Environmental Engineering Reference
In-Depth Information
estimation of pump efficiency is 55%, but with proper system design, a cen-
trifugal pump could achieve as much as 85% efficiency. The efficiency for
the pumping cycle or the turbine cycle can be optimized, but they cannot be
optimized simultaneously. For aquifer UPHES, the turbine efficiency must
be optimized. The assumed ranges of machine efficiency used in this text
are 70 to 85% for turbine operation and 65 to 80% for pump operation. These
numbers are estimates adopted from data on modern PAT pumps, centrifu-
gal pumps, and turbines.
Reaction type turbines, such as Kaplan or Francis designs, are capable
of accomplishing both pumping and turbine functions at efficiencies that
increase with unit size. Kaplan or propeller style turbines are used in low
head, high flow applications. Francis turbines and PAT designs are applied
in high head, high flow situations. Typical efficiencies for very large Francis
turbines can approach 95%.
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For smaller units, lower efficiencies in the range
of 70 to 90%, depending on head, flow, and specific speed, can be expected.
In standard Francis designs, the water enters or leaves a scroll-shaped vane
housing at a right angle to the rotation of the drive shaft. This characteris-
tic may pose a design challenge in installing such a unit in a vertical shaft
well.
Electric Motor Generator
Motor generator units that operate with relatively high efficiency represent
a mature and available technology. As with pump turbines, the efficiency
increases with size and rating. Some large motor generators can operate
above 96% efficiency. For the application in question, efficiencies between 88
and 94% may be achievable.
Well motor pumps commonly employ AC induction motors or synchro-
nous wound-rotor AC motors for larger machines. Although commercial
designs assume the unit will be used as a motor only, modifications can
produce efficient operation in generator mode In the case of a synchronous
wound-rotor AC machine, an interface to the machine's rotor windings that
provides excitation current during generating is needed. This modification
is relatively simple to implement with or without power electronics. If power
electronic control of the winding is used, the frequency, and therefore the
speed and torque, of the generator output can be regulated.
For AC induction machines, perhaps the most common and simple modi-
fication involves connecting excitation capacitors to the three-phase leads of
the machine.
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These capacitors provide excitation current that is 90 degrees
out of phase with the primary generation current waveform. This excitation
current induces currents in the rotor of the machine that allow it to operate
as a generator.
Another possible modification involves the use of power electronics to
synthesize the excitation current. The same electronics used to drive the
machine as a motor are used to control excitation while it is generating. To
