IEEE Electrification Magazine - September 2015 - 31

the cost of electricity generation by solar and wind is rapidly approaching grid parity in many regions of the world.
However, a question arises as renewable power generation on the grid increases: what supplies electric power
when the wind is not blowing or the sun is not shining? Several approaches can address this variability issue-e.g., balancing production and demand through forecasting; aggregating generation among different locations and renewable
sources, e.g., solar, wind, geothermal, and hydropower; and
ultimately, incorporating electric energy storage (EES). In this
article, we discuss some areas of energy-storage development and testing by the U.S. Department of Energy's (DOE's)
National Renewable Energy Laboratory (NREL).

Dealing with Variable Generation
In the future, energy usage will likely turn increasingly to
renewable resources and generation options that are
either no carbon or low carbon. With the sun as a source
of abundant energy, the challenge is
to reduce the cost of energy production using the solar resource and to
improve the dispatchability of the
renewable power supply to the grid.
Wind power and solar energy
technologies have developed rapidly
through the past several decades,
and the cost of capturing wind and
solar energy has decreased dramatically in recent years. However, the
generation variability of these
renewable technologies can create a
problem in large-scale applications.
Energy storage can be an important
bridge to ensure that the power supply meets demand and to allow
renewable energy to serve as a continuous, on-demand
power source.
Electricity is usually produced continuously without
electric-storage capability associated with the power
generation. Incorporating electricity storage can provide

significant benefits for high penetration of renewables
on the grid and for matching power supply and demand.
Energy storage can be used in transportation, mobile
power, and the power grid in the form of distributed generation (DG) or central utility power supply. Figure 1
shows a scenario of energy-storage technologies integrated with the electric grid that consists of renewable
and conventional power plants in both distributed and
utility-scale power generation.
Solar energy and wind power are the two largest sustainable sources of carbon-free energy. Storing the electricity from these variable-generation resources and shifting
the power supply to peak demand hours will greatly
improve the renewable generation reliability and grid stability. Discharging electricity from storage can smooth grid
fluctuations due to power production and surges in
demand. In utility-scale power generation tied to the electric grid, electricity storage reduces the amount of reserve
capacity and helps the grid energy
management. The utility-scale central storage can be achieved by bulk
or massive energy storage (MES). In
the mixed power-generation and
energy-storage scenario shown in
Figure 1, DG is any source of electricity at or near the point of load. Its generation capacity may match the load
and is small individually compared to
the centralized generation station. DG
can be connected to the utility's distribution lines or can simply provide
power to a stand-alone load. Energy
storage enables reliable DG for the
supply of uninterruptable power
locally; thus, a DG system with local
energy storage can form a stand-alone power supply without relying on the grid.
A trend in vehicle technology is to electrify transportation. The broad market penetration of electric vehicles
(EVs) using batteries will have a profound impact on grid

Energy storage
will be an enabling
technology to
allow utilities to
accommodate a
high penetration of
renewable energy.

Šistockphoto.com/karl-friedrich hohl

IEEE Electrific ation Magazine / S EP T EM BE R 2 0 1 5

31


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Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2015

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