Most Wave Energy Converters (WECs) produce highly distorted power due to the reciprocal motion induced by ocean waves. Some WEC systems have integrated energy storage that overcomes this limitation, but add significant expenses to an already costly system. As an alternative approach, this article investigates the direct export option that relies on aggregate smoothing among several WECs. By optimizing the positioning of the WEC devices with respect to the incoming waves, fluctuations may be mutually canceled out between the devices. This work is based on Fred. Olsen's WEC system Lifesaver, and a WEC farm consisting of 48 devices is designed in detail and simulated. The major cost driver for the electrical export system is the required oversize factor necessary for transfer of the average power output. Due to the low power quality, this number can be as high as 20 at the entry point of the electrical system, and it is thus crucial to quickly improve the power quality 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 a significant oversize factor will be required in the intermediate power system within the farm.

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.

Abstract:The future success of wave energy in the renewable energy mix depends on the technical advancements of the specific components and systems, on the grid access availability and, ultimately, on the economical profitability of the investment. Small and remote islands represent an ideal framework for wave energy exploitation, due both to resource availability and to the current high cost of electricity that mostly relies on diesel generation. Energy storage can be the enabling technology to match the intermittent power generation from waves to the energy needs of the local community. In this paper real data from La Palma, in the Canary Islands, are used as a basis for the considered test case. As a first step the study quantifies the expected power production from Wave Energy Converter (WEC) arrays, based on data from the Lifesaver point absorber developed by Fred. Olsen. Then, a stochastic optimization approach is applied to evaluate the convenience of energy storage introduction for reducing the final cost of energy and to define the corresponding optimal rating of the storage devices.