Renewable Energy Resources: Last year Question Paper Questions with Answer

With AKTU’s Question Paper, you may embark on an Exciting Renewable Energy Resources Journey. Improve your understanding, test your knowledge, and fly to exam achievement.

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Important Questions For Renewable Energy Resources:
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* Btech 4th Year

Section A: Short Question In Renewable Energy Resources

a. Describe photovoltaic effect.  

Ans. Photovoltaic Effect: When a solar cell is illuminated, electron hole pairs are generated and the electric current I is obtained. I is the difference between the solar light generated current I_L and the diode dark current I_j. 

Mathematically :  

I = I_L – I_j 

where, I_o = saturation current, 

e = electronic charge, 

T = absolute temperature, and 

K = Boltzmann’s constant

= 1.38 x 10^-23 J/K. 

b. Write down the properties of polycrystalline silicon cell. 

Ans.

• 1. They have a grainy bluish coating appearance. 
• 2. The conversion efficiency of polycrystalline silicon cells is between 14 % and 16 %.

c. Calculate the angle of declination for 7^th May of a leap year. 

Ans. Angle of declination on 7^th May in leap year is 128^th day of the year.

= 16.96°

d. Define solar constant and solar isolation. 

Ans. Solar Constant: At the mean distance from the sun, this is the amount of energy received from the sun in unit time on a unit area perpendicular to the sun’s rays. 

Solar Isolation: The solar isolation is the total quantity of solar energy received at a specific area over a specified time period, commonly expressed in kWh (m² day). 

e. What is meant by dry steam, wet steam and hot water in geothermal system ? 

Ans. A. Hot Water Fields: Hot water field, containing a water reservoir at temperature ranging 50-100 °C. 

B. Wet Steam Field: The wet steam fields contain pressurized water in reservoir at temperature higher than 100 °C. 

C. Dry Steam Field: These fields are similar to wet, steam fields but heat transfer from the depth is much higher. 

f. Write the chemical reaction which takes place in alkaline fuel cell.  

Ans. At Anode: H₂ + 2OH → 2H₂O + 2e^–

At Cathode:      O₂ + 2H₂O + 4e^– → 4OH^–

g. Write short note on HAWT and VAWT.

Ans. HAWT: In horizontal axis wind turbine (HAWT), the axis of rotation is perpendicular to the direction of the wind.  

VAWT: In vertical axis wind turbine (VAWT), the axis of rotation is parallel to the direction of the wind.  

h. State Seebeck effect and Peltier effect. 

Ans. Seebeck Effect: It asserts that an emf is created in a circuit if a closed circuit is formed of two dissimilar metals and the two junctions are kept at different temperatures. The size of the current is determined by the metals as well as the temperature difference between the junctions.

Peltier Effect: When an electric current passes through an isothermal junction of two dissimilar materials, heat is either evolved or absorbed at the junction. This is known as the Peltier effect.

i. What do you mean by recycling?  

Ans. Recycling entails a number of procedures, including the collection of recyclable materials, sorting them, and utilising them as raw material after palletizing. It also involves the processing, manufacture, and sale of finished goods. The collected materials are sorted and cleaned before being processed into two products.

j. Write the advantages and disadvantages for floating drum and fixed dome type biogas plant. 

Ans. Advantages of Floating Drum Biogas Plant : 

1. Floating drum plants are easy to understand and operate. 

Disadvantages of Floating Drum Biogas Plant : 

1. The steel drum is relatively expensive and the life time of the drum is short. 

Advantages of Fixed Dome Biogas Plant : 

1. Sturdy and possible to build from cheap, inexpensive materials.  

Disadvantages of Fixed Dome Biogas Plant : 

1. Skilled brick layers needed to construct a usable gas collection dome.

Section B : Long Questions of Renewable Energy Resources

a. Discuss the main features of various types of renewable and non-renewable energy sources. Also explain the importance of non-conventional energy sources in the context of global warming. 

Ans. A. Types of Renewable Energy Sources : 

• a.  Solar Energy :  
  • 1. Solar energy is a clean, affordable, and abundant renewable energy source. It is also the most essential of the non-traditional energy sources because it is non-polluting and thus helps to reduce the greenhouse effect.
• b. Biomass :
  • 1. Green plants use photosynthesis to capture solar energy and turn it into organic stuff. This organic substance is referred to as biomass.
  • 2. After burning, wood, charcoal, and agricultural waste provide bio-energy, and cow dung and garbage are anaerobically destroyed to obtain energy.
  • 3. Dried animal dung or cattle dung cakes are utilised directly as fuels in rural areas, but they emit smoke and have a low burning efficiency.
• c. Hydro Energy : 
  • 1. It is a renewable energy source, which is used to generate electricity. 
  • 2. Hydropower is obtained from water flow or falling water from a height. 
  • 3. Water contained behind a dam and at a high elevation contains a lot of potential energy, which is turned into mechanical and electrical energy.
  • 4. The water is gradually released and allowed to descend under the gravity force, which drives hydraulic turbines.
  • 5. The generators attached with turbine produce the electricity. 
• d. Wind Energy :
  • 1. Wind energy is a non-polluting renewable energy source with enormous potential that, if exploited, may easily meet a country’s energy needs.
  • 2. According to estimates, about 2% of total solar energy falling on Earth is transformed to kinetic energy in the atmosphere.
  • 3. 30% of total kinetic energy occurs in the lowest 1000 m of elevation, i.e., wind has the most kinetic energy in the lowest kilometre, which can be transformed into mechanical energy and used to create electricity or perform other useful tasks.
  • 4. Because wind has energy due to its speed, the equipment utilised to extract its energy should be capable of slowing down the wind.
• e. Tidal Energy :
  • 1. Gravitational pull by sun and moon result in the tides. 
  • 2. This type of energy can be harnessed by constructing the tidal barrage. 
  • 3. Energy can be harnessed from high as well as from low tides. 
  • 4. When high tides, sea water flows into the barrage’s reservoir and powers the turbine, which generates electricity by turning generators.
  • 5. When low tides, the reservoir’s stored water flows into the sea, reactivating the turbine.
  • 6. In this way the energy can be harnessed from high and low tides. 

B. Types of Non-renewable Energy Sources :

1. Conventional (Non Renewable) Energy Sources: These are the sources of energy which are exhaustible i.e., cannot be replaced if once they are used. 

Example: Coal, Petroleum products, Natural gas etc.  

Fossil Fuel as a Conventional Energy Source : Some of the fossil fuels are discussed below:

• a. Coal Energy :
  • 1. Coal is a conventional energy source. 
  • 2. It is formed due to degradation of trees and plants buried under layers of silt.
  • 3. It is composed of mainly carbon and hydrocarbons. 
  • 4. Coal is found in Jharkhand, U.P., M.P., Bihar etc. in India. 
• b. Natural Gas :  
  • 1. Natural gas formed by decomposition of dead animals and plants buried under the earth. 
  • 2. It is mainly composed of methane (CH₄) with small amount of propane and ethane.
  • 3. Natural gas is the cleanest fossil fuel. 

C. Importance of Non-conventional Energy Sources in the Context of Global Warming :

• 1. Global warming is the phenomenon of rise in temperature of environment due to the rise in concentration of the various gases like CH₄, CO₂, aerosols, NO_x, etc which is caused by the burning of conventional fossil fuels in industries. 
• 2. Non-conventional energy resources like solar energy, wind energy, ocean thermal energy, etc are eco-friendly resources and do not produce any kind of pollutants or harmful gases like CO₂, CH₄, SO_x, NO_x etc. 
• 3. Thus, we can say that non-conventional energy resources are playing an important role in context of global warming by not producing harmful gases which increases the temperature of environment. 

b. Describe the application and classification of hydrothermal resources.  

Ans. Classification of Hydrothermal Resources :

• a. Hot Water Fields : 
  • 1. Hot water below 100 °C emits from these areas as hot springs, with the geothermal aquifers being capped by confining layers to keep the hot water under pressure.
  • 2. Examples of hot water fields are Sahestra dhara near Dehradun, Sacred kund at Badrinath in Uttarakhand, Manikaran in Kullu valley (Himachal Pradesh) and Internationally known fields are Pannonian basin (Hungary), Po river valley (Italy) and Klamath Falls Oregon (USA).  
• b. Wet Steam Fields : 
  • 1. The pressured water in the geothermal reservoir (at 370 °C) is above 100 °C and contains modest amounts of steam and vapour.
  • 2. Fumaroles are areas where steam escapes via fissures in the surface.
  • 3. An impermeable cap-rock keeps the fluid from escaping into the atmosphere, and drilling is done to bring it to the surface.
  • 4. The fluid is used to produce steam and boiling water in predominant phase. 
  • 5. Examples of wet steam fields : Los Azufre (Mexico), Puna (Hawaii. USA), Deing (Indonesia), Azores (Portugal) and Latera (Italy).  

Applications of Hydrothermal Resources :

Depending on their temperature and depth, hydrothermal resources are exploited for a variety of energy uses. When the temperature of a hydrothermal resource reaches 50 degrees Fahrenheit or more, it can be used directly in spas or to heat buildings, cultivate crops, warm fish ponds, and for other purposes.

c. With the help of a schematic diagram, explain the operation of closed cycle MHD generating system. 

Ans. Closed Cycle MHID System :  

• 1. Fig. 3.8.2 shows the schematic diagram of closed cycle MHD 3 generator.  

• 2. In a closed cycle plant, very high thermal efficiency is attained with minimal cycle cost and produces more useable power at low temperatures of 1600 °C. Because of the high pressure, the duct of these devices is small.
• 3. Helium or argon is used as the working fluid, which is heated in a heat exchanger and ionised.
• 4. In closed cycle plants where recovery is possible, less ionised compounds such as alkali metal are combined with inert gas to produce the requisite conductivity.
• 5. There are two types of closed cycle plants: seeded inert gas systems and liquid metal systems.
• 6. In a closed cycle, the working fluid (argon or helium) is seeded with cesium and circulates in a closed loop.
• 7. The heat exchanger receives the gas burned in the combustor and transfers it to the working fluid.
• 8. To generate DC power, the ionized working fluid travels through the magnetic field.
• 9. Following heat removal in the heat exchanger, the combustion products are released to the environment.

d. What is the basic difference between thermoelectric and thermionic conversion system ? Also, explain the working of thermoelectric generators.  

Ans. A. Difference between Thermoelectric and Thermionic : 

B. Working of Thermoelectric Generators :

• 1. Consider a metal bar whose one side is kept at a higher temperature than the other.  
• 2. The kinetic theory of gases states that, if the free electrons in the metal were to act as a gas, the free electrons on the hot side of the bar would have more kinetic energy and would be moving more quickly than those on the cold side of the bar.
• 3. An accumulation of negative charge forms on the cold side of the bar as the faster-moving electrons go from the hot side to the cold side of the bar, preventing further charge buildup until the circuit is closed.
• 4. As long as the temperature gradient is kept constant, the current will flow in a closed circuit to reduce charge buildup.

e. Explain availability, conversion theory of biogas plant and energy conversion from biomass. 

Ans. A. Availability of Biogas Plant :

• 1. Biomass is derived from the plant, forest residues, animal dung etc. 
• 2. Fast-growing trees, sugar, starch, and oil-containing plants can all be farmed for use as biomass for energy.
• 3. The biomass developed under cultivation also comprises crops of sweet sorghum, sugar beets, cereals, herbaceous plants, aquatic plants grown in fresh water, sea water, muddy water, etc.
• 4. Renewable energy can also be obtained from algae. Organic material found in algae can be broken down to produce methane gas. Large-scale algae cultivation is possible.
• 5. Dry biomass products include wood and straw. This biomass can be burned to provide energy.
• 6. Wood in the form of cut logs, chips, sawdust is commonly used for domestic applications. 
• 7. Straw can be burn in straw burning stoves and furnaces. 
• 8. Organic wastes can also be converted into intermediate energy sources, such as heat or biogas fuels, to produce energy.
• 9. The wastes can be divided into urban, industrial, animal, fishing, poultry, forest, and human excreta wastes as well as agricultural wastes.
• 10. These wastes can be turned into heat, biogas, and biochemical by a variety of methods, including combustion (as with wood, straw, etc.), biochemical reactions, and biothermal processes.
• 11. Methane is present in biogas. Methane is created through anaerobic digestion in biogas facilities.
• 12. The human waste can also be used for production of biogas. 

B. Conversion Theory of Biogas Plant and Energy Conversion from Biomass :

• i. Biomass Conversion : 
  • 1. The following processes are used for the biomass conversion to energy or to biofuels :  
    • a. Direct combustion, 
    • b. Thermochemical conversion, and 
    • c. Biochemical conversion. 
• a. Direct Combustion :  
  • 1. The process of burning in the presence of oxygen to produce heat, light, and byproducts is known as combustion.
  • 2. Incineration is the process of completely burning to ashes.
  • 3. Pyrolysis can be used to create low calorific value gas from dried wood, manure and vegetable waste.
  • 4. During the pyrolysis process, organic material is heated to temperatures between 500 and 900 °C without the presence of oxygen, converting it to gases, solids, and liquids.
  • 5. Because biomass has a considerably higher moisture content than other fuels, combustion is more challenging.
  • 6. Biomass is free from toxic metals and their ash. 
  • 7. For the effective burning of forestry and agricultural waste materials including sawdust, wood chips, hog fuel, rice husks, straws, nutshells, and chips, “fluidized bed combustion” technology may be employed.
  • 8. In a fluidized bed combustion of biomass, the biomass is introduced into a bed of hot, inert particles, such as sand, that is kept in a fluidized state by air blown sufficiently quickly from below.
  • 9. The working temperature is often maintained between 750 and 950 °C; ideally, it should be kept as high as possible to maximise the rate of combustion and heat transmission while remaining low enough to prevent the issue of the bed particles sintering.
  • 10. The fluidized bed’s quick mixing and turbulence make it possible to accomplish efficient combustion with higher heat releases and more effective transfer than with a conventional boiler. This may lead to a boiler that is more compact and has fewer tubes.  
• b. Thermochemical Conversion :  
  • 1. Biomass is broken down through thermochemical processes under a range of pressures and temperatures.
  • 2. Gasification and liquefaction are the two types of thermochemical conversion.
  • 3. Biomass is heated with little oxygen to create low-heating-value gas, or it is reacted with steam and oxygen under high pressure and heat to create medium-heating-value gas.
  • 4. The latter can either be used directly as fuel or employed in the process of liquefaction by being turned into methanol (methyl alcohol), ethanol (ethyl alcohol), or high heating value petrol.
• c. Biochemical Conversion :  
  • 1. In biochemical conversion there are two principal conversion processes :
    • i. Anaerobic digestion, and 
    • ii. Fermentation 

Section 3 : Photovoltaic Solar Energy Conversion

a. Write a short note on PV arrays and system charge controllers. What are the advantages and disadvantages of photovoltaic solar energy conversion ? 

Ans. A. PV Arrays :

• 1. Solar cells are strung in series and thus form a solar module or array. 
• 2. They may be tracking arrays or fixed arrays. 
• 3. A tracking array is defined as one which is always kept mechanically perpendicular to the sun-array line so that all times it intercepts the maximum isolation. 
• 4. Such arrays must be physically movable by a suitable prime-mover and are considerably more complex than fixed arrays. 
• 5. A fixed array is usually oriented east-west and tilted up at an angle approximately equal to the latitude of the site. 
• 6. Fixed arrays are mechanically simpler than tracking arrays. Thus the array designs fall into two broad classes :
  • a. Flat-Plate Arrays 
  • b. Concentrating Arrays

B. System Charge Controllers : 

• 1. The power entering the battery bank from the solar array is controlled by a system charge controller.
• 2. It prevents the deep cycle batteries from being overcharged during the day and from having electricity go backwards overnight to the solar panels, which would otherwise drain the batteries.
• 3. Although some charge controllers are available with extra features like lighting and load management, their main function is to manage power.
• 4. There are two distinct technologies for a system charge controller: maximum power point tracking (MPPT) and pulse width modulation (PWM) (MPPT).

C. Advantages of Photovoltaic Solar Energy Conversion :  

• 1. Environment friendly. 
• 2. Operating and maintenance costs are low. 
• 3. PV panels are totally silent, producing no noise at all. 
• 4. PV panels can provide an effective solution to energy demand peaks, especially in hot summer months where energy demand is high. 
• 5. Residential solar panels are easy to install on rooftops or on the ground. 

D. Disadvantages of Photovoltaic Solar Energy Conversion :

• 1. Like all renewable energy sources, solar energy is intermittent; it may not shine at night and there may be cloudy or wet conditions during the day.
• 2. In order to be used on the power grid, solar energy panels need additional hardware (inverters) to change direct electricity (DC) to alternating electricity (AC).
• 3. When compared to other renewable energy systems, the efficiency levels of solar panels are rather low (between 14% and 25%).

b. Describe various direct and indirect application of solar energy. 

Ans. A. Direct Applications of Solar Energy : 

• a. Solar Pumping : 
  • 1. In solar pumping, the power generated by solar energy is utilized for pumping water for irrigation purpose. 
• b. Solar Furnace : 
  • 1. In a solar furnace, high temperatures are produced by focusing sun energy onto a specimen using a number of heliostats (rotatable mirrors) set on a sloping surface.
  • 2. Ceramic characteristics at very high temperatures are studied using the solar furnace.
• c. Solar cooking : 
  • 1. Solar cooking basically includes a solar cooker which works on solar radiations. 
  • 2. It consists of a well insulated metal or wooden box which is blackened from the inner side. 
• d. Solar Electric Power Generation

B. Indirect Applications of Solar Energy : 

• a. Wind Energy : 
  • 1. Due to the earth’s surface heating up during the day and cooling at night, a little percentage of the total solar radiation that reaches the planet’s surface creates wind.
  • 2. Wind energy, an indirect source of solar energy, can be used to produce electricity.
• b. Wave Energy :
  • 1. The movement of the sea’s surface caused by wind waves serves as an energy source.
  • 2. In shallow waters close to the coast, floating propellers are positioned. As the waves move, the propellers move as well, and this kinetic energy can be used to power turbines.
  • 3. This is cheap, clean and inexhaustible source of energy. 
• c. Ocean Thermal Energy :
  • 1. OTEC i.e., ocean thermal energy conversion plants convert the heat of the ocean into electrical energy, with the help of temperature difference. 
  • 2. The large temperature difference between warm surface sea water (28 – 30 °C) and cold deep sea water (5 – 12 °C) is used to generate electricity, with the help of ocean thermal energy conversion system. 
• d. Tidal Energy :
  • 1. Gravitational pull by sun and moon result in the tides.
  • 2. This type of energy can be harnessed by constructing the tidal barrage. 
  • 3. Energy can be harnessed from high as well as from low tides. 
  • 4. When high tides, sea water pours into the barrage’s reservoir and drives a turbine, which turns generators to generate power.
  • 5. When the tide is low, water from the reservoir pours into the water, turning the turbine once again.
  • 6. In this way the energy

Section 4 : Solar Thermal Collector

a. Classify different types of solar thermal collector and show the constructional details of a flat plate collector. What are its main advantages ? 

Ans. A. Principle : 

• 1. Visible sunlight is absorbed on the ground and converted into heat energy when solar radiation from the sun strikes the earth in the form of light (short wave radiation). The material warms up and stores the heat, conducts it to nearby materials (air, water, other solids or liquids), or reradiates it to materials with lower temperatures. 

B. Solar Collectors : 

• 1. Solar collectors are used to gather solar energy, which they then absorb and turn into thermal energy.
• 2. A collector fluid, such as water, oil, or air, is heated using this thermal energy.
• 3. The surface of the solar collector is built for optimum absorption and minimal emission.
• 4. Solar collectors are classified in two types : 

• a. Non-Concentrating Collector : 
  • 1. It is also known as flat plate solar collector.
  • 2. In these collectors, the area of collector to grasp the solar radiation is equal to the absorber plate and has concentration ratio of 1.  
• b. Concentrating Collector :  
  • 1. It is also known as focusing type solar collector.
  • 2. In these collectors, the area of the collector is kept less than the aperture through which the radiation passes, to concentrate the solar flux and has high concentration ratio. 
• c. Flat Plate Collector : 
  • 1. Flat plate collector is simplest in design and it is most important part of any solar thermal energy system. 
  • 2. In this collector both direct and diffuse radiations are absorbed and converted into useful heat. 
• d. Components of Flat Plate Collector : 
  • i. Absorber plate, 
  • ii. Transparent covers, 
  • iii. Insulation, and 
  • iv. Box.

• i. Absorber Plate : 
  • 1. Absorber plate is used to grasp and absorb solar radiation. 
  • 2. The plate is usually metallic (copper, aluminum or steel), sometimes plastics have been used in some low temperature applications. 
• ii. Transparent Covers :
  • 1. These are one or more sheets made of glass for trapping the heat received by the absorber plate. 
  • 2.  It helps in reducing the convective and radiative heat losses. 
• iii. Insulation: It minimizes the heat losses by conduction. 
• iv. Box: It contains the above components and keep them into desired position.

• E. Advantages of Flat Plate Collector :
  • 1. It absorbs both direct and diffuse radiations. 
  • 2. There is no need of tracking. 
  • 3. It has low cost and requires less maintenance.

b. Draw a schematic diagram for solar pond based electric power plant with cooling tower and explain its working. 

Ans.

• 1. The diagram for solar pond based electric power plant with cooling tower is shown in Fig. 1.

• 2. A solar pond is a solar energy collector which is fairly large in size. 
• 3. The variation in the water’s salt concentration is the primary feature of solar ponds that makes them good solar energy collectors.
• 4. As a result of this gradient, highly salinated water collects at the pond’s bottom, and as concentration decreases towards the surface, cold, fresh water accumulates on top of the pond.
• 5. The majority of the solar radiation that hits the pond is absorbed by its surface at the bottom. As a result, the dense salt layer’s temperature rises.
• 6. The lower layer’s temperature may increase to as much as 95 °C because the denser salt water at the bottom prevents heat from being transmitted to the fresh water layer above by natural convection.
• 7. The turbine can be operated using a straightforward Rankine cycle by using this high temperature.
• 8. The generator is linked to the turbine, which eventually produces electricity for us.

Section 5 : Geothermal Power Plants

a. Explain the working of geothermal power plants. Discuss the various technical developments.  

Ans. A. Working of Geothermal Power Plant : 

• 1. A production well is drilled into a known geothermal reservoir. 
• 2. An injection well is also drilled to return used geothermal fluids to the geothermal reservoir. 
• 3. To create energy, hot geothermal fluids (water) travel through pipelines to a power plant.
• 4. We let the hot, pressured water expand quickly.
• 5. The hot water is split into steam, which is forced to strike the turbine blade and cause it to begin revolving.
• 6. A generator that is connected to the turbine begins to spin and generate electricity.
• 7. The main machinery for converting geothermal energy to electrical energy is the turbine and generator.

B. Various Technical Developments :

a. Vapour-Dominated Power Plant : 

• 1. In a 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.

• 2. Steam is then used to power the turbine and generator at temperatures and pressures that are lower than those seen in typical steam cycle plants, namely 245 °C and 7 bar, respectively.
• 3. As a result, geothermal plants only operate at 20% efficiency.
• 4. The cooling tower is filled with the water that is created when exhaust steam from the turbine travels through a condenser.
• 5. It decreases environmental pollution brought on by the direct release of steam into the atmosphere and increases the turbine’s efficiency.
• 6. To assure a constant supply, waste water from the cooling tower sump is reinjected into the geothermal well.

b. Liquid-Dominated Power Plants : 

• 1. 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. 
• 2. There are two types of liquid-dominated power plants : 
  • i. Flashed steam system, and  
  • ii. Binary cycle system. 

b. Explain the working of molten carbonate fuel cells using appropriate diagram and write various chemical reactions involved in this type of fuel cell. 

Ans. A. Molten Carbonate Fuel Cell (MCFC) : 

• 1. It uses an electrolyte, which is a molten mixture of carbonate salts. 
• 2. Two mixtures commonly used are : 
  • a. Lithium carbonate and potassium carbonate, and 
  • b. Lithium carbonate and sodium carbonate. 
• 3. Since, these salts can act as electrolytes only in liquid phase; the operating temperature should be as high as 650 °C.

• 4. Due to high temperature, these salts melt and become conductive to carbonate ions (CO₃^–). 
• 5. These ions flow from the cathode to the anode where they combine with hydrogen to give water, carbon dioxide and electrons. 
• 6. The electrons flow through external circuit and reaches to cathode, generating electricity and byproduct heat.  
• 7. The reactions are given below : 
  • Anode reaction : CO₃^– + H₂ → H₂O + CO₂ + 2e^–
  • Cathode reaction : CO ₂ + 1/2 O₂ + 2e^– → CO₃^–
  • Total reaction : H₂ + 1/2 O₂ + CO₂ → H₂O + CO₂
• 8. The emf produced by each cell is theoretically 1 V and actual emf of 0.8 V at 700 °C and the expected efficiency is about 60 %.

Section 6 : Wind Energy Conversion

a. What is the principle of wind energy conversion ? What methods are used to overcome the fluctuating power generation of windmills ?  

Ans. A. Basic Principle of Wind Energy Conversion :

The basic principle of wind energy is to convert the kinetic energy of wind into rotational motion to operate an electric generator. 

B. Methods used to overcome the Fluctuation of Power Generation of a Wind Mill.  

• 1. As the wind speed varies, the speed of the generator varies and produces fluctuations in the electricity.  
• 2. The problems can be solved by following methods. 
  • a. To use turbines with constant speed that can adjust pitch by moving slightly to one side in response to wind speeds.
  • b. Using turbines with variable speeds, where the generator and blades vary speed in response to the wind and a power control system smoothes out output irregularities.
  • c. To use low-speed generators. 

The various mechanical controls provided with wind machine are as follow : 

• a. Tethering Control :  
  • 1. To prevent failure due to vibration (fatigue) during nacelle orientation, it has horizontal axis turbines with single and twin blades.
  • 2. The turbine’s axis is positioned so that the propeller blades rotate faster and on a slanting plane. At low speeds, the slants become smaller, while at greater speeds, they become larger.
  • 3. This type of control is not provided with three blade rotor. 
• b. Yaw Control : 
  • 1. The nacelle is automatically positioned in the direction of the wind with the aid of a hydraulic system, and the rotor is continuously oriented in the direction of the wind.
  • 2. The axis is positioned so that the swept area of the rotor is perpendicular to the direction of the wind, which may be upward or downward.
• c. Pitch Control :  
  • 1. The blade tips are automatically changed to provide a feathering effect. This lowers the turbine’s speed and power to match the generator’s speed. 
  • 2. The pitch angle has wide control between 0° – 30°. 

b. Using Betz model of a wind turbine, derive the expression for power extracted from wind. Under what condition does the maximum theoretical power can be extracted from the wind turbine ?

Ans. Expression for maximum efficiency : 

• 1. Allowing the wind to pass through moving wings that provide torque to a rotor enables the extraction of power from the wind.
• 2. The air density, the area swept out by the rotor, and the cube of the wind speed all directly relate to the amount of power delivered.
• 3. Fig. 4.12.1 shows the air flow diagram of rotor, with variation of wind speed at different sections.

4. The static pressure of the air decreases as it travels through the rotor disc, dropping to a level below atmospheric pressure when it exits the blade.

5. After the section (wake), the atmospheric pressure and air speed again rise. The wind speed also decreases in this part.

6. Let a_i and a_e = Inlet and outlet area of air enclosure, 

a_o = Rotor swept volume, 

v_i and v_e = Velocity of wind at inlet and outlet of enclosure, 

v_o = Velocity of rotor, 

ρ = 1.25, air density, and 

m = Mass flow rate of air over rotor. 

7. The thrust on the turbine by moving air as it passes over the rotor, 

F = m (v_i – v_e)                            …(4.12.1) 

8.  The power extracted by turbine, 

P_T = m(v_i – v_e) x v_o                   …(4.12.2)  

9. Instantaneous loss in kinetic energy of wind as it passes through rotor,

10. From equation (4.12.2) and equation (4.12.3), 

11. From equation (4.12.2) and equation (4.12.4), 

12. The mass flow rate through turbine rotor, 

13. From equation (4.12.5) and equation (4.12.6), 

14. For maximum power, 

17. The axial force on turbine, 

18. For a given turbine power, lower the angular velocity of rotor, higher the torque and conversely higher the angular velocity lower the torque. 

Section 7 : Gasification of Solid Biomass

a. Explain the process of gasification of solid biomass. What is the general composition of the gas produced and what is its heating value ? What are its applications ?  

Ans. A. Gasification :

The process of gasification involves the following four processes : 

a. Drying: Biomass fuels usually contain 10 %-35 % moisture. When biomass is heated to about 100°C, the moisture is converted into steam.  

b. Pyrolysis: The biomass passes through pyrolysis as the heating process continues after drying. It involves full oxygen-free combustion of biomass. Therefore, the biomass is broken down or divided into solids, liquids, and gases. The three components are solid (charcoal), liquid (tar), and gaseous (flue gases). 

c. Oxidation: The gasifier is then filled with air following the breakdown process. Charcoal or other solid carbonized fuel undergoes oxidation, which occurs between 700 and 1400 °C, when it combines with oxygen in the air to produce carbon dioxide and heat.

C + O₂ → CO₂ + heat

d. Reduction: At higher temperatures and under reducing conditions that is when not enough oxygen is available, the following reactions takes place forming carbon dioxide, hydrogen and methane.  

C + CO₂ → 2CO

C + H₂O → CO +H₂

CO + H₂O → CO₂ + H₂

C + 2H₂ → CH₄ 

B. Composition : 

Carbon Monoxide       – 18 – 22 %

Hydrogen                    – 13 – 19 % 

Methane                      – 1 – 5% 

Heavier Hydrocarbons – 0.2 – 0.4 %

Carbon dioxide           – 9 – 12%

Nitrogen                     – 45 – 55 %

Water vapour            – 4 %

C. Heating value : 

• 1. The gas produced in the gasifier is a clean burning fuel having heating value of about 950  1200 kcalm³.  

D. Application :

• 1. SI engines can be made to run entirely on producer gas. 
• 2. CI engines can be made to operate with about 60 – 80 % fuel oil replacement by producer gas. 
• 3. In an industrial oil fired boiler. 
• 4. In gas turbine.  

b. Explain the principle, working and efficiency of OTEC power plant. What are the environmental effects of OTEC ? 

Ans. A. Principle and Working of OTEC :

• 1. The water at the sea’s bottom and the water at its surface differ in temperature, which is the fundamental tenet of ocean thermal energy conversion (OTEC).
• 2. The majority of the radiation is being absorbed at the water’s surface layer, and this temperature difference can be exploited to power a heat engine.
• 3. Because there is no thermal convection between the hot and cold water layers, mixing of the two types of water is avoided. Thus, the top layer will serve as a source while the lower, colder layers will serve as a sink.
• 4. As a result, in order to generate work that may be turned into necessary applications, the reversible heat engine must be connected between the source and the cold sink.
• 5. The absorption of solar radiation in the water varies and can be expressed by Lambert’s law :

or,       I_y = I e^-μy
Where,  I_y = Radiation intensity at depth y from water surface and falls exponentially with depth, 

I = Radiation intensity at water surface, and  

μ = Extinction or absorption coefficient. 

B. Efficiency of OTEC :

1. If we want Carnot cycle efficiency, we have 

So this is the best efficiency that we can get.

2. Let’s say the cold water that is below has a temperature 5 °C and the water at the surface has temperature 25 °C. So,

η = 6.7%

3. So, if we do the calculation we arrive at about 6.7 %. So, that’s the best that we can get. But the practical kind of efficiency that we get out of an OTEC plant is 3%. This is remarkably low. 

C. Environmental Effect : 

Following are the environmental effects of OTEC: 

• 1. The marine environment gets affected by these plants through water heating. 
• 2. Release of toxic chemical and entrapment of small sea organism in intake pipes is common. 
• 3. The discharge of low and high water at the intermediate layer disturbs the thermal layer of sea water close to the plant.
• 4. Changes in salinity, dissolved gases, nutrients, carbonate, etc. have an impact on the marine ecosystem.
• 5. A large discharge of mixed water below the ocean’s surface over an extended period of time will alter the environment for egg hatching and slow the growth of fish, corals, and other organisms.
• 6. The plant may release toxic chemicals into the environment, killing nearby marine species.
• 7. The marine life gets affected because of change in pH value and dissolved oxygen.