IEEE Power & Energy Magazine - September/October 2020 - 52

Pumped storage units built today have undergone
significant technical changes since the Rocky River plant
was constructed in 1928.
pumps with a rating of not less than 112,500 gallons per minute (250 cubic feet per second) delivered
against a maximum head of 240 feet. When installed,
they were the largest in the United States. The generator was rated at 30,000 kilovolt-amperes, 24,000
kilowatts at 80 percent capacity factor, three-phase,
60-cycle, 13,900 volts, 36 poles, 200 revolutions per
minute. From the time it went into operation, it was
recognized that with some loss of efficiency, the station's two large pumps could be operated in reverse
to act as generators. In 1951, necessary modifications
were made, and the output capability of the plant was
increased to 31,000 kilowatts.

Pumped Storage Plants Worldwide
Pumped storage hydro is a common worldwide bulk power
energy storage facility in today's bulk power grids. The U.S.
Department of Energy (DOE) Energy Storage Database
reports that, worldwide, there are 352 pumped storage plants
with an installed capacity of 185 GW (see the "For Further
Reading" section).
In its "2018 Pumped Storage Report," the National
Hydropower Association (NHA) reported that there are 42
existing pumped storage projects in the United States, with
a total installed capacity on the order of 22 GW. The plants
range in size from the six-unit, 3,003-MW Bath County
plant in Virginia to the two-unit, 40-MW Lake Hodges project in California. The Lake Hodges plant began operation in
2012 and is the most recent U.S. project to enter commercial
service. To date, there are no grid-scale, doubly fed induction machine (DFIM), adjustable-speed pumped storage
units operating in the United States. There are several new
pumped storage plants in licensing and design, and some of
these will have adjustable-speed pumped units.
Japan has 38 pumped storage plants in operation, with
a total capacity of 22.6 GW. Of the 38 plants, nine have
DFIM, adjustable-speed pumped storage units. Since the
first adjustable-speed pumped storage units were installed in
Japan, there are now more than 30 adjustable-speed pumped
storage units operating worldwide.
China has 34 pumped storage plants in operation, with a
total capacity of 32 GW, and is constructing a new pumped
storage project that will be the largest in the world. When
completed, the 3,600-MW Feigning project will have two
300-MW variable-speed units.
An estimated 170 pumped storage plants, with a
total capacity of approximately 45 GW, are operating in
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Europe. Several existing plants have adjustable-speed
units, and others are in the design and construction stages.
The four-unit Goldisthal project in Germany, with two
adjustable-speed 351-MVA units with a nominal synchronous speed of 333 r/min, can operate over a speed range of
300-346.6 r/min.

The Evolution From Hydro
to Pumped Storage
Pumped storage units built today have undergone significant technical changes since the Rocky River plant was
constructed in 1928. Until 1956, pumped storage units
were constructed with motor/pumps and turbine/generators on separate shafts. A major cost factor of the first
pumped storage plants was that the turbine and generator
were on one shaft and the motor and pump on another
shaft. The technical debate asked, "Can a Francis turbine be operated as a pump turning in one direction and
then as a turbine when rotating in the other direction?"
A similar debate existed regarding a synchronous motor:
"Can a synchronous machine be operated both as a motor
and a generator?"

Hiwassee Unit 2-The First Reversible
Pump Turbine
The debate was resolved in 1956, when Allis Chalmers built
the first single-shaft reversible unit with a synchronous speed
motor/generator and pump/turbine, which was installed and
tested at the Hiwassee hydro plant in the Tennessee Valley
Authority (TVA) system. The Hiwassee Unit 2 is a reversible
pump/turbine and proves that a single runner can perform
as a pump and turbine and that a single-speed synchronous
generator can also operate as a motor.
ASME has included Unit 2 at the Hiwassee pumped storage project as project 67 in its list of dedicated mechanical engineering landmark projects. The following ASME
description summarizes the advancements that the Hiwassee
tandem unit accomplished:
The Hiwassee pump-turbine demonstrated to electric
power companies worldwide that reversible pumpturbines could be used in a pumped storage plant to
efficiently and economically store electrical energy
during periods of low demand to meet peak load demands. Prior to the installation of Hiwassee unit 2,
pumped storage plants used separate pumps and
conventional turbines for storage and generation. But
the size and reversible nature of the Hiwassee unit
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IEEE Power & Energy Magazine - September/October 2020

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