A Review on Power Electronic Topologies and Control for Wave Energy Converters

Ocean energy systems (OESs) convert the kinetic, potential, and thermal energy from oceans and seas to electricity. These systems are broadly classified into tidal, wave, thermal, and current marine systems. If fully utilized, the OESs can supply the planet with the required electricity demand as they are capable of generating approximately 2 TW of energy. The wave energy converter (WEC) systems capture the kinetic and potential energy in the waves using suitable mechanical energy capturers such as turbines and paddles. The energy density in the ocean waves is in the range of tens of kilowatts per square meter, which makes them a very attractive energy source due to the high predictability and low variability when compared with other renewable sources. Because the final objective of any renewable energy source (RES), including the WECs, is to produce electricity, the energy capturer of the WEC systems is coupled with an electrical generator, which is controlled then by power electronic converters to generate the electrical power and inject the output current into the utility AC grid. The power electronic converters used in other RESs such as photovoltaics and wind systems have been progressing significantly in the last decade, which improved the energy harvesting process, which can benefit the WECs. In this context, this paper reviews the main power converter architectures used in the present WEC systems to aid in the development of these systems and provide a useful background for researchers in this area

Performance Evaluation and Life Cycle Cost Analysis of the Electrical Generation Unit of a Wave Energy Converter

The main focus of this work is the performance and the economical assessment of a radialflux generator that is used in wave power applications. The wave energy converter (WEC) usedin this work is a point absorber, that is considered to move only in heave. The generation unitof the WEC consists of a permanent magnet machine and a power electronic converter.The straight and v-shaped interior mounted permanent magnet generators, surface mountedpermanent magnet generator and neodymium and ferrite assisted synchronous reluctance generatorsare selected as the main generator designs to be studied in this work. These designs areanalysed using finite element method (FEM) and the annual energy productions and losses arequantified. Furthermore, some design variations such as, different iron materials, stator slot geometriesand a SiC MOSFET based converter are investigated, in order to assess the impact ofa specific design variation on the energy efficiency. An economical evaluation of these variantsusing the life cycle cost (LCC) analysis is performed, in order to quantify the economical consequencesof the energy losses during the operational life time, as well as determining the costsof the initial generator investment. The results obtained suggest favorableWEC generator typesand design alterations for LCC improvements.An important finding is that the PM assisted SRM generator provides the best energy performance,given the same geometry and material limitations. The annual energy productionachieved by the SMPM generator is fairly similar to that of the IPM generator, despite not beingable to provide the required torque at high speed operations, since the high speed operationsoccur rarely. Moreover, it is found that the poor field weakening trajectory of the SMPM can beimproved by placing iron pieces at magnet sides. Another interesting result is that even thoughthe annual energy production is increased when the rotor material is replaced by a cobalt-iron,due to its high costs, this design was not found economically favorable. The design variationthat improves the electric generation system of theWEC to the highest degree is found to be theSiC MOSFET based converter design, rather than the IGBT variant. The annual energy lossesdecrease by 5 MWh, due to up to 3 times lower converter losses. Owing to the substantialenergy improvement, the SiC MOSFET case is the economically favorable choice compared tothe generation system that uses an IGBT converter, despite theMOSFET modules being 7 timesmore costly than its IGBT counterpart