Renewable Energy Resources
Unit-III
Geothermal Energy: Resources of geothermal energy, thermodynamics of geo-thermal energy conversion-
electrical conversion, non-electrical conversion, environmental considerations.
Magneto-hydrodynamics (MHD): Principle of working of MHD Power plant, performance considerations.
Fuel Cells: Principle of working of various types of fuel cells and their working, performance and limitations.
GEOTHERMAL ENERGY
3.1(a) Geothermal Energy: Geothermal energy is a form of renewable energy, having the source of natural
heat inside the earth. It is renewable since the earth’s interior will continue to energy having high energy
density. It is inexhaustible like solar or wind energy.
The geothermal energy in the form of heat energy can be utilized economically and efficiently for power
generation and other applications with the existing available technologies. Though the amount of thermal
energy within the earth is very large, useful geothermal energy is limited to certain site only in the world
because of the feasibility to access and extract heat. The sites from where the thermal energy can be extracted
are called geothermal fields. The commercial utilization of this energy started first in Italy in Larderello in
1904, Lardell (1913), Iceland (1930) and other part of the world by 2000. In India about 50 shallow wells are
drilled for steam production in J&K belt. The thermal springs are found in different parts of India i.e.
Himachal Pradesh, Maharashtra, Cambay of Gujarat, Uttar Pradesh and some parts of Bihar and near Jawai in
Assam.
3.1(b) Origin of Geothermal Energy:
The earth’s interior having mass of hot liquids, gases
and steam is cooling slowly and the temperature of the
earth’s core is estimated to be about 40000C.
The average geothermal heat dissipated from the land is
about 0.06 W/m2 as compare to solar heat energy of 1
kW/m2.
Below the solid crust is the molten mass called magma
which is still in the process of cooling.
The earth’s vibrations caused the magma to come near
the earth surface thus causes active volcanoes, hot
springs and geysers water exists.
The hot magma near the surface solidifies into igneous
rock. The heat of magma is conducted upwards into this
rock and ground water that finds its way down to this rock through fissures in it will be by the heat of the
rock or by mixing with hot gases and steam emanating from the magma.
The heated water will then rise upward by convection into porous and permeable reservoir above the
igneous rock. It is covered by the layer of solid impermeable rock. It traps the hot water in the
reservoir called hydrothermal field.
A well made by drilling hole in the upper crust in the region of geothermal field can be used to trap
steam for use in a conventional power plant. The steam originating from magma is called meteoritic
steam, largest source of geothermal steam.
3.1(c) Classifications of Geothermal Energy Resources/Systems:
Geothermal resources vary in temperature from 30-350 0C and consist of dry, a mixture of steam and water or
hot water only. The water is the transfer medium in order to extract geothermal heat from the earth. Table
given below lists the basic technologies normally utilized according to resources temperature.
The geothermal energy systems may be classified as:
1. Hydrothermal; 2. Hot dry rock (HDR); 3. Geo-pressured system;
3.1(d) Hydrothermal (Convective) Resources: These resources are located at shallow to moderate depths of
100 m to 5 km and in such, water is heated by contact with the hot rock and high pressurized steam is
generated up to about 3500C.
These resources further divided into following categories, depending upon whether steam or hot water
is the dominating product.
1. Dry steam fields (Vapour dominated); 2. Wet steam fields (Liquid dominated);
3.1(d.1) Dry Steam / Vapour Dominated Systems:
Such system delivers steam with little or no water of high pressure and temperature up to 2000C.
This steam is used to generate electrical energy by electromechanical energy conversion.
The steam extracted from such fields may contain some amount of water and solid particles; those are
removed in a centrifugal separator.
This pure dry steam is then directly supplied to a steam turbine, which is coupled to a generator.
The condenser is used to condense the exhaust steam from the turbine for recirculation purpose of cold
water.
This cold water is either sent to chemical recovery or it is reinjected into hot field with the help of
condensate pump.
The main drawbacks of these plants are the presence of corrosive gases and abrasive materials which
cause the environmental problems.
3.1(d.2) Wet Steam / Liquid Dominated Systems: The temperature range of such fields is in the range of
1000 - 3150C and these fields deposits hot water without much amount of steam. These can be further
classified and explained in following categories:
(i) High Temperature Wet Steam System:
The high temperature wet steam system
having resource temperature above 1800C,
suitable for power generation.
In such systems the hot water with steam at
high pressure and high temperature is
extracted by deep wells drilled into the
ground.
This hot water from underground from a
depth of about 1 km (point. A) at high
pressure (about 35 – 40 bar) reaches the well
head (point. B) at lower pressure, this
process produces two phase mixture having
the steam of low quantity.
This mixture is feed to flash chamber / brine separator which separates dry steam from brine (which is
collected from bottom).
This dry steam is supplied to steam turbine which is coupled to a generator, produces electrical energy.
The hot brine is re-injected into the ground along with steam condensate from the condenser.
(ii) Low Temperature-Binary Fluid System:
These hydrothermal resources are available in the temperature range of 900 – 1700C which is not
suitable for production of two phase mixture of steam.
In such situation to utilize this geothermal heat, a fluid of low boiling point (e.g. isobutane) is used
under pressure in a primary heat exchanger.
The fluid vapour so generated is passed through a turbine where it expands and this mechanical power
of turbine is converted into electrical energy by a generator.
The exhaust of the turbine is passed through a heat exchanger and condenser where it is cooled and
then condensed.
The thermal fluid after transferring heat is re-injected through a well into the ground.
3.1(e) Hot Dry Rock (HDR) or Petro-Geothermal Systems:
Hot dry rock receives heat energy from molten rock (magma).
These reservoirs are artificially created by fracturing deep compact rocks up to depth of 8 -15 km by
hydraulically / chemically or by nuclear explosion for injecting high pressure water.
The water is pumped into the fractured well (injection well) and withdrawn from another well
(production well) to extract heat to generate steam for power production.
To prevent mixing of water in the reservoir the depth of production well is kept slightly less than
reinjection well.
3.1(f) Geo-pressured Systems:
These reservoirs were developed by sand deposits carried down by rivers and entrapped brine and
natural gas at depth of 4-6 km and are located at different levels and at different places.
These systems comprise of heavily compressed sediments that contain heat, methane and pressurized
hot water.
When well is drilled in this zone the reserve energy comes out in the form of high pressurized water
having temperature about 1600C, heat and natural gas (methane) which are useful for power
generation.
These sources have been found in offshore wells in Texas and US Gulf Coast areas.
3.1(g) Main Features of Geo-thermal Energy:
(i) Advantages
1. It may be used in both energy supply, with power generation and direct heat use.
2. It is an inexhaustible source of energy.
3. It is almost free of pollution.
4. Independent of weather condition.
5. More reliable source of power generation than other renewable energy sources.
(ii) Disadvantages
1. Causes corrosion, erosion, and chemical deposition due to the presence of sufficient amount of CO2,
CH4, N2, NH3 and H2S.
2. Noise pollution because of well drilling, centrifugal separators.
3. Proper re-filling of water is necessary.
4. It causes settlement of land.
5. Life of plant is low compared to conventional power plants.
(iii) Environmental impact:
Some part of condensable gases (CO2, H2S, NH3, CH4, N2 and H2) and radio actives are escaped to
environment at various points and cause air pollution.
There is possibilities of seismic disturbance because of extraction and re-injection of saline.
Water pollution of rivers and lakes is a potential hazard as geo water contains high amount of
dissolved solid with silica.
The heat energy of turbine exhaust steam is rejected to atmosphere in cooling towers. It causes dense
fog, increase in moisture.
In such systems soil erosion takes place due to drilling of wells in geothermal fields.
The exploitation of geothermal energy may cause earthquakes also.
3.1(h) Potential of Geo-thermal Resources in India:
In India about 150 known geothermal sites which can produce working fluids up to 1700C. Main locations
of such fields are:
(i) Geothermal Fields of Puga-Chhumathang valley in J&K: A pilot project for space heating involved
construction of a shed at an altitude of 4000 m and 4400 m in
Ladakh district, Jammu & Kashmir and using steam at
1250C.
(ii) Manikaran (Himachal Pradesh): This project “cold
storage plant” has been recently commissioned by the
collaboration of Geological Survey of India, IIT Delhi, and
H.P. Government.
(iii) Geothermal Fields of Beas and Parbati Valleys,
Himachal Pradesh: Beas and Parbati valleys are well known
for their hot springs in Kulu district, Himachal Pradesh. The
spring’s temperature varies from 220C to 590C in Beas and
210C to 960C in Parbati valley at an altitude between 1300 m
to 3000 m.
(iv) Sohana near Delhi,
(v) Tuwa in Gujrat
Index: I-Himalayan Province, II- Aravali, Naga Lucha, West Coast, III- Gondwana, Son-Narbada,
Volcani Arc (Andaman Province), IV- Deep Sedimentry Basin,V- Radio Active Province
MAGNETO HYDRODYNAMIC (MHD)
3.2(a) Magneto Hydrodynamic Generator:
MHD is a device which converts heat energy of an
ionized fluid at high temperature directly through
the applied magnetic field, without a conventional
electric generator. In such systems power
generation is based on the Faraday’s law of
electromagnetic induction.
In such condition when plasma passes through a
strong magnetic field then force begin to act on its
ions, this force act on plate P1 for +ions and
towards P2 for –ions. Due to migration of these
charges a potential difference creates between the
plates. If a load is connected between these plates
then electricity begin to flow. This process takes
place in MHD channel.
3.2(b) Working Principle:
When a charged particle moving in a magnetic field, having charge q and velocity v, then the force acting on
it:
If is the electric field acts, then the total force:
The velocity to be used in above equation (1) is the vector sum of gas velocity v and the particle drift velocity
u. Thus force is given by:
Can be written as:
Where
From figure, the motion of gas is in x-direction, magnetic field B is in y-direction and the force on the particle
is in z- direction.
If the distance between the plates be d, then the electric intensity b/w the plates is
z= -V/d ………………………………………(4)
The total electric field is: = − z +B = (Bvd – V) ……..…..(5)
The electromagnetic fields Ez and B acting on the moving gas produce the same force on the ions as the
electromagnetic field EZ‘ and B produce on gas with zero average velocity. The term Bvd gives the internal
e.m.f. (or open circuit voltage) of the MHD generator.
If R0 and RL be the internal resistance of generator and load resistance respectively, then the current I through
the circuit be:
= ………………………………… (6)
So the power output: P = V0 I = I2R0 = ( )2 R0
The power delivered to the load is maximum when load resistance = RL = R0
3.2(c) MHD Cycles & Working Fluids:
(i) Open Cycle MHD Systems: In an open cycle system the working fluid is used on the once through basis.
The working fluid after generating electrical energy is discharged to the atmosphere through a stack.
Generally coal is used as fuel as it produces more conductive plasma; this is because of more carbon atom as
compared to hydrogen atom. The working temperature of such MHD generators lies approximately in the
range above 23000C.
The different parts of open cycle generator are:
(a) Compressor: Compressor is used to raise the pressure of the air supplied in combustion chamber for
efficient burning of fuel. The pre-cooled air is compressed in compressor before being supplied in the pre-
cooler.
(b) Pre-heater: The compressed air pre-heated in the pre-heater at 11000C from the exhaust gases of the
generator to increase the combustion efficiency of working fluid.
(c) Combustor: The fluid is burnt in this section of generator in the presence of hot air coming from pre-
heater & gas is seeded with cesium to ionize the gas.
(d) Nozzle: The high temperature gases after combustor pass through convergent-divergent nozzle to increase
its velocity of order of 1000 m/s.
(e) Generator Dust: It is made of heat resisting & insulating material. The high magnetic flux is applied in
this section of the duct at perpendicular to flow direction of gases. As the moving gas cuts the line of magnetic
flux the induced electric field produces DC current through generator in this section. The duct is water cooled
to remove the heat.
(f) Inverter: The generated DC is converted into AC by using the inverter before supply to grid.
(g) Gas Chamber: The exhaust hot gasses passes through the air cleaner to control the pollution & remove
harmful gases.
(h) Speed Recovery System: Seed material is expensive & need recovery system along with open MHD
power cycle for further use.
The removed seeds are supplied back in the combustor.
(ii) Closed Cycle MHD System:
In this system the very high thermal efficiency is achieved with low cycle cost in closed plant &
provides more useful power at low temperature at 16000C. The ducts of these units are small because
of high pressure.
Helium or argon is used as working fluid, heated in heat exchanger & get ionized, alkali metals is
mixed with inert gas to provide the necessary conductivity is closed cycle plant, where recovery is
possible.
The working fluid in closed cycle is seeded with Cesium and circulated in a closed loop. Gas is burned
in the combustor is supplied in the heat exchanger, where the heat is transferred to the working fluid.
The ionized gas passes through the magnetic field to produce DC power.
The combustion products are discharged to the atmosphere after removal of heat in heat exchanger
It shows the schematic of liquid metal MHD generator. The superheated metallic vapour is expanded
through the supersonic nozzle and enters in the generator in liquid form with velocity of 150 m/s.
The electrical conductivity of metallic vapour is poor. That brings the overall conversion efficiency
lower than that of gas as a working substance.
(iii) Hybrid MHD Generator:
The hybrid MHD generator is developed to improve the thermal efficiency of plant. The efficiency up
to 60% is achieved in the hybrid system.
The working gas enters in the MHD at 30000C and will leave at 20000C through the steam turbine
coupled with generator.
A steam power plant is connected along with MHD generator to use the remaining energy of exhaust
gases of MHD generator and converted it into electrical energy.
The MHD plant may be open or a closed type. This steam power plant works on Rankine cycle.
3.2(d) Advantages & Disadvantages of MHD System:
Advantages
1. These systems having higher efficiency (60-65%) because of higher operating temperature.
2. Requires less area and maintenance as compare to conventional power plants of same capacity.
3. Less pollution to the environment because of less fuel consumption as compared to other conventional
power plants.
4. They are compact in size, have low specific weight and high power density, having instant operation and
suitable as peak load
5. Provides direct conversion of heat directly into electrical energy.
Disadvantages
1. Power generation in MHD is directly proportional to square of magnetic flux, it needs very large power to
create it which increases to cost of the system.
2. High temperature gases causes corrosion and chemical erosion.
3. It is difficult to built equipments to withstand very high temperature, used in MHD.
4. High initial cost due to large sized magnets and equipments.
5. High thermal and frictional losses.
FUEL CELLS
3.3(a) Fuel Cells: A fuel cell is a device that converts the chemical energy from a fuel into electricity through
a chemical reaction with oxygen or another oxidizing agent. . Fuel cells are different from batteries in that
they require a constant source of fuel and oxygen/air to sustain the chemical reaction; however, fuel cells can
produce electricity continuously for as long as these inputs are supplied. There are many types of fuel cells,
but they all consist of an anode, a cathode and an electrolyte that allows charges to move between the two
sides of the fuel cell. Electrons are drawn from the anode to the cathode through an external circuit, producing
direct current electricity. As the main difference among fuel cell types is the electrolyte, fuel cells are
classified by the type of electrolyte they use.
3.3(b) Working Principle of Fuel Cells:
The basic design features of a fuel cell are:
The electrolyte substance. The electrolyte substance usually defines the type of fuel cell.
The fuel that is used. The most common fuel is hydrogen.
The anode catalyst breaks down the fuel into electrons and ions. The anode catalyst is usually made up
of very fine platinum powder.
The cathode catalyst turns the ions into the waste chemicals like water or carbon dioxide. The cathode
catalyst is often made up of nickel but it can also be a nanomaterial based catalyst.
Its working may be explained as:
The cell consists of two electrodes between which electrolyte (40% KOH) is filled and serves as a
carrying media for charged ions.
The fuel (usually hydrogen) is supplied at anode side and oxidant (oxygen or air) at the cathode side.
The electrolyte between the inner face of the both electrodes is kept stirred to prevent stay of ions for
long time at the surface.
The formed ions migrate through electrolyte to the opposite side of electrodes because of pressure
difference between the reactant gases and the electrolyte.
Sometimes catalysts (generally Platinum) are used to increase the reactivity at electrodes for breaking
the fuel molecules.
During chemical reaction H+ and OH- ions are produced as an intermediate product.
The H+ ions migrate to the cathode and reacts with oxygen to form water and vice-versa for oxygen
ions.
The electrons released at a anode travels through the external circuit to form the oxidizing products
and the reactions are taken place as:
3.3(c) Types of Fuel Cells:
(i) Phosphoric acid fuel cell (PAFC):
In these cells phosphoric acid (H3PO4) is used
as a nonconductive electrolyte to pass positive
hydrogen ions from the anode to the cathode.
PAFC is a non-conductive liquid acid which
forces electrons to travel from anode to cathode
through an external electrical circuit.
The hydrogen ion production rate on the anode is
small so platinum is used as catalyst to increase
this ionization rate.
These cells commonly work in temperatures of
150 to 200 degrees Celsius.
This high temperature will cause heat and energy
loss if the heat is not removed and used properly.
This heat can be used to produce steam for air
conditioning systems or any other thermal energy
consuming system.
The chemical reaction in this fuel cell are as
follows:
(ii) Molten Carbonate Fuel Cell (MCFC):
In these fuel cells mixture of H2 and CO is used as fuel
and O2 or air as oxidant.
MCFCs use lithium potassium carbonate salt as an
electrolyte, and this salt liquefies at high temperatures,
allowing for the movement of charge within the cell – in
this case, negative carbonate ions.
Working range of such fuel cells is 6500C – 7000C.
The hydrogen in the gas reacts with carbonate ions from
the electrolyte to produce water, carbon dioxide,
electrons and small amounts of other chemicals.
The electrons travel through an external circuit creating
electricity and return to the cathode.
Oxygen from the air and carbon dioxide recycled from
the anode react with the electrons to form carbonate ions
that replenish the electrolyte, completing the circuit.
The chemical reactions can be expressed as follows.
(iii) Proton Exchange Membrane Fuel Cells (PEMFCs):
These are also known as a "solid polymer
electrolyte fuel cell" (SPEFC).
In this type of fuel cell, solid polymer
membrane of an organic material such as
polystyrene is used as electrolyte.
On the anode side the H+ ions are
conducted through the membrane to the
cathode, but the electrons are forced to
travel in an external circuit (supplying
power) because the membrane is
electrically insulating.
On the cathode catalyst, oxygen molecules react with the electrons (which have traveled through the
external circuit) and protons to form water.
The chemical reactions can be expressed as follows.
(iv)Solid Oxide Fuel Cell (SOFC):
Solid oxide fuel cells (SOFCs) use a solid
material, most commonly a ceramic material (e.g.
zirconium dioxide) as the electrolyte.
They require high operating temperatures (800–
1000 °C), due to which no catalyst is required.
The anode in such cells is made up of porous
nickel and cathode is of metal oxide.
The output voltage is about 0.65V at about 8000C.
Generally H2 is uses as fuel in this cells but other fuel may be methanol, ammonia and hydrazine
(N2H4).
These cells having efficiency about 60%-70%.
(v) Regenerative Fuel Cells (RFC):
A regenerative fuel cell or reverse fuel cell (RFC) is a fuel cell
operates and run in a closed loop, where reactants are
regenerated from the products.
These cells produce electricity from fuel (H2) and oxygen and
generate energy and water as by a product.
The excess water is divided into H2 and O2 by electrolysis using
electricity from other sources like solar, wind etc.
The chemical reactions can be expressed as follows.
3.3 (d) Performance Analysis of Fuel Cell:
In fuel cell the electro-chemical reaction takes place and electrical power and produced. Fuel cell’s
performance may be describes with the help of Gibbs energy equation. The energy balanced equation may be
describes as: G = H – T.S … (1)
or for a constant temperature process: ………………....(2)
but from the first law of thermodynamics: Q + W = GH ……………….(3) (where W is work in, and is
negative)
∆S…………………….(4)
From second law of thermodynamics: Q = T∆
So from equations (3) & (4) : W = GH – T∆
∆S ……………...(5)
This applies for a chemical process at constant temperature and pressure, and is the maximum possible useful
work or available energy, known as the “free energy”.
So it is clear from equations (2) & (5): …....(6)
The ratio of work output to the energy supplied to the fuel cell is defined as the efficiency, η of energy
conversion of fuel cell. Therefore:
and its maximum efficiency:
At room temperature (250C), for H2-O2 fuel cell: ∆G = -237191kJ/kg mol (approx), and ∆H = -285838 kJ/kg
mol (approx).
⇒ Maximum efficiency of H2-O2 cell is:
3.3(e) Advantages, Disadvantages & Applications of Fuel Cells:
Advantages
1. Less pollution and higher efficiency.
2. Less maintenance required.
3. Since there is direct conversion of energy so efficiency is high.
4. Water is formed as product which may be used for many other applications.
5. Having fast starting time.
Disadvantages
1. High initial cost.
2. Short life because of degrading of electrodes.
3. Proper handling is required due to operate with hydrogen and oxygen.
4. Efficiency goes down with time.
Applications
1. Used in automotive vehicles.
2. Power generation upto 1 MW.
3. Used for space applications.
4. Used for military applications.
5. Used for submarine applications.
3.3 (f) Present Status in India: In India, fuel cell technology is in developing stage. It is supported by
Ministry of non-conventional energy sources and financially supported to import a 200 kW Phosphoric acid
fuel cells. Lots of research and developments are involved in this field. Bharat Heavy Electrical Ltd (BHEL)
is developing a 50 kW fuel cell power plants.
