Page 286 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

Marine and Hydrokinetic Power Generation and Power Plants 273

FIGURE 11.6 Verdant Power’s axial-flow turbine CEC, similar in design to today’s commercial-scale wind
turbines, being deployed in the East River near New York City [25]. (Photo from Verdant Power, NREL 17209,
New York, NY.)

11.2.2 CEC Technologies
Technologies that extract energy from ocean, tidal, and river currents are collectively called CECs.
Tidal currents are generally driven by the Earth’s rotation, the relative positions of the moon and the
sun to the Earth, and local bathymetry, and they consist of multiple constituents with varying peri-
ods. In most places, the dominant constituent is the principal lunar semidiurnal, which has a period
of 12 h and 25.2 min. The term ocean current refers to the wind-driven surface ocean current. The
Gulf Stream (in the North Atlantic Ocean) and Kuroshio (in the North Pacific Ocean) are the two
largest ocean currents.
Today, these technologies are significantly more mature than WECs because their designs draw
on decades of research and development experience from the wind energy and shipbuilding indus-
tries. Accordingly, most CECs currently under development resemble wind turbines that have been
adapted to operate in the ocean environment and marine propellers that have been modified to har-
ness energy. The common types of CECs include (1) conventional axial-flow turbines, similar to
today’s commercial-scale wind turbines, and (2) cross-flow turbines, similar to many small-scale
wind turbines. Figures 11.6 and 11.7 show the two prototype CEC designs currently being developed
by the industry. The performance characteristics of CECs are well understood: it can be shown math-
ematically that the maximum theoretical efficiency of a CEC in an unrestricted flow is 59% [24];
and efficiencies of 50% have been achieved by prototype devices operating in real-world conditions.

11.3 ELECTRICAL GENERATION
In this section, the electrical generation of MHK is presented. The focus is on system integration and
connecting the MHK generator to the grid. Different types of generators that can be used for MHK
generators are listed.