Exploring the Potential for Increased Production from the Wave Energy Converter Lifesaver by Reactive Control

Fred Olsen is currently testing their latest wave energy converter (WEC), Lifesaver, outside of Falmouth Bay in England, preparing it for commercial operation at the Wavehub test site. Previous studies, mostly focusing on hydrodynamics and peak to average power reduction, have shown that this device has potential for increased power extraction using reactive control. This article extends those analyses, adding a detailed model of the all-electric power take-off (PTO) system, consisting of a permanent magnet synchronous generator, inverter and DC-link. Time domain simulations are performed to evaluate the PTO capabilities of the modeled WEC. However, when tuned towards reactive control, the generator losses become large, giving a very low overall system efficiency. Optimal control with respect to electrical output power is found to occur with low added mass, and when compared to pure passive loading, a 1% increase in annual energy production is estimated. The main factor reducing the effect of reactive control is found to be the minimum load-force constraint of the device. These results suggest that the Lifesaver has limited potential for increased production by reactive control. This analysis is nevertheless valuable, as it demonstrates how a wave-to-wire model can be used for investigation of PTO potential, annual energy production estimations and evaluations of different control techniques for a given WEC device

Power Collection from Wave Energy Farms

Most Wave Energy Converters (WECs) produce highly distorted power due to thereciprocal motion induced by ocean waves. Some WEC systems have integrated energystorage that overcomes this limitation, but add significant expenses to an already costlysystem. As an alternative approach, this article investigates the direct export option thatrelies on aggregate smoothing among several WECs. By optimizing the positioning of theWEC devices with respect to the incoming waves, fluctuations may be mutually canceledout between the devices. This work is based on Fred. Olsen’s WEC system Lifesaver, anda WEC farm consisting of 48 devices is designed in detail and simulated. The major costdriver for the electrical export system is the required oversize factor necessary for transferof the average power output. Due to the low power quality, this number can be as high as20 at the entry point of the electrical system, and it is thus crucial to quickly improve thepower quality so that the downstream power system is efficiently utilized. The simulationsundertaken in this work indicate that a high quality power output can be achieved at the farmlevel, but that a significant oversize factor will be required in the intermediate power systemwithin the farm

Marine renewable energy conversion: Grid and off-grid modeling, design and operation

The global energy production from renewable sources is increasing, with high penetration of both wind and solar in key regions. Ocean Wave Energy is projected to contribute with an increasing share of the future power supply, and the focus of this work is to investigate the requirements for connecting wave energy to the power grid, in context of the Fred. Olsen (FO) Wave Energy Project. Most Wave Energy Converters (WECs) produce highly distorted power due to the reciprocal motion induced by the ocean waves. Some WEC systems have integrated energy storage that overcomes this limitation, but adds significant expenses. As an alternative approach, this work investigates direct power export that relies on aggregate smoothing among several WECs. By optimizing the position of the WEC devices with respect to the incoming waves, fluctuations may be mutually canceled out between the devices. FO has closely monitored the global development within wave energy for about two decades, and has worked actively on developing WECs since 2002. The latest WEC system, named Lifesaver, has been in operation since April 2012 and is the basis of this thesis work. The Lifesaver system is described in detail, and comprehensive data on operational performance is presented. The major cost driver for grid integration is the peak to average power ratio, which can be as high as 20 in the early power conversion stages. Thus, it is crucial to improve the power quality early in the conversion chain so that the downstream power system is efficiently utilized. The simulations undertaken in this work indicate that a high quality power output can be achieved at the farm level, but that significant oversize factors will be required in the intermediate power systems within the farm. Cost-benefit analysis of the system show that a grid connected system at the current technology level will return marginal profitability. Therefore, several alternative approaches are investigated that could serve as a bridge towards future large scale systems. This includes autonomous systems that could supply power to remote ocean based units such as measurement and surveillance buoys, aquaculture facilities and support systems for the off-shore oil and gas industry. In general, the findings show that the WEC system is well suited for grid integration, although it becomes clear that significant development remains before wave energy can become an important supplement in the energy mix. Moreover, there seems to be a market for autonomous systems that is economically viable at the current technology level that could allow for immediate deployment of commercial systems

Study and development of Solid State based Long Pulse Klystron Modulators for future Linear Accelerators at CERN

A new Klystron modulator is to be developed as a part of the new Linear Accelerator (LINAC4) project that is currently running at CERN. The Klystron modulator is required to supply a pulsed output voltage of -100 kV / 20 A with a repetition rate of 2 Hz and a pulse length of 800 us. In addition to this, the Klystron modulator must also handle arcing in the Klystron, and allow for no more than 10 J of energy to be dissipated in the arc in such a case. This thesis studies possible solid state based topologies that could be relevant for the Klystron modulator. A single switch topology, based on a 12 kV IGCT switch and a pulse transformer, is studied in detail and developed as a full scale prototype. Preliminary test results indicate that this will provide a satisfactory solution that meets the requirements. A second topology based on the Parallel Resonant Converter (PRC) was studied in detail through analysis and simulations. This showed to be a promising solution that could be an improvement compared to the single switch topology. The PRC is fully controllable and thus offers a flexible solution that can meet various demands. The topology also showed very good arc handling capabilities, and the PRC can be configured to protect the Klystron by its natural response

Power Collection from Wave Energy Farms

Most Wave Energy Converters (WECs) produce highly distorted power due to thereciprocal motion induced by ocean waves. Some WEC systems have integrated energystorage that overcomes this limitation, but add significant expenses to an already costlysystem. As an alternative approach, this article investigates the direct export option thatrelies on aggregate smoothing among several WECs. By optimizing the positioning of theWEC devices with respect to the incoming waves, fluctuations may be mutually canceledout between the devices. This work is based on Fred. Olsen’s WEC system Lifesaver, anda WEC farm consisting of 48 devices is designed in detail and simulated. The major costdriver for the electrical export system is the required oversize factor necessary for transferof the average power output. Due to the low power quality, this number can be as high as20 at the entry point of the electrical system, and it is thus crucial to quickly improve thepower quality so that the downstream power system is efficiently utilized. The simulationsundertaken in this work indicate that a high quality power output can be achieved at the farmlevel, but that a significant oversize factor will be required in the intermediate power systemwithin the farm

Analysis of the Power Extraction Capability for the Wave Energy Converter BOLT®

AbstractThe goal of this paper is to investigate the power extraction capability for the Wave Energy Converter (WEC) concept BOLT®. Specifically, the impact of different control strategies on the power extraction and their high sensitivity to the incoming waves will be presented. The BOLT® concept is based on a flat point absorber designed with a small mass and a Power Take-Off (PTO) solely controlling the amplitude of the WEC's motion, passive loading. It is reported that the small weight of the device makes passive loading suitable for most sea states. As the device is still on the pilot stage, there is room for exploring the potential improvement of the power extraction for different sea states by determining the optimal control strategy for sinusoidal waves of different amplitudes and frequencies. In addition, when considering realistic designs of PTOs, the constraint on the peak power should be taken into account. The power handled by the electro-mechanical system is limited by the ratings of the electrical components and the mechanical force limits. Imposing a constraint on the peak power will greatly affect the control strategies’ impact and thus the average extracted power

Stochastic Rating of Storage Systems in Isolated Networks with Increasing Wave Energy Penetration

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