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

Doktorsavhandling, 2018

The main focus of this work is the performance and the economical assessment of a radial
flux generator that is used in wave power applications. The wave energy converter (WEC) used
in this work is a point absorber, that is considered to move only in heave. The generation unit
of the WEC consists of a permanent magnet machine and a power electronic converter.
The straight and v-shaped interior mounted permanent magnet generators, surface mounted
permanent magnet generator and neodymium and ferrite assisted synchronous reluctance generators
are selected as the main generator designs to be studied in this work. These designs are
analysed using finite element method (FEM) and the annual energy productions and losses are
quantified. Furthermore, some design variations such as, different iron materials, stator slot geometries
and a SiC MOSFET based converter are investigated, in order to assess the impact of
a specific design variation on the energy efficiency. An economical evaluation of these variants
using the life cycle cost (LCC) analysis is performed, in order to quantify the economical consequences
of the energy losses during the operational life time, as well as determining the costs
of the initial generator investment. The results obtained suggest favorableWEC generator types
and 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 production
achieved by the SMPM generator is fairly similar to that of the IPM generator, despite not being
able to provide the required torque at high speed operations, since the high speed operations
occur rarely. Moreover, it is found that the poor field weakening trajectory of the SMPM can be
improved by placing iron pieces at magnet sides. Another interesting result is that even though
the 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 variation
that improves the electric generation system of theWEC to the highest degree is found to be the
SiC MOSFET based converter design, rather than the IGBT variant. The annual energy losses
decrease by 5 MWh, due to up to 3 times lower converter losses. Owing to the substantial
energy improvement, the SiC MOSFET case is the economically favorable choice compared to
the generation system that uses an IGBT converter, despite theMOSFET modules being 7 times
more costly than its IGBT counterpart.