EET322 RENEWABLE ENERGY

SYSTEMS

Module 3.3 – Small hydro power: Classification as micro, mini and small hydro
projects -Basic concepts and types of turbines - Classification, Characteristics and
Selection
Downloaded from Ktunotes.in
 SMALL HYDRO RESOURCE
• For harnessing hydropower, both major and minor hydro resources are important and needed
to be developed.
• However, major or large hydro projects involve construction of large dams which have many
social and environmental consequences.
• These include submerging and loss of forest or agricultural land, the need to rehabilitate or
relocate villagers from the submerged area, the risk of increasing seismicity as large amounts of
water is stored in the reservoir, excessive siltation at the dam site and the adverse effect on fish
population.
• Hydropower projects of ratings less than 10 MW are regarded are small hydro plant (SHP). Small
hydropower resources are considered as non-conventional and these resources have attracted
favourable attention after the oil crisis of 1973. The small hydropower projects are extremely
suitable for hilly, underdeveloped and remote areas as these resources eliminate the need of
long transmission system. These projects have also lower gestation period and lower investment
or cost as no large dam or reservoir is needed for such projects. Small hydro plants can be built
by local-staff and smaller organisations using locally made machinery. Hence, decentralised
small hydropower or mini hydel schemes are attractive option for energy supplies in rural areas.
Downloaded from Ktunotes.in
 CONVERSION OF HYDROPOWER
• Electric power is generated when water from height is made to flow through hydraulic turbine.
• The hydraulic turbine converts the potential energy of water or kinetic energy of flowing stream
into mechanical energy on its rotating shaft.
• The old-style water wheels used the impulse generated by the weight of falling water for their
rotation, but modern hydraulic turbines operated on the principle of impulse and reaction to
convert kinetic energy and potential energy respectively into mechanical energy.
• The work done per second or power given by the flowing water can be given by the following
expression:

Downloaded from Ktunotes.in

 TURBINES
• Turbines convert available energy in the form of falling water into rotating shaft power.
• They operate on the principle of either ‘impulse’ (equal pressure on each side of the runner) or
‘reaction’ (pressure drops across the runner). The turbines can work on the principles of
impulse and reaction.
1. Impulse Turbines
• In impulse turbines pressure energy is converted first in a nozzle into the kinetic energy of a
high-speed jet of water, which is then converted to rotation in contact with the runner blades by
deflection of water and change of momentum.
• The runner can operate in air and the water remains at atmospheric pressure before and after
making contact with runner blades.
• It needs casing only to control splashing and to protect against accidents.
• The three impulse turbines considered here are the
i. Pelton,
ii. Turgo and
iii. cross flow (also known as Banki, Mitchell or Ossberger turbine).
Downloaded from Ktunotes.in
 TURBINES (CONT..)
i. Pelton Turbine
• The Pelton Turbine consists of a wheel with a series of
split buckets set around its rim as shown in Fig. 11.2.
• A high velocity jet of water is directed tangentially at
the wheel.
• The jet hits each bucket and is split in half, so that each
half is turned and deflected back almost through 180°.
• Nearly all the energy of the water goes into propelling
the bucket and the deflected water falls into a
discharge channel below.
• Care must be taken to allow plenty of space on either
side of a Pelton runner to allow deflected water to exit
without splash interference.
• For optimum efficiency the jet velocity needs to be
about twice the speed of the bucket.
Downloaded from Ktunotes.in
 TURBINES (CONT..)
• The runner of such a turbine is large for the power produced.
• The use of two or more jets placed symmetrically around the rim will allow a smaller runner for a
given flow of water and hence an increased rotational speed. The required power can still be
attained.
➢Following options are available for control:
a) Replacement of nozzles: It is possible to divide the yearly flow variation in two, three or more
parts and make a nozzle for each flow. The turbine operator can then remove one nozzle and
replace it with the desired nozzle. This is very low cost method of controlling the flow.
b) Spear valves: A needle valve or spear valve (as shown in Fig. 11.2), which is so called because
streamlined spearhead, is arranged to move within the nozzle, allowing variation in effective
orifice cross section area without introducing energy loss.
c) Varying the number of jets: If multi-jet turbine has shut off valves fitted on each of its jets, it
can be run at different flow rates by simply altering the number of jets playing on the runner
d) Deflector plate: The water jet can be deflected away from the buckets of the runner if a jet
deflector plate (show in Fig. 11.2) is rotated into its path. This is very quick and does not
require the shutdown of the flow in the penstock, with consequent pressure surge danger.

Downloaded from Ktunotes.in

 TURBINES (CONT..)
e) Shut-off valves: It is usual to place a valve, either a gate valve or a butterfly valve, in the
turbine manifold. However, certain precautions are to be observed on its use. Pelton
wheels are often driven by long penstocks in which surge pressure effects, due to valve
closure, can be very dangerous and lead to damage caused by bursting of the penstock.
The valve must always be closed slowly, particularly during the last phase just before
shutoff. Gate valves are sometimes used mistakenly to regulate flow, by partially closing
them. This causes damage on the valve plate due to cavitation effects
ii. Turgo Turbine
• The Turgo turbine, shown in Fig. 11.3 is similar to the Pelton but the jet is designed to strike
the plane of the runner at an angle (typically 20°) so that the water enters the runner on one
side and exits on the other.
• Therefore, the flow rate is not limited by the spent fluid interfering with the incoming jet (as is
the case with Pelton turbines).
• As a consequence, a Turgo turbine can have a smaller diameter runner than a Pelton for an
equivalent power. It, therefore, runs at a higher speed. It shares the general characteristics of
impulse turbines listed for Pelton.
Downloaded from Ktunotes.in
 TURBINES (CONT..)
• Turgo does have certain disadvantages also. Firstly it is
more difficult to fabricate it as compared to a Pelton
wheel, since the buckets (or vanes) are complex in shape,
overlapping and more fragile than Pelton buckets.
Secondly, the Turgo experiences a substantial axial load on
its runner which must be met by providing a suitable
bearing on the end of the shaft.
iii. Crossflow Turbine
• Crossflow turbines are also called Banki, Mitchell or
Ossberger turbines.
• A Crossflow turbine, shown in Fig. 11.4, has a drum-
shaped runner consisting of two parallel discs connected
together near their rims by a series of curved blades.
• The shaft of the runner is always kept horizontal in all
cases (unlike Pelton and Turgo turbines which can have
horizontal as well as vertical orientations)
Downloaded from Ktunotes.in
 TURBINES (CONT..)
• In operation a rectangular nozzle directs the jet to
the full length of the runner.
• The water enters the top of the runner through the
curved blades imparting most of its kinetic energy.
• It then passes through the runner and strikes the
blades again on exit, imparting a smaller amount of
energy before falling away with little residual
energy.
• The effective head driving the cross flow runner can
be increased by inducement of a partial vacuum
inside the casing.
• This is done by fitting a draught tube below the
runner which remains full of tail water at all times.
• Careful design of valve and casing is necessary to
avoid conditions where water might back up and
submerge the runner.
Downloaded from Ktunotes.in
 TURBINES (CONT..)
• Because of symmetry of a crossflow turbine the runner length can theoretically be increased to
any value without changing the hydraulic characteristics of the turbine.
• Hence, doubling runner length merely doubles the power output at the same speed.
• The lower the head, the longer the runner becomes, and conversely on high heads the
crossflow runner tends to be compact.
• There are, however, practical limits to length in both cases.
• If the blades are too long they will flex, leading quickly to fatigue failure at the junction of blade
and disc.
• In case of short runner operating on high head, efficiency losses at the edges become
considerable.
• Two major attractions in the crossflow turbine are:
• Firstly, it is a design suitable for a wide range of heads and power ratings.
• Secondly, it lends itself easily to simple fabrication techniques, a feature which is of interest
in developing countries. The runner blades, for instance, can be fabricated by cutting a pipe
lengthwise in strips.
Downloaded from Ktunotes.in
 TURBINES (CONT..)
2. Reaction Turbines
• Reaction turbines exploit the oncoming flow of water to generate hydrodynamic lift forces to
propel the runner blades.
• They are distinguished from the impulse type by having a runner that always functions within a
completely water-filled casing.
• All reaction turbines have a diffuser known as a ‘draft tube’ below the runner through which the
water discharges.
• The draft tube slows the discharged water and reduces the static pressure below the runner and
thereby increases the effective head.
• The two main types of reaction turbine are:
i. Francis turbine and
ii. the propeller turbine (with Kaplan variant).
• In general, reaction turbines will rotate faster than impulse types given the same head and flow
conditions.
• The propeller type will rotate even faster than Francis.
Downloaded from Ktunotes.in
 TURBINES (CONT..)
• These high speeds have the very important implication that the reaction turbines can often be
directly couple to a generator without any speed-increasing drive system.
• Significant cost savings are made in eliminating the drive and the maintenance of the hydro unit
becomes very much simpler.
• On the whole, reaction turbines need more sophisticated fabrication than impulse types,
because they involve the use of large, more intricately profiled blades.
• The extra expense involved is offset by high efficiencies and the advantage of high running
speeds at low heads from relatively compact machines.
• However, for use in micro-hydro in developing countries, these turbines are less attractive due
to fabrication constraints.
• All reaction turbines are subject to the danger of cavitation and tend to have poor part flow
efficiency characteristics.
i. Francis Turbine
• Figure 11.5 illustrates the construction of Francis turbine.

Downloaded from Ktunotes.in

 TURBINES (CONT..)
• The inlet has a spiral shape.
• Casing is scrolled to distribute water around the
entire perimeter of the runner.
• The guide vanes, direct the water tangentially to the
runner.
• The runner blades are profiled in a complex manner.
• In operation, water enters around the periphery of
the runner through guide vanes, passes through the
runner blades before exiting axially from the center
of the runner.
• This radial flow acts on the runner vanes (blades),
causing the runner to spin.
• The guide vanes (or wicket gate) may be adjustable
to allow efficient turbine operation for a range of
water flow conditions.
Downloaded from Ktunotes.in
 TURBINES (CONT..)
• As the water moves through the runner its spinning radius decreases, further acting on the
runner.
• The water imparts most of its ‘pressure’ energy to the runner and leaves the turbine via a
draught tube.
• The guide vanes regulate the water flow as it enters the runner, and usually are linked to a
governor system which matches the flow to turbine loading.
ii. The Propeller Turbine and Kaplan
• Propeller type turbines are similar in principle to the propeller of a ship, but operating in
reversed mode.
• Typical construction is shown Fig. 11.6.
• It is often fitted inside a continuation of the penstock tube. Water flow is regulated by use of
swiveling gates (wicket gates) just upstream of the runner (propeller).
• The part flow efficiency characteristic tends to be poor.
• This kind of propeller turbine is known as a ‘fixed blade axial-flow’ turbine, since the geometry
of the turbine does not change.
Downloaded from Ktunotes.in
 TURBINES (CONT..)
• Although traditionally the propeller is profiled to
optimize the effect of pressure lift force acting on it,
designs have been produced with flat section blades
which offer less efficiency but are more easily
fabricated.
• This kind of design can be considered seriously for
micro hydro applications where low cost and ease of
fabrication are priorities.
• It is also possible to consider casting the propeller
casing in concrete.
• Large-scale hydro sites make use of more
sophisticated versions of propeller turbine.
• Varying the pitch of propeller blades simultaneously
with wicket gates adjustment has the effect of
maintaining high efficiency under part flow conditions.

Downloaded from Ktunotes.in

 TURBINES (CONT..)
• Such turbines are known as ‘variable pitch’ propeller
types or Kaplan turbines.
• Wicket gates are carefully profiled to induce
tangential velocity or ‘whirl’ in the water.
• Water enters radially or axially through these guide
vanes.
• Variable pitch designs involve complex linkages and
are usually not cost effective in any except the
largest of micro hydro applications.
• Propeller (Kaplan) turbine can be installed in
vertical, horizontal or inclined positions.
• A number of installation designs and arrangement of
drives are possible.
• Figure 11.7 shows three typical designs for
horizontal and inclined installation of the turbine.
Downloaded from Ktunotes.in
 TURBINES (CONT..)
• In ‘bulb type’ design the generator (and gear box if
any) is contained in a waterproof bulb, submerged in
the flow. Only electric cable duly protected leaves the
bulb.
• The ‘cross’ design requires a complex right angle drive
to transmit power to the generator, which is placed in
a separate chamber. ‘S’ design requires the bend in
the water passage to link the turbine with the
generator.
• A typical design for vertical installation of the turbine
is shown in Fig. 11.8
iii. Reverse Pumps or Pumps-as-Turbines (PATs)
• Centrifugal pumps can also be used as turbines.
• Potential advantages are: low cost owing to mass
production, availability of spare parts and wider
dealer/support networks.
Downloaded from Ktunotes.in
 TURBINES (CONT..)
• Because of high speed they can be directly coupled to generator without requiring coupling
drive.
• A PAT closely coupled to an induction motor sometimes referred to as ‘monobloc’ pump, is
commercially available.
• The motor runs as an induction generator.
• The disadvantages of PATs are: as yet poorly understood characteristics, no direct correlation
between pump characteristics and turbine characteristics, lower typical efficiencies, unknown
wear characteristics and poor part flow efficiency.
• In general, PATs are most appropriate for medium head sites.
• In many countries pumps are manufactured in large quantities for water supply and irrigation
purposes, whereas there may be no local manufacturer for water turbines.
• In these countries PATs may be economical for a wide range of heads and flows.

Downloaded from Ktunotes.in

 TURBINES (CONT..)
Speed Control of Turbines
• There is a tendency in turbine to speed up when the load on turbine falls and turbine slows
down when load is increased.
• It is necessary to run the turbine at a constant speed by using a governor.
• The governor can
i. reduce or increase the water flow through a nozzle of an impulse turbine,
ii. change the passage between the guide vanes to reduce or increase water flow in a radial
flow reaction turbine (Francis turbine) and
iii. change the passage between both guide and runner vanes to reduce or increase the
water flow through the axial flow reaction turbine (Kaplan).

Suitability of Turbines
• The Turbines are classified according to their specific speeds.
• The selection of turbine on the basis of specific speed is made in the following ways:
Downloaded from Ktunotes.in
 TURBINES (CONT..)
i. Low specific speed. Impulse turbines have a low value of specific speeds and these turbines
are suitable to work under high head and large discharge conditions. The specific speeds of
these turbines vary from 8 to 50.
ii. Medium specific speed. Reaction turbines such as Francis turbines have specific speeds
varying from 51 to 225. These turbines are suitable to work under moderate head and
discharge conditions.
iii. High specific speed. Reaction turbines such as Kaplan turbines have high specific speeds
varying from 250 to 850. These turbines are suitable to work under low head and large
discharge conditions.

Downloaded from Ktunotes.in

 TURBINE CLASSIFICATION, CHARACTERISTICS AND
SELECTION
CLASSIFICATION
• Turbines can be crudely classified as high-head, medium-head, and low-head machines, as shown in
Table 11.1.
• But this is relative to the size of machine: what is low head for a large turbine can be high head for a
small turbine; for example a Pelton Turbine might be used at 50 m head with a 10 kW system but
would need a minimum head of 150 m to be considered for a 1 MW system.
• Small turbines designed for micro
hydro applications often will have no
method of altering the flow rate of
water.
• On larger machines, some method of
altering the flow is normal.
• If flow control devices are fitted to the
turbine, then the same head of water
can be maintained above the turbine
while flow reduces.
Downloaded from Ktunotes.in
 TURBINE CLASSIFICATION, CHARACTERISTICS AND
SELECTION (CONT..)
CHARACTERISTICS
• Different turbine types respond differently to changed flow at constant head.
• Therefore an important aspect of their characteristics is their performance at part flow
conditions.
• Typical efficiency characteristics are given in Fig.
11.9.
• An important point to notice is that the Pelton and
cross flow turbines retain high efficiency when
running below designed flow.
• In contrast the Francis drops in efficiency, producing
very poor power output if run at below half the
normal flow.
• Fixed pitch propeller turbines perform very poorly
except at 80 to 100 per cent of full flow.
Downloaded from Ktunotes.in
 TURBINE CLASSIFICATION, CHARACTERISTICS AND
SELECTION (CONT..)
SELECTION
• Francis is one of the few turbines which turns at a reasonable speed at certain power and head
combination.
• An impulse turbine operated under these conditions of head and flow would be much larger,
expensive, cumbersomely slow turning and would need a greater speed increasing transmission.
• In addition to giving high speed at low head-to-power ratios, reaction turbines are particularly suited
to low head applications for a second reason.
• Since power conversion is caused partly by pressure difference across the blades, the drop in head
below the blades (known as ‘suction head’) is as effective in producing power as the head above the
turbine.
• It is generally difficult or expensive to place micro hydro turbine lower than about 2 meters above
the surface level of water down steam of the turbine.
• On a low head site of, say, 10 meters the suction head then represent 20 per cent of the power
available at the site.
Downloaded from Ktunotes.in
 TURBINE CLASSIFICATION, CHARACTERISTICS AND
SELECTION (CONT..)
• This is likely to be very significant in terms of the overall economy of the scheme.
• In contrast, impulse turbines do not usually make use of any suction head as their casing runs at
atmospheric pressure. However, sophisticated cross flows on low heads often use suction heads.
• Having noted the advantage of using a suction head, it should also be observed that the magnitude
of the usable suction is limited.
• This is because very low water pressures are induced on the blades of a reaction turbine running
under high suction.
• These can be low enough to vaporize the water in pockets (or ‘cavities’) of vapor attached to the
internal surfaces of the turbine.
• The cavities form and collapse at a very high rate which after a period of time can cause serious
pitting and cracking of the blades.
• The phenomenon is known as ‘cavitation’.
• In practical terms great care must be taken to situate the runner at a position which prevents the
possibility of damage to cavitation.
Downloaded from Ktunotes.in
 TURBINE CLASSIFICATION, CHARACTERISTICS AND
SELECTION (CONT..)
• Appropriate turbine is selected based on the
guidelines depending mainly on the available head
(H), discharge (Q) and power required (P).
• For a particular head they will tend to run most
efficiently at a particular speed, and require a
particular flow rate.
• The required speed at the generator shaft is
achieved using speed-increasing gear or pulley and
belt drive.
• The approximate ranges of head, flow and power
applicable to the different turbine types are
summarized in the chart of Fig. 11.10 (up to 500
kW power).
• These are approximate and dependent on the
precise design of each manufacturer.
Downloaded from Ktunotes.in
 SMALL HYDROPOWER PLANTS
How do you classify small hydropower plants?
• The hydropower plants having capacity below 10 MW are classified as small hydropower plants.
• As these plants have small generation capacity, there is no need for large reservoir or dam to
store water.
• Any seasonal variation in water flow in the water stream affects the power output from these
plants.
• Perennial streams flowing in hilly areas with steep gradients are the most suitable sites for such
plants.
• These plants can, therefore, meet the power requirements of most of the hill areas.
• Several international agencies are providing technical and financial assistance for the
construction of small hydropower plants in developing countries to improve the quality of life in
underdeveloped areas.
• Small hydro resources are largely free from any pollution and their potential is, therefore,
increasingly being utilized.

Downloaded from Ktunotes.in

 SMALL HYDROPOWER PLANTS (CONT..)
• Depending on the capacities, small hydropower plants can be classified as follows:
1. Micro hydel plants. The plants generating power up to 100 KW are called micro hydel
plants.
2. Mini hydel plants. The plants generating power above 100 kW but less than 1000 kW (1
MW) are classified as mini hydel plants.
3. Small hydel plants. The plants generating power in the range of 1-10 MW are classified
as small hydel plants.
• Depending upon available heads, the small hydropower plants (micro, mini and small) can also
be classified as follows:
i. Ultra-low heads up to 3
ii. Low heads from 3 to 30
iii. Medium heads between 30 and 75
iv. High heads above 75

Downloaded from Ktunotes.in

 CONVERSION OF HYDROPOWER (CONT..)
• The small hydel schemes can also be classified as follows:
a. Independent schemes. In these schemes, the stream flow is captured, regulated and
developed for the purpose of power generation only. The low head schemes are
unsuitable to be developed as independent power generation schemes.
b. Subordinate schemes. As the name suggests, the main purpose is not to generate
electricity, but to supply water for irrigation or drinking. These schemes are suitable for
micro and mini hydel plants because of the availability of small slopes in the canal system.
Demerits of Small Hydropower Sources
• The potential of small hydropower resources remains untapped for the following reasons:
i. Small hydel plants entail high cost of power generation per unit
ii. High managerial and administrative costs due to installation at isolated and remote areas
iii. Low load factor or utilization of power
iv. Unstable operation of isolated system due to changes in stream flow in different seasons.
Generation depends upon availability of flow
v. Susceptible to losses due to extreme climatic conditions leading to flooding, thereby
damaging the equipment
Downloaded from Ktunotes.in
 CONVERSION OF HYDROPOWER (CONT..)
Merits of Small Hydropower Resources
• The advantages of small hydel plants are as follows:
i. The plants can be built locally at low cost.
ii. It can be considered as a renewable energy resource.
iii. It is a non-polluting resource.
iv. Its installation does not require long gestation period. Installation may be also within 6-24
months.
v. Its operating costs are low as skilled manpower is not required for operation and
maintenance.
vi. It is an ideal decentralised power generation resource which is meant to supply energy to
local areas, thereby eliminating distribution losses and costs.
vii. The project neither submerges any area nor displaces any nearby villagers as necessary in
the case of a large hydropower with the construction of dam.
viii. Small hydropower plants can be developed to augment hydropower capacity of existing
irrigation dams.
Downloaded from Ktunotes.in
 THANK YOU

Downloaded from Ktunotes.in

10.2. Fuel Cells
a n electric
of cells) capable of generating
A cell (or combination of a fuel directly into electri-
the chemical energy
current by converting other electric cells in the respect
is similar to
cal energy. The fuel cell electro!yte
and negative electrodes with a n
consists of positive
that it 10.2.1.
suitable torm is supplied to the negative
between them. Fuel in
a
often trom air, to the positive electrode. When the
electrode and oxygen,
561
ntroc
Downloaded from Ktunotes.in
562 N n-Cc ventional Sources of Bnergy

provides the
cell operate: , the fuel is oxidised and thechemicalreaction
differ from conven.
energy that is converted into electricity. Fuel cells
the active material (i.e. fuel and
ti nal electric cells in the respect thatcell
the but are supplied from outside
oxygen) are not contained within
But for its costs, pure (or fairly pure) hydrogen gas would be
preferred fuel for fuel cells. Alternatively impure hydrogen obtained
from hydroe rbon fuels, such as natural gas or substitute natural gas
(methane), quified petroleum gus (propane and butane) or liquid
can be used in fuel cells.
Efforts are being made to
petroleum, oducts,
develop cel's that can carbon monoxide as the fuel ; if they are
use
successful, it should be possible to utilise coal as the primary energy
source. Man uses cells a r e in power production, utomobile
of fuel
vehicles and in special military use
fuel cell
Design and principle of operation of
a
10.2.2.
(with special reference to H2, O2 cell)
which the chemi-
As stated these are electro-chemical devices in
The chemical
cal energy of fuel is converted directly into electric energy.
energy is the free energy
of the reactants used. This conversion takes
and pressure. The basic feature of the
place at constant temperature
combined in the form of ions
fuel cell is that the fuel and its oxidant are
rather than neutral molecules.
demonstrated by Francis T.
The first practical fuel cells was
in 1959. As per the fuel
Bacin and J.C. Frost of Cambridge University
used the main types of fuel cells are
() Hydrogen (Ha) fuel cell,
(i) Hydrazine (N2H4) fuel cell,
iii) Hydrocarbon fuel cell, and
(iv) Alcohol (Methanol) fuel cell.
with reference
The operation of the fuel cell can best be described
be adopted to a variety of fuels by
to a specific device. Fuel cell can
cell 15
Here Hydrogen, Oxygen (Hydrox)
changing the catalyst.These and the most
described for example. types are the most efficient
highly developed.
The main components of a fuel cell a r e :
(i) a fuel electrode (anode),
() an oxidant or air electrode (cathode), and
(iii) an electrolyte.
In most fuel cells, hydrogen (pure or impure) is the active
material at the negative electrode and oxygen (from the oxygen or air
is active at the positive electrode. Since hydrogen and oxygen are gases

a fuel cell requires a solid electrical conductor

to current
serve as a

electrode. The slid electrode

collector and to provide a terminal at each
Downloaded
material from Ktunotes.in
is generally porous.
 Chemical Energy Sources 563

RL
www
H2
Electrolyte
J
typically--
.40% KOH-

Ha in Permeable O2 in
nickel electrode
Fig. 10.2.2.1. A hydrox (Hz, O2 cell)
Porous nickel electrodes and porous carbon electrodes are
generally used in fuel cells for commercial applications. Platinum and
other precious metals are being used in certain fuel cells which have
potentialutility in military and space applications. The porous electrode
has a larger number of sites, where the gas, electrolyte and electrode
are in contact; the electro chemical reactions occur at these sites. The
reactions are normally very slow, and catalyst is included in the
electrode to expedite them. The best electro chemnical catalysts are finely
divided platinum or platinum-like metal deposited on or incorporated
metals are
with the porous electrode material. Since the platinum
and silver (for
expensive, other catalysts, such a s nickel (for hydrogen)
a r e used where possible.
The very small catalyst particles
oxygen), electro-chemical
number of active sites at which the
provide a large
reactions can take place fairly rapid rate.
at a

details, the essential

Although practical fuel cells differ design
in
as indicated by the
schematic illustration in
principles a r e the same, and oxvgen
Fig. (10.2.2.1). Hydrogen gas is supplied to one electrodeof electrolyte.
the electrodes is a layer
(or air) to the other. Between
gas belbw about 200°C; the
cells operate at temperature
Most existing fuel of a n alkali o r acid. The
then usually and aqueous solution
is
electrolyte in a porous membrane ; but it
electrolyte is generally retained
liquid Different electric current is
drawn
in some cells.
may be free flowing by connecting a load
between the
theusual manner
from the cell in
electrode terminals. of a
reactions occurring at the electrodes
The electro chemical but
vary with the nature of the electrolyte,
hydrogen-oxygen may
cell
electrode, hydrogen gas
follows. At the negative
basically they are with a positive
as
1ons (H) :.e. hydrogen
into hydrogen
n u m b e r of electrons (i.e. ) ; thus
(H) is converted a n equivalent
electric change, plus H 2 H +27
(10.2.2.1)

Downloaded from Ktunotes.in

564 Non-Conventional Sources of Energ

At this electrode, hydrogen is diffused through the permeable nickel in

which is embedded a catalyst. The catalyst enables the hydrogen
molecules, H2 to be absorbed, on the electrode surface as hydrogen
afoms, which react with the hydroxyl ions (OH) in the electrolyte to
form water.
When the cell is operating and producing current, the electrons
flow through the external load to the positive electrode ; here they
interact with oxygen (O,) and water (H,0) from theelectrolyte to form
negatively charged hydroxyl (OH)ions; thus
O+HO +2720H ...(10.2.2.2)
The hydrogen and hydroxyl ions then combine in the electrolyte to
produce water
H'+OH -H20 .(10.2.2.3)
The electrolyte is typically 40% KOH solution because of its high
electrical conductivity and it is less corrosive than acids.
The above equations show that hydroxyl ions produced at one
electrode are involved in the reaction at the other. Also electrons are
absorbed from the oxygen electrode and released to the hydrogen
electrode. Addition of the three forgoing reactions show that when the
cell is operating, the overall process is the chemical combination of
hydrogen and oxygen (gases) to form water that is
H2+O2 H20 .(10.2.2.4)

The oxygen and hydrogen are converted to water, which is the waste
product of the cell. The reactants are stored outside the cell (note
difference from storage battery), and the electrodes and electrolyte are
not consumed in the overall process. These properties lad to the design
of convenient small size and long life power units.
Ifthe electrodes are on open cireuit (Fig. 10.2.2.2), the hydrogen
electrode accumulatesa surface layer ofnegative charges. These attract
potassium ions, K', of the electrolyte, providing an electrical double

=OH
OH
OHER
OH
R OH
Fig. 10.2.2.2. Electrodes on open circuit.
Downloaded from Ktunotes.in
 Chemical Energy Sources 565

layer. Similarly, the loss of electrons from the oxygen electrode results
in a layer of positive charges, which in turn attracts
hydroxyl
from the electrolyte. These electrical double layer at the electrodes build ions, Ofi
up until the potentials are such that they inhibit only other further
reactions between the electrolyte and the fuel gases. This situation is
illustrated in the figure showing that an open circuit voltage 1s
developed between the electrodes. The magnitude of this emf is 1.23
volt at 1 atm and 25°C.
Ifthe cireuit is closed (Fig. 10.2.2.3), the electrons can now leave
the electrodes pass through the connecting circuit to the oxygen

www

2 f
H20 H,o
02
Ha
OH OH

10.2.2.3. Electrodes on closed curcuit.

Fig.
(10.2.2.2) above.
and take part in the reaction of equation
electrodes, current passing through a n
movement of electrons constitutes a
This obtained directly
useful electrical work is
external load. In this way flow is from the
chemical process.
Note that the electrons
from the
electrode.
hydrogen to the oxygen
cells (Hydrox) a r e of two types:
Hydrogen fuel
temperature is 90°C. It
temperature cell. The electrolyte
1. Low usually say upto
but not by a great amount,
is sometimes pressurised,
4 atmospheres. about 45 atmospheres
and
cell. Pressure is upto
2.Highpressure A single "Hydrox"
fuel cell c a n produce
300°C say. number of
temperatures upto 25°C. By connecting
a
a t l atn. and and
1.23 volts of 100 to 1000 volts
a n e.m.f. of useful potential
to create upon the
cells, it is possible to 100 MW nearly. The current depends
levels of 1 kW varies directly with
power The output of the fuel cell
oi he cell. p r e s s u r e is
raised.
physical size increase the
cell output, the gas
3 to perkW.
so is about 0.27 c u . m .
thepressure,
.

cell at present
The optimum siz of
the carbon-
must be free from
in the hydrogen oxYgen cell hydroxide
the potassium
The gases
this gas can combine with electrical
because If this occurs, the
carbonate.

dioxide,
form potassium
electrolyte to

Downloaded from Ktunotes.in

566 Non-Conventional Sources of Energy

resistance of the cell is increased and its output voltage is decreased.

Consequently, when air is used to supply the required oxygen, carbon
dioxide must first be removed by scrubbing with an alkaline medium
i.e. lime).
Fuel cells are particularly suited for low vollage and high current
applications.
Hydrogen-oxygen fuel cells have been proposed for propulsion of
electric vehicles, with the hydrogen provided by a metal hydride.
However this use is limited by the heavy weight of the hydride andthe
st of the relatively pure hydrogen required. It appears that for the
present, at least the use of hydrogen oxygen cells will be restricted
mainly to manned space vehicles. Such cells with porous nickel
electrodes and potassium hydroxide electrolyte have been used to
provide electric power for the Apollo and shuttle space craft. The
hydrogen and oxygen for operating the cell are stored in liquid form to
minimize the volume occupied.
10.2.3. Classification of Fuel Cells
Before describing the different types of fuel cells, it is necessary
to have some method of classification ofvarious types of fuel cells, which
are either in existence or are being invented. Several methods of
classification of fuel cells have appeared in the literature. One of the
difficulties in arriving at a systematic classification is that several
operational variables exist.
For example, fuel cells may be classified according to the
temperature range in which they operate,
Low temperature 25-100°C
Medium temperature 100-500°C
High temperature 500-1000°C
and very high temperature above-1000°C
Another method would be according to the type ofelectrolyte, e.g,
aqueous, non-aqueous, molten or solid. One could also classify the fuel
cells according to the physical state of the fuel:
Gas-hydrogen, lower hydrocarbons
Liquid--alcohols, hydrazine, higher hydrocarbons
Solid-Metals etc.
n the present discussion, a broad division is first made according
to weather the fuel cell system is a primary or secondary one. A primary
fuel cell may be defined a s one in which the reactants are passed
through thecellonly once, the products of the reaction being discarded.
e.g., Hz-O2 fuel cell. A secondary fuel cell is one in which the reactants
are passed through the cell many times because
they are regenerated
from the products by thermal, electrical, photochemical methods, e-g
Nitric oxide-chlorine fuel cells.
Downloaded from Ktunotes.in
 Chemical Energy Sources 567
10.2.4. Types of Fuel Cells
Following fuels are mostly used in fuel
(1) Hydrogen,(2) Fossil fuel, (3)
cells:
fuel, (5) Hydrazine fuel. Hydrocarbon fuel, (4) Alcoho
There are discussed in the
following sections.
(1)Hydrogen, oxygen (H2, O2) cell of primary systems is already
described using 40% KOH solution as electrolyte. Here
exchange membrane cell will be described which uses membrane now lon
electrolyte.
lon BExchange Membrane cell. The basic design of the cell,
which
consists of a solid electrolyte lon-exchange membrane, electrocatalysts
and gas feed tubes is represented in Fig. (10.2.4.1). The distinctive

Load

ELECTRODEP ELECTRODE

H2 2H2 4H
AIR

-02
2H20

Ion Exchange
Membrane cel1
Fig. 10.2.4.1 form of a n
that it uses a solid electrolyte in the
feature of this cell is non-permeable to the r e a c -
The membrane is from
thus prevents them
membrane.
ion-exchange which
and oxygen, hydrogen
hydrogen to
tant gases, membrane is however, permeable
contact. The
coming into current
carriers in the electrolyte. membrane
the ideal ion-exchange
ions which are
an
properties of
The desired
electrolyte are:
conductivity.
ionic
) High
conductivity.
electronic
(ii) Zero oxidant.
of fuel and
permeability
(iii) Low e l e c t r o - o s m o s i s .

degree of
(iv) Low to
dehydration.
resistance

(u) High oxidation or or hydrolysis

hydrolysis aand,
n
resistance
to its
High
(vi) Mechanical
(vii) stability.

Downloaded from Ktunotes.in

 Non-Conventional
Sources of Enersy
568
carried out in a
research has been
amount of andd
A
considerable
Interpolymers
of p o l y f l u r o c a r b o n
ideal
membrane.
quite satisfactory. In
search for the been found to
be
sulfonic acids have a thin sheet of
polystyrene low a s possible,
resistance be a s
electrolyte electrolyte. The use
order that used a s the
t h i c k n e s s ) is of gas
(0.076 cm. by problems
this material sheets is prevented
thinner electrolyte feature of this
advantageous
of e v e n etc. An
m e c h a n i c a l stability
permeability, of w a t e r and rejects
limited quantity
that it retains only
a
electrolyte is
fuel cell.
waterproduced in the and a
excess
consist of the electrocatalyst
which
The two electrodes, electrode a r e in
the form of fine
the
for water-proofing side of the electrolyte
plastic material a r e bonded
o n either
s c r e e n s . They Metallic
metallic wire titanium o r platinum.
material is
layer. The wire
screen com-
each electrode. The hydrogen
ribbed onto
c u r r e n t collectors
are
enters this compart-
the hydrogen gas
partment the cell is enclosed;
of circulates throughout the
ribbed c u r r e n t
inlet and
ment through a small electrode. On the
o v e r the
collectors and
distributes itself evenly
coolant tub s run
enters the compartment,
or air the
opposite side oxygen collectors. On the oxygen side,
the current
through the ribs of the product of
aiso hold wicks which absorb water,
current collectors,
o v e r by capillary
action. The water leaves
fuel-cell reaction and carry it
is
a n exit from
the oxygen compartment. Oxygen
the cell through
the inclusion of a differen-
prevented from leaving its compartment by
tial pressure water-separation system.
is acidic and the current
The ion-exchange membrane electrolyte
ions. The hydrogen ions a r e produced
carriers in solution a r e hydrogen
by the reaction at anode according to,
2H -
4H* +4e
the
These ions a r e then transported to the cathode through
circuit.
electrolyte and the electrons reach the cathode via the external
At the cathode, oxygen is reduced producing water as represented by
:

O2 + 4H' + 4 2 H , 0
Thus the overall cell reaction is,
2H2+Og 2H,0
This cell operates at about 40-60°C. The thermodymamic
reversible potential for the reaction is 1.23 volts at 25°C.
(2) Fossil Fuels Cells. The most interesting fuel cells for the near
future are modified hydrogen-oxygen (or air) cells, in which a gaseous
or liquid hydrocarbon is the source of hydrogen. Eventually, coal may
serve as the primary energy source for fuel cells. Cells based on fossil
fuels have three main components
(1) The fuel processor which converts the fossil fuel into a
hydrogen-rich gas,
Downloaded from Ktunotes.in
 Chemical Energy Sources 569

(2) The power

section consisting of the actual fuel cell (or com-
bination of cells),and
(3) The inverter for
fuel cell into changing the direct current generated by the
alternating
current to be transmitted to user.
Main components of fuel cell
system
Fig.(10.2.4.2). The most highly developed fossil fuel cellsschematically
are shown in

acid cells, molten carbonate cells, solid

are
phosphoric
oxide electrolyte cell.
AIR

FOSSIL FUEL HYDROGENPOWER SECTIOND.CINVERTERA

FUEL PROCESSOR (MAINLY)(FUEL CELL)
STEAM

Fig. 10.2.4.2. Main components of fuel cell system

In phosphoric acid cell utilizes a concentrated acqueous solution
of phosphoric acid as the electrolyte. The primary fuel is light hydrocar-
naphtha. The operating temperature is 150
to
bon, such natural gas or
cell unit is only
200°C and the discharge voltage is 0.7 to 0.8 volt. Each
so that a large number can
be stacked in a
a few millimeters thick
the desired voltage and power.
package of reasonable size to produce
These are high temperature fuel
Molten Carbonate Cells. the
as the electrolyte, offer
with molten carbonate mixture
cells, a
of fssil fuels, including coal. A special
for u s e with a variety
prospect operation they c a n oxidize
carbon-
cells is that during
feature of these water. Hence gaseous
a s hydrogen to
to carbondioxide a s well
monoxide
carbon monoxide,
which are relatively inex-
and
mixture of hydrogen of carbon-
can be used
in the cell, the presence
pensive to manufacture, minor effect.
dioxide would
have only a
These
available for fuel processing.
are
Several methods those used for the
commercial
same as
essentially the known a s
methods a r e and c a r b o n - m o n o - o x i d e ,
hydrogen
production, of
mixtures of made from coal by
reactions with
c a n be
Synthesis gas In order to prevent
synthesis gas. v a r i o u s petroleum
products.
o r from
must be
the gas mixture
air and steam e l e c t r o d e s in
the fuel cell,
the section.
poisoning of supplied to the power
before being
desulfurized
temperature fuel cells under develop-
the high and potas-
electrolyte in (lithium, sodium
The alkali
metal
m i x t u r e of in aan
molten 00-700°C. This is retained
of 600
ment is a temperature
electrodes.
at tWo porous nickel
a
carbonates Derween

sium) sandwicned

matrix
porous
inert
Downloaded from Ktunotes.in
570 Non-Conventional Sources of Energy

electrochemical catalysts are

Because of the high temperature,
mixture of hydrogen
apparently not necessary on the electrodes. The
electrode and oxygen
and carbon monoxide is supplied to the negative
e.m.f. of the cell
(from the air) to the positive electrode. The discharge
is about 0.8 volt.
The electrolyte is usually held in a sponge-like ceramic matrix,
contact with this solid
Metallic electrodes a r e placed in direct
methane or kerosene is used.
electrolyte. A hydrocarbon fuel, such as and CO. At the fuel
The fuel is reacted inside the cell to produce H2
the electrolyte, releasing
electrode, H2 and CO react with COg ions in
electrons to the electrode, and forming H20
and CO2 (Fig. 10.2.4.3). At
the oxygen electrode O2 reacts with the returning electrons and CO,

H,0+CO2 N2
LOAD

4e

H20
2e ..c00.
H2 .co
H2t
Ca C01 2e 2e
cO2 - AIR OR

CO2 02
CO
co2
CO2

C02
Fig. 10.2.4.3. Molten carbonate fuel cell.
divertei from the fuel electrode to form CO3 ions. These CO3 ions then
migrate through the electrolyte to the fuelelectrode. Thus the reactions
are as follows.
At the fuel electrode
H2 +CO3 = H20 + CO2 + 2e
CO + CO= 2C02 + 2e
At oxygen electrode
O2 +2 C02 + 4e = 2 CO3
Overall cell reaction
H2+ C0 + O2 =
H,0 +CO2
An important aspect of the molten carbonate fuel cells is that the
discharged gases, consisting mainly of the steam and carbondioxide
products and nitrogen from the air, are at a temperature exceeding

Downloaded from Ktunotes.in

 Chemical Energy Sources571
540°C. The hot
gases could be used to provide industrial process heat,
to operate a gas turbine
or to
produce steam in a waste-heat
ue be
exchanger) boiler to drive a steam turbine. The turbine would
attached to a generator to produce additional electric power. The overall
efficiency for fuel use would thus be substantially increased.
Solid Oxide Electrolyte Cells. Certain solid, ceramic ox
oxides
are able to conduct electricity at high temperature and can serve as
electrolytes for fuel cells. These cells could utilize the saune fossil fuels
as the molten carbonate cells. The processing operation would then be
the same as described above. Solid-oxide cells are in the early stages of
development. A possible electrolyte is zirconium dioxide containinga
small amount of another oxide to stabilize the crystal structure ; this
material is able to conduct oxygen ions (O2) at high temperatures. 'The
electrode material might be porous nickel and the operatingtempera
ture in the range of 600-1000°C. Electrochemical catalysts would not
be required.
stored
Other energy fuels, that can be conveniently
sources or
ammonia and
and transported in liquid form, such as methanol, can be
cells. Methanol
hydrazine, have been proposed for the fuel of
about 200°C to yield a mixture
catalytically reformed with steam at This can be
and carbondioxide. gas
hydrogen (75 volume per cent) the positive
to the negative electrode of a fuel cell with air at
supplied acid solution as the
electrode. The cell with aqueous phosphoric
described.
might be similar to those already
electrolyte,
oxygen (air)
fuel cell, ammonia gas,
In the ammonia (NH3) into
stored liquid, is decomposed catalytically
obtained from the is
volume cent) and nitrogen. Part of the hydrogen
per
hydrogen (75 the heat required for the
decomposition. The
burned in air to provide to the negative
electrode of a
is then supplied
bulk of the hydrogen most suitable electrolyte
would probably be
air fuel cell. The formed in the decomposi-
hydrogen solution. The nitrogen
of
potassium hydroxide no role in the
cell. The advantage
is inert and plays form. Dis-
tion of ammonia be stored in the liquid
is that it c a n
ammonia a s fuel pressure and
cell does not operate
vapour
include higher
advantages
temperatures.
low vehicle
satisfactorily at mobile source, possibly for
fuel cell for
a
A compact and hydrogen peroxide
utilizes the liquid
hydrazine (N2H4) is injected as required
propulsion, source.
Hydrazine
provide the
active
a s the
energy
electrolyte to
(HO) o r air
potassium hydroxide for the positive
electrode
into the aqueous
The oxygen
the negative electrode. of hydrogen peroxide
material at catalytic decomposition
the electrode may
electrode m
either by he
the ambient air. Each
is obtained from ambient silver (pos
or
temperatures
nickel o r silver
(negative) or
kel (negative) (posi-
m a t r i x with
at ordinary reaction is the
screen
cell
catalyst. The
nickel overall
consist of a
electrochemical

tive) as the Downloaded from Ktunotes.in

572 Non-Conventional Sources of Energy
discharge emf is
to water and nitrogen, but the
oxidation of hydrazine
oxygen cell.
similar to that of the hydrogen
Basic reactions a r e :
4 H20 + 4e
NH2 NH2 + 4
OH - N2 +

+ 4e 4 OH
O2 + 2H20
Overall cell reaction:

N2 + 2H,0
NH2NH2 +O2 is that Hydrogen
and Oxygen are
cell
Basic drawback of H-O2
Liquid fuel hydrazine is highly
there is storage difficulty. for this
gases, so circuit voltage (OCV)
reactive. Power output is
m o r e as open
cell.
1.23 volt of Hz-O2
case is 1.56 a s compared to toxic as well
fuel is that it is highly
Main drawback of hydrazine
as costly. It is being
Cell (or battery).
Aluminium-Oxygen (Air)
Livermore National Laboratory
(U.S.A.),
the Lawrence
developed by is unusual in the respect
for electric vehicle propulsion. This cell
mainly the fuel which is
consumed
that the metal aluminium is effectively
aluminium (A) forms
a s required. The
during operation and replaced
and oxygen (from air) is the positive
the negative electrode of the cell,
solution of sodium hydroxide.
electrode; the electrolyte is an aqueous
The overall cell reaction is symbolically

Al+O2 (air) +H0 Al (OH)3

( (+)
and water (from the electrolyte)
so that aluminium, oxygen (from air),
The aluminium
combine to form aluminium hydroxide (Al) (OH)3.
metal containing a small amount
(negative) electrodes are made of the
carbon coated with an
of gallium, and the air (positive) electrodes are
electro chemical catalyst, possibly silver. Before entering the battery,
the air is scrubbed to remove CO2. The operating temperature
of the
battery is about 50 to 60°C.
Regenerative Systems. A regenerat1v» fuel cell is one in which
the fuel cell product (e.g., water in the hy drogen-oxygen fuel cel) 1s
re-covered into its reactants (e.g., hydrogen and oxygen) by one ot
several p0ssible methods-tliermal, chemical, photochemical, electrical
or radio echemical.
Since there are two stages in a regenerative fuel cell:
(1) Conversion of fuel cell reactants into products while produc-
ing electrical energY, and
(2) Reconversion of fuel cell products into reactants,it is clear
that the overali efficieney of a regenerative fuel cell is the product of the
efficiencies of these two stages. In the following section, the principle of

Downloaded from Ktunotes.in

 Chemical Energy Sources 573
-

the regenerative systems is explained by considering the

mode of regeneration. photochemia
Photochemicallyregenerative fuel cells. In this method, the
products of the fuel cell reaction are transformed into its reactants by
light. Because of the ample availability of solar energY, this method
should be valuable, provided that there are suitable substances which
can undergo photo chemical dissociation.
The sequence of reactions which are taking place in this fuel cell
canbe represented as follows
Electrochemical : A+ B AB + electricity
A+ B
Photochemical : AB + light
Overall Light + electricity
reaction
The nitric oxide-chlorine fuel cell, in which the overall
1s,
2 NO +Cl 2 NOCI
Photochemically to chlorine
the product nitrosyl chloride is decomposed
represented in
and nitrous oxide. The system is schematically
Fig. (10.2.4.4).

ABSORBER

Cl2 NO
AND UNCONVE- STRIPPER SEPARATOR
NO, Cl2
RTED NOCI Cl2 NO
Cl2 STORAGE
$TORAGE
LOAD

ELECTROLY TE

SOLAR
REGENERATOR

NO
NOCI Cl2
NOCI
EVAPORATOR

Regenerative fuel cell.

10.2.4.4.
Fig.

Downloaded from Ktunotes.in

 574 Non-Conventional Sources of Energy

The cell has a standard reversible-cell potential of 0.21 volt. The

reactants may be regretted from NOCl, in the liquid phase by light.
Under these conditions, the quantum yield is low because the back
reaction is also rapid. In the gas phase regeneration is easier
although
there is some problem of separating the NO and Cl2. The currents
obtainable are low, probably as a result of the low value of the reversible
potential.
10.2.5. Advantages and Disadvantages of Fuel Cell
Advantages. (1) It has very high conversion efficiencies as high
as70 per cent have been observed, since it is a direct conversion
process
and does not involve a thermal process. In the conventional thermal
process for generating electricity, heat energy produced by combustion
of the fuel is converted partially into mechanical energy in a steam
turbine and then into electricity by means
of a generator. The etficiency
of a heat engine is limited by the operating temperatures, and in the
large modern steam-electric plants above 40 per cent of the heat energy
of the fuel is converted into electrical energy. Fuel cells, on the other
hand, are not heat engines and are not subjected to their temperature
limitations.
(2) Fuel cells be installed near the use point, thus
can
reducing
electrical transmission requirements and accompanying losses. Conse-
quently considerably higher efficiencies are possible.
(3) They have few mechanical components; hence, they opera
fairly quietly and require little attention and less maintenance.
(4) Atmospheric pollution is small if the primary energy source
is hydrogen, the only waste product is water ; if the source is a
hydrocarbon, carbon dioxide is also produced, Nitrogen oxides, suchas
accompany combustion of fossil fuels in the air, are not formed in the
fuel cell. Some heat is generated by fuel cell, but it be
to the atmosphere or
a can dissipated
possibles used locally.
(5) There is no requirement for large volumes of cooling water
such as are necessary to condense exhaust system from a turbine in
conventional power plant.
(6) As fuel cells do not make noise, they can be readily accepted
in residential areas.
(7) The fuel cell takes little time to go into operation.
(8) The space requirement for fuel cell power plant is consider-
ably less as compared to conventional power plants.

Disadvantages The main disadvantages of fuel cells are their

high initial costs and low service life.

Downloaded from Ktunotes.in

Downloaded 1from Ktunotes.in
Downloaded 2from Ktunotes.in
Downloaded 3from Ktunotes.in
Downloaded 4
from Ktunotes.in
16.3. Mechanical Energy Storage
16.3.1. Pumped Hydroelectric Storage
Electric power in excess of the immediate demand is used to
pumpwater from a supply fe.g. lake, river or reservoir) at a lower level
to a reservoir at a higher level. When the power demand exceeds the
supply,the water is altowed to flow
back down through a hydraulic
turbine which drives an electric generator. The overall efficiency ot
pumped slorage, thrat is, the percentage of the electrical energy used to
pump the water that is recovered as electrical energy, is about T0 per
cent.
Pumped hydroelectric storage is the most economical means
presently available to electrical utilities. It could also be used for storing
electrical energy produced from solar and wind energy.
There are relatively few suitable sites where there is a water
supply at a lower level and reservoir be constructed at
can a higher
level, but the use of natural or excavated underground caverns as lower

Downloaded from Ktunotes.in

 Energy Storage And Distribution 781

being developed,
reservoirs, now be should greatly increase the number of
possible sites
Pumped storage is an indirect method for temporarily storing
eutbstantial amounts ofelectrical energy by pumping water from a lower
substant

toa higher level. Pu

umped storage can be used in conjunction with

lectrical generating plants ofall types, regardless of the energy source.

In a pumped-storage fac1lity, generated in excess of the
the power
river
demand is used to pump water from a lower reservoir (e.g. lake,
a

cavern) to an upper reservoir. During periods of peak

ar underground
exceeds the normal generating plant
demand, when the power demand flow through a
capacity, water from upper level is allowed to
the
at the lower level. (Fig. 16.3.1.1) the turbine then
hydraulic turbine
a generator to produce electricity in the usual way.
drives
Upper reservoir

Moter/Generator To turbineand

Storage
generator 7777
F r o m m o t o ra n d

pump

Lower reservoir

Pump/tur bine
(Lake river
etc.)
TTT77
Fig. 16.3.1.1

is
the turbine generator system
pumped-storage plants,
In most the lower to the upper
s e r v e to pump
water from mode, the
reversible and
can the pumping
electricity. In a gener
evel, as well a s to generate
electricity produced by
driven by Start
becomes a motor, operates a s
a pump.
senerator turbine then
and the to turbine/gen-
ator in the main plant, motor/pump
orreversal from after
turbine/generator could be stored
pot the a few
minutes, s o
that power

crator requires only in the main plant.

the of a failure
event
4 short delay in levels (i.e. the
and lower
between upper
difference
from less than
30 m too
The altitude
facilities ranges
storage reversible
water head) in pumped rule, Francis type
300 m. As a general turbines, are
m o r e than type
heads propeller
sOme what but for
low
urbines a r e used,

Preferred. Downloaded from Ktunotes.in

782 Non-Conventional Sources of EnergY
In the pumped-hydro system the high heads are desirable come
and thus pumped-hydro systems
topographies do not allow them,
which includes the preferred high h d
often classified asabove-ground,
and medium head, and underground.
AccesS

Upper
reservoir

Nent
Shaft
Water
Conductor
Power
Plant

Lower reservoir

Fig. 16.3.1.2. Schematic of an underground pumped hydro storage system.

In underground pumped hydro system, the upper reservoir may

be at or near ground level. The lower reservoir is placed underground
in natural caverns, old mines, or other underground cavities. This type
of system overcome the requirement of a suitable topography. Sucha
system is shown in Fig. 16.3.1.2. In all systems, a reversible pump-tur-
bine or motor-generator set is a principal piece of equipment.

Pumped hydro, like compressed air, is a potential-energy storage

system suitable for large utility energy storage. It is the most developed
and used of all storage systems. The principle behind pumped hydro is
and follows the law
simple
of mass to
of potential energy (PE) that is, the raisimg
an elevation, height or head H. It is given by
PE = Pg Qo l1,
where Qo is the natural flow rate of water at the site,
PE =potential energy (Joule)
gravitational acceleration 9.81 m/sec 2
H = the vertical distance through which water falls, m.

Also PE =g ml ...(16.3.1)
where mn = mas (kg/sec)
= p Qo

Downloaded from Ktunotes.in

 Bnergy Storage And Distribution 783
The operating heads on the pump turbine, in the pumping
H.and in the turbine-generating mode 1r are different and are mode
made
up of
two components each.
1, = H + H .(16.3.2)
H =H-H . .(16.3.3)
where H is the static head or height and Il, represents the losses during
ow conditions (which are different because of different flow rates).
The pumping and generating powers are given by replacing the
mas in equation (16.3.1) with the mass flow rate kg/s and using the
proper head, o r with
Pp 8p Q, Hp ...(i6.3.4)
and
Pr = 8 p r Hr ..(16.3.5)
respectively (W)
where Pp and PT =pumping and turbine powers

P density of water (kg/m*)

volumetric flow rates in pumping and generation,
Qand QT =

respectively, m'/s.
will store
that 1000 kg raised 100
m
Equation (16.3.1) shows m a s s e s must be
elevated to
0.2725 kWh. Thus large
9.81x 10" J or of energy. Fortunate-
heights to store large quantities
sufficiently large the elevation
a r e available
in pumped hydro systems by
One o r
ly large masses

of water from a
lower to a n upper
reservoir.
quantities may be
a
of large excavated or

these reservoirs may be artificially

both of
lake. 65
natural river o r from
of a pumped-storage
plant is commonly
The efficiency 25 to 35 per cent,
pumped storage
the loss of
to 75 per cent.
In spite of power at
times of peak
stored water generates
economical. The turbine o r diesel
c a n be be supplied by a gas
would
otherwise from
demand when it the otherhand, apart
On
to operate. pumped-
engine that is expensive the costs of
operating a

maintenance charges,
capital and
small. storage
are P l a n t s . Pumped
storage plants Storage
Advantages
of Pumped

plants have the

following advantages: pumped storage
plants
peaking units, of
to other
source
economical
compared
(2) As cost and
thus a n

low capital
have relatively as
dependable
peaking capacity. is as rugged and in a
storage plant load rapidly
The pumped can pick up
() station and
conventional hydel power well as
automation as
minutes.
matter of few adoptable to
are
readily
Such plants
(iii)
pollution.
remote control. e n v i r o n m e n t a l

from
effects
of
It is free
(iv) Downloaded from Ktunotes.in
784 Non-Conventional Sources of Energy

() These types of plants allow a great deal of flexibility in the

operational schedules of the system.

(vi) The power required for pumping is available at cheaper rate.
(vii) The pumped storage plants allow the entire thermal
r
nuclear power generation to take up the base load. Thus the load fant
actor
of these units improve giving rise to overall greater system efficiency.
cy
(viii) Standby capacity is available on short notice. Power en-
gineers in utilities having pumped-storage installations have long
g
realised the benefit of quick sitching on and off capability of these
installations. Pumped storage plants need a starting time of only 2 to
3 seconds and can be loaded fully in about 15 seconds. In the event of
an outage on a unit, a pumped storage plant can be called upon to meet
the generation deficiency, (occurring due to outage) thus ensuring
reliable supply and avoiding the necessity of load shedding.
(ix) Since the base load plants need not be used to supply peak
loads, the forced and maintenance outages of these plants are likely to
be reduced.
(c) Pumped storage plants can be used for load frequency control.

16.3.2. Compressed Air Storage

This type of storage is analoguous to pumped hydro
storage.
Whereas in pumped hydro system excess energy generated by a base-
load plant during periods of
low demand is used to increase the potential
energy of hydrostatic pressure of water, compressed-air energy storage
compresses and stores air in reservoirs, aquifiers, or caverns. The stored
energy is then released during periods of peak demand by expansion of
the air through an air turbine. In general, the
air storage is comparable to that of
efficiency of compressed
pumped-hydro storage.
In a gas-turbine, roughly 60 per cent of the power output is
consumed in compressing air for combustion of the
gas. In the com-
pressed air storage system, electrical energy in excess of the demand is
sed
to compress air which is stored ina reservoir for later use in a gas
turbine to generate electricity. Compressed- air storage could serve for
electric utility load
levelling or for storing electrical energy generated
from solar or wind energy. The overall
to be about 65 to 75
recovery efticiency
is
estimateu
per cent. A wind turbine, for example, could
created which would De
directly pump air into a suitable pressurizea
storage tank. Then later when the wind is not
in the air could be utilized to drive blowing, the energystore
an air turbine whose shaft
then drive a generator, this woula
the wind is not blowing a
supplying the needed electrical power when
compressed air storage may also be applicable
to other solar-electric conversion
systems.
Downloaded from Ktunotes.in
 Energy Storage And Distribution 785
In a
conventional
aannected. In a
gas turbine, the
compressor and turbine are
con.
compressed-air energy storage system, however, the
t+a1rbine
and
compressor are
uncoupled so that they can operate
separately. Furthermore, the electric generator, normally connected
sepa
he turbine, must also be capable of to
functioning
electricity is supplied. (refer Fig. 16.3.2.1). as a motor when
Air
Compressor
Exhaust
Clutch
Clutch
H Motor Generator
H
Gas
turbine
Cooler
Fuel Air
heater
Valve Valve

Compressed air
s1orage

Compressed-air-energy storage.
Fig. 16.3.2.1.

the immediate demand is supplied to

Electric power in excess
of compressed air
thhe compressor ; the
which drives below).
the motor/generator suitable reservoir (see
is stored in a
at about 70 atm (7 MP), may and have to be cooled prior
compression When additional
air is heated during the reservoir walls.
The damage to and
to storage to prevent compressed air is released
the
is needed to meet the demand, compressed air is then expanded
power fuel. The hot which n o w acts
o r oil
using gas m o t o r / g e n e r a t o r unit
neated to the
connected
n a gas turbine
too large
as a generator. would probably be
reservoir
storage hence, underground
underground
Compressed-air construction; ; hence, the
above ground c o n s i d e r e d . Among
and too expensive for a r e being oil
preferably
existing ones,
aquifiers, depleted
gas or
reservoirs, caverns,
deep abandoned
mines.
natural and
possibilities are
or salt c a v e r n s ,
mined-out rock petroleum
reservoirs, the past to store
used in the
have been storage loading for
Caverns
Salt compressed-air
under
stable
products. They are
Downloaded from Ktunotes.in
 786 Non-Conve: tioral Scurces of Ener

duration of plant life. 'he major concerns are cavern geometry, size :
of rock salt, and airleaka
acing, long-term cre ep and creep-rupture formation.
are naturally occuring porous
rock They have h
A ifiers
serl for natural gas storage. Hard-rock caverns type of reservoi
quire water- compensating surface, due to their size. It is to maintain
n
a. pr ssure. Therefore these types are mostly compare the two pes
me tioned above, but they are stable.
When the air is ompressed for storage, its temperature will ris
se.
The heat of compress: on may be retained in the compressed air or in
another heat-storage medium and then restored to the air before
expanding through the turbine. This is called adiabatic storage and
results in high storage efficiency. Restoring the heat to the air also
prevents the turbine parts from freezing if low temperature air is
allowed to dissipate, additionalheat could be added by fuel combustion
to retain high storage efficieney, but the results would be extra
the
expense and maintenance problems. This is called a hybrid system.
16.3.3. Energy Storage via Flywheels
The basic idea of flywheel energy-storage, sometimes referred to
as a
"super flywheel, is to accelerate a suitably designed physical rotor
to a very high speed in a vacuum, as via an electric motor, at which
state high energy storage densities are achieved. The energy is stored
as kinetic
energy most of which can be electrically retrived when the
flywheefis run as a generator, (armature is rotated by the flywheel).
Flywheel couldconceivably be used for electric utility peaking units, for
storage ofsolar andwind energy, and for
vehiclepropulsion.
recovery efficiency is estimated to be upto 90 per cent. The
The energy
of super flywheel energy proponents
storage claim this storage method has
energy storage per kg than conventional higher
batteries.
flywheel or lead acid
storage
More recently interest in flywheel energy
generated by motor vehicle designers. In the storage has been
so-called
automobile for example, the hybrid
flywheel stores some of the energy o
gasoline engine during periods of low vehicle demands and
during periods of high releaseshill
demands, such
climbing, etc, and thus operates as
during acceleration,
the engine at a
more efficient output. more steady and hence
Flywheels have been used
pulses from reciprocating engines. extensively to smooth out power
energy.The fluctuations in speedThey store off-peak energy as kinetic
reduced to a minmum by the use ofcaused by torque variations are
flywheels., As kinetic energy
proportional to the mass times velocity squared,
from the addition or subtraction of the changes in veiocity
kinetic energy are reduced by the
Downloaded from Ktunotes.in
 Energy Storage And Distribution 787
e of a large mass.
Conversely the energy stored in a flywheel can be
use
creased
inc by increasing the velocity.
Energy stored in flywheel (E) is
a
equal to the kinetic energy, given by
E =; mv
m(2rn)2
=2 mR22
...(16.3.3.1)
where velocity of the flywheel = 2rtRn
E = energy (Joule)
m = mass of the flywheel (kg)
R =radius of gyration (m)
0.7071 Ro
Downloaded from Ktunotes.in
Ro outer radius
n revolutions per second
revolutions/min.
60
The energy E absorbed or released by a flywheel between speed
ofrotationni and n2 is thus given by.
AE 2 mR* 0n22 n1 -
..(16.3.3.2)
The ratio of the variation in rotational speed to the mean speed
n is called the coefficient of speed fluctuation ks, given by
n2 - n1
ks
2(n2 n1 .(16.3.3.3)
n1+n2
n +n2
where n =-
2
Now AE = 27 mnR" (n2 + n1) n2-n)
16.3.3.2 and 16.3.3.3)
= 2T mR 2n kg n (from Eqs.
..(16.3.3.4)
4 ks mnR* n
The value of the coefficient ks depends upon the desired close
it varies from 0.005 for fine to 0.2
ess of speed regulation. For example absorption AE, m and or
Or coarse regulation. Thus for a given energy
Maximum energy densily of
must be high for close speed regulation.
a rotating steel disk is
W 1/2I2
m
 Non-Conventional
Sources of Energ
788
rotation of an object is Ë which is equal+
The kinetic energy of of the object about its avi
1/2I o, where I is the of inertia
moment
(rad/sec). In the simplest case, where th
and o is its angular velocity
in a rim of radius r
concentrated
=
a, then l =ma2. For a
mass m is I is lower (ma/2) because the mae.
ass
uniform disk of the same mass,
to .
nearer the shaft contributes less
Now W ma2 m
.(16.3.3.5)
For a flywheel to be a useful store of energy (and not just a
Downloaded from Ktunotes.in

that it must rotate

smoothing device), it follows from Equn. (16.3.3.5),
as fast as possible. However, its angular velocity is limited by the
the centrifugal forces
strength of the material which has to resist
tending to fling it apart.
For a uniform wheel of density p, the maximum tensile stress is
Omap o a ...(16.3.3.6)
Ingenerall = km a for a particular shape, where k is a constant
-1, so
W- 2
.(16.3.3.7)
R Omax
..(16.3.3.8)
and
Wm 2p
Conventional materials such as steel give rather low energ
densities. For a rotating steeldisk, maximum energy density with k =1,
Wm 1x(10 N/m (from Equn. 16.3.3.8)
2x 7800 kg/m
- 0.06 MJ/m
Much higher energy densities can be obtained by using light
weight fibre composite materials, such as fibreglass in epaxy resius
which have higher tensile strength omax and lower density p. To man
the best use of these
materials, flywheel should be made in unconven
tional shapes with the
strong fibres aligned in the direction of maximum
stress. Such systems can have energy densities of 0.5MJ/kg(better thau
lead acid batteries) or even
higher.
Materials for energy storage
flywheels must have
strengths, high strength density ratio, high resistance to high tensie
growth, and high strength density to costratios. Those undercyclic crac
cônsidera"
tion include some alloys, such as so-called
maraging steels, and mo
 Bnergy Storage And Distribution 789
oromising, composites such as fibre-reinforced plastics. One
that shows particular promise is a 62
composite
volume per cent S-glass in epoxy
composite. Other attractive composites are graphite-epoxy and kelvar.
epoxy.
Flywheels for energy storage are systens that include, besides
he flywheel it self, a number of subsystems. These are a housing;
bearings with ball bearings believed the most suitable; a vacuum pump
to minimize windage losses inside the housing ; seals to minimize oil
and air leakage into the vacuum chamber ; and sometimes a contain-
ment ring to protect nearby personnel and equipment from flying
fragments in case of flywheel rotor fracture.
Losses in a flywheel energy-storage system include windage,
bearing and real friction, vacuum pump input power, and eddy current
Downloaded from Ktunotes.in
(hysteresis) and other inefficiencies in the motor generator (or in
transmission systems). In early designs these losses were prohibitively
arrive at
development work still needs to be done to
a
large, and much
technically attractive system.
For use in smoothing demand in large electricity networks,
that they can
flywheelhave the advantage over pumped hydro-systems
little land a r e a . Units with a 100
be installed anywhere and take up
of about 10 MWh. Larger
tonne flywheel would have a storage capacity such
best be met by cascading many
storage demands would probably
small' units. bat-
interesting alternative to storage
Flywheel also offer an
vehicles especially since the energy
teries for use in electrically powered than in a battery.
m o r e quickly
c a n be replenished
in a flywheel
acid battery
16.4. Electrical Storage : The lead
Storage batteries
and therefore great
Electricity is a high
grade form of energy,
for storing it. A device
and efficient m e a n s
effort is made to find cheap is called a n (electrical)
both a input and output
that has electricity Usualy the combination of
or (electrical) accumulator. storage, however.
storagebattery included a s
'electrical
fuel cell is not photovoltaic and
electrolyzer and of almost all
essential component
form an development of battery
Batteries
there is steady
Small wind electric
system, and
powered vehicles. reversible in
reactions are
electrochemical
wil be
Although m a n y storage battery, which
suitable for a practical and discharging
theory, few
are charging
times b e t w e e n is the lead
required to cycle
hund eds of used storage battery
Tle most widely
France in 1860.
currents of 1-100A. Plante in source
invented by it to a
acid battery, charged, by connecting
storage
battery is o c c u r in
the battery
When a chemical changes
electric current,
of direct
 Non-Conventional
Sources of Ener
790
materials. As a result, electrical energy 1s converted into stored chem
g.
emi-
load (e a a
battery is discharged,
by connecting
When the a r e reversed an
calenergy. chemical
the
reactions
and
the terminals, into electrical he
motor) between
energy
reconverted
energy is
the stored chemical varies with the type of
ni
recovery effieieney of a
storage battery
cent should be attainable
energy but 76 per
battery and
the rate of discharge,
often lower.
efticiencies are
However the
that is to say, they are built
modular in nature;
Batteries a r e m e a n s that the
moderate size. This ergy
individual units of
up of over a wide range simply by varying
the
can be vatied
storage capacity connected together. Consequently,
batteries are
number of units that
are
is in electric
of in which the energy input
adaptable to any type storage shaving (and possibly load
form. Potential applications
are utility peak
Downloaded from Ktunotes.in
and storage ofelectrical energ generated
levelling), vehiclepropulsion, rapid operation
from wind energy solar cells. The capability of
or
makes batteries especially con-
reversal, from charge to discharge,
venient for electric utility applications.
Moreover, they permit dispersed
load centres.
distribution by locating storage facilities n e a r
is the lead acid
By far the most common type of storage battery and
ignition)
battery (see Art. 10.3.6) used in the SLI (Starting, Lighting
and other road vehicles. These
systems of essentially all automobiles condition by a
batteries are usually maintained in a fully charged
when the engine is running.
generator which produces direct current
conditions
For utility applications or vehicle propulsion, the operating
are quite different. In such uses, the battery is subjected to a deep (i.e.
almost complete) discharge and then recharged roughly once a day.
When subjected to such repeated cycling, the lead acid batteries used
for automobile SLI systems have a short life time.
Studies in the progress will undoubtedly result in the production
of lead-acid batteries/more-suited to utility and vehicle applications
However, there are some basic limitations to lead-acid batteries (8
their heavy mass per unit of stored energy) that can not be overcome
Consequently, several other kinds of storage batteries, usually, entirey
different materials and some operating athigh temperatures, are under
development in the hope of finding a type (or types) that has a low (or
moderate) cost, a
The
longlife time, and is lighter than the lead- acidbattery
low (or
battery requirements for stationary (utility) applications are
mnoderate) cost and
long life time; the mass is less impor
a
thantor vehicle propulsion. For stationary storage, a battery shoula
capable of atleast
3000 deep discharges over a life time of 10 to 15
The discharge time for peak years
power supply would be 8 to 10 hours an
the charge time roughly 10 hours. No existing stbrage battery can e e t
these requirements, but there is u
hope that one or more of the new P
 Energy Storage And Distributioi 791
ill eventually do
transmit
s0.
Central-station
alternating current (ac),
power
but direct
plantsusually
generate and
hattery charging. current (dec) is required for
Furthermore,
de which must be converted upon discharge, the batteries
into ac for produce
Hence, the battery test facilities will feeding into transmission lines.
involve the
for converting ac de, and de back to ac in an testing of equipment
into
Basic Battery Theory. A efticient manner.
battery is a
cells. A cell is the elemental ecombination of thecombination of individual
constituting the basic electro-chemical energy materials and electrolyte
storer. Abattery also can
he thought of as a black box into which
electrical is put, stored
electrochemically, and later regained a s electricalenergy
energy.
A generalized cell
consisting of two electrodes called the anode
and cathode immersed in a suitable electrolyte. When an electrical load
is connected between the electrodes charge separation occurs at the
Downloaded from Ktunotes.in
interface between one electrode and the electrolyte, freeing both an
electron and an ion. The electron flows through the external load and
the ion through the electrolyte, recombining at the other electrode.
The polarity and magnitude of the cell terminal voltage is, in
general, a function of the electrode materials, electrolyte, cell tempera-
ture, and other factors. By combining appropriate numbers of cells in
both series and parallel, the battery c a n deliver the desired voltage and
current. An appropriate combinations of cells can provide the desired
power output. Fig. (16.4.1). Charge
SNNN1
Discharge C
H20
Pb Pb02
HSO
acid cell.
Schematic diagram of lead reaction.
Fig. 16.4.1. direction
shown during the discharge
move in the during charging.
The charge carriers movements
are
reversed
and carrier closed.)
The reaction and S
(Switch S, open
of which is shown
built up from cells,
one
battery is cells, there are
The lead acid in all electrochemical
16.4.1. As In this
schematically in Fig. solution (electrolyte).
conducting
immersed in
a
two solid plates
 792 Non-Conventional Sources of Energy
case the plates are in
the forn of grids holding pastes of lead and lead
is sulfuric acid, which ionizes as
dioxide respectively. The electrolyte
follows.
H2SO H' +HS04
...(16.4.1)
During discharge, the reaction at the negative electrode is
Pb+ + HSO04 PhSO +H' +2
..(16.4.2)
Lead (Pb) is oxidized to Pb*" which is deposited as PbS04. The
sulfate takes the place of the Pb paste in the plate. The electrons so
liberated travel through the external circuit to the positive electrode,
where they contribute to the reaction:
PbO2+HSO4 + 3H* +2e- PbSO4 + 2H,0 ..(16.4.3)
Downloaded from Ktunotes.in

This PbSO likewise replaces the PhOg in that plate. The electri
cal current through the solution is carried by H° and HSO," ions from
the sulfuric acid, which themselves take part in the plate reactions.
Knowing the reactions involved and the corresponding standard
electrode potentials (given in the chemical tables), the theoretical
energy density of any proposed battery can be calculated. Theoretical
energy density of lead acid battery (Wm") is calculated and its value
omes out to be 0.60 MJ/kg of active material as shown below:
The reactions (16.4.1) and (16.4.2) show that to transfer 2
mol of
electrons requires:
1 mol Pb 207 gram
1 mol PbO2 239 gram
2 mol HoSO4 196 gram
=
Total active material = 642
gram.
But 2 mol of electrons a represents charge
(2mol)(-1.60 10- C(6.02 102 mol)
x x
= (2) (9.6) (10) C 1.93 10
-
= -
x C
The standard electrode
potential for Pb/PbSO, is 0.30 V
andfor (PbSO,/Pb4*) is-1.62 V.
So the theoretical cell emf for
Ecell = + 1.92 V. (Pb/PbSO,/H,SO PbS0,/PbO,) is
With the PbO2 plate positive, according
convention. to the IUPAC sign
The actual cell
and can be calculated
EMF depends on the concentration of reagents,
the voltage of a cell by standard electrochemical
methods. In general,
operating
cent from the theoretical at low currents differs
by only a few per
cell voltage. In particular, lead acid batteries
are usually set to give 2.0 V per cell.
 Energy Storage And Distribution 793

Therefore the work done in moving 2 mol of electrons is (1.93 x

10 C)(2.0 V) = 0.386 x 10" J. Thus the energy stored in 1 kg of active
105
Tedients is, in theory,
W,n = (0.386 x 10" J/0.642 kg) = 0.60 MJ/kg.
Unfortunately, the energy density W, of any practical battery is
lways
always much
m less than the theoretical value Wm, if the total m a s s of
he whole battery is considered. Most commercial batteries have Wm
o15 Wn', although more careful (and more experience) designs can
be expected to achieve energy densities up to 25% of the
rea reasonably
theoretical values.
for
In the specific case of the lead acid battery, the main reasons
this'under achievement' are active meterials,
(1) A working battery necessarily contains non
the electrodes short circuit-
eg. the case, the separators (which prevent
water in which the acid is dissolved. (The
acid concentration
ing) and the
will discharge itself). Since the
mass

must not be too high or the battery active ingredients, the

of an actual battery exceeds the m a s s of the calculated from the
theoretical value
energy density is less than the
active mass alone.
If all the
allowed to go to completion.
(2) The reactions cannot be be n o electrode left
consumed by reaction (16.4.2), there would
lead w a s
could not be cycled.
reaction to operate at i.e. the battery
for the the
is allowed to fall too low,
r e v e r s e
of
concentration H2SO4
Similarly, if the the battery
adequate conductor. In practice,
ceases to be an
electrolyte m o r e than
about 50% ofits stored energy,
cannot be allowed to discharge
called a 'deep discharge.
Such a discharge is
or it will be ruined.
batteries is similar to all
car owners

A further limitation
a s dense a s
of real
the PbSO4
forever. Solid Pb is almost twice
they do not last Therefore it is difficult
to fit
reaction (16.4.2). in
found in the discharge occupied by the Pb paste
the space originally
the PbSO crystals into some PbSO,
falls to the bottom
of
negative electrode. In practice, irreversible loss of active
the This constitutes a n
The cell in every discharge. is allowed to fully
discharge;

material. This loss is worse if the battery

to recharge
the battery.
become impossible
1deed, it may rapidly well-maintained
ofeven a
limiting the life
The other main factor electrode. This is
particularly
of the positive Pb but a
battery is self-discharge the grid is pure not
batteries in which stand the
actute in vehicle (SLI)which is stronger and better able to
ead-antimony alloy, antimony promotes
motion. Unfortunately
nechanical stresses during
the reaction.
(SbO)2 SO4 + 5PbSO, + 6H,0
6HS04 ..(16.4.4)
5PbO2 +2Sb +

Downloaded from Ktunotes.in

 Non-Conventional
Sources of Energy
794
renmoves
active material from #
irreversibly,
which also slowly, but
battery. (e-B. photovoltaic lightine
Batteries for stationary
applications
life (upto 7 years) if no
years) ifno
Sb-free plates and have longer
systems) c a n u s e
excessively discharged.
the current at which i
The performance
of a battery depends on
depth to which it is
and discharged, and the ularly
is charged
discharged.
16.5. Chemical Storage
16.5.1. Introduction
Downloaded from Ktunotes.in

chemical compounds
bonds of many
Energy can be held in the
and released by exothermic reactions,
notably combustion. Some times
other catalysts (e.g. enzymes) to promote
it is necessary to apply heat or
a r e a special c a s e . Here w
the desired reaction. Biological compounds
discuss the most important inorganic
compounds which have been
means of their combustion in
suggested practical energy storers by
as
air.
16.5.2. Energy Storage via hydrogen
which
Energy can be both stored and transported hydrogen,
or
serves as a secondary fuel.
The input energy, usually electrical but
electro-chemical
possibly thermal, serves to decompose water (H20) by
(or chemical) reaction into its
constituents elements hydrogen and
substances can then be recombined to release the
stored
oxygen. These
energy as required instead of using the oxygen produced from water in
commonly
this energy recovery process, oxygen from the air is
sold for industrial
employed. The pure oxygen from water can then be
applications (e.g. in iron and steel fabrication). Hydrogen_can_be
transported either compressed hydrogen gas, as liquid hydrogen(a
as
metals
low temperature), or in the form ofa solid compound with certain
of storing
alloys. Consequently, hydrogen may be useful as a means
and transporting energy generated in remote locations far
from l0au
centres.
The most convenient means for producing hydrogen and oxyee
from wateris by electrolysis, that is, by passinga direct electric curre
through water containing an acid or alkali to make it an electr
conductor. The input energy is then in the form of electricalenergy
al
may also be possible to decompose water by heat (i.e., with therieol
energy input) as a result of a series of chemicalreactions. The chem
energy in hydrogen (and oxygen) can be converted into thera
mechanical, or electrical energy. One possibility is to burn hydroge
air, in amanner similar to natural gas, to produce heat (thermalener&
 Energy Storage And Distribution 795
r 115e in the home or in industry. Hydrogen can also serve as the
n Dlace of gasoline in automobile truck, and even air craft fuel,
engines.
Electrical energy can be obtained from hydrogen in several ways.
For example, steam from a water boiler heated by burning hydrogen
eould be used to drive a conventional steam turbine with attached
electric generator. Alternatively, hydrogen can provide the fuel for a
gas (combustion) turbine which in turn drives a generator. The maxi
mum overall efficieney (possibly 55 to 60 per cent) for recovery of the
mu
input energy, however, would be obtained by means of a fuel cell ; in
such a cel, electrical energy 1s generated directly from hydrogen and
OXVgen. Fuel cells could also be used in homes and apartments houses,
by industry, for peak saving by utilities, and in electric vehicles.
Downloaded from Ktunotes.in
Energy from various solar electric systems can be stored in
with
hydrogen. A wind-electric or photovoltaic system, for example a
tank which
d.c. output, the power can be fed directly into an electrolyzer
produces hydrogen and oxygen from ordinary water as already stated.
The gases may be either under pressure or near atmospheric
produced desired
via external pumps compressed to the
pressure and then
however requires auxiliary energy. The
approach,
pressure. The latter in
can be stored either gas liquid
or
hydrogen and oxygen gas produced and easily
forms for time. It can when needed, be quickly
a long fuel
electrical energy via the well known
converted again directly into stores the sun's energy
thus effectively
cell (see Art. 10.2). The system smooth reliable power
and from this storage a
as hydrogen and oxygen,
the hydrogen storage
for a limited time set by
output may be taken
capacity. Most
quantities is not
common.
To store hydrogen in large such as those from which
underground caverns,
promisingg is the u s e of of gas-even if compressed-is
extracted. But storage
natural gas is n o w since its boiling point
is 20 Kthese
be liquefied, but a s metal hydrides,
Dulky. Hydrogen c a n Chemical storage
to maintain.
Stores a r e awkward be released by heating,
can
is manage- more
rom which the hydrogen
volumes of H2 to
be stored. For example:
aDle and allows large
T 50°CC
>FeTiHo.1 +0.8 H2
FeTiH.7 so that a portable hydride
store can
reversible, The heat
This reaction is central filling station'.
a
hydrogen a s and the portable
replenished with distrie heating,
e can be used for
vehicle. of
The main
this process tank a
e d in the fuel also
be used a s metals u s ed.
d. Hydrogen c a n
Hydrogen
can
dride store can
and cost
of the for natural
networks used
difficulty is the weight extensive pipeline
through the to convert H2 very efficiently
E distributed countries. It is also possible described.
already
ss n many
means offuel cells as
toe Clectricity, by