Q. 1 Describe briefly what Biogas is?

Biogas is a renewable energy source produced by the breakdown of organic matter by certain
bacteria under anaerobic conditions. It is a mixture of methane, hydrogen, and carbon dioxide. It
can be produced by agricultural waste, food waste, animal dung, manure, and sewage. The process
of biogas production is also known as anaerobic digestion.
Biogas recycles the waste products naturally and converts them into useful energy, thereby,
preventing any pollution caused by the waste in the landfills, and cutting down the effect of the
toxic chemicals released from the sewage treatment plants.
Biogas converts the harmful methane gas produced during decomposition, into less harmful
carbon dioxide gas.
The organic material decomposes only in a wet environment. The organic matter or the waste
dissolves in water and forms a sludge which is rich in nutrients and can be used as a fertilizer.

Q. 2 What is a Biogas Plant? Explain the process of biogas production.

Biogas production is carried out in anaerobic digesters known as Biogas plant. These have five
components:
1. An inlet to feed the slurry
2. The fermentation chamber where the biogas is produced with the activity of
microorganisms,
3. The gas storage tank to store the gas produced,
4. The outlet for the used slurry,
5. The exit pipe for removing the gas produced.
The organic matter if fed into the digesters which are completely submerged in water to provide it
with an anaerobic environment. These digesters are hence called ANAEROBIC DIGESTERS. The
micro-organisms breakdown the organic matter and convert it into biogas. The biogas thus
produced is supplied to the respective places through the exit pipes.

Process of Biogas Production

1. The first stage involves the breakdown of organic polymers, such as carbohydrates,
making it available to the next stage of bacteria known as the acidogenic bacteria.
2. The acidogenic bacteria then convert the sugar and amino acids into carbon dioxide,
ammonia, hydrogen, and organic acids.
3. The organic acids are now converted into acetic acid, hydrogen, ammonia, and carbon
dioxide.
4. These are finally converted into methane and carbon dioxide by the action of
methanogens.
Methane is a combustible gas, i.e., it can be burnt. This gas is supplied to various places and is
used in cooking and lighting. It is an eco-friendly gas and reduces various environmental problems
like, it reduces the reliance on fossil fuels.

Q. 3 Describe Deenbandhu Biogas Plant with a neat sketch.

Deenbandhu model was developed in 1984, by Action for Food Production (AFPRO), a voluntary
organization based in New Delhi. About 90 percent of the biogas plants in India are of the
Deenbandhu type.
The Deenbandhu biogas plant has a hemispherical fixed-dome type of gas holder, unlike the
floating dome of the KVIC-design. The dome is made from pre-fabricated ferrocement or
reinforced concrete and attached to the digester, which has a curved bottom. The slurry is fed
from a mixing tank through an inlet pipe connected to the digester. After fermentation, the biogas
collects in the space under the dome. It is taken out for use through a pipe connected to the top
of the dome, while the sludge, which is a by-product, comes out through an opening in the side of
the digester.

Q.4 What is Anaerobic Digestion? Discuss the Advantages and

Disadvantages of Biogas.
Anaerobic Digestion (AD) is a natural process where plant and animal materials (biomass) are
broken down by micro-organisms in the absence of air. The Anaerobic Digestion process begins
when biomass is put inside a sealed tank called digester.

Advantages of Biogas:
1. Renewable Source of Energy: Organic materials are derived from plants, animals, and humans.
Raw materials can be reproduced, making biogas a green energy source. It also lessens the
damaging impact and improper wastes disposal.
2. Utilization of Waste: Instead of letting the wastes rot in landfills, it is more advantageous to
utilize and turn them into biogas. An environmental hazard is reduced due to lesser methane,
carbon dioxide, and other greenhouse gases produced. Wastes are turned into energy to utilize for
electricity, heating, cooking, and as fertilizers.
3. Produces a Circular Economy: Animal manure, food wastes, wastewater, and crop residue are
wastes produced by humans and animals. These wastes can cause harm if not processed
correctly. By turning these organic wastes into biogas, the wastes are converted into a more useful
form. The wastes are made into biogas for electricity and heating use, natural gas for cars and
cooking, and digestate as fertilizers.
4. A Good Alternative for Electricity: Some areas in the developing countries have limited access
to electricity, hampering their way of living. Biogas can provide them a good alternative. It is
economical to set up and possible both for small- and large-scale production.

Disadvantages of biogas:
1.Few Technological Advancements: The biogas industry is not yet advanced. Additional research
is needed to develop new technology and make production efficient. Also, governments provide
more support on established energy sources such as solar, geothermal, wind, and hydropower.
2.Weather Dependence: Like other intermittent energy sources (solar, wind), biogas production is
also affected by the weather. Anaerobic digestion happens in an environment with a temperature
of 37°C. Heat energy is required in cold climates to produce biogas continually.
3.Foul Odour (Smell) Emitted from Biogas Power Plant: Biogas plant emits foul odour from the
wastes they process. Power plants should be built in a location away from residences and other
industrial areas.

Q.5 Define Solar Constant.

Solar constant is defined as the amount of energy received per unit time on a unit area
perpendicular to the direction of propagation of radiation at the average distance between the Sun
and Earth.
Solar Constant, Isc = 1367 W/m2.

Q. 6 Describe the principle and the working of solar cooker.

The conversion of sunlight into heat energy occurs when the photons of light waves interact with
molecules of a receiving substance. The electromagnetic radiation emitted by the Sun possesses
energy in them. When they strike, the energy causes the molecules of the matter to vibrate. The
molecules get excited and jump to higher levels generating heat.
A mirror surface with high specular reflection is used to concentrate and channelise light from the
sun into a small cooking space. The sunlight can be concentrated to a very high magnitudes,
enough to melt salt and metal. The concentrated sunlight is focused onto a receiver such as a
cooking pan. The interaction between the light energy and the receiver material helps to convert
light into heat by the process called conduction.
The conversion is maximised by making use of materials that conduct and retain heat. Pots and
pans used in solar cookers should be matte black in colour to maximise absorption.
The occurrence loss of heat due to convection is reduced by isolating the air inside the cooker
from the air outside. Using a glass lid on the pot enhances light absorption from the top of the pan
and decreases the convection energy loss along with improving the heat holding capacity of the
cooker.

Q. 7 Describe box-type solar cooker with a neat sketch.

Box-type solar cookers consists of an insulated box, metallic cooking pots inside the box, double
glass lid on the cooking tray, and a reflecting mirror fitted on the underside of the lid of the box.
The cooking tray is insulated on the sides and bottom. The incoming solar radiation falls onto the
double glass lid and passes through it to strike the blackened cooking pots and the cooking tray.
The heat is absorbed by the blackened surface and gets transferred to the food inside the pots to
facilitate cooking. The mirror reflector is set in such a way to reflect the solar radiation falling on
it to the cooker box. Up to four black painted vessels are placed inside the box. The box type solar
cooker takes 1½ to 2 hours to cook vegetables, rice, dals, cake etc.

Q. 8 What are Fuel Cells?

A fuel cell is a device that can generate electrical energy from the chemical reactions occurring in
it, or use the electrical energy supplied to it to facilitate chemical reactions in it. These devices are
capable of converting chemical energy into electrical energy, or vice versa. Fuel cells require a
continuous input of fuel and an oxidizing agent (generally oxygen) in order to sustain the reactions
that generate the electricity. Therefore, these cells can constantly generate electricity until the
supply of fuel and oxygen is cut off.
Despite being invented in the year 1838, fuel cells began commercial use only a century later when
they were used by NASA to power space capsules and satellites. Today, these devices are used
as the primary or secondary source of power for many facilities including industries, commercial
buildings, and residential buildings.
Advantages:
1.High Efficiency: When utilizing co-generation, fuel cells can attain over 80% energy efficiency

2.Good reliability: Quality of power provided does not degrade over time.
3.Noise: Offers a much more silent and smoother alternative to conventional energy production.
4.Environment- friendly: Greatly reduces CO2 and harmful pollutant emissions.
5.Size reduction: Fuel cells are significantly lighter and more compact.
Disadvantages:
1.Expensive to manufacture due the high cost of catalysts (platinum).
2.Lack of infrastructure to support the distribution of hydrogen.
3.A lot of the currently available fuel cell technology is in the prototype stage and not yet validated.
4.Hydrogen is expensive to produce and not widely available.

Q.9 Describe and explain the working of Wind Energy Conversion

System (WECS) with a neat diagram.
A wind energy conversion system (WECS), or wind energy harvester, is a machine that is powered
by the Kinetic Energy of the wind and generates mechanical energy that can be used to directly
power machinery (mill, pump etc.) or to power an electrical generator for making electricity. The
term can thus refer to windmills, windpumps as well as wind turbines.
The Kinetic energy of the wind rotates the turbine. This energy is supplied to electrical generator
through fixed gear ratio, stepped up gears and coupling according to generator requirements. The
speed is maintained by pitch control of wind turbine. The electrical generator may be a DC
generator or it may use a synchronous or induction type of generator to produce AC power which
is interfaced with the grid.
The function of controller is to derive the reference voltage and frequency signals from the grid
and to sense the wind speed and its direction. The controller will process the power output of the
generator and accordingly send the signal to various blocks for taking the suitable correction
measures.
Q. 10 Describe the main considerations in selecting site for Wind
Generators.
1. High annual average wind speed: The speed generated by the wind mill depends on cubic values
of velocity of wind, the small increases in velocity markedly affect the power in the wind. For
example, Doubling the velocity, increases power by a factor of 8. It is obviously desirable to select
a site for WECS with high wind velocity.
2. Availability of wind V(t) Curve at the proposed site: This important curve determines the
maximum energy in the wind and hence is the principal initially controlling factor in predicting the
electrical output and hence revenue return of the WECS machines.
3.Wind structure at the proposed site: The ideal case for the WECS would be a site such that the
V(t) Curve was flat, i.e., a smooth steady wind that blows all the time; but a typical site is always
less than ideal. Wind specially near the ground is turbulent and gusty, and changes rapidly in
direction and in velocity. This departure from homogeneous flow is collectively referred to as “the
structure of the wind”.
4. Altitude of the proposed site: It affects the air density and thus the power in the wind and hence
the useful WECS electric power output. Also, as is well known, the wind tends to have higher
velocities at higher altitudes. One must be carefully to distinguish altitude from height above
ground. They are not the same except for a sea level WECS site.
5. Nearness of site to local centre/users: Transmission line length and hence losses and cost can
be greatly reduced by the site selected near to the consumers.
6.Nature of ground: Ground condition should be such that the foundation for a WECS is secured.
Ground surface should be stable. Erosion problem should not be there, as it could possibly later
wash out the foundation of a WECS, destroying the whole system.

Q. 11 What is OTEC system? Describe Open and closed cycle OTEC

systems.
Ocean Thermal Energy Conversion (OTEC), refers to the use of temperature difference between
the deep parts of the sea, which are cold and the shallow parts of the sea, which are cold, to run a
heat engine and produce useful work. The deeper parts of the ocean are cooler because the heat
of sunlight cannot penetrate very deep into the water. The efficiency of the system depends on the
temperature difference. Greater the temperature difference, the greater the efficiency. The
temperature difference in the oceans between the deep and shallow parts is maximum in the
tropics, 20o C to 25o C.

Closed Cycle OTEC: Closed cycle Ocean Thermal Energy Conversion systems use a working fluid
with a low boiling point, Ammonia, for example, and use it to power a turbine to generate
electricity. Warm seawater is taken in from the surface of the oceans and cold water from the deep
at 5o. The warm seawater vaporises the fluid in the heat exchanger, turning the generator’s
turbines. The fluid now in the vapour state is brought in contact with cold water, which turns it back
into a liquid. The fluid is recycled in the system, which is why it is called a closed system.
Closed Cycle OTEC System

Open Cycle: Open cycle OTEC directly uses the warm water from the surface to make
electricity. The warm seawater is first pumped into a low-pressure chamber, where it undergoes a
drop in boiling point due to the pressure drop. This causes the water to boil. This steam drives a
low-pressure turbine which is attached to an electrical generator. The advantage this system has
over a closed system is that, in the open cycle, desalinated water is obtained in the form of
steam. Since it is steam, it is free from all impurities. This water can be used for domestic,
industrial, or agricultural purposes.

Open cycle OTEC system

Q. 12 What are MHD systems. Describe open and closed cycle MHD
system.
The MHD generation or, also known as magneto hydrodynamic power generation is a direct
energy conversion system which converts the heat energy directly into electrical energy, without
any intermediate mechanical energy conversion, unlike all other power generating plants.
Therefore, in this process, substantial fuel economy can be achieved due to the elimination of the
link process of producing mechanical energy and then again converting it to electrical energy.
The principle of MHD power generation is Faraday’s law of electromagnetic induction, which states
that when a conductor and a magnetic field moves relative to each other, then voltage is induced
in the conductor, which results in flow of current across the terminals.
In a Magneto Hydro dynamic generator, a pressurized, electrically conducting fluid flows through
a transverse magnetic field in a channel or duct. Pair of electrodes are located on the channel
walls at right angle to the magnetic field and connected through an external circuit to deliver power
to a load connected to it. Electrodes in the MHD generator perform the same function as brushes
in a conventional DC generator. The MHD generator develops DC power and the conversion to AC
is done using an inverter.

Open Cycle MHD System: In open cycle MHD system, atmospheric air at very high temperature
and pressure is passed through the strong magnetic field. Coal is first processed and burnet in the
combustor at a high temperature of about 2700oC and pressure about 12 ATP with pre-heated air
from the plasma. Then a seeding material such as potassium carbonate is injected to the plasma
to increase the electrical conductivity. The resulting mixture having an electrical conductivity of is
expanded through a nozzle, so as to have a high velocity and then passed through the magnetic
field of MHD generator. During the expansion of the gas at high temperature, the positive and
negative ions move to the electrodes and thus constitute an electric current. The gas is then made
to exhaust through the generator. Since the same air cannot be reused again hence it forms an
open cycle and thus is named as open cycle MHD.

Closed Cycle MHD: The working fluid in a closed cycle MHD is circulated in a closed loop. Hence,
in this case inert gas or liquid metal is used as the working fluid to transfer the heat. The liquid
metal has typically the advantage of high electrical conductivity, hence the heat provided by the
combustion material need not be too high. Contrary to the open loop system there is no inlet and
outlet for the atmospheric air. Hence, the process is simplified to a great extent, as the same fluid
is circulated time and again for effective heat transfer.

Q. 13 What is a thermoelectric power generator?

Thermoelectric power generators are the devices which can convert temperature difference
between two junctions into electrical energy.
The working of thermoelectric generator is based on See-back effect. According to which, a loop
of two dissimilar metal develops an emf when the two junctions are kept at different temperature.
That is why it is also referred to as See-back Power Generation. A thermo-electrical generator
basically consists of heat source, which is kept at high temperature and a heat sink, which is
maintained at a temperature less than the heat source. The temperature difference between heat
source and heat sink causes direct current to flow through the load. In this type of energy
conversion there is no intermediate energy conversion like in case of most of the conversion.
Q.14 Explain Thermo-ionic emission.
The process by which free electrons are emitted from the surface of a metal when external heat
energy is applied is called thermionic emission. Thermo-ionic emission occurs in metals that are
heated to a very high temperature. When heat energy applied to the metal is increased to a higher
value, the free electrons gain enough energy and overcome the attractive force of the atomic
nucleus, which holds the free electrons in the metal. The free electrons, which overcome the
attractive force of the nuclei, break the bonding with the metal and jumps into the vacuum. The
free electrons, which are escaped from the surface of a metal when heat energy is supplied, are
called thermions. Thermionic emission process plays a major role in the operation of electronic
devices.

Q. 15 Define Work function.

The minimum amount of external energy required to remove free electrons from a metal is called
work function or threshold energy. The work function of metals is measured in electron volts (eV).
Metals that have low work function will require less amount of heat energy to cause the free
electrons to escape from the metal. Hence, the metals with low work function emit large number
of free electrons at high temperature.
On the other hand, metals that have high work function will require more amount of heat energy to
cause the free electrons to escape from the metal. Hence, the metals with high work function emit
a smaller number of free electrons at high temperature.
Thus, the emission of free electrons from the metal is inversely proportional to the work function
of a metal.

Q. 16 What is Geothermal Energy and what are the natural sources of

Geo-thermal energy?
Geothermal energy is the thermal energy generated and stored inside the Earth’s crust.
Geothermal Energy Resources are found in three different types:
a) Hydrothermal b) Geo-pressured c) Hot Dry Rock d) Magma.