This design is used for comparison as the radial-flux PM machines are well suited to both flooded and conventional architectures. Both of these generators are assumed to be permanent magnet (PM) radial-flux direct-drive generators, unless otherwise stated. The qualitative comparison is based on the pros and cons of each generator type, followed by their structural differences. This chapter gives a qualitative comparison between a conventional airgap and a flooded generator for tidal stream turbines. We discuss some design aspects of the flooded generators in this chapter. Compared to the conventional airgap generators, the design of flooded generators has been little addressed in literature. ![]() This will be illustrated later in the chapter. To begin with in a flooded generator a stator and a rotor can (or sleeve) are required to protect the active machine parts from corrosion. As a result, the design of a flooded generator has to be different from the airgap generator. In a flooded generator, instead of an airgap the stator-rotor gap is filled with the seawater. One possible way of minimizing the O&M expenses and improving the capacity factor is to use a flooded (or wetgap) generator rather than the sealed airgap generator. Likely reduction in per unit LCoE for 20 and 50% reduction in cost component (Source: ). Consequently, the focus of this chapter will be also on HATTs. As is evident, most of them prefer horizontal axis tidal turbines. Some of the main tidal energy developers are listed in Table 1. This is primarily because of their higher technology readiness level (TRL), and similarity to commercial wind turbines. Although vertical axis tidal turbines and other topologies such as oscillating hydrofoil, enclosed tip turbine and tidal kites are also used to harness tidal energy, most of the research and development efforts are focused on HATTs see Figure 3. Ī typical tidal stream turbine is shown in Figure 2, which is an example of a horizontal axis tidal turbine (HATT). Furthermore, the potential for tidal stream turbines is expected to be more than the tidal range technology. ![]() The commercial success of wind turbines is to a large extent responsible for this shift in harnessing tidal energy. Recently tidal stream turbines have become a preferable mode of harvesting tidal energy over tidal range (or dams) technology. The focus of the chapter is, however, to give a basic insight into the design aspects of the flooded generators, and compares it with the currently used sealed airgap generators in tidal turbine systems. The chapter begins with a brief description of the generator systems used in current tidal stream turbines. This architecture has the potential to improve cooling and reduce reliance on ancillary systems (e.g., bilge system), thereby improving reliability. ![]() Inside flooded generators, the gap between the stator and rotor is filled with the seawater. A possible way of minimizing the LCoE and improving the availability is to use a flooded (or a wetgap) generator rather than a conventional airgap generator. A major reason for this is the high operation and maintenance costs for submerged installations. The main issue for low utilization of tidal energy is the high levelized cost of energy (LCoE) from tidal stream turbines. Recently, tidal stream turbines have become a preferable mode of harvesting tidal energy.
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