IEEE Power & Energy Magazine - September/October 2017 - 84

several technologies are in use, such as alkaline and solidoxide fuel cells. Operating temperature, corrosion potential,
and maintenance costs are among the decisive factors to
consider when choosing the appropriate technology.
HFCESSs offer several advantages, such as adequate dy--
namic response and no emissions, producing only water as
the by-product of the electrochemical reaction. These systems have a longer lifetime compared to BESSs, allowing
up to 20,000 charge/discharge cycles; in addition, hydrogen
can be stored for long periods. On the other hand, HFCESSs
have a considerably lower efficiency, with a net fuel-cell and
electrolyzer efficiency of around 50%.

Flywheel Energy Storage Systems
FESSs store energy as the kinetic energy of a massive cylinder, spinning up to 50,000 r/min; the cylinder is coupled with
an electric machine that acts as a motor during charge and
as a generator during discharge. Friction and wind shear are
reduced by the use of bearings and a vacuumed containment
ring. In most cases, permanent magnet electric machines are
used in FESSs, although it is also possible to use induction
machines. The frequency of the electricity produced by the
permanent magnet machines depends on the speed of rotation; thus, FESSs are connected to the grid via two back-toback ac/dc and dc/ac converters.
FESSs provide a fast, dynamic response and are ideal for
primary regulation provisions. However, they have a high selfdischarge rate, approximately 20% of the nominal capacity
per hour, which makes them inappropriate for long-term storage applications. The advantages of FESSs include long lifespan and high cycling rate, high power and energy density, and
high energy-conversion efficiency.

Thermal Energy Storage Systems
TESSs store thermal energy converted from electrical energy
during off-peak hours, which they discharge to meet the
thermal demand. There are several kinds of TESSs based on

Duration

Frequency of Operation

0.1 s
1/Month

1s

15 s

15 min

1h

8h

Fault Ridethrough and Grid-Forming Applications

1/Day
12/Day

1 min

the storage medium, placement of storage, and conversion of
electric energy. The storage medium can be liquid or solid.
The storage can be located directly in the room that is heated
or cooled or centrally located to heat/cool the whole house
or commercial building. These systems basically comprise
three components: a heating or cooling element, a storage and
heat exchanger, and an air-handling unit.
Brick-core electric thermal storage (ETS) systems are
widely used as TESSs for space heating in microgrids, where
the conversion of electric energy is carried out by heating rods
placed between high-density bricks that store thermal energy;
these bricks are in a well-insulated box to ensure a low selfdischarge. To discharge the heat from the ETS, a controlled
fan blows air through the air channels in the bricks. For different
room sizes in residential buildings, different sizes of brick-core
ETS systems (which differ in the electric power drawn and thermal storage size) are available. Standard room heating TESSs
are available in sizes ranging from 1.32 to 10.8 kW and 13.5
to 40 kWh, while central-house-heating TESSs are available
in sizes ranging from 14 to 46.5 kW and 120 to 240 kWh.
Ice-storage air conditioning systems are a type of TESSs
used for space cooling, in which the conversion of electric
energy is carried out by a condenser that condenses the refrigerant. A heat exchanger made of copper coils is placed inside
an insulated storage tank to circulate the refrigerant through
the condenser, storage tank, and evaporated coil. The condensed refrigerant produces ice inside the storage tank during
charging, while the discharge takes place via an evaporating
coil in the air-handling unit. Residential or commercial cooling TESSs are available in sizes of approximately 14 kW, with
storage capacity ranging from 70 to 105 kWh.
The charging of TESSs units can be controlled locally
by the user and/or centrally by grid operators using control
signals. The cost of TESSs ranges around US$50-150/kWh,
which is comparatively much less expensive than that of
BESSs. The efficiency of TESSs can be close to 100%.
Because of their large storage capacity and rapid response
rate, they can be used for frequency regulation, emergency
support, coordination with RESs,
and load shifting.

Energy Shifting
Primary Regulation

30/h
30/min

Power Quality

5/s

figure 3. The time duration and frequency of operation of ESSs in microgrids.
(Based on "Electrical Energy Storage," white paper, IEC, Dec. 2011.)
84	

ieee power & energy magazine	

Applications
ESSs play multiple roles in is--
landed/isolated microgrids with
high penetrations of RESs, in--
cluding ancillary services such as
frequency and voltage regulation.
In fossil-fuel-based microgrids,
ESSs can considerably reduce
fuel consumption and the runtime
of the generation assets; for RESbased microgrids, they can en--
sure continuous and stable system operation. Figure 3 shows
september/october 2017



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