IEEE Power & Energy Magazine - September/October 2020 - 50

The introduction of hydro generation
marked the beginning of a second industrial revolution
based on electrification.
In the United States, Lefferts Tide Mill was an early example of how water could be stored at one time of day and used at
another time. The mill was constructed in 1795 and located on
the south bank of Huntington Harbor in Suffolk County, New
York. Lefferts Tide Mill stored ocean tidal energy at high
and low tides, and a water wheel converted the stored energy
to mechanical torque, which powered a flour mill operation.
While this use of tidal power does not involve electric machinery, it does illustrate that the energy storage concept was being
used to operate machinery before it was electric. Today, the
time shifting of energy from off to on peak is a basic function
of pumped storage plants.
Basic hydrodynamic principles were not understood until
the 1600s, when Leonard Euler recognized that dynamic
laws for fluids could be expressed in a simple form only if
the fluid was assumed to be incompressible. This principle
was followed by the evolution of the modern hydropower
turbine in the mid-1700s, when a French hydraulic and military engineer, Bernard Forest de Bélidor, wrote a technical series, L'architecture hydraulique. Based on this work,
three types of turbines were developed for use in hydro generation plants: impulse (Pelton), reaction (Francis), and axial
flow (Kaplan).
With contributions by James Watt and with the industrial
revolution in the mid-1800s and into the 1900s, engineers
took up the challenge of providing mechanical and electric
power to operate machines in mines, mills, and factories. In
some cases, steam power was used, and in other regions
with fast-flowing rivers, hydropower was the preferred
power source. The introduction of hydro generation marked

Upper
Reservoir
Transmission
Connection
Upper Water
Conductor

Lower
Water
Conductor

Lower
Reservoir

Powerhouse

figure 1. The pumped storage concept. (Courtesy of
Argonne National Laboratory, managed and operated
by UChicago, Argonne, LLC, for the U.S. Department of
Energy under Contract DE-AC02-O6CH11357.)
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ieee power & energy magazine

the beginning of a second industrial revolution based
on electrification.
Early applications used dc technology. The breakthrough
of ac allowed power to be transmitted longer distances
at higher voltages and ushered in early installations of
ac hydropower.
Electric power engineering technology was advanced
by engineers who grappled with the challenge of matching
generation output to meet and follow changing load demand
while maintaining voltage with dc and voltage and frequency with ac. Lurking in the background was the realization that large-scale electric energy storage was a long-term
goal worth pursuing.

Energy Storage Technologies
In the past century, hydropower has experienced several
major advances that have made hydroelectric power an integral part of the renewable energy mix throughout the world.
One of these advances was the development of single- and
adjustable-speed pumped storage hydro technology.
A list of energy storage technologies includes mechanical, electrical, electromechanical, chemical, and thermal.
Among these technologies, pumped storage hydro and
electrochemical battery storage are the dominant storage
technologies in use today. Each of these storage technologies involves a charge-and-discharge cycle that can convert
three-phase ac energy to another form of energy for storage and then reverses the process to return the stored energy
as three-phase ac electric energy. With mechanical storage,
energy is stored in the form of potential energy and then
returned as kinetic energy.
A pumped storage plant requires two bodies of water (reservoirs) separated vertically and connected by water conductors. With this configuration, sufficient hydraulic head can be
provided to effectively convert potential energy into kinetic
energy in the form of water flow that drives hydro turbines
and synchronous generators. Unlike chemical-based storage
technologies, which require the procurement of chemicals
(involving cost and environmental consequences), pumped
storage hydro uses water and gravity, which are available,
plentiful, and convenient (Figure 1).
In the charge mode, a pump driven by a synchronous
motor converts surplus three-phase alternating power,
whenever it is available, to pump water from a lower reservoir
to an upper one where the water, with its potential energy, is
stored until it is required in the discharge mode. In the discharge mode, water stored in an upper reservoir has potential
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IEEE Power & Energy Magazine - September/October 2020

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