Environmental Engineering Reference
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grid technology. An example is flexible AC transmission systems (FACTS):
electronic-based equipment to control AC transmission system parameters to increase
power-transfer capability. A key component of FACTS is technology to stabilize the
voltage, to reduce the presence of reactive power on the lines which exists when the
current and voltage are not in phase. Volt-ampere reactive (var) is a unit used to measure
reactive power in an AC power system, and reactive power compensation, or what is
called static var compensation (SVC), is used to regulate and stabilize grid voltage. The
stabilization function of SVCs is critical to prevent voltage breakdown, particularly when
new kinds of electricity generation such as wind are added to the transmission system.
Substation automation applications for voltage control, synchronism, load and bus transfer,
load curtailment, and fault detection are also sometimes considered to be important aspects
of smart grid technology.
3.3.3 Storage Technologies for Smart Grid
The capacity to store electricity so that generation does not have to always meet real-time
demand for electricity has huge potential for reducing the costs of satisfying peak demand
and allowing variable renewable energy to contribute more to meeting baseload demand.
Because the wind does not always blow and the sun does not always shine, integrating
some kinds of electricity storage into electricity systems seems to be a necessary
component of transitioning to a renewable-based system. Enhancing storage capacity is
particularly important when integrating more renewable sources, because peak wind does
generally not coincide with peak demand. While some energy storage technologies have
been around for decades, such as the widely used pumped hydro storage, other forms
of electricity storage such as compressed air storage, batteries, or flywheels have only
been applied in electricity systems in a few specific instances. These different storage
technologies harness either potential or kinetic energy to enable use of the energy at a later
time.
Currently the U.S. electricity system only has capacity to store about 2.3 percent of total
electricity production capacity, which is about 24.6GW (DoE
2013
). Most of this storage
(~95 percent) is from pumped storage hydro, while the combination of other storage
technologies, including compressed air storage (CAES), thermal energy storage, batteries,
and flywheel, make up the other 5 percent. Other regions, including Europe and Japan,
have larger storage capacity in their electricity systems. Different storage technologies are
more appropriate for different applications, and the benefits offered by storage are varied.
Among the different services that storage provides are balancing out the time between
energy generation and electricity use (loads), providing adequate reserves, damping the
variability of wind and solar PV, reduction in new upgrade and new capacity build, and
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