Energy Storage
Energy is stored to use it at a different time than when it
was generated.
The process of converting the energy to storable form
means that some energy is lost.
Additional energy is lost when the energy is released or
recovered.
Ideally, storage is avoided to have a more efficient process.
Renewable energy, however, is often intermittent (like wind
and sun), and storage allows use at a convenient time.
Prof. R. Shanthini
09 Feb 2013
Types of Energy Storage
Electricity can be stored by converting it into another form
such as potential, kinetic or chemical energy.
Electrical energy storage technologies include the following
types of storage media:
- Battery electric storage system (BESS)
- Flow batteries
- Fuel cells
- Flywheel energy storage (FES)
- Superconducting magnetic energy storage (SMES)
- Super capacitor energy storage (SCES)
- Compressed air energy storage (CAES)
- Pumped hydro energy storage (PHES)
Prof. R. Shanthini
09 Feb 2013
Energy and Buildings 42 (2010) 2124–2130
Pumped Hydro Energy Storage (PHES)
Electricity is produced from this 27-billion gallon reservoir using six
turbines that drive electric generators. The same turbines double as
giant water pumps to fill the reservoir with water from Lake Michigan.
Real-life example:
1872 MW generating capacity
Prof. R. Shanthini
09 Feb 2013
www.consumersenergy.com/content.aspx?id=1830
Pumped Hydro Energy Storage (PHES)
Reversible pump-turbine can spin an alternator to generate
electricity.
It also can act as a pump to pump water uphill using electricity.
This reversibility allows excess electrical energy to be used to
pump water to a higher storage reservoir to be used as an
energy source later.
PHES is the most widespread high-energy storage
technique.
PHES provides rapid response (<1 minute) to support
intermittent renewable generation.
Prof. R. Shanthini
09 Feb 2013
Pumped Hydro Energy Storage (PHES)
The energy used to pump a water volume (V) to a height (h)
with a specific pumping efficiency (ηp) is given by:
Epumping =
·g·h·V
ηp
The energy supplied to the electrical network by a generator
of efficiency (ηg) can be obtained by:
Egenerator =  · g · h · V · ηg
Overall efficiency of the energy storage system
= Egenerator / Epumping
Prof. R. Shanthini
09 Feb 2013
http://www.sc.ehu.es/sbweb/energias-renovables/temas/almacenamiento/almacenamiento.html
Pumped Hydro Energy Storage (PHES)
Prof. R. Shanthini
09 Feb 2013
http://sun.anu.edu.au/pumpedhydro
Pumped Hydro Energy Storage (PHES)
Ireland plan: A €3.45 billion project will comprise a 2GW peak power
plant, including 18 onshore wind farms, a hydro station and a grid
transmission connection.
Prof. R. Shanthini
09 Feb 2013
http://www.forumforthefuture.org/greenfutures/articles/irelands-3-billion-euro-bid-energy-independence
Pumped Hydro Energy Storage (PHES)
Hybrid wind-hydro power plant
Prof. R. Shanthini
09 Feb 2013
http://www.sc.ehu.es/sbweb/energias-renovables/temas/almacenamiento/almacenamiento.html
Compressed Air Energy Storage (CAES)
Off-peak electricity is used compressed air, which is stored.
During peak hours, compressed air is used to generate electricity.
This system operates in the same way as a conventional gas turbine,
but compression and expansion operations are independent and they
occur at different times.
Prof. R. Shanthini
09 Feb 2013
Conventional gas turbine (recalling)
fuel
compressed
air
Compressor
Combustion
Chamber
hot
gases
Gas
Turbine
Gen
fresh
air
Prof. R. Shanthini
09 Feb 2013
gases
to the
stack
Compressed Air Energy Storage (CAES)
The above shown CAES system that operates with the excess
generation of a wind farm.
Prof. R. Shanthini
09 Feb 2013
http://www.sc.ehu.es/sbweb/energias-renovables/temas/almacenamiento/almacenamiento.html
Compressed Air Energy Storage (CAES)
Air is compressed and stored in reservoirs, aquifers or
underground cavities.
The stored energy is released during periods of peak demand,
expanding the air through a turbine.
When air is compressed for storage, its temperature will
increase according to
 P2 
T2  T1 
 P1 
( k 1)
k
where T is the temperature, P is the pressure, k is the polytropic index of
the expansion, and subscripts 1 and 2 indicate the initial and final state
of compression, respectively.
Prof. R. Shanthini
09 Feb 2013
http://www.sc.ehu.es/sbweb/energias-renovables/temas/almacenamiento/almacenamiento.html
Compressed Air Energy Storage (CAES)
The heat generated can be retained in the compressed air or
in another heat storage medium.
This way, heat can be returned to the air before its expansion
in the turbine.
This method is called adiabatic storage system and achieves
a high efficiency.
However, if the heat generated in compression is lost,
additional heat must be supplied to the air by burning a fuel.
In general, for a given pressure ratio, the work of the turbine is
directly proportional to the absolute temperature of admission.
Prof. R. Shanthini
09 Feb 2013
http://www.sc.ehu.es/sbweb/energias-renovables/temas/almacenamiento/almacenamiento.html
Flywheel Energy Storage (FES)
Conventional flywheel
Prof. R. Shanthini
09 Feb 2013
Flywheel Energy Storage (FES)
Flywheels store electric power as kinetic energy of the rotor
mass spinning at very high speeds, according to
1
2
E  I 
2
where I is the inertia momentum of the flywheel and  is the
angular speed.
Prof. R. Shanthini
09 Feb 2013
http://www.sc.ehu.es/sbweb/energias-renovables/temas/almacenamiento/almacenamiento.html
Flywheel Energy Storage (FES)
The charging/discharging of the device is carried out through
an integrated electrical machine operating
- either as a motor to accelerate the rotor up to the
required high speeds by absorbing power from the
electric grid (charge mode)
- or as a generator to produce electrical power on
demand using the energy stored in the flywheel mass
by decelerating the rotor (discharge mode).
Prof. R. Shanthini
09 Feb 2013
Flywheel Energy Storage (FES)
The system has very low rotational losses due to the use
of magnetic bearings which prevent the contact between
the stationary and rotating parts, thus decreasing the
friction.
In addition, because the system operates in vacuum, the
aerodynamic resistance of the rotor is outstandingly
reduced.
These features permit the system to reach efficiencies
higher than 80%.
Prof. R. Shanthini
09 Feb 2013
Flywheel Energy Storage (FES)
Flywheels have the ability to charge and discharge rapidly,
and are almost immune to temperature fluctuations.
They take up relatively little space, have lower maintenance
requirements than batteries, and have a long life span.
Flywheel devices are relatively tolerant of abuse, i.e. the
lifetime of a flywheel system will not be shortened by a deep
discharge unlike a battery.
Prof. R. Shanthini
09 Feb 2013
Flywheel Energy Storage (FES)
The stored energy is directly proportional to the flywheel
rotor momentum and the square of the angular momentum,
a reason why increments in the rotation speed yield large
benefits on the storage energy density.
1
2
E  I 
2
Prof. R. Shanthini
09 Feb 2013
Battery Electric Storage System (BESS)
- have high energy densities
- technology is matured
- relatively easy to use
Let us consider the following four types:
- Lead-acid
- Nickel cadmium (NiCd)
- Sodium sulphur (NaS)
- Nickel metal hydride (NiMH)
- Lithium ion (Li-ion)
Prof. R. Shanthini
09 Feb 2013
Lead-acid (PbA) Batteries
Cathode: Lead peroxide (PbO2) on a lead (Pb) lattice support
Anode: Sponge lead on a lead lattice support
Electrolyte: Sulphuric acid and water
Prof. R. Shanthini
09 Feb 2013
Energy Conversion and Management 58 (2012) 134–148
Lead-acid (PbA) Batteries
Anode (oxidation):
Pb( s)  SO42 (aq)  PbSO4  2e
Cathode (reduction):
PbO2 ( s)  SO42 (aq)  4H   2e  PbSO4  2H2O(l )
Prof. R. Shanthini
09 Feb 2013
Lead-acid (PbA) Batteries
Prof. R. Shanthini
09 Feb 2013
Lead-acid (PbA) Batteries
The lead-acid battery is the oldest and most mature
technology that has been used for electrical energy
storage.
They are ideal for small-cycle renewable energy
integration applications.
They can be discharged repeatedly by as much as 80%
of their capacity and hence are suited for grid connected
systems where users sell power back to the grid through
net metering.
Prof. R. Shanthini
09 Feb 2013
Energy and Buildings 42 (2010) 2124–2130
Lead-acid (PbA) Batteries
They provide a cost-competitive and proven solution to a range
of storage requirements owing to
- low investment costs
- lowest self-discharge of all rechargeable battery
systems
- relatively ease of maintenance
Drawbacks:
- limited cycle life
- poor performance at low and high ambient temperatures
- failure due to deep and continuous cycling
- environmentally unfriendly lead content and acid electrolyte
which result in a large eco-footprint
Prof. R. Shanthini
09 Feb 2013
Energy and Buildings 42 (2010) 2124–2130
Nickel cadmium (NiCd) Batteries
Cathode:
Anode:
Electrolyte:
Prof. R. Shanthini
09 Feb 2013
Nickel oxide hydroxide (Nickel hydroxy oxide)
on a nickel foam, graphite or iron substance
Metallic cadmium pressed onto a nickel wire
mesh
Potassium hydroxide (KOH)
Energy Conversion and Management 58 (2012) 134–148
Nickel cadmium (NiCd) Batteries
Cathode and anode
are rolled up and
placed in a steel
case.
The cathode and
anode are separated
by a porous polymeric separator strip
(three thin layers that
consist of nylon/polypropylene/nylon),
with the electrolyte
free to diffuse
through it.
Prof. R. Shanthini
09 Feb 2013
Energy Conversion and Management 58 (2012) 134–148
Nickel cadmium (NiCd) Batteries
The chemical reactions during discharge are:
At the cadmium electrode:
At the nickel electrode:
The net reaction during discharge is
During recharge, the reactions go from right to left.
The alkaline electrolyte (commonly KOH) is not consumed in this
reaction and therefore its specific gravity, unlike in lead–acid
batteries, is not a guide to its state of charge.
Prof. R. Shanthini
09 Feb 2013
Nickel cadmium (NiCd) Batteries
Nickel cadmium batteries are a robust and proven alternative
to lead-acid batteries and rank alongside them in terms of
their maturity.
Longer cycle life, higher energy densities and low
maintenance requirements highlight superiority of NiCd
batteries in comparison with lead-acid batteries.
NiCd batteries offer many advantages in PV applications and
factors such as their cycling ability, durability, long life and
reliability make them ideal for operating under adverse
conditions.
Prof. R. Shanthini
09 Feb 2013
Energy and Buildings 42 (2010) 2124–2130
Nickel cadmium (NiCd) Batteries
Drawbacks:
- contain toxic heavy metals
- suffer from severe self-discharge
Prof. R. Shanthini
09 Feb 2013
Energy and Buildings 42 (2010) 2124–2130
Sodium sulphur (NaS) Batteries
Cathode: molten sulfur
Anode: molten sodium
Electrolyte: solid beta-alumina.
Beta-alumina solid electrolyte
(BASE) is a fast ion conductor
material used as a membrane in
several types of molten salt
electrochemical cell. Currently
there is no known substitute
available.
Prof. R. Shanthini
09 Feb 2013
Sodium sulphur (NaS) Batteries
During discharge, sodium ions produced at the negative
electrode are transported through the porous separator to the
liquid sulphur positive electrode, reducing some of the sulphur
to polysulphide ions.
Prof. R. Shanthini
09 Feb 2013
Sodium sulphur (NaS) Batteries
If a load is connected to terminals, electric power is
discharged through the load.
During the discharge, sodium ions converted from
sodium in a negative electrode pass through solid
electrolyte then reach to sulfur in positive electrode.
The electrons finally flow to outside circuits.
The electric power is generated by such current flow.
With the progress of the discharge, sodium polysulfide
is formed in positive electrode; on the contrary, sodium
in negative electrode will decrease by consumption.
Prof. R. Shanthini
09 Feb 2013
Sodium sulphur (NaS) Batteries
For the battery to operate effectively it is necessary to
maintain its temperature at about 300 to 350oC and this
requires the use of sophisticated methods of construction for
the battery pack, together with auxiliary heating during
charging and periods of non-use.
It is essential that the battery is not allowed to cool below
200oC, as the sodium electrolyte freezes solid at this
temperature.
Reactivation then becomes slow and difficult and the freezing
of the sodium can damage the battery because of the
mechanical stresses introduced.
Prof. R. Shanthini
09 Feb 2013
Sodium sulphur (NaS) Batteries
The sodium-sulphur battery has a high-energy density of
110 Wh/kg, a high power density of 150 W/kg and is
capable of 1000 cycles of deep discharge in use.
Drawback: Sodium polysulfides are highly corrosive.
Prof. R. Shanthini
09 Feb 2013
Sodium sulphur (NaS) Batteries
During the charge, the electric power supplied from outside
form sodium in negative electrode and sulfur in positive
electrode by following the reverse process of the discharge.
Because of this, the energy is stored in the battery.
Prof. R. Shanthini
09 Feb 2013
Nickel metal hydride (NiMH) Batteries
Cathode:
Anode:
Nickel hydroxyl oxide on a nickel foam substrate
Mischmetal (Mm) hydrides on a nickel or a nickel
plated steal mesh
Electrolyte:
Potasium hydroxide
H2O  Mm  2e  OH  (1/ 2)H2 (stored as Mm-Hx)
charged in the right direction of the equation above and
discharged in the left direction.
Mischmetals (Mm) is the name given for a group of metals
from lanthanide series (or rare earths), ytterium, titanium,
zirconium, nickel and vanadium.
Prof. R. Shanthini
09 Feb 2013
Energy Conversion and Management 58 (2012) 134–148
Nickel metal hydride (NiMH) Batteries
Used in electric vehicles
Prof. R. Shanthini
09 Feb 2013
Lithium ion (Li-ion) Batteries
Anode:
Cathode:
Electrolyte:
Graphite or lithium salt of titanium oxide (TiO)
Metal oxide such as CoO2
Lithium salt is an organic solvent
They have a large ratio of power to weight and are used in
cell phones and laptops.
They achieve energy storage efficiencies of close to 100%
Prof. R. Shanthini
09 Feb 2013
Energy and Buildings 42 (2010) 2124–2130
Lithium ion (Li-ion) Batteries
Drawbacks:
- high investment costs
- Lithium metal is very reactive and explosive
Prof. R. Shanthini
09 Feb 2013
Energy and Buildings 42 (2010) 2124–2130
Energy Storage
Prof. R. Shanthini
09 Feb 2013
http://www.sc.ehu.es/sbweb/energias-renovables/temas/almacenamiento/almacenamiento.html
Energy Storage
Prof. R. Shanthini
09 Feb 2013
http://www.sc.ehu.es/sbweb/energias-renovables/temas/almacenamiento/almacenamiento.html
Energy Storage
Prof. R. Shanthini
09 Feb 2013
http://www.greentechmedia.com/research/report/grid-scale-energy-storage-technologies-and-forecasts-through-2015