IEEE Power & Energy Magazine - September/October 2017 - 82

Microgrids are considered a critical link in the evolution
from vertically integrated bulk power systems to a set of
smart decentralized distribution networks.
is bidirectional between generators and "prosumers." In particular, with decreasing RESs costs, these technologies are
becoming attractive solutions to bring energy to remote communities and/or replace expensive fossil-fuel-based generators. However, RESs such as wind and solar are intermittent
sources of energy, difficult to predict, and prone to large output fluctuations-therefore, significantly affecting system
voltage and frequency.

Introduction to Energy Storage
in Microgrids
Microgrids are considered a critical link in the evolution
from vertically integrated bulk power systems to a set of
smart decentralized distribution networks. These are defined
as a cluster of interconnected distributed energy resources
(DERs), which include energy storage systems (ESSs) and
loads, that operate as a part of the grid or in an islanded/
isolated mode. Microgrids are characterized by a high penetration of RESs, easily scalable structures, and increased
reliability. However, microgrid operation poses certain challenges, especially with high RES penetration given their
intermittent nature. In addition, most DERs in microgrids are
connected through inverters, reducing the mechanical inertia
in the system. As a result, microgrids face a problem in maintaining stable, optimal operation. In this context, ESSs are a
key enabling element for the operation of microgrids because
they compensate for generation and demand fluctuations,

while providing various ancillary services, particularly frequency control.
Prior to the 1980s, storing electrical energy was significantly costly and inefficient, and it had little market demand.
Today, ESSs are attracting considerable attention and investment, with microgrids creating a large market as high-power
and economically feasible power electronic systems have made
storing electrical energy viable both financially and technologically. Thus, the use of ESSs in various microgrid sectors has
exhibited a considerable increase; for example, their use in the
commercial and industrial sector more than tripled between
2015 and 2016, as illustrated in Figure 1. Microgrids have
included widespread deployment of various ESS technologies,
such as battery ESSs (BESSs), flow-battery ESSs (FBESSs),
flywheel ESSs (FESSs), and hydrogen-based fuel-cell ESSs
(HFCESSs). Thermal ESSs (TESSs), also a part of ESSs, represent a less expensive option than other ESS technologies and
can be controlled to meet thermal heating/cooling demand.
Because of their high functionality as well as the decreasing
costs of these technologies, ESSs are expected to become a key
component of future microgrids.
In this article, we focus on ESS technologies and applications relevant to microgrids. Thus, the types of ESS technologies typically used in microgrids are described in some
detail first, followed by several practical applications of
ESSs in isolated microgrids, which is where these systems
are most necessary.

60
Fourth Quarter 2015
Second Quarter 2016

50
40
30
20
10

Total

Direct Current

Remote

Military

Institutional

Utility Distribution

Community

0
Commercial
and Industrial

ESS Utilization in Various
Microgrid Sectors (%)

Energy Storage System Technologies

figure 1. The use of ESS globally by microgrid sectors.
(Based on "Microgrid Deployment Tracker 2016," Navigant
Research, 2016.)
82

ieee power & energy magazine

Depending on the functional, technical, and economic criteria, various ESS technologies have been developed and used
in microgrids. These technologies can be classified according to the forms of energy used, as shown in Figure 2, with
pumped-hydro storage (PHS) and compressed-air energy storage (CAES) limited to certain geographical areas and mostly
to interconnected grids. (Supercapacitors and superconducting magnetic coils are expensive and not widely used; therefore, we do not discuss these technologies here.)

Battery Energy Storage Systems
In BESSs, the electrical energy is transformed and stored
in the form of chemical energy through an electrochemical
process. Typical BESSs consist of the battery cell, a dc/dc
converter, and a dc/ac inverter. The electrochemical process
takes place within the battery cell, where the electrolyte provides a medium for the anode and cathode to exchange positive and negative ions. The dc/dc converter, along with the
september/october 2017

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - September/October 2017