Page 287 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

274 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
®

FIGURE 11.7 A cross-flow turbine developed by Ocean Renewable Power Company being prepared for an
open-water test [26]. (Photo from Ocean Renewable Power Company, NREL 24507, Portland, ME.)

11.3.1 MHK Generators

Because the wind power industry has accumulated many years of experience, many types of genera-
tors used for wind power generation are adaptable to MHK generators.
A major difference between MHK generation and wind generation is the nature of the energy
source. In addition to the salinity of the ocean, the specific mass density of water is 1000 times that
of the air. The normal wind speed range for a wind turbine generator is between 4 and 25 m/s; on the
other hand, the speed of the water in the current turbine is very low, between 0.5 and 3 m/s. Thus,
the design and material used to build the hardware of the MHK generation must be carefully chosen
to withstand corrosion, mechanical loads, and mechanical stresses. The electrical generator in wind
generation mostly converts rotational motion into electrical energy, whereas in MHK generation,
some linear motion is converted into electrical energy. However, from the perspective of electri-
cal machine design and control, the linear generator and rotational generators have very similar
principles—for example, the higher the rotational or translational speeds, the smaller the dimension
of the generator.
Four types of generators are commonly used in wind generation and are being studied for MHK
power generation.

11.3.1.1 Fixed-Speed Induction Generator: Type 1 MHK Generator
This type of generator is very simple, rugged, and requires no maintenance. It is directly connected
to the grid, and it operates within a small range of slip (~1%) variation, so the rotational speed
is practically constant. For 60 Hz grid frequency, this generator operates at 1800 revolutions per
minute (rpm); thus, it requires a gearbox to reduce the speed to match the turbine speed for wind
turbine, tidal turbine, and ocean current turbine applications (see Figure 11.8).
In addition, by its nature, it requires reactive power compensation (accomplished by installing
parallel capacitor banks) to deliver unity power factor to the grid. An example application for this
generator can be found in an installed pilot wave energy plant of the OWC type at Vizhinjam, on the
west coast of India [27].