Page 84 - Fluid Power Engineering

62 Chapter Four

Betz Law in Fig. 2-8 indicated that the maximum power coefficient is
16/27. Comparison of power coefficient of this drag turbine with Betz
limit indicates that the maximum efficiency of drag-based machines
is substantially lower than the maximum theoretical efficiency. Here,
efficiency is used to mean efficiency in converting wind energy to
rotor energy. Even if a shape with highest drag coefficient is chosen—a
hemispherical cup in which air fills the cup—the value of C D is 1.3,
which makes the maximum efficiency of a drag-based device = 0.193.
Therefore, drag-based machines are rarely used because of low
efficiency. In addition, the amount of rotor area used in a drag-based
machine is, at most, one-half of area of the rotor in other configura-
tions. For these reasons, most efficient modern turbines are lift-based
that have a small component of drag and a large component of lift.

References
1. Burton, T., Sharpe, D., Jenkins, N., and Bossanyi, E. Wind Energy Handbook,
Wiley, Hoboken, NJ, 2001.
2. Talay, T. A. Introduction to Aerodynamics of Flight, NASA, Langley, 1975
(NASA SP-367).
3. Hansen, M. O. L. Aerodynamics of Wind Turbines, Earthscan, Sterling, VA,
2008.
4. Shankar, L. N. Course Notes Low Speed Aerodynamics, Georgia Institute of
Technology, Guggenheim School of Aerospace Engineering [Online] 2008.
http://www.ae.gatech.edu/people/lsankar/AE2020/.