Environmental Engineering Reference
In-Depth Information
Wind Flow Augmentation
Another category of innovative concepts encompasses methods of augmenting (accele-
rating) the wind stream. For example, a diverging exit cone or diffuser extending downwind
from the rotor plane can augment the mass low into and through the turbine. To prevent
separated low the angle of divergence in a conventional diffuser must usually be rather
shallow, which would make the exit cone very heavy and expensive. Research at Grumman
Aerospace Corporation [Foreman and Gilbert 1979, Loefler 1981] showed that placing
annular slots in the cone allowed external air to energize the boundary layer and delay sepa-
ration. This could lead to a shorter exit cone with an augmentation factor ( i.e. , maximum
augmented power divided by the maximum power of an unducted rotor of the same size)
of two to three. Even with these favorable aerodynamic results, the cost of the exit cone
and its supporting structure has been estimated to exceed that of using a larger conventional
rotor to obtain the additional power.
An attractive alternative, conceived at the University of Delft (Netherlands), is the use
of tip vane dynamic inducers to provide the same low effect as a diffuser. Considerable
research on dynamic inducers was also undertaken in the U.S. by AeroVironment, Inc.
[Lissaman et al. 1980]. Performance was found to be extremely sensitive to tip-vane
operating parameters, and this has prevented signiicant augmentation over a practical
operating range, at least to-date.
Atmospheric Energy Conversion
Several laboratory-scale research efforts examined methods of extracting the latent heat
of vaporization existing in the humidity in the atmosphere (the same energy source that
causes thunderstorms) and the possibility of tapping the energy of tornado-like vortices .
It turned out that these would require structures several thousands of meters high to achieve
reasonable eficiency. Cost and institutional issues precluded further consideration.
Another “tower-type” energy concept that is re-discovered at intervals utilizes
atmospheric convection currents, either natural or augmented. The most-notable attempt
in recent times was the construction of a large tower in Spain by a German-Spanish team.
Several acres of black Mylar plastic sheet were suspended a meter above the ground around
the tower, forming a solar “oven” to heat the air below. Convection forces then drew the
heated air to the base of the tower and through an internal turbine, exhausting upward
through the tower. The predictable result of such experiments is a inding that the system
has a low eficiency and a high capital cost.
A more esoteric scheme utilized electrostatic generators incorporating charged screens
which could extract kinetic energy from the wind by electric ields operating on minute
charged particles in the air. In theory, that could lead to a “no-moving-parts” wind power
device. In experiments at the University of Dayton and at Marks Polarized, Inc. power was
actually extracted, but not enough to justify further research and development.
Unconventional HAWT Subsystems
An interesting innovative concept developed with private funds was the 50.3-m
diameter Schachle-Bendix HAWT shown in Figure 3-19, sponsored by the Southern Cali-
fornia Edison Company and tested at its Devers site near Palm Springs [Rybak 1981]. One
of its innovative features was a tripod truss tower rotating on a track, there being no yaw
drive at the nacelle. This approach was similar to that of some early French HAWTs and
the pioneering Russian Balaclava turbine of the 1930s (Fig. 1-21). A second unusual fea-
ture was the use of a variable-speed hydraulic drive in the power train. Fourteen ixed-
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