IEEE Power & Energy Magazine - September/October 2017 - 83

Microgrids have included widespread deployment of various
ESSs technologies, such as battery ESSs, flow-battery ESSs,
flywheel ESSs, and hydrogen-based fuel-cell ESSs.
dc/ac inverter, forms the power conditioning system (PCS),
which maintains the dc-link voltage and regulates the active
and reactive power injection/absorption of the BESSs.
A wide range of technologies exist for the fabrication of
the battery cell, including lead-acid, nickel-cadmium, zincair, lithium-ion, and so forth. BESSs are among the most
fully developed and used types of ESSs because they respond
quickly, are highly efficient, have no harmful emissions, and
require relatively low maintenance due to the absence of
mechanical parts. On the other hand, these technologies are
costly and have a shorter lifespan compared to other types
of ESSs, especially when used for high-cycling applications,
such as primary regulation.
Lithium-ion is among the most widely adopted battery
technologies for BESSs. These have higher cell voltages
compared with other battery technologies, thus the required
number of series cells needed to create a certain voltage level
is lower. In addition, they have a high round-trip efficiency
of approximately 95-98%. State-of-the-art commercial lithium-ion batteries maintain acceptable performance for up to
5,000 cycles; however, their cost of ~$US1/Wh ranks them
among the most expensive of ESSs technologies. Moreover,
because safety is an issue with these batteries, they require
a thermal management system and protection equipment to
prevent thermal runaways.

Flow-Battery Energy Storage Systems
Typical FBESSs consist of flow-cell stacks and at least one
pair of tanks. The tanks contain the positive and negative liquid electrolytes; two closed-loop circuits connect the tanks to
the flow-cell stacks. Within each flow cell, an ion exchange
membrane separates the positive and negative electrolytes;
electrochemical active species are dissolved in the two electrolytes. The positive and negative electrodes provide the surface area on which the electrochemical reaction takes places;
thus, energy is not stored in or around the anode and cathode
but rather in the electrolyte itself, with its volume determining the storage capacity of a flow battery. The charging and
discharging power is determined by the size and the number
of the flow cells. This decoupling of storage capacity and
charging/discharging power is unique to FBESSs and makes
these systems easily scalable.
FBESSs are expected to handle more than 10,000 charging/discharging cycles over their lifespan. For large-scale
mid- to long-term storage, the cost of a flow battery depends
on the cost of the electrolyte and is expected to reduce with
increasing production volume.
september/october 2017

The comparatively low round-trip efficiency of approximately 75-80% is one disadvantage of FBESSs. In addition,
FBESSs have relatively low energy densities; the most common system, employing vanadium in both electrolytes, yields
an energy density of approximately 20 Wh/l. However, for
stationary applications, such as those required in microgrids,
this is a minor drawback because the size of the battery is not
a critical factor.

Hydrogen-Based Fuel-Cell
Energy Storage Systems
Typical HFCESSs consist of a water electrolyzer, hydrogen storage tank, and fuel cell. In these systems, hydrogen is produced
through an electrochemical process in the electrolyzer, during
which the electric current splits a water molecule into hydrogen
and oxygen. State-of-the art electrolyzers can produce highpressure hydrogen of up to 6,000 psi; the produced hydrogen
can then be stored as gas in metal tanks, as liquid in low-temperature containers, or in metal and/or complex hydrides. However,
in small islanded or remote microgrids, hydrogen is typically
stored in above-ground gas bottles or tanks at pressures of up to
900 psi. Note that the O2 produced by the electrolysis process is
not stored because of economic and technical issues.
Electricity is generated in HFCESSs through a reverse
electrochemical reaction within the fuel cell, during which
the stored hydrogen reacts with the oxygen in the air, producing water and heat. Depending on the electrolytic material,

ESS Technologies
Mechanical

Electrochemical

Electrical

PHS

BESS

Supercapacitor

CAES

FBESS

Superconducting
Magnetic Coil

Chemical

Thermal

HFCESS

TESS

FESS

figure 2. A classification of ESS technologies. [Based
on -"Electrical Energy Storage," white paper, International
-Electrotechnical Commission (IEC), Dec. 2011.]
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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - September/October 2017