Environmental Engineering Reference
In-Depth Information
Changing the system coniguration to one utilizing advanced structural concepts and
active aerodynamic, electronic, and electrical controls reduces loads and increases energy
capture, causing the cost of energy to decrease again. This assumes that the system is large
enough to afford such controls. Finally, as rotor size continues to increase, the square-cube
law eventually dominates, and further increases in rotor size lead to increases in the cost
of each unit of energy produced. This establishes the second minimum of energy cost .
In practice, of course, many other factors come into play. Technical risk , at today's
stage of technology, is higher in larger machines. The higher capital investment required
for manufacturing large-scale turbines represents a major commitment that is unlikely to be
made unless a very irm market and market advantage are discerned. For a given capital
investment, more small units can be fabricated, and the advantages of production tooling
and improvements based on production experience can be realized. Small- and medium-
scale turbines may also be able to use standard off-the-shelf components , such as gearboxes
or low-cost automotive brakes. A number of large-scale prototype turbines have been suc-
cessful in both European and U.S. government programs, and many private ventures have
occurred in this size range. The size of new commercial wind turbines has, been steadily
increasing over time.
Tip Speed Ratio and Solidity
Tip-speed ratio (tip speed divided by wind speed) is a key coniguration variable that
has increased over several centuries and continues to do so today, albeit at a slower rate.
The reasons are numerous. A higher tip-speed ratio reduces rotational wake losses , and
hence increases the theoretical peak power coeficient. More importantly, a higher tip-speed
ratio means a higher turbine shaft speed (for a given rotor diameter) and thus a lower
torque for a particular power output. This in turn means smaller gearbox shafts, cases,
bearings, and gears. Higher turbine shaft speed may permit fewer step-up stages in the
gearbox to reach an eficient generator speed or a smaller generator, if a direct-drive
concept is used. This can be signiicant in anything other than the smaller wind turbines.
For highest aerodynamic eficiency, tip-speed ratio and rotor solidity (total blade
planform area divided by swept area) are inversely related, so higher tip-speed ratios lead
to lower blade area and hence less blade material. This is particularly important at the
larger sizes in which the blades form a larger percentage of total system cost.
Factors that make higher tip speed ratios dificult to achieve include the added starting
dificulties associated with a low-torque rotor, particularly in moderate or low wind
conditions and the potential for acoustic noise from the tips. The starting problem can be
solved by using a generator which can motor the rotor up to operating speed. This involves
a small amount of energy consumption and added equipment cost, but motor-starting is an
attractive solution. The Darrieus VAWT, for example, does not produce torque (of any
signiicance) when stopped, and it is essentially always motored up to a rotational speed
where aerodynamic forces can take over.
Number of Blades
At very low solidities, blade dimensions become suficiently small that it becomes
dificult to design them with adequate structural strength and stiffness. For a given solidity,
dividing the total planform area amongst fewer blades maximizes the cross-sectional size
and strength of each blade. Thus, modern wind turbine rotors have almost universally either
two or three blades. Some experiments have been undertaken on one-bladed (counterbal-
anced) machines as the logical limit of this trend. One-bladed rotors appear to have peak
power coeficients only 5 percent to 10 percent below those with two or three blades. The
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