IEEE Power & Energy Magazine - September/October 2017 - 86

One core application of ESSs is enabling an islanded/isolated
microgrid to operate with no fossil-fuel-based generation,
providing voltage and frequency for the system.
outages. For example, following 2012's Superstorm Sandy in
the northeastern United States, diesel-based backup generators
could not operate because all gas stations were inoperable; hence,
more than 1 GW of New Jersey's installed PV was unusable due
to the absence of a voltage source. In this case, microgrids with
grid-forming ESSs could have prevented the outage of PV units,
providing continuous backup for critical loads.

Voltage and Frequency Regulation
ESSs can effectively compensate for the rapid output fluctuations in RESs, mitigating the negative impact on system frequency and enhancing stability. Thus, frequency regulation is
a critical task of ESSs in islanded/isolated microgrids with a
high share of RESs because, in such systems, PV units or wind
turbines may experience large output fluctuations (up to 80%
of their nominal power) in a few seconds, which can cause
rapid frequency changes. For example, on 22 December 2005,
the Danish Bornholm Island distribution system was isolated
due to a failure in the high-voltage transmission cables. During that period, local regulators were not able to compensate
for fast variations of wind generator output power; as a result,
the whole distribution system was forced to shut down. High
ramping-rate ESSs such as FESSs, BESSs, and FBESSs could
have prevented the problem. Specifically, FBESSs are particularly suitable for such applications, due to their high cycling
rate and overload capacity. Also, TESSs, which are commercially available as grid-interactive ETS, could be used in this
case, as they can measure and respond to changes in the grid
frequency by activating an appropriate number of heating elements to cope with rapid changes in RES generation.
ESSs can also participate in voltage regulation and reactive power compensation in islanded/isolated microgrids. In
industrial and commercial islanded microgrids, there might
be large reactive power consumers such as industrial-sized
induction motors and/or electric furnaces. In such systems,
quickly responsive ESSs can considerably improve the power
quality, effectively compensating for reactive power variations by regulating the system voltage.
Size and control techniques for ESSs should be carefully
chosen to ensure an acceptable power quality and guarantee
system stability. Thus, optimization techniques are usually
used to minimize the investment, operation, and maintenance costs, considering security and reliability constraints.

Energy Shifting
ESSs can be utilized to store surplus energy and use it
when needed. Such service is incentivized for various
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reasons, depending on the microgrid operating mode and
DER types. In off-grid systems fully supplied by RESs,
energy shifting is a critical application that ensures continuous operation. For such systems, careful advance planning
is required to determine the optimal capacity of the ESSs
based on historical data of solar irradiance, wind profiles, and
electric loads.
In islanded/isolated microgrids that include fossil-fuel-based
generation, ESSs can be used to reduce the consumption of
fuel and increase the efficiency of fuel-based generators. Thus,
in such systems, appropriate optimization models can determine the optimal contribution of DERs, including ESSs, to
supply demand at minimum operating cost. In stand-alone diesel-based microgrids, generators often operate inefficiently,
some even suffering from low-load operation that causes wetstacking and breakdowns. In this case, ESSs can perform
peak load shaving and valley filling to increase the efficiency
of the generator and reduce operating and maintenance
costs. For larger microgrids in which electricity market principles apply, peak-shaving and valley-filling services may
be provided by ESSs to store energy during off-peak hours,
when the electricity price is low, and inject it during periods
of high electricity demand and price.
In all these energy-shifting scenarios, ESSs are expected
to have adequate capacity to supply the nominal load for a
few hours, depending on the load and RES supply profiles.
Well-suited ESS technologies for these applications include
BESSs, FBESSs, HFCESSs, and TESSs (note that TESSs can
only provide peak shaving services because they absorb electrical energy to transform it for thermal energy injections).

Long-Term Storage
In islanded/isolated microgrids with large seasonal variations in generation and/or demand, ESSs with the capacity
to store and supply energy for a few months can significantly
reduce the operating cost of otherwise oversized generation
units. However, ESS technologies with such energy capacity are limited to PHS and HFCESSs with large hydrogen
tanks; PHS is further limited to certain geographical areas
with a large water reservoir close by, while transportation
and maintenance costs could be an issue with large hydrogen
tanks, especially in remote areas.

Case Studies
Aboriginal Affairs and Northern Development Canada reports
that there are approximately 200,000 Canadians living in
280 northern and remote off-grid communities (N&RCs)
september/october 2017

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