This presentation is on the topic " Renewable energy sources" which is based on the Chemistry Undergraduate syllabus of Mumbai University.

1. Renewable Energy Sources
Dr.Pravin U. Singare
Department of Chemistry,
N.M. Institute of Science, Bhavan’s College,
Andheri (West), Mumbai 400 058

2. Introduction
• Energy sources are broadly classified as Renewable and Non-renewable energy sources.
• Non-renewable energy sources like oil and coals produce energy when they are burned.
• Such non-renewable energy sources after they are burned do not naturally replenish in a
short span for human use.
• Hence supply of such energy sources are limited.
• They also have tremendous negative environmental impact.
• Renewable energy sources often referred as an alternative energy sources serve as a fuel
option replacing conventional non-renewable fossil fuels like oil and coal.
• They are called renewable energy sources because they are constantly getting replenished
through natural process.
• Renewable energy resources have very less or no negative environmental impact as
compared to conventional non-renewable energy sources.
• Technologies using solar energy, wind power are becoming more common globally as a
renewable energy sources.

3. Semiconductors, Conductors & Insulators
• In case of a regular metallic solid, very large number of atoms are closely packed together.
• In such case electrons in any orbit can have a range of energy levels rather than a single
energy level.
• This range of energy levels processed by an electron in a solid is called energy bands.
• The energy band which is occupied by an valence electrons is called valence band.
• This valence band has electrons of highest energy.
• This band may be partially or completely filled.
• In case of metals, the valence electrons are loosely attached to the nucleus.
• As a result, some of the valence electrons will get detached from the nucleus to become
free electrons.
• This electrons are called conduction electrons and will occupy the conduction band.
• The energy gap between the conductance band and valence band is called forbidden
energy gap.

4. Semiconductors, Conductors & Insulators (continued------)
• In case of metallic conductors, the valence band and the conductance band overlap each
other.
• As a result electrons can easily slip from the valence band to the conductance band.
• For the same reason, the metals are good conductors of electricity at all the temperatures
(even at room temperature).
• In case of an insulators, the conduction band and valence band are separated by a very
large energy gap
• This energy gap also known as forbidden energy gap is in the order of 5eV to 15 eV.
• As a result practically no electrons can cross such a large forbidden energy gap and go to
the conductance band.
• For the same reasons, insulators can not conduct electricity even by increasing the
temperature.

5. Semiconductors, Conductors & Insulators (continued------)
• Semiconductors are the materials having small conductance at room temperature.
• With rise in temperature, the conductance of semiconductor increases.
• In case of a semiconductors, the forbidden energy gap between the conductance band and
valence band is small with the energy difference of 1 eV.
• Hence small amount of energy which is supplied externally will be sufficient for the
transfer of electrons from valence band to the conductance band.
• As a result the conductance capacity of semiconductors will increase with rise in
temperature.
• Semiconductors can be doped by externally adding the impurities.
• Such semiconductors are called extrinsic semiconductors.
• The extrinsic semiconductors are of two types
 p-type semiconductors
 n-type semiconductors

6. P-type extrinsic semiconductors
• Consider Silicon (Si) semiconductor lattice as shown in Fig 1, in which
each Si has 4 valency electrons which form 4 covalent bonds with the 4
neighboring Si atoms.
• Consider that one of the Si atom in the semiconductor lattice is
replaced by an impurity element like Boron (B) from Group 3 (Fig. 2) .
• The impurity element B will contribute 3 valence electrons to the
lattice (forming 3 covalent bonds with the neighboring 3 Si atoms) ,
leaving one positively charged hole.
• When an electric field is applied across the dopped crystal, an electron
from the neighboring covalent bond will jump into the hole leaving
behind hole in the previous position.
• This hole is again filled by an electron from the next bond and so on.
• Since the hole arising due to addition of group 3 impurities will accept
free electrons, Group 3impurities are also called an acceptor.
• Because an acceptor donates excess of positively charged holes, a
semiconductor which has been doped with an acceptor is called P-type
semiconductor.
• Thus P-type semiconductor is a material having holes as a majority
carrier while electrons as a minority carriers.
Fig. 1
Fig. 2

7. N-type extrinsic semiconductors
• Consider Silicon (Si) semiconductor lattice as shown in Fig 1, in
which each Si has 4 valency electrons which form 4 covalent
bonds with the 4 neighboring Si atoms.
• Consider that one of the Si atom in the semiconductor lattice is
replaced by an impurity element like Arsenic (As), phosphorous
(P) etc from Group 5 (Fig 3).
• The impurity element P will contribute 5 valence electrons to
the lattice (forming 4 covalent bonds with the neighboring 4 Si
atoms) , giving one excess electron.
• When the electric field is applied across such doped Si crystal,
electron travel in a direction opposite to that of electric field
and the electric current is set up.
• Since electrical conductivity is due to the migration of
negatively charged electrons, they are called N-type
semiconductor.
• Because the impurity donate electron, it is called donor
impurity.
• Donor impurity donate negatively charged electrons to the
lattice, so the semiconductor that has been doped with a donor
impurity is called N-type semiconductor.
• In such conductor, electrons are majority carriers while holes
are the minority carriers.
Fig. 1
Fig. 3

8. Solar energy & Solar Cells
• Solar energy is the most common form of energy available from the
sun.
• This energy is available in the form of heat and light radiations.
• The solar energy reaching the earth surface is in the order of 1016
watts.
• The device which converts solar energy into electrical energy is
called solar cell.
• Solar cells are small semi-conductor devices which convert solar
radiations (radiant energy) into electrical energy based on the
photovoltaic effect.
• Hence they are also known as photovoltaic cells.

9. Construction of a Solar Cell
• Conventional silicon solar cells is made up of a
thin wafer of silicon.
• The silicon wafer is having thickness of 0.2 to 0.3
mm and size of 1 x2 cm.
• The thin wafer is prepared from either n or p
type of silicon which is doped with the
impurities.
• A very thin layer of p-type silicon is formed on n-
type silicon wafer or vice versa.
• This gives rise to p-n junction which provides
light sensitive face.
• A narrow conducting Ti-Ag electrode in the form
of a grid is placed on the top of the cell.
• The entire bottom surface of the cell is also
covered with the same Ti-Ag electrode material.

10. Working of the Solar Cell
• The electrode material grid on the top of the cell
allows the solar radiations to pass through and
reach the silicon layer below the grid.
• When the radiations are incident on the thin silicon
layer due to photovoltaic effect, the electrons are
dislocated from their normal position leaving
positively charged holes.
• Thus due to photovoltaic effect a pair of positively
charged holes and negatively charge electrons are
generated.
• At the p-n junction majority carriers from both p-
type and n-type silicon materials combine together
forming potential barrier across the junction.
• This positively charged holes and negatively
charged electrons are available for conductance of
electricity provided their recombination is avoided.
• The positively charged holes are pulled towards p-
side of junction while electrons are pulled towards
n-side of the junction.
• The electrons travel through the external load and
generate current in the circuit.

11. Limitations of Solar Cells
• The power output of a single solar cell is very less.
• The operational efficiency of solar cell is only 15-20%.
• For large scale power production, solar panel is used which
consists of array of solar cells.
• For construction of solar cells extra pure silicon is required
thereby increasing the cost of the solar cell and the cost of
energy generation.

12. Advantages of solar cells
• They have no moving part.
• They require low maintenance.
• The solar cells work satisfactorily with normal radiations.
• They are free of pollution.

13. Video on Construction and working of solar cell
•https://www.youtube.com/watch?v=L_q6LRgKpT
w&feature=emb_logo

14. Video on Construction and working of solar cell
•https://www.youtube.com/watch?v=UJ8
XW9AgUrw&feature=emb_logo

15. Hydrogen as a Fuel
• Hydrogen is emerging as the best alternative to fossil fuels (non-
renewable energy sources) as an energy carrier.
• It can be easily generated by direct electrolysis of water or by gasification
of biomass.
• Because hydrogen can be easily generated from renewable energy
sources and from water, it has a good potential as an energy source.
• Hydrogen may be stored as a gas or as a liquid.
• However, storage of hydrogen in liquid form is difficult due to the fact
that a very low temperature is required for its liquification.
• Hydrogen has a good properties as a motor fuel and it can be used in
internal combustion engines of automobiles.
• Hydrogen can also be directly converted into electricity using fuel cells.

16. Advantages of Hydrogen as an universal energy medium
• It is possible to produce hydrogen from electrolysis of water or by
gasification of biomass.
• It is most suitable fuel for use in fuel cells.
• Hydrogen on combustion produces water which can be used effectively.
• Amount of energy produce is very large.
• The cost of biomass which is used as a raw material in production of
hydrogen is very less. Hence have low production cost.
• Burning of hydrogen as a fuel produces negligible level of greenhouse
gases and other pollutants like CO2, CO, soot or unburned hydrocarbon.
Hence it is environmentally safe and cleanest source of energy
production.

17. Hydrogen production by direct electrolysis of water
• Extremely pure hydrogen as required in some fuel cells is
prepared by direct electrolysis of water using electricity
from solar and wind energy resources.
• The apparatus for electrolysis of water consist of a tank
filled with electrolyte which is 20-30% KOH solution.
• The metal electrodes are suspended in the electrolyte.
• Generally nickel (Ni) plated steel is used as a electrode.
• The electrode surface area is increased by depositing
porous Ni on the electrode surface.
• The electrodes are suspended in a porous diaphragms
(partitions) which are permeable to the electrolyte but not
to the gas molecules.
• The porous diaphragms (partitions) are made upof
asbestos which is supported on fine Ni wire.
• The electrodes are alternately connected to the same
terminal.

18. Hydrogen production by direct electrolysis of water (Continued-----)
• During electrolysis, hydrogen is liberated at the
cathode and oxygen gas is liberated at anode.
• The theoretical potential required to break the water
into H2 & O2 is 1.23 V.
• However, practically higher voltage of 1.7 V is required
to break the water into H2 & O2 .
• This extra voltage which is essential for decomposition
of water is called Over voltage which depends on
current and temperature.
• Therefore the required higher voltage is attained by
increasing the current density.
• With increase in the current density, the rate of H2
production also increases.
• The reaction taking place during the electrolysis of
water are
At cathode: 4H2O + 4e-  2H 2 + 4OH- (Reduction)
At anode: 4OH-  2H2O + O2 (Oxidation)

19. Videos on mass production of H2 as a fuel
• https://www.youtube.com/watch?v=g243uT_r4ZQ&feature=emb_logo
• https://www.youtube.com/watch?v=lQWIubQyaao&feature=emb_logo
• https://www.cnbc.com/video/2017/10/12/a-hydrogen-generation-
plant-that-turns-water-into-car-fuel.html