AKTU B.Tech 7th Sem Renewable Energy Resources Notes

Renewable Energy Resources is an important B.Tech 7th Semester subject that explains conventional and non-conventional energy sources and the technologies used to convert renewable energy into useful heat and electrical power. The subject begins with basic energy-resource classification and solar photovoltaic concepts before moving into solar thermal systems, geothermal power, direct energy-conversion methods, wind energy, biomass, and ocean energy.

These AKTU B.Tech 7th Sem Renewable Energy Resources Notes are organized according to the five-unit structure followed in the provided study material for Renewable Energy Resources (BOE074). The notes focus on the working principles, construction, classifications, advantages, limitations, applications, and important energy-conversion systems discussed in the source material.

The subject is strongly concept-oriented. For examination preparation, students should understand not only definitions but also the energy-conversion sequence of important systems such as solar collectors, geothermal plants, MHD generators, fuel cells, thermoelectric generators, wind turbines, biogas plants, OTEC systems, and tidal power plants.

Subject Overview

Renewable Energy Resources studies energy sources that can reduce dependence on limited conventional fuels such as coal, petroleum, and natural gas. Renewable sources discussed in the notes include solar, wind, hydro, biomass, tidal, wave, geothermal, and hydrogen-based energy systems.

The subject combines several areas of engineering:

• classification and availability of energy resources
• environmental effects of conventional energy
• solar photovoltaic and solar thermal conversion
• geothermal power generation
• magnetohydrodynamic power generation
• fuel-cell technology
• thermoelectric and thermionic conversion
• wind-energy conversion systems
• biomass conversion and biogas
• ocean thermal, wave, and tidal energy

A recurring idea throughout the subject is energy conversion. Solar radiation may be converted into heat or electricity, wind kinetic energy into mechanical and then electrical energy, biomass chemical energy into heat or fuel, and geothermal heat into mechanical and electrical power.

Unit-Wise Detailed Study Guide

Unit 1: Introduction

Unit 1 introduces energy, conventional and non-conventional resources, their classification, environmental importance, and solar photovoltaic technology. It establishes the foundation required for the later renewable-energy systems.

Energy and Its Forms

Energy is the capacity to do work. It can appear in several forms, including:

• mechanical energy
• heat energy
• light energy
• electrical energy
• chemical energy
• nuclear energy

The form of energy may change during an energy-conversion process. For example, a turbine converts fluid or wind energy into mechanical rotation, while a generator converts that mechanical energy into electricity.

Renewable and Non-Renewable Energy Resources

Non-renewable energy resources are available in limited quantities and cannot be replenished quickly after use.

Important examples include:

• coal
• petroleum
• natural gas
• nuclear fuels

Coal is extensively used for power generation and industries. Petroleum is refined into fuels such as petrol, diesel, kerosene, and LPG. Natural gas, whose major component is methane, is used for cooking, transport, industries, and electricity generation. Nuclear energy is produced through fission of fuels such as uranium and thorium.

These sources offer large amounts of energy but are limited and may create environmental problems.

Renewable energy resources, in contrast, are naturally replenished. Examples discussed in the notes are:

• solar energy
• wind energy
• hydropower
• biomass
• tidal energy
• wave energy
• geothermal energy
• hydrogen energy

Renewable sources are important because they can provide sustainable energy while reducing dependence on fossil fuels.

Environmental Effects of Conventional Energy

Different conventional resources produce different environmental impacts.

Coal:

• creates smoke and dust
• releases carbon dioxide
• may release sulphur dioxide
• contributes to global warming and acid rain
• mining can damage land and forests

Petroleum:

• releases carbon dioxide when burned
• vehicle emissions cause air pollution
• oil spills damage marine ecosystems
• refineries may release harmful gases

Natural gas:

• burns cleaner than coal and petroleum
• still produces carbon dioxide
• methane leakage creates safety and environmental concerns

Nuclear energy:

• does not produce conventional combustion-based air pollution
• creates radioactive waste
• requires safe waste disposal
• carries the risk of radiation-related accidents

Major Renewable Energy Sources

Solar Energy

Solar energy is obtained from sunlight. It can be converted into:

1. Solar thermal energy – sunlight is used for heating.
2. Solar photovoltaic energy – sunlight is converted directly into electricity.

Applications include solar cookers, water heaters, street lights, photovoltaic systems, and solar power plants.

Wind Energy

Wind energy uses the kinetic energy of moving air. Wind rotates turbine blades, and the turbine drives a generator to produce electricity.

Its major advantages are clean operation and low running cost after installation. Its availability, however, depends on wind conditions.

Hydropower

Hydropower uses flowing or falling water to rotate a turbine connected to a generator.

Large hydro projects can supply substantial electrical power, while small and micro-hydro systems can serve remote locations.

Biomass Energy

Biomass energy is obtained from organic material such as:

• wood
• agricultural waste
• animal waste
• municipal organic waste
• crop residues

Biomass can be burned directly or converted into biogas and biofuels.

Tidal Energy

Tidal energy is obtained from the periodic rise and fall of ocean tides. Tidal turbines or tidal power plants use moving water to generate electricity.

Wave Energy

Wave-energy converters capture the motion of ocean waves and transform it into useful mechanical and electrical energy.

Geothermal Energy

Geothermal energy is obtained from heat inside the Earth. Underground hot water or steam can be used for heating or electricity generation.

Hydrogen and Fuel Cells

Hydrogen can be used in fuel cells. Hydrogen and oxygen participate in electrochemical reactions to generate electricity, with water produced as a major reaction product in hydrogen-based fuel cells.

Classification of Energy Resources

Energy resources can be classified in several ways.

Based on Usability

Primary energy is directly available from nature.

Examples:

• coal
• crude oil
• natural gas
• sunlight
• wind

Secondary energy is obtained by converting primary energy into another usable form.

Examples include:

• electricity
• petrol
• hydrogen fuel

Based on Traditional Use

Conventional energy includes sources that have been widely used for a long period.

Examples:

• coal
• petroleum
• natural gas
• large hydropower

Non-conventional energy includes newer or alternative sources such as:

• solar
• wind
• tidal
• biomass
• geothermal
• fuel cells

Based on Long-Term Availability

Renewable resources are naturally replenished.

Non-renewable resources exist in limited stock and cannot be rapidly replaced.

Based on Commercial Application

Commercial energy is bought and sold in the market, such as electricity, coal, oil, and natural gas.

Non-commercial energy includes traditionally available resources such as firewood, animal waste, and agricultural waste.

Merits of Renewable Energy Resources

Major advantages include:

• naturally replenished supply
• low pollution compared with conventional fuels
• reduced greenhouse-gas emissions
• improved energy security
• potential employment generation
• suitability for decentralized generation
• usefulness in rural and remote areas

Limitations of Renewable Energy Resources

Important limitations include:

• high initial installation cost
• dependence of some sources on weather
• significant land requirements
• relatively low energy density for some resources
• energy-storage requirements
• location-specific availability

Renewable Energy and Global Warming

Global warming refers to the gradual increase in the average temperature of Earth associated with greenhouse gases such as carbon dioxide and methane.

Combustion of coal, petroleum, and natural gas adds greenhouse gases to the atmosphere. Greater use of renewable resources can help reduce fossil-fuel consumption and associated emissions.

Renewable energy also contributes to:

• cleaner air
• reduced dependence on imported fuels
• decentralized power generation
• sustainable long-term energy supply

Ministry of New and Renewable Energy

The notes identify the Ministry of New and Renewable Energy (MNRE) as the Government of India ministry concerned with developing and promoting renewable-energy resources.

Functions discussed include:

• policy making
• financial support
• research and development
• awareness
• international cooperation

Programs and initiatives mentioned in the notes include solar, bio-energy, wind-solar hybrid, hydrogen, rooftop solar, and international solar cooperation.

Solar Cell and Photovoltaic Effect

A solar cell converts sunlight directly into electrical energy through photovoltaic action.

A typical solar cell contains:

• transparent top layer
• anti-reflective coating
• semiconductor layers
• metallic contacts

The semiconductor region is the important part where incoming light contributes to generation and movement of electrical charge.

Solar Cell Materials

Materials discussed in the notes include:

• silicon
• thin-film materials
• perovskite materials
• organic photovoltaic materials
• gallium arsenide

Silicon is presented as the most commonly used solar-cell material.

Main Elements of a Photovoltaic System

A basic photovoltaic system may contain:

1. PV modules – generate DC electricity from sunlight.
2. Charge controller – manages battery charging.
3. Battery – stores electrical energy when required.
4. Inverter – converts DC into AC.
5. Electrical load – consumes the generated power.
6. Wiring and switching equipment – connects and controls the system.

Solar Cell, Module, and Array

The hierarchy is:

Solar cell → Solar module/panel → Solar array

A single solar cell produces a limited electrical output. Multiple cells are connected to make a module, while several modules are combined to create an array.

Connections may be:

• series
• parallel
• series-parallel

Series connection is used when greater voltage is required, while parallel connection is used when greater current capacity is required.

Solar Cell Power Plant

A photovoltaic power plant converts sunlight directly into electrical power using large groups of solar modules.

Major parts include:

• solar-cell arrays
• mounting structures
• inverter station
• transformer and switchyard
• control and monitoring system
• optional battery storage

Advantages of Photovoltaic Solar Energy

• renewable source
• no fuel requirement during operation
• low running cost
• quiet operation
• modular and scalable
• useful in remote areas
• can be installed at different capacities

Limitations of Photovoltaic Systems

• output depends on sunlight
• high initial cost
• significant area may be required
• battery storage increases cost
• no direct solar generation at night
• manufacturing of equipment also has environmental implications

Unit 2: Solar Thermal Energy

Solar thermal energy uses solar radiation as a source of heat. Instead of converting sunlight directly into electricity as in photovoltaic cells, solar thermal systems collect solar heat for water heating, cooking, industrial processes, or power generation.

Solar Radiation

Solar radiation reaching Earth may be considered in different forms.

Direct or Beam Radiation

Radiation travelling directly from the Sun to a surface without being scattered is called direct or beam radiation.

Diffuse Radiation

Solar radiation scattered by atmospheric particles before reaching the ground is called diffuse radiation.

Reflected Radiation

Part of the solar radiation may be reflected from surrounding surfaces and reach a collector indirectly.

Solar Constant

The notes define the solar constant as the solar-energy flux received per unit area outside Earth’s atmosphere on a surface normal to the Sun’s rays.

The approximate value given is:

Solar constant ≈ 1361 W/m²

It is an important reference value in solar-energy calculations.

Solar Collectors

A solar collector captures solar radiation and converts it into useful thermal energy.

Major types discussed include:

• flat plate collectors
• evacuated tube collectors
• integral collector storage systems
• concentrating collectors
• air collectors

Flat Plate Collector

A Flat Plate Collector (FPC) is a non-concentrating solar collector used to absorb solar radiation and transfer the resulting heat to a working fluid.

Main Components

1. Transparent cover or glazing
   Allows solar radiation to enter and helps reduce heat loss.

2. Absorber plate
   Usually dark or selectively coated to absorb maximum solar radiation.

3. Fluid tubes or channels
   Carry water, air, or another heat-transfer medium.

4. Insulation
   Reduces heat loss from the back and sides.

5. Casing
   Supports and protects the collector components.

Working of a Flat Plate Collector

1. Solar radiation passes through the transparent cover.
2. The absorber plate absorbs radiation and becomes hot.
3. Heat passes from the absorber to the fluid flowing through tubes.
4. Heated fluid travels to a storage or utilization system.
5. Cooler fluid returns to the collector for reheating.
6. Glazing and insulation reduce heat losses.

Useful Heat Gain and Collector Efficiency

The performance of an FPC is evaluated from the useful heat obtained compared with the solar energy incident on the collector.

The notes describe useful heat gain as the heat transferred to the working fluid.

Collector efficiency is expressed conceptually as:

[ \eta = \frac{\text{Useful heat output}}{\text{Solar energy incident on collector}} ]

Higher useful heat gain with lower thermal losses results in better collector performance.

Factors Affecting Flat Plate Collector Performance

Important factors include:

• absorber material
• absorber surface coating
• glazing
• thermal insulation
• fluid flow rate
• collector tilt and orientation
• ambient temperature
• wind conditions
• intensity of solar radiation

Applications of Flat Plate Collectors

• domestic water heating
• space heating
• low-temperature industrial heating
• crop drying
• other low- and moderate-temperature thermal applications

Concentrating Solar Collectors

A concentrating solar collector uses mirrors or lenses to focus radiation from a relatively large collection area onto a smaller receiver.

Concentration allows the receiver to reach significantly higher temperatures than a typical flat plate collector.

Main Components of Concentrating Collectors

• reflector or mirror
• receiver or absorber
• tracking arrangement where required
• heat-transfer fluid
• supporting structure
• insulation
• transparent enclosure in some designs

Working Principle

1. Solar radiation falls on the reflector.
2. The optical surface redirects radiation toward a receiver.
3. The receiver absorbs concentrated solar energy.
4. Heat is transferred to a working fluid.
5. Heated fluid can be used directly or used for steam generation and power production.

Types of Concentrating Collectors

Parabolic Trough Collector

A parabolic trough uses a long curved reflective surface.

Solar radiation is concentrated onto a receiver tube positioned along the focal line. Fluid flowing through the tube absorbs heat.

Applications include:

• industrial heating
• steam generation
• solar thermal power production

Mirror Strip Reflector

Multiple mirror strips are arranged so reflected sunlight is directed toward a common receiver.

The configuration concentrates incoming solar radiation and heats a fluid passing through the receiver.

Fresnel Lens Collector

A Fresnel lens consists of specially shaped sections that can focus sunlight while using a thinner optical structure than a conventional thick lens.

The concentrated radiation is directed onto a receiver to obtain higher temperatures.

Compound Parabolic Collector

A Compound Parabolic Collector uses specially shaped reflecting surfaces to direct radiation toward a receiver.

An advantage is its ability to accept radiation over a wider angular range than strongly focusing systems.

Materials Used in Concentrating Collectors

Important material groups include:

• highly reflective mirror surfaces
• heat-resistant receiver materials
• selective absorber coatings
• glass envelopes
• heat-transfer fluids
• structural materials
• thermal insulation

Applications of Concentrating Solar Systems

• electricity generation
• industrial process heating
• desalination
• solar cooking
• solar cooling
• water and space heating

Advantages of Concentrating Collectors

• higher operating temperatures
• useful for electricity generation
• renewable heat source
• scalable systems
• reduced conventional-fuel consumption

Limitations

• strong dependence on direct solar radiation
• tracking may be required
• higher installation complexity
• mirrors require cleaning and alignment
• large areas may be required

Performance of Concentrating Collectors

Important performance considerations include:

• incident solar radiation
• optical efficiency
• receiver heat absorption
• thermal losses
• useful heat output
• collector efficiency

Better reflectivity, correct focusing, good receiver coatings, effective insulation, and proper tracking improve overall performance.

Solar Thermal Power Plants

A solar thermal power plant collects solar heat and uses it for useful thermal or electrical output.

The notes discuss systems according to operating-temperature range:

• low-temperature systems
• medium-temperature systems
• high-temperature systems

In power-generation applications, concentrated solar heat can be used to heat a working fluid, generate vapour or steam, operate a turbine, and produce electricity through a generator.

Limitations of Solar Thermal Power

• availability changes with sunlight
• energy storage may be required
• large installations need substantial space
• high-temperature systems require accurate tracking
• initial cost can be high
• collector performance is affected by weather and cleanliness

Unit 3: Geothermal Energy

Geothermal energy is heat energy obtained from inside the Earth. Underground heat can appear as hot water, steam, hot rocks, geysers, or hot springs and may be used directly or for electricity generation.

Sources of Geothermal Energy

The notes discuss several geothermal-resource categories:

1. hydrothermal systems
2. vapour-dominated systems
3. hot dry rock resources
4. geopressured resources
5. magma-based systems

Hydrothermal Systems

Hydrothermal resources contain naturally heated underground water or steam.

They can broadly occur as:

• hot-water systems
• vapour-dominated systems

Their usefulness depends on temperature, pressure, fluid availability, and geological conditions.

Vapour-Dominated Resources

In vapour-dominated geothermal fields, steam is the major usable fluid.

Steam can be brought to the surface and used to drive a turbine.

Hot Dry Rock

Hot Dry Rock resources contain very hot underground rock but insufficient naturally circulating fluid.

The heat stored in the rock can potentially be extracted using engineered fluid-circulation systems.

Geopressured Resources

Geopressured resources contain hot underground water at very high pressure.

Such resources contain both thermal and pressure-related energy.

Magma-Based Geothermal Resources

Magma contains extremely large amounts of thermal energy. The notes discuss it as a geothermal-energy source, although practical utilization is technologically difficult.

Geothermal Power Plant

A geothermal power plant converts underground thermal energy into electrical power.

Major Components

• production well
• steam or hot-fluid handling arrangement
• separator where required
• turbine
• generator
• condenser
• cooling system
• reinjection well

General Working

1. Hot geothermal water or steam is brought to the surface through a production well.
2. Steam is separated if the resource contains a steam-water mixture.
3. Steam rotates a turbine.
4. The turbine drives a generator.
5. Exhaust steam is cooled and condensed.
6. Condensed fluid or geothermal brine is reinjected underground.

Reinjection helps dispose of the fluid and supports sustainable use of the geothermal reservoir.

Types of Geothermal Power Plants

Dry Steam Plant

A dry-steam system uses geothermal steam directly.

The steam:

1. comes through the production well
2. flows toward the turbine
3. rotates the turbine-generator system
4. is condensed after expansion
5. is returned underground where appropriate

This is suitable where natural geothermal steam is available.

Flash Steam Plant

A flash system uses high-temperature geothermal water.

When pressure is reduced, part of the hot water changes rapidly into steam. This steam is separated and sent to the turbine.

Major stages are:

1. hot fluid extraction
2. pressure reduction
3. flash steam formation
4. steam separation
5. turbine expansion
6. condensation
7. reinjection

Binary Cycle Plant

Binary-cycle plants are suitable for geothermal water of comparatively moderate temperature.

Geothermal fluid transfers heat through a heat exchanger to a second working fluid with a lower boiling point.

The secondary fluid:

1. vaporizes
2. drives a turbine
3. is condensed
4. is circulated again

The geothermal fluid remains in a separate loop and can be reinjected.

Developments in Geothermal Technology

The notes mention developments such as:

• dry-steam technology
• flash-steam systems
• binary-cycle plants
• Enhanced Geothermal Systems
• hybrid configurations
• improved drilling technologies
• better emission-control techniques

Applications of Geothermal Energy

Geothermal energy may be used for:

• electricity generation
• direct heating
• hot-water applications
• industrial thermal uses
• other applications where naturally available underground heat is suitable

Environmental Problems of Geothermal Energy

Potential problems discussed include:

• land subsidence
• water consumption
• water contamination
• release of underground gases
• noise
• induced seismic activity
• thermal pollution
• visual impact

Reinjection and improved plant design can help reduce some of these effects.

Magnetohydrodynamic Power Generation

Magnetohydrodynamic (MHD) generation converts the energy of a high-temperature electrically conducting gas directly into electrical energy.

It differs from conventional turbine-generator systems because electrical power can be obtained from the motion of an ionized conducting fluid in a magnetic field.

Principle of MHD Generation

When an electrically conducting fluid moves through a magnetic field, an electromotive force is produced.

A high-temperature gas is made electrically conducting, often by ionization and the addition of suitable seed material.

Working of an MHD Generator

1. Hot conducting gas enters the MHD channel.
2. The gas moves at high velocity.
3. A strong magnetic field is applied across the channel.
4. Charged particles in the gas experience electromagnetic force.
5. Positive and negative charges move toward opposite electrodes.
6. A voltage develops between the electrodes.
7. Electrical current flows through the external circuit.

Open-Cycle MHD System

An open-cycle system uses hot combustion gases.

Important elements discussed include:

• combustion chamber
• nozzle
• MHD duct
• electrodes
• magnetic system
• inverter
• seed recovery unit
• preheater
• pollution-control equipment

The gas passes through the system and is ultimately discharged after heat recovery, seed recovery, and treatment.

Closed-Cycle MHD System

A closed-cycle system circulates a working gas repeatedly through the plant.

Working gases discussed include inert gases such as argon or helium.

Major components include:

• MHD generator
• diffuser
• heat exchanger
• compressor
• gas heater
• inverter

Because the working fluid is recirculated, the configuration differs fundamentally from the open-cycle system.

Fuel Cell

A fuel cell is an electrochemical device that converts the chemical energy of continuously supplied fuel and oxidant directly into electricity.

A fuel cell can continue producing electrical energy as long as the required reactants are supplied.

Major Components of a Fuel Cell

• anode
• cathode
• electrolyte
• external electrical circuit
• fuel supply
• oxidant supply

General Working of a Hydrogen Fuel Cell

1. Hydrogen is supplied to the anode.
2. Electrochemical reactions release electrons.
3. Electrons travel through the external circuit and provide electrical power.
4. Ions pass through the electrolyte.
5. Oxygen is supplied at the cathode.
6. Reaction products are formed, including water in hydrogen-oxygen fuel cells.

Advantages of Fuel Cells

• direct electrochemical energy conversion
• quiet operation
• no conventional combustion process
• modular operation
• useful for continuous power when reactants are supplied

Fuel-cell characteristics depend strongly on the specific electrolyte and cell technology.

Unit 4: Thermo-Electrical and Thermionic Conversions

Unit 4 covers direct conversion of thermal energy into electricity and then develops wind-energy conversion systems.

Seebeck Effect

The Seebeck effect is the production of an electromotive force when two different conducting materials form junctions maintained at different temperatures.

Working

1. Two dissimilar materials form a circuit.
2. One junction is heated.
3. The other junction remains comparatively cold.
4. Charge carriers respond to the temperature difference.
5. A potential difference is produced.
6. Electrical current can flow through a connected circuit.

This effect forms the basic principle of a thermoelectric generator.

Peltier Effect

The Peltier effect is the absorption or liberation of heat at the junction of two dissimilar conductors when electric current passes through the junction.

Depending on current direction, one junction may absorb heat while another releases heat.

This principle is used in thermoelectric heating and cooling.

Joule Effect

The Joule effect refers to heating produced when electrical current flows through a conductor with electrical resistance.

Electrical energy is converted into heat.

Applications include:

• electric heaters
• irons
• electric bulbs
• fuses
• cooking appliances
• industrial furnaces

Thomson Effect

The Thomson effect occurs when current passes through a homogeneous conductor in which a temperature gradient already exists.

Depending on the material, current direction, and temperature gradient, heat may be absorbed or released along the conductor.

Relationship Between Thermoelectric Effects

The Seebeck and Peltier effects are related thermoelectric phenomena.

The notes discuss the relationship between the Seebeck coefficient and Peltier coefficient, showing that these effects are not independent but are thermodynamically connected.

Thermoelectric Generator

A thermoelectric generator (TEG) converts heat directly into electricity using the Seebeck effect.

Construction

Important elements include:

• p-type thermoelectric material
• n-type thermoelectric material
• hot junction
• cold junction
• electrical load
• electrical interconnections

Working

1. Heat is supplied to the hot side.
2. A temperature difference develops across thermoelectric elements.
3. Charge carriers move because of the temperature difference.
4. A voltage is generated.
5. Current flows through the external load.

Maintaining a strong temperature difference between hot and cold sides is essential.

Thermoelectric Materials

Good thermoelectric materials should provide favorable combinations of:

• high Seebeck coefficient
• good electrical conductivity
• low thermal conductivity
• stability over the operating temperature range

The effectiveness of a thermoelectric material is commonly discussed through its figure of merit.

Performance of Thermoelectric Generators

Performance depends on:

• hot-side temperature
• cold-side temperature
• temperature difference
• thermoelectric material properties
• thermal losses
• electrical resistance
• quality of cooling

A better thermoelectric figure of merit indicates more favorable conversion performance.

Applications of Thermoelectric Generators

Applications mentioned include:

• recovery of waste heat
• automobile exhaust-energy recovery
• specialized power-generation situations
• systems where mechanical moving parts are undesirable

Limitations of Thermoelectric Generators

• relatively low efficiency
• expensive materials
• limited operating-temperature range
• comparatively low output for many applications
• heat losses
• need for an effective cooling system

Thermionic Converter

A thermionic converter converts thermal energy directly into electrical energy using emission of electrons from a heated surface.

Construction

The basic converter contains:

1. Cathode or emitter – heated to a high temperature.
2. Anode or collector – receives emitted electrons.
3. Vacuum or controlled inter-electrode region.
4. External load circuit.

Working

1. Heat is supplied to the cathode.
2. Electrons gain sufficient energy to leave the cathode surface.
3. Electrons travel across the gap toward the collector.
4. The electron flow produces current through the external circuit.
5. Heat is rejected from the collector side.

Advantages

• direct heat-to-electricity conversion
• no conventional mechanical turbine is necessary
• compact construction is possible

Limitations

• high emitter temperature requirement
• material challenges
• heat losses
• comparatively limited conversion efficiency

Wind Energy Conversion

Wind is moving air. Wind is produced largely because solar heating of Earth’s surface is uneven, creating temperature and pressure differences.

Formation of Wind

1. The Sun heats different regions unevenly.
2. Warmer air expands and rises.
3. Cooler, denser air moves into its place.
4. This atmospheric movement produces wind.

Site Selection for a Wind Farm

Important factors include:

• wind speed
• open terrain
• elevation
• site accessibility
• distance from electrical grid
• land availability
• environmental effects

A good site should provide strong and sufficiently consistent wind.

Principle of Wind-Energy Conversion

The fundamental sequence is:

Wind kinetic energy → Rotational mechanical energy → Electrical energy

Wind acts on turbine blades and rotates the rotor. The rotor drives a shaft, directly or through a gearbox, and a generator converts mechanical rotation into electricity.

Wind-Power Dependence on Wind Speed

The notes emphasize that wind power varies approximately with the cube of wind speed:

[ P \propto V^3 ]

Therefore, relatively small increases in wind speed can produce a large increase in available wind power.

For example, if wind speed doubles, the theoretical available wind power increases by a factor of eight.

Fluctuating Wind-Power Generation

Wind speed changes with weather, location, season, and time. Therefore wind power is inherently variable.

Methods discussed for dealing with fluctuations include:

• energy-storage systems
• hybrid power systems
• variable-speed turbines
• careful site selection
• electrical-grid connection

Types of Wind Energy Conversion Systems

Two major categories are:

1. Horizontal Axis Wind Energy Conversion System
2. Vertical Axis Wind Energy Conversion System

Horizontal Axis Wind Turbine

In a horizontal-axis wind turbine, the main rotor shaft is approximately horizontal.

Major components include:

• blades
• hub
• main shaft
• gearbox
• generator
• brake
• nacelle
• anemometer
• tower
• foundation
• transformer

Working of a Horizontal Axis Wind Turbine

1. Wind produces aerodynamic force on the blades.
2. Blades rotate the rotor and hub.
3. The main shaft transfers mechanical power.
4. A gearbox may increase rotational speed.
5. The generator produces electricity.
6. A transformer adjusts voltage for transmission.
7. Electricity is supplied to the grid.

Horizontal-Axis Turbines Based on Number of Blades

The notes discuss:

• single-blade turbines
• two-blade turbines
• three-blade turbines
• multi-blade turbines

Three-blade turbines provide smooth and stable rotation and are widely suitable for electrical power generation.

Multi-blade turbines produce high torque at low rotational speeds and are useful for applications such as mechanical water pumping.

Vertical Axis Wind Turbine

In a vertical-axis wind turbine, the rotating shaft is vertical.

An important characteristic is its ability to accept wind from different directions without continuously turning the rotor toward the wind.

Major parts include:

• blades
• hub
• vertical shaft
• cross arms
• bearings
• support arrangement
• housing
• gearbox
• generator
• foundation
• transformer
• grid connection

Types of Vertical Axis Turbines

Savonius Turbine

The Savonius turbine uses curved bucket-like blades and works mainly on the drag principle.

Advantages:

• simple design
• operates at low wind speeds
• accepts wind from different directions
• high starting torque

Limitations:

• low rotational speed
• lower efficiency
• unsuitable for large-scale electrical generation

Darrieus Turbine

The Darrieus turbine uses curved blades and works mainly on aerodynamic lift.

Advantages:

• higher efficiency than drag-type turbines
• suitable for higher rotational speeds
• accepts wind from varying directions

Limitations:

• difficult self-starting in conventional designs
• more complex structure
• maintenance can be difficult

H-Rotor

The H-rotor is a straight-bladed form related to the Darrieus concept.

Its straight vertical blades generate lift and rotate the vertical shaft.

It provides comparatively simple construction and may be useful in smaller or urban wind projects.

Wind Characteristics

Important wind characteristics include:

• wind speed
• wind direction
• air density
• height above ground
• terrain roughness
• daily and seasonal variation

Wind speed normally increases with height because the influence of surface friction becomes smaller.

Terrain also matters. Smooth open regions generally permit stronger and steadier wind than heavily obstructed areas.

General Limitations of Energy-Conversion Systems

The notes identify common limitations such as:

• conversion losses
• high capital cost
• intermittent renewable supply
• storage difficulty
• maintenance
• site restrictions
• environmental effects
• technological limitations

No real energy-conversion system converts all available input energy into useful output.

Unit 5: Biomass

Biomass is organic plant- or animal-derived material that can be used for energy production. It is considered renewable because biological material can be regenerated.

Sources of Biomass

Examples include:

• agricultural residues
• forest residues
• animal waste
• urban organic waste
• industrial organic waste
• wood
• crop residues

India has significant biomass availability because of agricultural, rural, livestock, and agro-industrial activities.

Biomass Conversion

Biomass can be converted into useful energy using several routes.

The notes discuss:

• direct combustion
• thermochemical conversion
• biochemical conversion

Direct Combustion

Direct combustion means burning biomass in the presence of air to release heat.

The heat may be used:

• directly for heating
• to produce steam
• to generate electricity through a steam-power cycle

Although technically simple, uncontrolled biomass burning can produce smoke and other emissions.

Thermochemical Conversion

Thermochemical conversion uses heat to transform biomass into more useful fuels or energy products.

Important processes include:

• pyrolysis
• gasification
• liquefaction

Pyrolysis

Pyrolysis involves heating biomass with little or no oxygen.

The process breaks complex organic material into products that may include:

• gases
• liquids
• solid carbon-rich material

Gasification

Gasification converts solid biomass into a combustible gaseous fuel using a controlled amount of oxidant.

Producer gas contains combustible components such as:

• carbon monoxide
• hydrogen
• methane

Main Gasification Stages

The notes describe four important zones:

1. Drying
2. Pyrolysis
3. Oxidation
4. Reduction

Drying

Moisture is removed from incoming biomass.

Pyrolysis

Heat decomposes biomass into volatile matter, gases, liquids, and char.

Oxidation

A controlled amount of oxygen reacts with fuel components and provides heat for the gasification process.

An example reaction given is:

[ C + O_2 \rightarrow CO_2 + \text{Heat} ]

Reduction

Hot carbon and gases undergo reactions that produce combustible gases.

One reaction given in the notes is:

[ CO_2 + C \rightarrow 2CO ]

The resulting gas can be used for heat or power applications after suitable cleaning and handling.

Biochemical Conversion

Biochemical conversion uses microorganisms to convert biomass into useful fuels.

Two major methods are:

1. anaerobic digestion
2. fermentation

Anaerobic Digestion

Anaerobic digestion is decomposition of organic material by microorganisms in the absence of oxygen.

Feed material may include:

• cow dung
• food waste
• sewage
• other biodegradable organic material

Major products are:

• biogas
• digested slurry

Biogas contains methane as the main combustible constituent along with carbon dioxide and smaller quantities of other gases.

Anaerobic Digestion Process

1. Organic material is mixed and placed inside an airtight digester.
2. Anaerobic microorganisms decompose the material.
3. Biogas is produced and collected.
4. Digested slurry leaves the plant.
5. The slurry can be used as organic manure.

A simplified representation is:

[ \text{Organic waste} \xrightarrow{\text{anaerobic bacteria}} CH_4 + CO_2 + \text{slurry} ]

Fermentation

Fermentation uses microorganisms such as yeast to convert sugars into ethanol.

Biomass containing sugar or starch can be processed and fermented.

The resulting ethanol can be used as a fuel or fuel component.

Advantages of Biochemical Conversion

• renewable process
• useful waste treatment
• fuel production
• reduced organic-waste accumulation
• useful manure may be obtained from anaerobic digestion

Limitations

• biological processes may be slow
• operating conditions need control
• temperature can affect microorganisms
• large-scale output may be limited by feedstock supply

Deenbandhu Biogas Plant

The Deenbandhu biogas plant is described as a low-cost fixed-dome biogas system developed for practical use in India.

Main Parts

1. mixing tank
2. inlet pipe
3. underground digester
4. fixed gas-storage dome
5. gas outlet and valve
6. displacement or outlet chamber

Working

1. Cow dung or other suitable organic material is mixed with water.
2. The slurry enters the digester through the inlet.
3. Anaerobic bacteria decompose organic matter.
4. Biogas accumulates inside the dome.
5. Gas pressure displaces digested slurry toward the outlet chamber.
6. Biogas leaves through the gas pipe for use.
7. Digested slurry can be used as fertilizer.

Advantages of Deenbandhu Plant

• relatively low construction cost
• simple underground structure
• useful for rural applications
• converts organic waste into fuel
• produces useful manure

Biomass Gasification

Biomass gasification is a thermochemical process that converts solid biomass into combustible producer gas.

The overall process involves drying, pyrolysis, oxidation, and reduction.

Proper control of air supply is necessary because complete combustion would convert most fuel into carbon dioxide and heat instead of retaining combustible gas components.

Ocean Thermal Energy Conversion

Ocean Thermal Energy Conversion (OTEC) generates energy using the temperature difference between:

• warm surface seawater
• cold deep-ocean water

The temperature difference operates a thermodynamic conversion system.

Advantages of OTEC

• renewable ocean-energy source
• can operate continuously where suitable temperature difference exists
• low direct combustion emissions
• open-cycle systems can also produce fresh water

Limitations

• very high capital cost
• relatively low conversion efficiency
• suitable mainly in specific warm-ocean regions
• large seawater flow is required
• marine equipment is difficult to maintain
• environmental effects must be managed

Types of OTEC Systems

Two principal systems discussed are:

1. Closed-cycle or Anderson-cycle OTEC
2. Open-cycle or Claude-cycle OTEC

Closed-Cycle OTEC

A closed-cycle system uses a low-boiling working fluid circulating in a closed loop.

Main Components

• warm seawater intake
• cold seawater intake
• evaporator
• secondary working-fluid circuit
• turbine
• generator
• condenser
• pumps
• piping and control equipment

Working

1. Warm surface seawater enters the evaporator.
2. Its heat vaporizes the low-boiling working fluid.
3. Working-fluid vapour drives the turbine.
4. The turbine drives the generator.
5. Cold deep seawater cools the vapour inside the condenser.
6. Working fluid becomes liquid again.
7. A pump returns it to the evaporator.
8. The cycle repeats.

The ocean water and working fluid remain in separate circuits.

Open-Cycle OTEC

In open-cycle OTEC, warm seawater itself is used as the working fluid.

Working

1. Warm seawater enters a low-pressure flash chamber.
2. Reduced pressure causes part of the water to evaporate.
3. The resulting low-pressure steam drives a turbine.
4. Steam is condensed using cold deep-ocean water.
5. The condensate can provide fresh water.

Environmental Effects of OTEC

Potential positive effects include:

• renewable electricity
• freshwater production in open-cycle plants
• possible productive use of nutrient-rich deep water

Possible negative effects include:

• changes in local seawater temperature
• disturbance to marine organisms
• risk associated with working-fluid leakage in closed-cycle systems
• noise
• visual effects from large offshore structures

Wave Energy

Wave energy is obtained from the movement of sea waves.

Principle

Ocean-wave motion contains kinetic and potential energy. A wave-energy converter captures this motion and transforms it into mechanical energy, which can then drive an electrical generator.

General Energy-Conversion Process

1. Sea waves move a floating or oscillating structure.
2. Mechanical motion is created.
3. The motion operates a turbine, hydraulic system, or another mechanical arrangement.
4. A generator converts the mechanical power into electricity.

The feasibility of wave-energy systems depends strongly on wave conditions and marine-environment requirements.

Tidal Energy

Tidal energy is obtained from periodic changes in ocean level caused by tides.

Both potential energy from differences in water level and kinetic energy of moving tidal water may be utilized.

Tidal Power Plant

A tidal power plant generally uses a basin or barrage together with turbines and generators.

Important components include:

• tidal basin
• barrage or dam
• gates
• turbine
• generator
• water passages

Single-Basin Tidal Power Plant

A single-basin system uses one basin to store seawater.

Working

1. During high tide, seawater enters the basin.
2. Gates are controlled to retain water.
3. As sea level falls, a head difference develops.
4. Water is released through turbines.
5. Turbines drive electrical generators.

Advantages

• relatively simple configuration
• renewable tidal resource
• tidal timing is predictable

Limitations

• generation is intermittent
• large civil structures are required
• output depends on tidal range
• marine ecosystems may be affected

Double-Basin Tidal Power Plant

A double-basin arrangement uses two water basins at different levels.

Water can be transferred from a higher-level basin toward a lower-level basin through turbines.

Compared with a basic single-basin arrangement, the system can provide better control over the timing and continuity of generation.

Important Topics for Exam Preparation

Unit 1

• renewable and non-renewable energy resources
• classification of energy resources
• merits and demerits of renewable energy
• environmental impact of conventional energy
• role and importance of renewable energy
• solar-cell working principle
• solar-cell materials
• PV-system components
• solar cell, module, and array
• solar-cell power plant
• advantages and limitations of photovoltaic systems

Unit 2

• direct, diffuse, and reflected solar radiation
• solar constant
• flat plate collector
• construction and working of FPC
• factors affecting collector performance
• concentrating solar collectors
• parabolic trough collector
• Fresnel collector
• compound parabolic collector
• concentrating-collector materials
• solar thermal power plants
• advantages and limitations of solar thermal systems

Unit 3

• geothermal-energy resources
• hydrothermal, vapour-dominated, hot dry rock, and geopressured systems
• geothermal power-plant working
• dry-steam plant
• flash-steam plant
• binary-cycle plant
• environmental problems of geothermal energy
• MHD principle
• open- and closed-cycle MHD systems
• fuel-cell construction and working

Unit 4

• Seebeck effect
• Peltier effect
• Joule effect
• Thomson effect
• thermoelectric generator
• thermoelectric materials and figure of merit
• thermionic converter
• wind-farm site selection
• wind-energy conversion principle
• fluctuating wind-power generation
• horizontal-axis wind turbines
• vertical-axis wind turbines
• Savonius turbine
• Darrieus turbine
• wind characteristics
• relation (P \propto V^3)

Unit 5

• biomass and biomass availability
• direct combustion
• thermochemical conversion
• pyrolysis
• gasification
• biochemical conversion
• anaerobic digestion
• fermentation
• Deenbandhu biogas plant
• biomass gasification stages
• OTEC principle
• closed- and open-cycle OTEC
• environmental effects of OTEC
• wave-energy conversion
• tidal-energy principle
• single- and double-basin tidal plants

Quick Revision Points

• Renewable energy resources are naturally replenished; non-renewable resources have limited reserves.
• Solar energy can be converted either directly into electricity through photovoltaic cells or into heat through solar thermal collectors.
• A PV system commonly contains modules, a controller, battery where required, inverter, wiring, and electrical loads.
• Flat plate collectors are suitable mainly for lower-temperature thermal applications.
• Concentrating collectors focus solar radiation onto a smaller receiver to obtain higher temperatures.
• The solar constant used in the notes is approximately 1361 W/m².
• Geothermal power uses underground heat in the form of steam or hot water.
• Dry-steam, flash-steam, and binary-cycle systems are important geothermal plant configurations.
• MHD generation converts energy of an electrically conducting fluid directly into electricity in a magnetic field.
• Fuel cells convert chemical energy directly through electrochemical reactions.
• The Seebeck effect produces voltage from a temperature difference.
• The Peltier effect produces heating or cooling when current passes through dissimilar-material junctions.
• Thermoelectric generators use the Seebeck effect.
• Thermionic converters use electron emission from a heated emitter.
• Wind turbines convert wind kinetic energy into mechanical rotation and then electricity.
• Available wind power varies approximately as the cube of wind speed.
• Horizontal-axis and vertical-axis turbines are the two major wind-turbine arrangements.
• Savonius turbines mainly use drag; Darrieus turbines mainly use lift.
• Biomass conversion can be thermal, thermochemical, or biochemical.
• Anaerobic digestion produces biogas from organic matter in the absence of oxygen.
• Fermentation can convert biomass-derived sugars into ethanol.
• Biomass gasification commonly includes drying, pyrolysis, oxidation, and reduction zones.
• OTEC uses the temperature difference between warm surface seawater and cold deep water.
• Closed-cycle OTEC uses a secondary low-boiling working fluid.
• Open-cycle OTEC flashes warm seawater into steam and can produce fresh water.
• Wave energy utilizes sea-wave motion.
• Tidal energy utilizes periodic changes in sea level and tidal water flow.

How to Use These Notes Effectively

1. Study the subject unit by unit instead of memorizing isolated definitions.
2. For every power-generation system, learn the principle, components, working sequence, advantages, limitations, and applications.
3. Draw and practice labelled diagrams for major systems such as solar collectors, geothermal plants, MHD systems, fuel cells, wind turbines, biogas plants, OTEC plants, and tidal plants.
4. Revise important relationships such as the dependence of wind power on (V^3).
5. Compare similar systems, especially dry/flash/binary geothermal plants, horizontal/vertical wind turbines, and open/closed OTEC.
6. Before examinations, use the Quick Revision Points and then revisit detailed sections for topics you cannot explain without looking at the notes.

For official examination notices and syllabus updates, visit the AKTU official website.

Frequently Asked Questions