Energy storage systems notes module 3 ktu

Module III
Overview on Energy storage technologies

Electrochemical energy storage
• Primary batteries
• Secondary batteries
• Fuel cells
• Primary and secondary batteries utilise the chemical
components built into them, whereas fuel cells have
chemically bound energy supplied from the outside in the
form of synthetic fuel.
• Unlike secondary batteries, primary batteries cannot be
recharged when the built-in active chemicals have been
used, and therefore strictly they cannot be considered as
genuine energy storage.

Battery
• The battery has two circuits, one external and the other
internal.
• The internal circuit is comprised of the battery cell itself,
and provides the path through which the resultant ions
flow.
• The electrical circuit is closed by adding the external
circuit, thus providing the path through which the
electrons resulting from the oxidation reduction
reactions can flow.
• This external path is provided by the external system
(either a load or an energy source) to which the battery
is connected.

Battery
• The battery comprises of the following components:
– The electrodes
– Two pairs of electrochemically active substances
– The electrolyte
– The separator.
– The container

Battery
• Open circuit voltage V0 - The electric potential derived from
the chemical reactions by the two pairs of electrochemically
active substances, measured in the full charge state of the
cell when disconnected from any circuit.
• The open-circuit voltage is different from the voltage
measured in the cell during the discharging process, as
commonly presented in manufacturers’ datasheets.

Battery
• The discharge voltage
profile of the cell
depends on many
factors, such as
pressure and
temperature.
• The cell voltage and
the discharge current
rate are related and
determine the energy
capacity.

Battery parameters
• The energy capacity of batteries is defined as the usable
energy at a defined discharge rate(unit is Ah).
• The battery C rating is the measurement of current
which the battery is charged/discharged at. The
capacity of a battery is generally rated at 1C.
• Eg: A 10 Ah battery rated at 2C will give 20 A for 30 min
A 10 Ah battery rated at 0.2Cor C/5 will give 2 A for 5
hours
A 10 Ah battery rated at 0.5Cor C/2 will give 5 A for 2
hours

Battery parameters
• The ratio between the remaining energy capacity and
the rated capacity of the battery (at a defined discharge
rate) determines the state of charge (SoC) of the system.
• The energy efficiency can be formulated in terms of the
cell voltage and the theoretical maximum voltage
produced in the chemical reaction in the cell; the so-
called thermodynamic voltage.
• While charging, the energy efficiency in the cell is
defined as μc = V/Vth
• While discharging, the energy efficiency in the cell is defined
as μd = Vth/V
where Vth is the thermodynamic voltage and V is
the voltage between the cell terminals.

Battery parameters
• The discharging efficiency increases with the SoC.
• Likewise, the charging efficiency decays with the SoC.
• Therefore, to maximize the overall efficiency throughout
a full charge and discharge loop, an optimal average
SoC should be calculated.
• This duty is usually assigned to battery management
systems(BMS).

Battery parameters
• The depth of discharge(DoD) is defined as the maximum
fraction or percentage of a battery's capacity which can
be removed from the charged battery on a regular basis.
• Eg: A DoD of 80% means 80% charge can be discharged
during normal operation.
• DoD can also be used to express the currently drained
charge of the battery. Here DoD is the complement of
SoC: as one increases, the other decreases.

Battery parameters
• The specific energy capacity of a battery is
where n is the number of electrons transferred in a chemical reaction,
F is the Faraday constant in coulombs per mole, Vth is the theoretical
thermodynamic voltage of the cell, and ΣMi is the sum of the
molecular weights of the reactants of the cell.
• To optimize the specific energy, we must maximize Vth and
minimize ΣMi.
• The former is obtained by building up the two pairs of
electrochemically active substances with a highly
electropositive element and a highly electronegative element.
When that is done, the chemical reactions will yield a high
thermodynamic potential Vth.
• The latter is achieved by configuring cells based on reactants
with low atomic weights.

Battery parameters
• The specific power determines the maximum power that
the cell can deliver in relation to its weight.
• It depends on the open-circuit voltage of the cell, V0,
internal resistance Rint and the ohmic resistance of
conductors, Rc.
• This variable internal resistance, usually called the
overpotential, models a voltage drop in the cell and
depends on the amount of current drawn.

Lead-acid battery
• The lead-acid battery is the oldest chemical storage
device.
• The main drawbacks of these are comparatively low
energy density, long charge time and the need for careful
maintenance.
• It consists of alternate pairs of
plates, one lead and the other lead
coated with lead dioxide, immersed
in a dilute solution of sulphuric acid
that serves as an electrolyte.
• During discharge, both electrodes
are converted into lead sulphate.
• Charging restores the electrodes.

Lead-acid battery
• The performance of such a battery deteriorates gradually
because of irreversible physical changes in the electrodes;
ultimately, failure occurs between several hundred and
2000 cycles, depending on battery design and duty cycle.
• The reaction at the electrodes is given

Alkaline electrolyte batteries -
Ni-Zn battery
• The Ni-Zn battery is considered as a possible medium-
term EV system.
• The Ni-Zn cell’s main drawbacks are its short life cycle,
separator stability, temperature control, high cost and
mass production problems.
• Its short life cycle is the result of the high solubility of
the reaction products at the zinc electrodes.

Ni-Fe battery
• The Ni-Fe battery is an alkaline storage battery using
KOH as the electrolyte.
• The main drawback of Ni-Fe batteries for electric vehicle
applications is their low energy density.
• The battery has a low peaking capability and other
drawbacks are low cell voltage and the low hydrogen
overvoltage of the iron electrode, which results in self-
discharge and low cell efficiency.

Li-ion battery
• Lithium is the lightest of all metals, has the greatest
electrochemical potential and provides the largest specific
energy per weight.
• The Li-S battery comprises liquid lithium, sulphur
electrodes and electrolyte of molten LiCl–KCl eutectic at an
operating temperature in the 380–450oC range.
• Highly corrosive liquid lithium attacks the ceramic
insulators and separators and shortens the cell’s life.
• Efficiency is not very high because of self-discharge caused
by lithium dissolving in the molten LiCl–KCl electrolyte.

Li-ion battery
• The use of lithium–aluminium alloys and iron sulphide
as electrodes has led to the development of more
efficient Li-S cells with reasonable energy densities.
• The reactions in the cell can be written as follows:
or

Li-ion battery
• The term lithium-ion is the name of batteries that share
similarities, but have different chemical fundamentals.
– Lithium Cobalt Oxide
– Lithium Manganese Oxide
– Lithium Nickel Manganese
– Lithium Nickel Cobalt Aluminium Oxide
– Lithium Iron Phosphate
– Lithium Titanate

The Flow Battery Energy Storage
System
• Flow batteries differ from conventional ones in the fact
that the electrolyte is not permanently stored in the cells
but, instead, two aqueous electrolytic solutions (A and
B) are contained in separate tanks.
• During the charging process, these aqueous solutions
are pumped through the electrochemical cells, where
the electrochemically active material dissolved in
electrolyte A is oxidized at the anode, and the
electrochemically active material in electrolyte B is
reduced at the cathode.
• The discharge cycle comprises the reverse process.

The Vanadium Redox Battery
• Both the anode and cathode of the cells are composed of
catalyzed graphite.
• The aqueous electrolyte for both the anodic and the
cathodic regions is based on sulfuric acid, in which
vanadium sulfates are dissolved as active chemical
species.
• The anodic and cathodic regions are separated by a
polymer membrane, which permits ion exchange
between the two electrodes.

The Zinc–Bromine Battery
• The electrodes of ZBB cells are based on a carbon–
plastic composite.
• The separator between the anodic and cathodic regions
is made up of polyolefin sheets.
• The electrolyte is aqueous, containing dissolved zinc
bromide salts.
• Zn is the active chemical species at the anode (the
negative electrode), while Br is located at the cathode
(the positive electrode).

The Polysulfide–Bromide Flow
Battery
• In PSBs, the electrodes are based on a carbon–plastic
composite.
• The separator between the anodic and cathodic regions
is made up of polyolefin sheets.
• The aqueous electrolytes are based on sodium
polysulfide Na2Sy in the anodic region, and sodium
bromide NaBr in the cathodic one.

Flow Battery
• Flow batteries are easily scalable, since the volume of
the stored electrolyte determines the energy capacity of
the system.
• The power capacity depends on the number and size of
the electrochemical cells.
• They are suitable for storing energy over long periods of
time due to their very low self-discharge.
• Flow batteries can be fully discharged without any
damage, and in terms of cyclability, they present better
characteristics than conventional batteries.
• They require very low maintenance.

Fuel cells
• A fuel cell is an electrochemical cell that can
continuously change the chemical energy of a fuel and
oxidant to electrical energy with high efficiency.
• Fuel cells are comprised of two electrodes and an
electrolyte, which enables ion exchange between them.
• The anodic and cathodic regions are separated by a
polymeric membrane.
• To keep the reactions that create electricity going, these
cells need a constant supply of fuel and an oxidizing
agent.
• The types of electrolyte are diverse and determine the
performance of the cell; for example, the pressure of the
hydrogen produced and the operating temperature.

Fuel cells
• Electrolytes can be liquid or solid.
• Conventional electrolyzers use liquid alkaline
electrolytes, while modern ones use solid electrolytes.
The latter type is known as the proton exchange
membrane fuel cell (PEMFC).
• The synthetic fuels such as hydrogen, methanol,
ammonia and methane are used.

Fuel cells
• Fuel cells are distinguished from other secondary
batteries by their external fuel store.
• The mobile positive ions in the electrolyte may be metal
ions, produced by the metal anode supplying electrons
to the external circuit in the battery, while in a fuel cell
they may be Hþ (H3Oþ), produced by the hydrogen
supplied to the anode.
• The EMF of such a cell is E – (RT /nF) ln(a1/a2) – Nernst
eqn.
– where n is the number of electrons needed to get one atom or
molecule of X into its ionic form in the electrolyte, and a1 and a2
are the activities at electrodes 1 and 2.

Fuel cells
• The essential functions of a fuel cell are
1. the charging (or electrolyser) function in which the
chemical AB is electrochemically decomposed to A and B;
2. the storage function in which A and B are held apart
3. the discharge (or fuel-cell) function in which A and B are
reunited, with the simultaneous generation of electricity.
• In secondary batteries, the electrolyser and fuel-cell
functions are combined within the same cell. This is a
convenient arrangement but is not essential.
• Fuel cells are attractive because of their high energy
density, lack of pollution and high cell efficiency.
• A good hydrogen or oxygen conductor has to be used as
the electrolyte.

Superconducting Magnetic Energy
Storage (SMES)
• The energy is stored in a magnetic field created by a DC
current flowing through a superconducting coil at
cryogenic temperatures.
• Superconductor materials present almost negligible
resistance while at cryogenic temperatures, so the
magnetic field in the coil can be created and maintained
with a very small amount of current flowing through it;
very little energy is dissipated by ohmic losses.
• The energy stored is determined by the self-inductance
of the coil L and the square of the electric current I.

Storage (SMES)
• There are two types of SMES systems, depending on the
working temperature of the coil: SMES systems based
on high-temperature coils (HTS) and low-temperature
coils (LTS).
• The former work at temperatures around 70 K, while the
latter work at temperatures around 5 K.
• Due to the very low energy consumption of the system’s
cryocoolers, the energy efficiency of SMES systems is
very high, at around 90%.

Superconducting coils
• Technical superconductors are normally made of NbTi
or Nb3Sn multi-filaments embedded in a copper or
aluminium stabilisation matrix.
• At present, Nb3Ti superconductors are mainly used for
reasons of ease of manufacture.
• If the superconductor
becomes normally
conducting, the
current transfers to
the stabilisation
matrix, thus avoiding
destruction of the
superconductor by
overheating.

Superconducting coils
• Since ferromagnetic materials do not apply to inductions
higher than 3T, coils for SMES are usually placed in air
or vacuum with μ = 1.
• To increase the magnetic energy stored, Wm with a given
current I, which is limited by the applied superconductor,
the total self-inductance L of the storage has to be made
as high as possible by choosing a suitable coil geometry.
• There are three concepts of SMES design:
– circular-shaped single solenoid
– series connection of coaxial solenoids
– circular, oval or D-shaped torus comprising series-connected single
coils.

Cryogenic systems
• The cryogenic system of an SMES device comprises a
refrigerator, where a coolant has to be prepared, and a
cryostat-storage vessel in which a superconducting coil is
placed to be refrigerated and thermally isolated from the
environment.
• The cooling system is usually based on liquid helium as
coolant, either in a helium bath or by forced circulation.
• It ensures that the superconductor temperature does not
exceed its critical value.
• Coil refrigeration and thermal insulation are extremely
difficult technical problems, since low temperatures
(around 1.8 K) are needed to enhance the superconductor’s
current-carrying ability.

Power extraction
• A superconducting coil is a source of variable direct
current.
• To couple this source to a constant voltage AC power
system, a special power transformation system is
required.
• By adjusting the thyristor’s delay angle, smooth and
rapid change of charge or discharge rate becomes
possible within one cycle of the power system frequency.
• A typical SMES configuration comprises two 6-pulse
thyristor Graetz bridges series-connected to the
superconducting coil on the DC part of the bridge and
coupled through an AC transformer to a power system
on the AC side of the bridge.

Storage (SMES)
• An attractive feature of such a PTS is its high efficiency:
losses attributable to the solid-state bridge conversion
are estimated to be between 3% and 8% of the total
stored energy.
• When the phase delay angle p is less than 90, the bridge
operates in rectifier mode and acts as a load for the AC
power system.
• If p is set above 90, the average convertor voltage
becomes negative and the bridge becomes a kind of
power source for the AC system. So it operates in
inverter mode.

Storage (SMES)
• The major advantage of SMES systems is related to their
ability to inject or absorb vast amounts of energy in a
very short time.
• The cyclability of the system is very high, at up to 105
cycles at 100% of DoD.
• The use of SMES devices is limited to short-time storage
applications, as the self-discharge rates of the system
are relatively high, in the range of 10–15% of the rated
energy capacity per hour.
• It becomes completely discharged in a very short time,
discharging at full load in a matter of minutes or
seconds.

The Supercapacitor Energy Storage
System
• Supercapacitors are based on electrochemical cells that
contain two conductor electrodes, an electrolyte and a
porous membrane that permits the transit of ions
between the two electrodes.
• The main difference between supercapacitors and
batteries lies in the fact that no chemical reactions occur
in the cells, but the energy is stored electrostatically in
the cell.
• In supercapacitors, the electrodes and the electrolyte are
electrically charged. By applying a voltage between the
electrodes, both the electrodes and the electrolyte
become polarized.

System

System
• “Electrical double layers” are formed at the electrode
surfaces.
• The mechanism behind the operating principle of such a
double layer can be explained using the Helmholtz
model.
• The model establishes that the two layers are separated
by a layer of solvent molecules of the electrolyte.
• This layer of solvent molecules actually separates the
positive and negative charges of the electrode and
electrolyte, thus acting as a dielectric.
• Hence the double layer can be taken to resemble a
capacitor.

System
• The energy stored in the supercapacitor is
• The voltage generated in the cell is dependent on the
strength of the electric field between the layers building
up each of the “electrical double layers”.
• This electric field is proportional to the amounts of
positive and negative ions located at the electrode
/electrolyte interface.
• To avoid transfer of ions between the two layers of
positive and negative ions, thus decreasing the voltage
within the double layers, the breakdown voltage of the
dielectric should be maximized.

System
• The selection of the electrolyte is key to ensuring the
maximum energy capacity.
• Both aqueous and organic electrolytes are commonly
found, the latter being the most common type.
• The second factor affecting the energy capacity of
supercapacitors is the capacitance of the cell.
• The capacitance is given by
where 𝜀 is the dielectric constant, 𝜀0 is the permittivity of a vacuum,
A is the effective area of the surface of the electrode, and d is the
dielectric thickness.

System
• In order to maximize the capacitance, different metal-
oxide electrodes, electronically conducting polymer
electrodes and activated carbon electrodes, are used.
• These materials are porous, so as to maximize the
effective area of the electrode in which ions can be
allocated.
• The most common types are the ones based on activated
carbon, since they can lead to supercapacitors with a
high energy density and capacitances around 5000 F.
• The cell represented can be considered to resemble two
capacitors in series.

System
• With reference to the supercapacitor dynamics, one
defining parameter is the so-called charge/discharge
time constant, 𝜏.
• This is given by the product of the equivalent series
resistance (ESR) of the supercapacitor and its
capacitance.
𝜏 = RC
• The ESR weights the losses in the supercapacitor while
charging and discharging.
• The third characteristic parameter for the supercapacitor
is the leakage resistance, which weights the self-
discharge of the cell.
• This resistance is much higher than the ESR.

System
• Supercapacitors are characterized by offering high ramp
power rates, high cyclability, high efficiency and a high
specific power and power density. (10 times more than
for conventional batteries).
• The latter characteristic defines supercapacitors as well
suited for applications that impose major volumetric
restrictions.
• Major drawbacks of the technology are related to its
high self discharge rates and its limited applicability to
situations where high power and energy are needed.
• As a short-timescale ESS, supercapacitors are
unsuitable in that they are expensive in comparison
with other competitors such as flywheels.

Energy storage systems notes module 3 ktu

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