Chapter-2
Energy Sources
Introduction:
Man
has needed and used energy at an increasing rate for his sustenance and
well-being since he came on the earth a few million years ago. Primitive man
required energy primarily in the form of food. He derived this by eating plants
and animals he hunted. Subsequently he discovered fire and his energy needs increased
as he started to use wood and other biomass to supply the energy needs for
cooking as well as for keeping himself warm.
With the advent of science and technology demand for energy increased at
a very fast rate and with the availability of cheap coal, petroleum oil and
natural gas, the dependence on these energy sources also increased at an
alarming rate. Due to ever increasing rate of consumption of energy day by day,
the rate of depletion of these sources has been very rapid resulting in their
reserves reaching very low levels. This irreversible situation of today in the
energy front has directed us to search for the alternate sources of energy like
solar, wind, biomass, tidal, ocean, geothermal, etc.,.
Energy and its forms:
Energy is the capacity to do work or it can
also be defined as the ability to cause change.
Energy
exists in various forms like kinetic energy (energy possessed by bodies in
motion), potential energy (energy possessed by bodies at an elevation), chemical
energy (internal energy i.e., the energy possessed by virtue of motion and
forces of individual atoms and molecules of the system). The various other forms of energy are
mechanical, thermal, electrical, chemical (storage batteries), radiant and
atomic. All forms of energy are inter-convertible by appropriate process.
Energy can be
classified into several types based on the following criteria as:
• Primary and Secondary energy
• Commercial and Non commercial energy
• Renewable and Non-Renewable energy
Primary and Secondary Energy sources:
Primary energy sources are those that are either
found or stored in nature. Common primary energy sources are coal, oil, natural
gas, and biomass (such as wood). Other primary energy sources found on earth
include nuclear energy from radioactive substances, thermal energy stored in
earth’s interior, and potential energy due to earth’s gravity. The major
primary and secondary energy sources are shown in Figure 1.1
Primary energy
sources are mostly converted in industrial utilities into secondary energy sources;
for example coal, oil or gas converted into steam and electricity. Primary
energy sources can also be used directly as well.
Commercial Energy
The energy sources that are available in the market
for a definite price are known as commercial energy. By far the most important
forms of commercial energy are electricity, coal and refined petroleum
products. Commercial energy forms the basis of industrial, agricultural,
transport and commercial development in the modern world. In the industrialized
countries, commercialized fuels are predominant source not only for economic
production, but also for many household tasks of general population.
Examples: Electricity,
lignite, coal, oil, natural gas etc.
Non-Commercial
Energy
The energy sources that are not available in the
commercial market for a price are classified as non-commercial energy.
Non-commercial energy sources include fuels such as firewood, cattle dung and
agricultural wastes, which are traditionally gathered, and not bought at a
price, used especially in rural households. These are also called traditional
fuels. Non-commercial energy is often ignored in energy accounting.
Example: (i)Firewood, agro waste in
rural areas; (ii)solar energy for water heating, electricity generation, for
drying grain, fish and fruits;(iii) animal power for transport, threshing,
lifting water for irrigation, crushing sugarcane; (v) wind energy for lifting
water and electricity generation.
Renewable and Non-Renewable Energy
Renewable energy is energy obtained
from sources that are essentially inexhaustible. Examples of renewable
resources include wind energy, solar energy, geothermal energy, tidal energy
and hydroelectric energy. The most
important feature of renewable energy is that it can be harnessed without the
release of harmful pollutants.
Non-renewable energy is the
conventional fossil fuels such as coal, oil and gas, which are likely to
deplete with time.
Advantages
1.
Available in plenty
in nature and inexhaustible.
2.
These can be built
as close as possible to the point of consumption and transmission losses can be
minimised.
3.
The available
technologies are more flexible and highly diverse.
4.
A good amount of
variation is possible in the energy quality.
5.
Locally available
renewable energy can be fully utilised.
Disadvantages
1.
Concentration of
these resources is limited to certain regions.
2.
Supply is
intermittent and also varies seasonally.
3.
Cost of the
equipment to harness these resources is comparatively higher than those of the
conventional energy sources.
4.
Some of the
components like solar cells, auto tracking systems, concentrators etc., all
require very high technology.
5.
Need for storage
system to store energy when the supply is available in plenty and to use when
the supply is inadequate or absent.
Another way of classifying the energy sources is conventional and
non-conventional energy sources. Fossil fuels, hydro and nuclear energies are
conventional whereas direct solar energy, tidal, geothermal, wind energy and
ocean thermal energy are non-conventional.
Fossil Fuels:
Fossil fuels are energy rich substances that have been formed from
long-buried plants and micro-organisms. Fossil fuels include petroleum, coal
and natural gas. Chemically fossil fuel consists largely of hydrocarbons, which
are compounds of hydrogen and carbon. Hydrocarbons are formed from ancient
living organisms that were buried under layers of sediment millions of years
ago. As accumulating sediment layers exerted increasing heat and pressure, the
remains of the organisms gradually transformed into hydrocarbons.
Coal Formation : Coal is a solid fuel formed from ancient plants- including trees, ferns
and mosses that grew in swamp and bogs or along the coastal shore lines.
Generations of these plants died and were gradually buried under layers of
sediment. As the sedimentary overburden increased, the organic material was
subjected to increasing heat and pressure that cause the organic material to undergo
a number of transitional states to form coal. The mounting pressure and
temperature caused the original organic material, which was rich in carbon,
hydrogen and oxygen, to become increasingly carbon-rich and hydrogen and
oxygen-poor. The successive stages of coal formation are peat, lignite,
bituminous and anthracite.
Coal Reserves:
The proven global coal reserve was estimated to be 9,84,453
million tonnes by end of 2003. The USA
had the largest share of the global reserve (25.4%) followed by Russia (15.9%), China (11.6%). India was 4th
in the list with 8.6%.
Petroleum and Natural Gas formation:
Petroleum is formed chiefly from ancient microscopic plants and bacteria
that existed in the ocean and seas. When these micro-organisms died and settled
to the seafloor, they mixed with sand the silt to form organic –rich mud. As
layers of sediment accumulated over this organic ooze, the mud was gradually
heated and slowly compressed into shale, chemically transforming into
petroleum. The petroleum fills the tiny holes within nearby porous rocks. The
liquid petroleum and gases which are less dense than water and lighter move
upwards through the earth’s crust. A portion of these petroleum eventually
encounter an impermeable layer of rock which traps the petroleum, creating a
reservoir of petroleum and natural gas. Because of its low density relative to
petroleum, natural gas forms a layer over the petroleum.
Oil Reserves:
The global proven oil reserve was estimated to be 1147 billion
barrels by the end of 2003. Saudi
Arabia had the largest share of the reserve
with almost 23%. (One barrel of oil is
approximately 160 litres)
Gas Reserves:
The global proven gas reserve was estimated to be 176 trillion
cubic metres by the end of 2003. The Russian Federation had the largest
share of the reserve with almost 27%.
Reserves/Production (R/P) ratio- If the reserves remaining at the end of the year are divided by
the production in that year, the result is the length of time that the
remaining reserves would last if production were to continue at that level.
The
Reserves-to-production ratio (or R/P) is the remaining amount of a non-renewable resource, expressed in years. While applicable to all natural resources, the
R/P is most commonly applied to fossil fuels,
particularly petroleum and natural gas. The reserve
portion (numerator) of the ratio is the amount of a resource known to exist
in an area and to be economically recoverable (proved reserves). The production portion (denominator) of the
ratio is the amount of resource used in one year at the current rate.
R/P = (amount of
known reserves) / (amount used per year)
World oil and gas reserves are estimated at just 45 years and 65 years
respectively. Coal is likely to last a little over 200 years
This
ratio is used by companies and government agencies in forecasting the future
availability of a resource to determine project life, future income,
employment, etc, and to determine whether more exploration must be undertaken
to ensure continued supply of the resource.
Advantages and disadvantages of conventional energy
sources
Advantages:
1.
Fully developed
technology is available to harness this energy.
2.
Cost of generation
has been brought down to affordable levels.
3.
They can be easily
transported to any place.
4.
Ideal for small
applications.
Disadvantages
1.
They are polluting
because of their emissions.
2. Their availability is reducing as they are in limited quantity in nature
and these sources are depleting at a fast pace.
3.
They are leading to
lot of ecological imbalances.
4.
They are usually
far off from the point of consumption.
Solar
Energy:
Solar energy is a very
large, inexhaustible source of energy. The power from the sun intercepted by
the earth is approximately 1.8 x 1011MW, which is many thousands of
times larger than the present consumption rate on the earth of all commercial
energy sources. Hence solar energy can supply all the present and future energy
needs of the world on a continuing basis. This makes it one of the most
promising renewable source of energy.
The sun provides earth
with the radiant energy which has two distinctive properties viz, lighting and
heating resulting from nuclear fusion reactions at its core. Some solar
equipment is designed to use the light property of solar radiation while few
others are designed to use the heating property.
The sun constantly
delivers 1343 W/m2 of power on an average to the earth out of which
a maximum of 1000 W/m2 of power is received on the earth’s surface
after passing through the earth’s atmosphere. It is environmentally clean
source of energy and is freely available. However the solar energy is very
diffuse, cyclic and often undependable. Therefore it needs systems and
components that can collect and concentrate it efficiently.
Solar energy conversion:
The
solar energy can be utilised in direct form as well as indirect form.
The
direct form of utilisation of solar energy are (i) Helio–electrical and (ii)Helio-
thermal processes.
The
indirect forms of solar energy are (i) Biomass energy (Helio-chemical) (ii)
Wind energy (iii) Tidal energy (iv) ocean thermal energy (v) Hydel energy etc.,
Helio-electrical Process
:
Solar
energy can be directly converted into direct current by photovoltaic cells.
The
devices used in photovoltaic conversion are called solar cells. When solar
radiation falls on these devices it is converted directly into DC electricity.
The principal advantages associated with solar cells are that, they have no
moving parts, require little maintenance and work quite satisfactorily with
beam and diffuse radiation.
Description and Principle
of working of solar cell
Single
Crystal silicon cell:
|
|||
|
|||
Figure 2(a): Structure and
principle function of a solar cell Figure 3: Photovoltaic cell.
Figure 2(b)
Single
crystal silicon cells are thin wafers about 300µm in thickness, sliced from a
single crystal of p-type doped silicon. The silicon with added impurity such as
boron or gallium is called p-type Silicon. A shallow junction is formed at one
end by diffusion of the n-type impurity. The silicon with added materials such
as arsenic or Phosphorus is called n-type silicon. Metal electrodes made from-
Ti-Ag solder are attached to the front and back side of the cell. On the front
side, the electrode is in the form of a metal grid which permit the sunlight to
pass through, while on the back side, the electrode completely covers the
surface. A typical cell develops a
voltage of 0.5 to 1 V and a current density of 20 to 40 mA/cm2. In
order to obtain higher voltages and currents, individual cells are connected in
series and parallel to form a module. In turn, a number of modules are
interconnected to form an array.
Principle of working of a solar cell.
When a p-n junction of a semiconductor is exposed to sunlight some of
the photons are absorbed in the vicinity of p-n junction. The photons absorbed
at the p-n junction will have high energy to dislodge an electron from the
fixed position in the material and gives it enough energy to move freely in the
material. The electron evicted from its customary bond can travel through the
entire crystalline solid and capable of responding to electric field and other
influences. The bond from which the electron was ejected is short of one
electron creating a hole which is also mobile. Thus the ejected free electron
and the hole form an electron –hole pair. The electrons and the holes being of
opposite charge will be pushed in different directions by the electric field
which already exists in the vicinity of the junction if they come into the
region near the p-n junction. The permanent electric field which is already
built-in near the p-n junction pushes the hole into the p-region and the electron
into the n-region. Thus p-region becomes positively charged and the n-region
becomes negatively charged. If an external load is applied, this charge
difference will drive a current through it. The current will flow so long as
the sunlight keeps generating the electron-hole pairs.
Helio-thermal
Process.
The
heating property of solar radiation is used in the devices to meet the thermal
energy needs. It is necessary to collect
and concentrate the solar radiation in an efficient manner to arrive a
reasonably high-temperature heat source. The collectors gather the sun’s energy
and direct it onto receivers that contain the working fluid.
Basically
two types of collectors are used and they are flat plate collectors and
concentrating collectors.
Figure
4: Flat plate collector
Figure
4: Flat plate collector (cut-view)
In
flat plate collectors the incident solar radiation is absorbed by the
collectors surface itself, which are usually coated with black paint (usually electroplated),
covered with transparent glass cover on top and insulated all around to prevent
the heat loss from the collector surface. The black collector surface gets
heated up and then in turn transfers the heat to the fluid passing through the
tubes which are either welded or soldered or are integral part of the collector
plate. Flat plate collectors are usually sloped and oriented in one particular
direction and are capable of collecting both diffuse and beam radiation. Since
there are no moving parts in it, the repair and maintenance cost is also nil or
negligible. A maximum of 100°C can be easily achieved using flat plate
collectors and are more popularly used in solar water heating applications and
solar air heaters as they are relatively cheaper as compared to the cost of concentrating
collectors.
In
concentrating collectors the incident solar radiation falls on a large curved
surface from where it is reflected and focused on to focal point or line
depending upon the type of the geometrical construction of the concentrating
collector. When temperatures higher than
100°C are required, it becomes necessary to concentrate the radiation. This is
achieved using focusing or concentrating collectors. A schematic diagram of a
typical concentrating collector is shown in figure 5. The collector consists of
a concentrator and a receiver. The concentrator shown is a mirror reflector
having the shape of a cylindrical parabola. It focuses the sunlight onto its
axis, where it is absorbed on the surface of the absorber tube and transferred
to the fluid flowing through it. A concentric glass cover around the absorber
tube helps in reducing the convective and radiative losses to the surroundings.
In order that the sun’s rays should always be focussed onto the absorber tube,
the concentrator has to be rotated. This movement is called tracking. In the
case of cylindrical parabolic concentrators, rotation about a single axis is
generally required. Fluid temperatures upto 400°C can be achieved in
cylindrical parabolic focussing collector systems. The generation of still higher temperature is
possible by using paraboloid reflectors (shown in figure) which have a point
focus. These require two-axis tracking so that the sun is in line with the
focus and the vertex of the paraboloid.
One
of the major problems associated with the utilisation of solar energy is its
variability. For this reason, most applications require some type of energy
storage system. The purpose of such a system is to store when it is in excess
of the requirement of an application and to make it available for extraction
when the supply of solar energy is absent or inadequate.
Figure
5: Concentrating collectors
Helio-chemical Process / Photosynthesis
The
most important chemical reaction on the earth is the reaction of sunlight and
green plants. Radiant energy of the sun is absorbed by the green pigment chlorophyll
in the plant and is stored within the plant in the form of chemical bond
energy.
The
visible light having wavelength below 700A° is absorbed by the green
chlorophyll which becomes activated and passes its energy on to water
molecules. A hydrogen atom is released
and reacts with the carbon dioxide molecules, to produce H2CO and
oxygen. H2CO is the basic molecule forming carbohydrate. The oxygen librated is from H2O
molecule and not from CO2.
This process is called as carbon fixation or carbon assimilation.
The process of photosynthesis has two
main steps:
(i) Splitting of H2O molecules
into H2 and O2 under the influence of chlorophyll and
sunlight. This phase of reaction is called the light-reaction. In this
reaction, light absorbed by chlorophyll causes photolysis of water. O2
escapes and H2 is transformed into some unknown compounds. The solar
energy is converted into potential chemical energy.
(ii) In the second step, hydrogen is
transferred from this unknown compound to CO2 to form starch or
sugar. Formation of starch or sugar is dark reaction not requiring sunlight.
However,
photosynthesis concepts is less attractive as the average efficiency of solar
energy conversion in plants is about 1% and overall efficiency of the
conversion of sunlight to electricity would be about 0.3% compared to 18 to 21%
for photo-voltaic cells. Still worldwide photosynthetic activity can store more
than 15times as much energy as consumed by all nations of the world.
Wind
Energy:
Wind
energy is an indirect form of solar energy. It is caused by the uneven heating
of the earth’s surface by the sun. Since the earth’s surface is made of very
different type of land and water, it absorbs the sun’s heat at different rates.
During the day, the air above the land heats up more quickly than the air over
water. The warm air over the land expands and rises, and the heavier, cooler
air rushes in to take its place, creating wind. At night, the wind is reversed
in direction because the air cools more rapidly over the land than over water.
Although solar energy is cyclic and predictable, wind energy, however is
erratic, unsteady. However, there are many locations where the wind direction and
velocity are relatively constant over long periods.
Wind
possesses energy by virtue of its motion. Any device capable of slowing down
the mass of moving air, like a sail or propeller, can extract part of the
energy and convert it into useful work. Three factors determine the output from
a wind energy converter:
i)
The wind speed,
ii) The
cross-section of wind swept by the rotor and
iii) The
overall conversion efficiency of the rotor, transmission system and generator
or pump.
The
wind turbines or windmills are used to convert the kinetic energy of the wind
into mechanical work, which may be converted into electrical energy. No device,
however well-designed, can extract all the wind’s energy because the wind would
have to be brought to a halt and this would prevent the passage of more air
through the rotor. The most that is possible is for the rotor to decelerate the
whole horizontal column of intercepted air to about one-third of its free
velocity.
The
power in the wind can be computed by using the concepts of kinetics. The wind
mill works on the principle of converting kinetic energy of wind to mechanical
energy. We know that the power is equal to energy per unit time. The energy
available is the kinetic energy of the wind.
The
kinetic energy of any particle is = ½ mV2 joules
Where
m = mass of the particle and
V = velocity
The
amount of air passing in unit time, through an area A, with velocity V, is =
A.V m3/s
and its mass m is equal to its volume
multiplied by its density ρ of air, or
i.e. m
= vol X ρ = ρ AV kg/s
(m is the mass of air traversing the area A
swept by the rotating blades of a wind mill type generator)
Substituting
this value of the mass in the expression for the kinetic energy, we obtain, power available in wind = ½ ρ AV. V2
watts
=
½ ρ A V3 watts.
Since A = (π D2) / 4, where D =
diameter of the rotor.
Available wind power = 
=
watts
watts
Hence
the wind power is proportional to square of the diameter of the rotor and to
the cube of wind velocity.
Figure
6 shows the schematic diagram of a horizontal axis windmill. It consists of
tall tower with a large propeller on the top. The wind blows the propeller to
rotate, which turns a generator to produce electricity. Some of the generally
used propeller for horizontal axis windmills is: Multiblade type, sail type and
propeller type. The vertical axis windmill propellers are: Savonius type and
Darrieus type. One advantage of vertical windmills is that they operate in all
wind directions and thus need no yaw adjustments. But they have relatively low
tip-to wind speed ratios and lower power outputs per given rotor size, weight
and cost.
Figure
6: Windmill
Multi-blade Windmill:
Usually consists of 12 to 20 blades made from metal sheets as shown in figure 7.
Windmill having many blades is usually of low speed system. The Multiblade
arrangement will be very heavy but develops a high torque. It is mainly used to
drive water pumps. Example: Sail type, Propeller type
Fig.7:
Multiblade windmill Fig.8: Sail type Fig.9: Propeller type
Figure 10: Savonious rotor Figure
11: Darrieus Rotor
Advantages
and disadvantages of wind energy
Advantages:
1.
It is a renewable resource.
2.
Wind energy is free,
non-polluting and inexhaustible energy.
3.
Windmills are highly desirable to the rural
areas, which are far from the existing grids
Disadvantages:
1.
Large size conversion
machines are necessary due to dilute form of energy.
2.
The wind velocity is neither
constant in magnitude nor in direction and also velocity varies from the bottom
to top of the large rotor. This imposes cyclic loads on the turbine blades.
3.
Necessitates the energy
storage device. i.e., store the energy when the wind is good and use it when it
is inadequate or absent.
4.
High initial cost and low
power co-efficient
Ocean Thermal Energy Conversion
The
solar energy stored as heat in the ocean can be converted into electrical
energy by making use of the temperature difference between the warm surface
water and the colder deep water. The operation of Ocean Thermal Energy
Conversion plant is based on the thermodynamic principle. It is possible to run
a heat engine (prime mover), by utilising the temperature difference, if a heat
source at higher temperature and a heat sink at lower temperature are
available. The prime mover can convert a part of the heat taken from the source
into mechanical energy and hence into electrical energy. The residual heat is
discharged to the sink at lower temperature. Warm surface water is the heat
source and the deep colder water provides the sink in OTEC systems.
The
tropical oceans acts as built-in solar collectors and solar radiation is
absorbed by the top layers of sea water and constitutes an infinite heat
storage reservoir. Solar energy absorption by water takes place according to
Lambert’s law of absorption, which states that each layer of equal thickness
absorbs the same fraction of light that passes through it.
Mathematically,
I(x)
= I0 e-kx
Where
I0 and I(x) are the intensities of radiation at the
surface (x = 0) and at a distance x below the surface. K is an extinction
co-efficient that has the unit L-1. K has values of 0.05m-1
for very clear fresh water, 0.27m-1 for turbid fresh water and 0.5m-1
for very salty water. Thus the intensity decreases exponentially with the depth
and depending upon the K, almost all of the absorption occurs very close to the
surface of deep waters. Because of the heat and mass transfer at the surface
itself, the maximum temperatures occur just below the surface. There will be no
thermal convection currents between the warmer, lighter water at the top and
deep cooler, heavier water. The temperature difference between warm surface
water and deep cool water can exceed more than 25 K.
The
surface temperature vary both with latitude and season, both being maximum in
tropical, subtropical and equatorial water making these the most suitable for
OTEC systems.
Ocean thermal electric power generation using closed cycle system
It
consists of an evaporator, condenser, turbine, pump and electric power
generator. The ammonia (or propane or a Freon) is used as the working fluid
which executes a closed cycle.
Fig.16: Schematic layout of closed
cycle Ocean Thermal Energy Conversion plant
The
warm surface water is pumped to an evaporator (a surface heat exchanger) where
the working fluid is evaporated to high pressure vapour to drive the turbines
which are essentially very small in size as compared to that of open cycle
system turbine which handles low pressure steam. The exhaust of the turbine is
condensed by the cold water drawn from the deep ocean through a pump. The
condensate is pumped at high pressure to the evaporator to re-execute the
rankine cycle. The schematic layout of the plant is as illustrated in the
figure16.
The
electricity produced could then be transmitted inexpensively to land by
submarine cables or can be utilised at the plant site to produce
energy-intensive materials in case of offshore OTEC plants.
