an introduction of the MLS and a presentation of the results from a finite-element analysis used to find the magnetic forces. Furthermore, the force per magnet surface area measure is presented as a better alternative to the force density measure, which is often used for linear magnetic devices. Based on this, the overall design of a 500-kN MLS is presented focusing on the bearing supports used to compensate for the magnetic attraction forces and the resulting deflection of the rotor. In addition, in order to avoid some of the assembling-related disadvantages of using surface-mounted magnets, an embedded-magnet topology is proposed. To demonstrate the technology, a scaled 17-kN MLS is presented together with experimental results.

Control Performance Assessment and Design of Optimal Control to Harvest Ocean Energy

Different approaches exist to design the optimal control strategy to harvest ocean energy with a point absorber. However, the control paradigm changes if the efficiency of the power takeoff (PTO) is taken into account. The motivation of this paper is twofold. The first objective is to develop a framework that includes the PTO efficiency to determine the optimal control strategy. The second objective is to assess the performance of any control strategy given the PTO efficiency. The performance assessment is based on the upper bound of the deliverable electrical energy of the optimal control strategy. As an example, different PTO efficiencies were considered for a given point absorber model, and extensive simulation results show the annual electrical energy delivered to the grid by the optimal control strategy. The performance of a nonoptimal control strategy is assessed with respect to the optimal control strategy.

Analysis of discrete pressure level systems for Wave Energy Converters

Within the research field of harvesting the energy of ocean waves, fluid power has been identified as a crucial technology in the Power Take-Off (PTO) design, due to the high torque densities required in Wave Energy Converters (WECs). The PTO is the technology converting the captured wave motion into electricity. However, conventional fluid power systems are characterized by offering poor efficiencies, rendering current PTO designs inefficient. This paper investigates the feasibility of a fluid power system based on implementing the force control of hydraulic cylinders by switching between a few fixed system pressures. The proposed design is optimized at multiple levels, as evaluating the feasibility of a solution highly depends on finding the optimum trade-off between e.g. harvested wave energy and losses in the PTO system.

Modeling and Control of a Teletruck Using Electronic Load Sensing

In mobile hydraulic application the actuating fluid power system is most commonly controlled using a hydro-mechanical control scheme called Hydraulic Load Sensing (HLS). However, with the demands for increased efficiency and controllability the HLS solutions are reaching their limits. Motivated by availability of electronic controllable fluid power components and the potential of increased dynamic performance and efficiency, this paper investigates how HLS can be replaced with electronic control, i.e. Electronic Load Sensing (ELS). The investigation is performed by taking a specific application, a teletruck, and replace the HLS control with ELS. To aid the controller design for the ELS system, a complete model of the teletruck’s articulated arm and fluid power system is developed. To show the feasibility, a preliminary control structure for the ELS system is developed. The controller is tested on the machine, validating that features such as pump pressure control, flow sharing and over pressure protection can be implemented using ELS and with improved energy efficiency.© 2010 ASME

Simulation of Utilisation of Pressure Propagation for Increased Efficiency of Secondary Controlled Discrete Displacement Cylinders

A key component of upcoming secondary controlled fluid-power systems for e.g. wave energy is the implementation of discrete force control of cylinders by discrete variation of the cylinder displacement. However, as the discrete control is implemented by shifting between fixed system pressures in multiple cylinder chambers using on/off valves, the energy efficiency of the performed shifts is essential for the total system efficiency. However, pressure shifting on a volume, where the dynamics of pressure propagation in the pipelines is negligible have been proved to have an unavoidable minimum loss due to the compressibility of the fluid. This paper performs a simulation study, showing that an improved energy efficient shift may be implemented by utilising the pressure propagation in the line between valve and cylinder chamber.

Control of a 420 kN Discrete Displacement Cylinder Drive for the Wavestar Wave Energy Converter

To improve the power production of their 1 MW wave energy converter, Wavestar is developing a new transmission based on discrete hydraulics. The discrete hydraulic system allows all cylinders to supply a common accumulator storage while maintaining low-loss individual force control of the 20 absorbers. The system is implemented using multi-chambered cylinders, where the different chambers may be switched between three pressure lines using a manifold with fast on/off valves. Resultantly, a Discrete Displacement Cylinder (DDC) is obtained, where force control is implemented by shifting between different area/pressure combinations. Currently, a 420 kN DDC prototype has been implemented and tested at the newly commissioned full size wave energy testbench at Aalborg University. The initial design and control of the DDC had poorly damped switching transients. These issues treated in this paper. This leads to a new control, which gives a smooth operating DDC, while meeting the requirements to the efficiency of the drive.Copyright

Influence and Utilisation of Pressure Propagation in Pipelines for Secondary Controlled Discrete Displacement Cylinders

Efficient discrete force control of cylinders may be realised by having multi-chambered cylinders, where the pressure of the chambers are shifted between fixed pressure levels of a secondary controlled system. However, the pressure shifting on a volume where the dynamics of pressure propagation is negligible have been be proven to have an unavoidable minimum loss due to the compressibility of the fluid. This paper investigates the effect of the pressure propagation in the connection and concludes that except more complex shifting schemes are introduced, the minimum loss remains unchanged. The paper analysis however also demonstrates and suggests, that with a clever valve control and shifting scheme, increased efficiency of a shift is obtainable by utilising the pipeline inductance.