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Energy Storage
References
1-ENERGY STORAGE FOR POWER SYSTEMS by Ter-Gazarian
2-Energy storage by Prof. Dr. Robert A. Huggins
3-THERMAL ENERGY STORAGE by ˙Ibrahim Dinc¸er and Marc A. Rosen
1.1 Introduction
In addition to the inevitable depletion of the fossil fuels that are now the major sources of energy, and
the relatively smaller current alternatives, there is another matter that is very important in considering
the effective use of the energy that is available. This is the relationship between the several types of
energy supplies and the various uses of energy. Worldwide energy consumption is between 500 and 600
EJ (5–6 _ 1020 J). In terms of consumption rate, this is 15–18 TW (1.5–1.8 _ 1013 W). The United States
consumes about 25% of the total, although its share of the World’s population is about 5%. A recent
estimate of the major United States sources of energy is shown in Table 1.1. The current distribution of
energy use, by major category, is indicated in Table 1.2. These different types of applications have
different requirements for access to energy, and different characteristics of its use. One of the
important problems with the effective use of available energy supplies is that the schedule of energy use
is often not synchronous with its acquisition, even from natural sources. Thus, buffer, or storage systems
are necessary. This requirement for storage mechanisms is highly dependent upon the type of use.
Those that use fossil fuels, or their derivatives, for combustion purposes, such as for space heating or
internal combustion-powered automobiles, require one type of storage and distribution system.
Another, quite different, category involves the various applications that acquire their energy from the
large-scale electric power transmission and distribution (T&D) grid. In that case, there are two types of
storage systems to consider. One has to do with the electric power grid system itself, and the time
dependence of its energy supplies and demands, and the other has to do with storage mechanisms
applicable to the various systems and devices that acquire their energy from the grid.
Two important physical media for storage and regulation of thermal energy are
water and hydrogen. Table 8.1 lists some thermo physical properties of water and
hydrogen, including the heats of formation of water in liquid and vapor states.
Their material and thermo physical properties make them unique in not only
storing and transforming energy but also regulation of the temperature of the earth.
Hydrogen represents a store of potential energy which can be released by nuclear
fusion in the sun in the form of light. Solar energy may be stored as chemical
energy by photosynthesis after it strikes the earth. Also, when water evaporates
from oceans and is deposited high above sea level, it drives turbine/generators to
produce electricity, hence sunlight may again be stored as gravitational potential
energy.
1. Water
Approximately 80% of the earth’s surface is water. Water has a high specific
heat capacity of 4.185 kJ/kg _C, which is much higher than that of soil and rock
(see Table 8.1). Water can retain and store a large amount of heat per volume
Figure 1: water cycle
making the oceans the world’s
massive temperature regulator. Therefore, the
temperature of water changes more slowly than that of the earth’s surface. In
addition, water has large heat of vaporization of around 2,257 kJ/kg at 20_C.
This means that a large amount of latent heat can be transferred over long
distances in vapor form and released after condensing as rain (see Fig.1).
Accumulation of rain at high elevation reservoirs stores of large amounts of
potential energy, which drives turbines to produce electricity. The oceans control
the earths’ energy balance as they absorb nearly four times more insolation than
the earth.
Liquid water also helps to control the amount of carbon dioxide in the atmosphere
by dissolving it into the form of ‘‘carbonic acid’’ (soda water). Some of the
dissolved carbon dioxide combines with minerals in the water and settles to the
ocean floor to form limestone. Water is also essential for all life-forms as most
animals and plants contain more than 60% of water by volume, which helps
organisms to regulate their body temperature. Therefore, water regulates the
temperature of the earth as well as the temperature of living systems.
2.Hydrogen
Hydrogen is a versatile storage medium due to its conversion to useful work in a
fuel cell to produce electricity as well as its usage in an internal combustion
engine. Hydrogen is also being developed as an electrical power storage medium.
It must first be manufactured by other energy sources and may become a significant
energy storage medium in using renewable energies. Hydrogen can be
manufactured by steam reforming of natural gas. When manufactured by this
method it is a derivative fuel like gasoline. When produced by electrolysis of
water, it is a form of chemical energy storage. About 50 kWh (180 MJ) of solar
energy is required to produce 1 kg of hydrogen. At an electricity cost of $0.09/
kWh, hydrogen costs around $4.50/kg. Large quantities of gaseous hydrogen can
be stored in underground caverns and in depleted oil or gas fields for many years,
and can function as multipurpose energy storage.
Types of Energy Storage
The method of energy storage may be chosen on the basis of stability, ease of transport, energy capture,
and release. The main energy storage technologies involve the storage of energy in thermal, electrical,
chemical, and mechanical forms:
• Thermal. Thermal energy can be stored by sensible and latent heats at temperatures
above or below the ambient temperatures. Heat is also stored for short or long-term applications.
Thermal energy from the sun, for example, can be captured by solar collectors and stored in a reservoir
for daily or seasonal use. Thermal energy for cooling can be stored in ice.
• Electrical. Dams can be used to store hydroelectricity by accumulating large
amounts of water at high altitudes. The water then turns a turbine and generates
electricity. Also by using pumped-storage hydroelectricity stores energy by
pumping water back into the reservoir. A capacitor can store electric charge as a
part of electrical energy production system. Batteries can also store electric
energy to be used in portable electrical devices.
• Chemical. Stable chemical compounds such as fossil fuels store chemical
energy. Biological systems can store energy in chemical bonds of energy-rich
Thermal Energy Storage
Thermal energy storage technologies balance the energy demand and energy production. For example,
the solar energy would be available during the night time or in the winter season if it is stored
previously. Also a stored and transformed energy can be used somewhere else different from the
location where it is captured or produced. The thermal storage medium may be maintained at a
temperature above (hotter) or below (colder) that of the ambient temperature of 25_C. In hot climates,
the primary applications of thermal energy storage are cold storage because of large electricity demand
for air conditioning. Thermal energy storage may be planned for short term or seasonal. Some
advantages of utilizing thermal energy storage are :
• Reduced energy consumptions and carbon footprint.
• Reduced initial equipment and maintenance costs.
• Reduced pollutant emissions such as CO2.
• Increased flexibility of operation, efficiency, and effectiveness of equipment
utilization.
• Process application in portable and rechargeable way at the required
temperature.
• Isothermal and higher storage capacity per unit weight.
• Energy from any source (thermal or electrical) when needed.
Thermal energy is generally stored in the form of sensible heat and latent heat.
Figure 8.2 compares the characteristics of heat storage by sensible and latent heats.
Temperature keeps increasing as a material absorbs and store sensible heat until it
reaches a temperature where a phase changing occurs. Under their melting points,
the solid—liquid phase changing materials store sensible heat and their temperature
rises as they absorb the heat (see Fig. 8.2a). When phase changing materials reach their melting
temperature they absorb large amounts of latent heat at an almost constant temperature until melting
is completed. If the outside heat supply continues, then the melted material starts to store sensible
heat. A material may freeze at a temperature lower than its actual freezing temperature. This is called
subcooling. Also, a material may melt at a temperature higher than its actual melting temperature. This
is called superheating. Subcooling and superheating of a phase changing material create an interval for
temperature control (see Fig. 8.2b). This temperature interval between the subcooling and superheating
may be reduced by adding nucleating agents to phase changing material. Stored heat is usually
transferred to a heat transfer fluid such as water or air in a heat exchanger system as seen in Fig. 8.3.
When air is used as heat transfer fluid, the cold inside room air extracts the heat as it flows through a
heat storage system and the warm air out of the storage is fed directly into the room.
Night time low-cost electricity may be used to store energy as sensible heat or/ and latent The stored
energy is then used for the cooling/heating needs during the peak demand hours. Thermal energy
storage systems can be designed using
There are three basic methods for storing thermal energy:
Methods of Thermal Energy Storage
1. Heating a liquid or a solid, without changing phase: This method is called sensible heat storage. The
amount of energy stored depends on the temperature change of the material and can be expressed in
the form
𝑇2
E= m ∫𝑇1 𝐶𝑝𝑑𝑇
where m is the mass and Cp; the specific heat at constant pressure. T1 and T2 represent the lower and
upper temperature levels between which the storage operates. The difference (T2 – T1) is referred to as
the temperature swing.
2. Heating a material, which undergoes a phase change (usually melting): This is called latent heat
storage. The amount of energy stored (E) in this case depends upon the mass (m) and latent heat of
fusion (λ) of the material. Thus,
E=mλ
The storage operates isothermally at the melting point of the material. If isothermal operation at the
phase change temperature is difficult, the system operates over a range of temperatures T1 to T2 that
includes the melting point. The sensible heat contributions have to be considered and the amount of
energy stored is given by
where Cps and Cpl represents the specific heats of the solid and liquid phases and T* is the
melting point.
Sensible Heat Storage
cooling a liquid or a solid, which does not change its phase during this process. A variety of substances
have been used in such systems. These include liquids like water, heat transfer oils and certain inorganic
molten salts, and solid like rocks, pebbles and refractory. In the case of solids, the material is invariably
in porous form and heat is stored or extracted by the flow of a gas or a liquid through the pores or voids.
The choice of the substance used depends largely on the temperature level of the application, water
being used for temperature below 100°C and refractory bricks being used for temperatures around
1000°C. Sensible heat storage systems are simpler in design than latent heat or bond storage systems.?
However they suffer from the disadvantage of being bigger in size. For this reason, an important
criterion in selecting a material for sensible heat storage is its (ρCp) value. A second disadvantage
associated with sensible heat systems is that they cannot store or deliver energy at a constant
temperature. We will first take up for consideration the various materials used.
Performance of a THS is characterized by storage capacity, heat input and output rates while charging
and discharging, and storage efficiency. The storage capacity of an SHS with a solid or liquid storage
medium is given by
Q=mcdT =Vpcpcdt
where m is mass, V is volume, c is specific heat, ρ is density and ΔT = Tmax-Tmin is maximum
temperature difference between maximum and minimum temperatures of the medium. This expression
can be used to calculate the mass and volume of storage material required to store a given quantity of
energy.
For a packed bed used for energy storage, the porosity of the bed must be taken into
consideration and neglecting the heat capacity of the energy transferring medium in the storage the
volume of the packed bed storage is written as
where ε is the porosity of the packed bed. The storage energy density per unit mass and the
storage energy density per unit volume are respectively, defind as:
Example:
Sensible heat storage calculations
Problems
1.
(a) Estimate sensible heat stored in 6 m3 water and 6 m3 granite heated
from 20 to 40_C.
(b) Estimate the heat stored in 500 lb of water heated from 20 to 30_F.
2. (a) Estimate sensible heat stored in 1.1 m3 Dowtherm A and 1.1 m3
therminol 66 heated from 20 to 240_C.
(b) Estimate the heat stored in 500 lb of molten salt heated from 150 to
300_C.
3. (a) Estimate sensible heat stored in 2.5 m3 mixture of 50% ethylene glycol
and 50% water and 2.5 m3 water heated from 20 to 65_C.
(b) Estimate the heat stored in 1,500 lb of draw salt heated from 250 to
400_C.
Latent Heat Storage by Phase Changing Material
Latent heat storage can be achieved through solid–solid, solid–liquid, solid–gas, and liquid–gas phase
changes. Liquid–gas transitions have a higher heat of transformation?? than solid–liquid transitions.
However, liquid–gas phase changes are not practical because of the large volumes or high pressures
required to store the materials when they are in their gas phase states. Solid–solid phase changes are
typically very slow and have a rather low heat of transformation. Therefore, the main phase change
used for heat storage is the solid–liquid change. Figure 8.6 shows the change of enthalpy of water
from solid to vapor state on enthalpy temperature diagram. Between -30 and 0_C the sensible heat of
water increases. At 0_C the water starts to melt by absorbing heat of melting. After the melting is
completed, the liquid water is heated until 100C.
Phase changing materials used for storing heat are chemical substances that undergo a solid–liquid
transition at temperatures within the desired range for heating and cooling purposes. During the
transition process, the material absorbs energy as it goes from a solid to a liquid state and releases
energy as it goes back to a solid from liquid state. A phase changing material should possess:
• Melting temperature in the desired operating temperature range.
• High latent heat of fusion per unit volume.
• High specific heat, density, and thermal conductivity.
• Small changes of volume and vapor pressure on phase change at operating
temperatures.
• Congruent melting (solid compositions is the same as the composition of the
liquid melt).
• Chemical stability and complete reversible freeze/melt cycles.
• Noncorrosive, nontoxic, nonflammable, and nonexplosive.
• Low cost and availability.
Latent heat storage technology reduces temperature fluctuations and offers a higher heat storage
capacity per volume/mass. The temperature and the amount of energy stored can be adjusted by
selecting a specific phase changing material.
Total heat stored by a solid-to-liquid phase changing material between initial and final temperatures
would be estimated by
Ice Storage
Ice storage is the thermal energy storage using ice. It is practical because of the
large heat of fusion of water. One metric ton of water (one cubic meter) can store
334 MJ or 317,000 Btu, 93 Wh, or 26.4 ton-hours. The original definition of a
‘‘ton’’ of cooling capacity was the heat to melt one ton of ice every 24 h. One ton
HVAC capacity is known as 12,000 Btu/h. A small storage unit can hold enough
ice to cool a large building for a day or a week, by using, for example, off-peak
power and other such intermittent energy sources.
Molten Salt Technology
Molten salt can be employed as a heat store to retain heat collected by a solar
tower so that it can be used to generate electricity. The molten salt is a mixture
of 60% sodium nitrate and 40% potassium nitrate, by mass, called saltpeter. It is
nonflammable, nontoxic, and is used in the chemical and metals industries as a
heat transfer fluid. The salt is kept as liquid at 288_C (550_F) in an insulated
storage tank and is pumped through panels in a solar collector where the
focused sun heats it to 566_C (1,051_F). It is then sent to a well-insulated
storage tank. When electricity is needed, the hot salt is pumped to a
conventional steam generator to produce superheated steam for the
turbine/generator of a power plant.
Seasonal Thermal Energy Storage
A seasonal thermal storage retains heat deposited during the hot summer
months
for use during colder winter weather for heating. For cooling, the winter is
used to
store cold heat to be used in the summer. The heat is typically captured
using solar
collectors, although other energy sources are sometimes used separately or
in
parallel. Seasonal (or ‘‘annualized’’) thermal storage can be divided into
three
broad categories:
• Low-temperature systems use the soil adjoining the building as a heat
store medium. At depths of about 20 ft (6 m) temperature is naturally
‘‘annualized’’ at a stable year-round temperature. Two basic techniques can be
employed:
– In the passive seasonal heat storage, solar heat is directly captured by the
structure’s spaces through windows and other surfaces in summer and
then passively transferred by conduction through its floors, walls, and sometimes
roof into adjoining thermally buffered soil. It is then passively returned by
conduction and radiation as those spaces cool in winter.
– The active seasonal heat storage concept involves the capture of heat by
solar collectors or geothermal sources and deposited in the earth or other storage
masses or mediums. A heat transfer fluid in a coil embedded in the storage
medium is used to deposit and recover heat.
• Warm-temperature systems also use soil or other heat storage mediums
to store heat, but employ active mechanisms of solar energy collection in summer
to store heat and extract in winter. Water circulating in solar collectors transfer
heat to the storage units beneath the insulated foundation of buildings. A
ground source heat pump may be used in winter to extract the heat from the
storage system. As the heat pump starts with a relatively warm temperature the
coefficient of performance may be high.
• High-temperature systems are essentially an extension of the building’s
HVAC and water heating systems. Water or a phase change material is normally
the
storage medium.
Advantages of seasonal storage systems:
• Seasonal storage is a renewable energy utilization system.
• Stored thermal energywould be available in the desired amount, time,
and location.
• Nontoxic, noncorrosive, and inexpensive storage mediums can be
selected for a required application.
• Seasonal heat storage needs relatively moderate initial and maintenance
costs.
• Size and capacity of heat storage system can be adjusted based on the
size of the space to be conditioned.
• It can be used as preheater and precooler of existing heating and cooling
systems.
• It is possible to use water as heat transfer fluid during charging the
storage with water solar heaters, and airflow for discharging mode.
Third lecture
Seasonal Solar Thermal Energy Storage for Greenhouse
Heating
The three common processes in a seasonal thermal energy storage for
greenhouse
heating are:
• Charging: the charging is for capturing solar energy by solar air heaters and
feeding the warm air to the storage unit through the coils embedded in the heat
storage tank.
• Storing: the storage process is for storing the captured solar energy by sensible
and latent heat using a phase change material in a well-insulated heat storage
tank.
• Discharging: the discharge process, on the other hand, is for recovering the
stored heat by the cold air flowing through the heat storage tank and delivering
the warm air to the greenhouse directly when necessary.
Figure 8.7 shows the schematics of the seasonal solar energy storage systems
by paraffin for greenhouse heating with a data acquisition and control system to
activate the required process, such as discharging process [3].
Figure 8.8 shows the charging and discharging operations within the three units of
• Unit 1: solar air heaters
• Unit 2: heat storage
• Unit 3: greenhouse
Ambient air is heated to around 60–75_C by the solar radiation using solar air
collectors. The warm air from the solar collectors flows in spiral coils embedded
inside the storage tank and heats the paraffin. When the temperature of the
greenhouse drops below a set point of around 14_C, the discharge process is
activated so that cold air from the greenhouse flows through the hot and melted
paraffin, recovers a part of the latent heat, and delivers it to the greenhouse until
raising the temperature to a required level. The data acquisition system controls
the
activation and deactivation of the discharge process. Energy losses occur in
various
levels in all the three units.
Lect.4
Capacitor bank storage
A capacitor is a device for storing electric charge and used as parts of electrical
systems. Figure 8.10 shows a flat plate model for a capacitor. The forms of
practical capacitors vary widely, but all contain at least two conductors separated
Energy can also be stored in the form of an electrostatic field. Let us consider
an electrical capacitor, i.e. a device which can collect electric charge which is
establishing an electric field and hence storing energy. The capacitance C of a
capacitor is defined by the amount of charge q it can take up and store per unit
of voltage:
C= q/Vc
where Vc is the voltage of the capacitor.
As an example, let us take a plate capacitor with plate area A and distance d
between its plates, as shown in the following figure.
From the definition of capacitance it follows that the capacitance of the capacitor
is: C = K A/d
So-called dielectric materials can be polarised and, if used as a medium, the total
charge stored in a capacitor using such a material will be increased.
The properties of this medium may be described by the constant K called the
permittivity, which is measured in farad/m since both A and d have metres as
the basic unit. The electric field E is homogeneous inside the plate capacitor only
when tile distance between the plates is small. In order to keep the charges at
the plates divided, and thereby to maintain the electrostatic field, the dielectric
medium must have a low electronic conductivity; so we are looking for dielectric
materials with high permittivity. Let us consider a series RC circuit which is
connected to a constant voltage source V as shown in Fig. 8.2. When the switch is
closed a transient process which can be described by Kirchhoff's circuit equation
begins:
V- i(t)R - Vc = 0
According to the basic definitions voltage and charge are described by V, = q/C
and q = i(t)dt; then Kirchhoffs circuit equation will be given by:
V- i(t)R - i(t)dt/C = 0
Starting with I =dq/dt
Therfore, we can get
1- I =I0e-t/R.C
2- V=V0 e-t/R.C
3- Q=Q0 e-t/R.C
Were I, V and Q are the current, voltage and charge of the capacitor in the
discharging state while I0, V0 and Q0 are the initial quantities.
The charging curve represented by Q takes the form;
Q=Vs.C[1- e-t/R.C]
……………..can you prove how did you get this relation??
Here V s is the source voltage
The current at t=0 is limited by total resistance R such that; I0=V/R
Examples
Example 1
A capacitor of 1000 F is with a potential difference of 12 V across it is discharged through a 500 
resistor. Calculate the voltage across the capacitor after 1.5 s
V = Voe-(t/RC) so V = 12 e-1.5/[500 x 0.001] = 0.6 V
Example 2
A capacitor is discharged through a 10 M resistor and it is found that the time constant is 200 s. Calculate
the value of the capacitor.
RC= 200 Therefore C = 200/10 x 106 = 20 F.
Example 3
Calculate the time for the potential across a 100 F capacitor to fall to 80 per cent of its original value if it is
discharged through a 20 k resistor.
V = 0.8 V0. Therefore 0.8 = e – t/20000 x0.0001
Therefore
ln(1/0.8) = 20 000 x 0.0001
This gives t= 2 x In (1/0.8) = 0.45 s.
New lect. 31-12-2016
Hydroelectric Energy Storage
Pumped-storage hydroelectricity is a type of hydroelectric power generation used
by some power plants for load balancing. The pumped-storage hydroelectricity
stores energy in the form of water, pumped from a lower elevation reservoir to a
higher elevation. Low-cost off-peak electric power is used to run the pumps. The
stored water is released through turbines to produce electric power. Although the
losses of the pumping process makes the plant a net consumer of energy overall,
the system increases revenue by selling more electricity during periods of peak
demand, when electricity prices are highest.
Example
One method of meeting the additional electric power demand at peak usage is to
pump some water from a source such as a lake back to the reservoir of a
hydropower plant at a higher elevation when the demand or the cost of electricity
is low. Consider a hydropower plant reservoir with an energy storage capacity of
0.5 *106 kWh. This energy is to be stored at an average elevation of 40 m relative
to the ground level. Estimate the minimum amount of water has to be pumped
back to the reservoir.
Do checking for the units in the previous example
Mechanical Energy Storage
Energy may also be stored in pressurized gases or alternatively in a vacuum.
Compressed air, for example, may be used to operate vehicles and power tools.
Large-scale compressed air energy storage (CAES) facilities may be used to
supplement demands on electricity generation by providing energy during peak
hours and storing energy during off-peak hours. Such systems save on expensive
generating capacity since it only needs to meet average consumption
1. Compressed Air Energy Storage
CAES technology stores low cost off-peak energy, in the form of compressed air in
an underground reservoir. The air is then released during peak load hours and
heated with the exhaust heat of a standard combustion turbine. This heated air is
converted into energy through expansion turbines to produce electricity. Energy
storage systems often use large underground caverns due to their very large
volume, and thus the large quantity of energy that can be stored with only a small
pressure change. The cavern space can be compressed adiabatically with little
temperature change and heat loss. CAES can also be employed on a smaller scale
such as exploited by air cars and air-driven locomotives. CAES application using
sequestered carbon dioxide is also possible for energy storage. Compression of air
generates heat and the air gets warmer. If no extra heat is added, the air will be
much colder after decompression, which requires heat. If the heat generated
during compression can be stored and used again during decompression, the
efficiency of the storage improves considerably. CAES can be achieved in
adiabatic, diabatic, or isothermic processes:
• Adiabatic storage retains the heat produced by compression and returns it to
the air when the air is expanded to generate power. Heat can be stored in a fluid
such as hot oil (up to 300_C) or molten salt solutions (600_C).
• Diabatic storage dissipates the extra heat with intercoolers (thus approaching
isothermal compression) into the atmosphere as waste. Upon removal from
storage, the air must be reheated prior to expansion in the turbine to power a
generator which can be accomplished with a natural gas-fired burner.
• Isothermal compression and expansion approaches attempt to maintain
operating temperature by constant heat exchange to the environment. They are
only practical for low power levels and some heat losses are unavoidable.
Compression process is not exactly isothermal and some heat losses will occur.
The ideal gas law for an isothermal process is
PV = nRT = constant
WAB = nRT ln(VB/VA)= nRT ln(PA/PB)
where A and B are the initial and final states of the system and W represents the
maximum energy that can be stored or released. In practice, no process is
perfectly isothermal and the compressors and motors will have heat losses.
Compressed air can transfer power at very high flux rates, which meets the
principal acceleration and deceleration objectives of transportation systems,
particularly for hybrid vehicles. Examples 8.5 and 8.6 illustrate the simple analysis
of mechanical energy storage by compressed air.
Q.1
Energy Storage in Biofuels
Various biofuels, or biomass can be helpful in reducing the use of hydrocarbon
fuels. Some chemical processes, such as Fischer–Tropsch synthesis can convert the
carbon and hydrogen in coal, natural gas, biomass, and organic waste into short
hydrocarbons suitable as replacements for existing hydrocarbon fuels. Many
hydrocarbon fuels have the advantage of being immediately usable in existing
engine technology and existing fuel distribution infrastructures. A long-term high
oil price may make such synthetic liquid fuels economical on a large-scale
production despite some of the energy in the original source being lost in the
conversion processes [8]. Carbon dioxide and hydrogen can be converted into
methane and other hydrocarbon fuels with the help of energy from another
source preferably a renewable energy source. As hydrogen and oxygen are
produced in the electrolysis of water,
2H2O = 2H2 + O2
hydrogen would then be reacted with carbon dioxide in producing methane and
water in the Sabatier reaction
CO2 + 4H2 = CH4 + 2H2O
Produced water would be recycled back to the electrolysis stage, reducing the
need for new pure water. In the electrolysis stage oxygen would also be stored for
methane combustion in a pure oxygen environment (eliminating nitrogen oxides).
In the combustion of methane, carbon dioxide and water are produced.
CH4 + 2O2 = CO2 + 2H2O
Produced carbon dioxide would be recycled back to the Sabatier process.
Methane production, storage, and adjacent combustion would recycle all the
reaction products, creating a cycle. Methane is the simplest hydrocarbon with the
molecular formula CH4. Methane can be stored more easily than hydrogen and
the technologies for transportation, storage, and combustion infrastructure are
highly developed. Methane would be stored and used to produce electricity later.
Besides that, methanol can also be synthesized from carbon dioxide and
hydrogen. If the hydrogen is produced by electrolysis using electricity from wind
power, then the electricity is stored by the production of methane or methanol.
Batteries
Batteries are referred to as electrochemical or galvanic cells, due to the fact that
they store electrical energy in the form of chemical energy and because the
electrochemical reactions that take place are also termed galvanic. Galvanic
reactions are thermodynamically favorable (the free energy difference, Δ G , is
negative) and occur spontaneously when two materials of different positive
standard reduction potentials are connected by an electronic load (meaning that
a voltage is derived). The material with the lower positive standard reduction
potential undergoes an oxidation reaction providing electrons by the external
circuit to the material with the higher positive standard reduction potential,
which in turn undergoes a reduction reaction. These half reactions occur
concurrently and allow for the conversion of chemical energy to electrical energy
by means of electron transfer through the external circuit. It follows that the
material with the lower positive standard reduction potential is called the
negative electrode or anode on discharge (since it provides electrons), while the
material with the higher positive standard reduction is called the positive
electrode or cathode on discharge (since it accepts electrons). In addition to the
electrodes, the two other constituents that are required for such reactions to take
place are the electrolyte solution and the separator. The electrolyte is an ion
conducting material, which can be in the form of an aqueous, molten, or solid
solution, while the separator is a membrane that physically prevents a direct
contact between the two electrodes and allows ions but not electrons to pass
through; it therefore ensures electrical insulation for charge neutralization in both
the anode and cathode once the reaction is completed. Two final parts required
to complete a commercial galvanic cell are the terminals. They are necessary
when applying the batteries to electrical appliances with specific holder designs in
order to prevent short - circuit by battery reverse installation, and they are
shaped so as to match the receptacle facilities provided in the appliances. For
example, in cylindrical batteries, the negative terminal is either designed so as to
be flat, or to protrude out of the battery end, while the positive terminal extends
as a pip at the opposite end. A simple galvanic cell is illustrated in Figure 1.1 a,
while Figure 1.1 b shows terminal designs for cylindrical batteries.
Voltage
The theoretical standard cell voltage, E0 (cell) can be determined using the
electrochemical series and is given by the difference between the standard
electrode potential at the cathode, E 0 (cathode), and the standard electrode
potential at the anode, E0 (anode) as
E0 (cathode) −E0 (anode) = E0 (cell)
The standard electrode potential, E0 , for an electrode reaction, written (by
convention) as a reduction reaction (i.e., involving consumption of electrons), is
the potential generated by that reaction under the condition that the reactants
and the products are in their standard state in relation to a reference electrode.
In order to obtain a true estimate of the actual open circuit cell voltage in the
fully charged state for operation of the battery, the theoretical cell voltage is
modified by the Nernst equation, which takes into account the nonstandard state
of the reacting component as
E = E0 −RT lnQ
(1.2)
The Nernstian potential in Eq. (1.2) will change with time due to the self –
discharge by which the activity (or concentration) of the electroactive component
in the cell is modified. Thus, the nominal voltage is determined by the cell
chemistry at any given point of time. The operating voltage produced is further
modifi ed as a result of discharge reactions
actually taking place and will always be lower than the theoretical voltage due to
polarization and the resistance losses ( IR drop) of the battery as the voltage is
dependent on the current, I , drawn by an external load and the cell resistance, R ,
in the path of the current. Polarization rises in order to overcome any activation
energy for the electrode reaction and/or concentration gradients near the
electrode. These factors are dependent upon electrode kinetics and, thus, vary
with temperature, state of charge, and with the age of the cell. Of course the
actual voltage appearing at the terminal needs to be sufficient for the intended
application.
Tafel Curves for a Battery
In a battery there are two sets of Tafel curves present, one for each electrode
material. During discharge one material will act as the anode and the other as the
cathode. During charging the roles will be reversed. The actual potential
difference between the two electrodes for a given current density can be found
from the Tafel curve. The total cell potential is the difference between the anodic
potential, Ea , and the cathodic potential, Ec . In a galvanic cell, the actual
potential, V ′ cell, discharge , is less than the Nernst potential
Vcell,discharge = Ec − ηc +Ea – ηa
(1.28)
Upon discharge the cell potential may be further deceased by the ohmic drop due
to the internal resistance of the cell, r . Thus, the actual cell potential is given by
Vcell, discharge =Vcell, discharge – i
Ar
(1.29)
where A is the geometric area relevant to the internal resistance and i is the cell
current density.
Similarly, on charging the applied potential is greater than the Nernstian
potential, and can be calculated by the equation
Vcell, charge = Ec + ηc +Eα + ηa
(1.30)
The cell charging potential may now be increased by the ohmic drop
The cell charging potential may now be increased by the ohmic drop, and the final
actual cell charging potential is given by Vcell, charge =Vcell charge + iAr
(1.31)
In summary, it can be stated that in order to maximize power density, it is
important to achieve the most optimum value of cell potential at the lowest
overpotentials and internal resistance. Usually at low current densities,
overpotential losses arise from an activation energy barrier related to electron
transfer reactions, while at a higher current density, the transport of ions
becomes rate limiting giving rise to a current limit. Ohmic losses increase with
increasing current, and can be further enhanced by the increased formation of
insulating phases during the progress of charging. Power is a product of voltage
and current; therefore, decreasing the current density by increasing the true
surface area can also in principle result in a higher power density. However,
unwanted side reactions may also be enhanced.
Capacity
The bar graph of Figure 1.8 shows the difference between the theoretical and
actual capacities in mAh g − 1 for various battery systems. The theoretical molar
capacity of a battery is the quantity of electricity involved in the electrochemical
reaction. It is denoted as Q charge and is given by
Qcharge = xnF
(1.32)
where x is the number of moles of a chosen electroactive component that take
place in the reaction, and n is the number of electrons transferred per mole of
reaction. The mass of the electroactive component can be calculated as
M= xMr
(1.33)
where M denotes the mass of the electroactive component in the cell and M r the
molecular mass of the same component. The capacity is conventionally expressed
as Ah kg − 1 thus given in terms of mass, often called specific capacity, C specific
C specific= nF/Mr
1.34
If the specifi c capacity is multiplied by the mass of the electroactive component
in the cell, one will obtain the rated capacity of a given cell. It is important to note
that the mass may refer to the fi nal battery mass including packaging or it may be
reported with respect to the mass of the electroactive components alone. It is
quite straightforward to recalculate the capacity in terms of the mass of the cell
by
dividing the rated capacity with the total mass of the cell.
In practice, the full battery capacity could never be realized, as there is a signifi cant mass contribution from nonreactive components such as binders &
conducting particles, separators & electrolytes and current collectors and
substrates, as well as packaging. Additionally, the chemical reactions cannot be
carried out to completion; either due to unavailability of reactive components,
inaccessibility of active materials or poor reactivity at the electrode/electrolyte
interface. The capacity is strongly dependent upon the load and can decrease
rapidly at high drain rates as defined by the magnitude of current drawn, due to
increased overpotential losses and ohmic losses which can exacerbate the
problems with completion of the reaction. At higher drain rates denoting high
operating currents, a battery will be discharged faster.
1.2.4
Shelf - Life
A cell may be subject to self discharge in addition to discharge during
operation.Self - discharge is caused by parasitic reactions, such as corrosion, that
occur even when the cell is not in use. Thus, the chemical energy may slowly
decrease with time. Further energy loss may arise as a result of discharge during
which nsulating products may form or the electrolyte may be consumed.
Therefore, shelf – life
1.2.5
Discharge Curve/Cycle Life
The discharge curve is a plot of the voltage against the percentage of the capacity
discharged. A flat discharge curve is desirable as this means that the voltage
remains constant as the battery is used up. Some discharge curves are illustrated
in Figure 1.9 , where the potential is plotted against time as the battery is
discharged through a fi xed load. In the ideal mode, the cell potential remains
steady with time until the capacity is fully exhausted at the same steady rate and
then it falls off to a low level. Some of the primary lithium cells display this type
of nearly ideal flat discharge characteristics. In most other real batteries, the
voltage may slope down gently with time as in primary alkaline cells or do so in
two or more stages during discharge as in Lechlance cells.
is limited by factors related to both nonuse and normal usage.