UNIT-3

RENEWABLE ENERGY RESOURCES

(KOE074)
Contents
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 and limitations.

Fuel Cells: Principle of working of various types of fuel cells and their
working, performance and limitations.
GEOTHERMAL ENERGY
INTERNAL STRUCTURE OF EARTH
GEOTHERMAL ENERGY

GEO : Earth
THERMAL: Heat
 INSEPTION OF GEOTHERMAL
ENERGY

Thermal Energy
Primordial Thermal
produced from
Energy which was GEOTHERMAL
decay of heavy
present when the ENERGY
radioisotopes
earth was formed
within the earth
CATEGORY OF GEOTHERMAL
ENERGY
HYDROTHERMAL ENERGY
• Associated with steam and hot springs

PETROTHERMAL ENERGY
• Associated with hot rocks

GEO PRESSURED SYSTEM

• High Temp. water under high Pressure
Hydrothermal
Petro thermal Geopressured
Geothermal

Large Deep
Dry Steam
Hot dry rocks Sedimentary
Field
Basin

Wet Steam
Volcanos
Field

Magma
Hot Water
Deposits
GEOTHERMAL ENERGY
Geothermal Power Plant
THERMODYNAMICS OF
GEOTHERMAL ENERGY
CONVERSION,
ELECTRICAL
CONVERSION,
NON ELECTRICAL
CONVERSION
 Systems used for Generating Power
1. Vapour-Dominated Power Plant:
 Systems used for Generating Power
1. Vapour-Dominated Power Plant:
 Systems used for Generating Power
1. Vapour-Dominated Power Plant:
• Steam is extracted from geothermal wells, passed through a
separator to remove particulate contents and flows directly to a
steam turbine.
• Steam operates the turbine coupled with the generator is at a
temperature of about 245 °C and pressure 7 bar which are less than
those in conventional steam cycle plants.
• The efficiency of geothermal plants is low i.e., about 20%.
• Exhaust steam from the turbine passes through a condenser and the
water so formed circulates through the cooling tower.
• It improves the efficiency of the turbine and controls environmental
pollution associated with the direct release of steam into the
atmosphere.
• Waste water from the cooling tower sump is re-injected into the
geothermal well to ensure continuous supply.
 Systems used for Generating Power
2. Liquid-Dominated Power Plant:
• These plants are also called wet steam plants because they give wet
steam. i.e., a mixture of hot water and steam under high pressure.
• There are two types of liquid-dominated power plants:
i. Flashed steam system
ii. Binary cycle system
 Systems used for Generating
Power
2i Flashed Steam System:
 Systems used for Generating Power
2i Flashed Steam System:
• This system is preferred for high temperature mixture of geothermal
brine and steam with low dissolved impurities.
• Geothermal fluid (i.e., mixture of brine and steam) passes through a
flash chamber where a large part of the fluid is converted to steam.
• Dry saturated steam passes through the turbine coupled with the
generator to produce electric power.
• Hot brine from the flash chamber and the turbine discharge from
the condenser are re-injected into the ground and reinjection of the
spent brine ensures a continuous supply of geothermal fluid from
the well.
 Systems used for
Generating Power
2ii Binary Cycle System:
 Systems used for Generating Power
2ii Binary Cycle System:
• A binary cycle is used where geothermal fluid is hot water with
temperature less than 100 °C.
• This plant operates with a low boiling point working fluid in a
thermodynamic closed Ranking cycle.
• Hot brine from underground reservoir circulates through a heat
exchanger and is pumped back to the ground.
• In heat exchanger, hot brine transfers its heat to the organic fluid
thus converting it to a superheated vapour that is used in a standard
closed Ranking cycle.
 Applications of Geothermal Energy
• Generation of electric power
• Industrial process heat
• Space heating for buildings
• Production of salt from sea
• Extraction manufacturing
• Textile Industry
• Sewage heat treatment
• Geothermal water is utilized for greenhouse
cultivation using discharge water from a
geothermal drill hole
 Geothermal Power Plant Vs
Thermal Power Plant
It uses inexhaustible source of It uses exhaustible source of
energy. energy.
It is more environment friendly. It is less environment friendly.
These power plants in some There is no such problem.
dangerous cases can cause
earthquakes.

It is mainly used for power It can be used for various

generations process. industrial processes.
Setup cost is high. Setup cost is low.
Byproducts of these plants are Byproducts of these plants can
not used be used.
These plant are less flexible. These plants are more flexible.
Specified area is required. No such restriction.
ENVIRONMENTAL
CONSIDERATIONS
 Environmental Considerations
• Generation energy is not completely pollution free energy.
• The main adverse environmental effects are air pollution (waste steam is
sometimes vented directly to the atmosphere), thermal pollution (pumping more
thermal energy to the atmosphere), surface disturbance, physical effects (land
subsidence) caused by fluid withdrawal.
• At geothermal site, the air pollution is the major problem because of emission of
poisonous gases such as hydrogen sulphide (H2S), ammonia, methane, Carbon
dioxide (CO2) etc.
• The extraction of energy from hot dry rocks or molten magma, it is necessary to
force water down boreholes as a working fluid and return it to surface to use in
turbine.
• If the underground reservoir is highly permeable, there is no way to know how
much water will need to be injected before a useful amount of steam or hot
water is returned to the surface.
• A large volume of flash steam escaping into the atmosphere could cause dense
fog to occur.
 Environmental Considerations

• At geothermal site, some harmful substances may escape into the air.
• These may contain radioactive materials, therefore systematic monitoring is
advisable.
• Geothermal water contains dissolved solids.
• The amount of dissolved solids is in the range of 300-1500 ppm of which
silica amounts to 25-50%.
• The possible solution is reinjection or disposal into sea through ducts and
channels and also the use of evaporator ponds.
MAGNETO-HYDRODYNAMICS
(MHD)
 Magneto Hydrodynamics (MHD)
• The magneto hydrodynamics deals with generation of electric field, when an
ionized fluid at high temperature passes through an applied magnetic field.
• The direct current is generated from the system with the expense of thermal
energy.
• MHD power generation is based on the Faraday’s law of electromagnetic
induction.
• The MHD generator should meet the following requirements:
• The magnet material should have high melting point.
• The electrodes are made of SiC or ZrC material to withstand high
temperature for preventing the chemical erosion.
• To prevent the chemical erosion from hot gases, the ceramics are chosen to
construct the duct.
• Duct material should have high electrical & thermal insulation.
• The insulation and conducting materials should be able to withstand high
temperature around 2500 ⁰C.
 Principle of MHD Power Plant
• The MHD power generation conversion process depends upon
Faraday’s law of electromagnetic induction, which states that when
a conductor and a magnetic field move relative to each other, a
voltage is induced in the conductor. This induced voltage produces
an electric current.
• The conductor may be solid, liquid or gas.
• In MHD generator, solid conductors are replaced by hot ionized gas.
• The hot ionized gas of temperature 3000 C is passed throgh the
MHD duct across which a strong magnetic field is applied.
• Since the gasses are hot and ionized, they form an electrically
conducting medium moving in a magnetic field, thus a voltage is
generated.
• The power generated by MHD generator is in the direct current
form.
• Now, if the electrodes are placed in a suitable position then
generated current can be extracted.
Principle of MHD Power Plant
:
 Classification of MHD System
1. Open Cycle MHD System
The working fluid after generating electrical energy
is discharged to atmosphere through the stack. The
working fluid used is air.

2. Closed Cycle MHD System

The working fluid is recycled to the heat source
used again and again. Helium or argon is used as
the working fluid
Open Cycle MHD System
 1. Open Cycle MHD System
• The open cycle MHD generator uses coal as a fuel as it
produces more conductive plasma. This is because of more
carbon atom as compared to hydrogen atom (as the
presence of hydrogen is undesirable in MHD)
• The working temperature in the open cycle MHD generator
lies approximately in the range above 2300 ⁰C.
• This is a lower temperature limit and below this the effective
electrical conductivity becomes zero.
• There may be no limit in the upper working temperatures; so
far the materials can stand with the high heat fluxes under
high electric field.
Closed Cycle MHD System
 2. Closed Cycle MHD System
• High thermal efficiency is achieved with low cycle cost in closed cycle plant and
provides more useful power at low temperature at 1600 ⁰C. The duct of these units
is small because of high pressure.
• Helium or Argon is used as a working fluid which is heated in heat exchanger and
gets ionized.
• Less ionized substances e.g., alkali metal is mixed with inert gas to provide the
necessary conductivity in closed cycle plant .
• The closed cycle plant is further classified as seeded inert gas system and liquid
metal system.
• The working fluid (Argon or Helium) in closed cycle is seeded with cesium and
circulates in a closed loop.
• The gas burned in the combustor is supplied in the heat exchanger, where the heat
is transferred to the working fluid.
• The ionized working fluid passes through the magnetic field to produce DC power.
• The combustion products are discharged to the atmosphere after removal of heat in
heat exchanger.
 2i. Closed Cycle Liquid Metal
MHD Generator

• The superheated metallic vapor is expended through the supersonic nozzle and
enters in the generator in liquid form with velocity of 150 m/s.
• The electrical conductivity of metallic vapor is poor which brings the overall
conversion efficiency lower than that of gas as a working substance.
• However, it has the advantage to supply AC current directly and there is no need of
inverter.
 2ii. Closed Cycle nuclear fired Metal
MHD Generator

• In nuclear fired MHD generator, the high temperature of nuclear reactor is used to utilize
solid fuel elements to meet the requirements.
• The ceramic coated electrodes are film cooled by hydrogen to protect them from unusual
build up of Uranium droplets.
• The cyclonic separators are used to remove the uranium droplet from the hydrogen gas and
the hydrogen flows in the compressor expands through the turbine, then it is cooled in heat
exchangers in multistage compression.
 System
• It has no moving parts.
• Conversion efficiency is better.
• Plant size is smaller than conventional power plants.
• It has ability to full power level generation immediately
when started or small start-up time.
• It is a direct method of heat to electrical energy
conversion.
• Less pollution.
• Good candidate for peak power savings and for
emergency
 System
• Material confinement:- To find materials that can survive
high operating temperatures of these generators. Both
insulator and conducting materials should sustain
temperature of 2500 ⁰C for prolong duration.
• Erosion–corrosion problems:- Electrode materials are
chemically eroded by combustion gases.
• Seeding material potassium attacks insulating materials and
make them conducting.
• They generate high current and low voltage DC.
• Expensive:- The additional investment in the magnet,
generator, duct, compressors, scrubbers, seed recovery
plant and DC to AC converters increase the plant cost and it
may be much higher than conventional plant.
FUEL CELLS
Working Principle
The biggest difference between the two is that a
battery stores energy, while a fuel cell
generates energy by converting available
fuel. A fuel cell can have a battery as a system
component to store the electricity it's generating.
Reaction in Acid Electrolyte Fuel
Cell
Reaction in Alkali Electrolyte Fuel
Cell

Anode: 2H2 + 4 OH- = 4H2O + 4 e-

Cathode: O2 + 2H2O + 4e- = 4 OH-

Types of Fuel Cell
Alkaline Fuel Cell
Proton Exchange Membrane Fuel Cell
(PEMFC)
Phosphoric Acid Fuel Cell (PAFC)
Molten Carbonate Fuel Cell
(MCFC)
Direct Methanol Fuel Cell (DFMC)
Zinc Air Fuel Cell (ZAFC)
Solid Oxide Fuel Cell (SOFC)