(SEM VII) THEORY EXAMINATION 2021-22 RENEWABLE ENERGY RESOURCES

RENEWABLE ENERGY RESOURCES – KOE 074

B.Tech (Semester VII) – Detailed Answers

SECTION A – Descriptive Answers (2 Marks Each)

a) Photovoltaic Effect

The photovoltaic effect is the phenomenon by which electrical energy is generated when sunlight falls on a semiconductor material. Photons with sufficient energy create electron–hole pairs; the built-in electric field of the p–n junction separates these charges, producing a DC voltage and current.

b) Properties of Polycrystalline Silicon Cell

Polycrystalline silicon cells are made of multiple silicon crystals. They have moderate efficiency (lower than monocrystalline), lower manufacturing cost, good thermal stability, longer life, and a characteristic bluish appearance due to light scattering at grain boundaries.

c) Angle of Declination for 7th May (Leap Year)

Declination (δ) is the angle between the line joining the centers of the Earth and Sun and the Earth’s equatorial plane.

δ=23.45∘sin⁡(360366(n−81))\delta = 23.45^\circ \sin\left(\frac{360}{366}(n-81)\right)δ=23.45∘sin(366360​(n−81))

For 7th May, n=128n = 128n=128:

δ≈23.45∘sin⁡(360366×47)≈+16.9∘\delta \approx 23.45^\circ \sin\left(\frac{360}{366}\times 47\right) \approx +16.9^\circδ≈23.45∘sin(366360​×47)≈+16.9∘ 

d) Solar Constant and Solar Insolation

Solar constant is the average solar radiation received outside Earth’s atmosphere on a surface normal to the Sun’s rays, approximately 1367 W/m².
Solar insolation is the total solar energy received per unit area on Earth’s surface over a given time (kWh/m²/day).

e) Dry Steam, Wet Steam and Hot Water (Geothermal)

Dry steam: Steam directly drives the turbine.

Wet steam: Mixture of steam and water; separation is required.

Hot water: High-temperature water flashed into steam or used in binary cycles.

f) Chemical Reactions in Alkaline Fuel Cell (AFC)

Anode:

2H2+4OH−→4H2O+4e−2H_2 + 4OH^- \rightarrow 4H_2O + 4e^-2H2​+4OH−→4H2​O+4e−

Cathode:

O2+2H2O+4e−→4OH−O_2 + 2H_2O + 4e^- \rightarrow 4OH^-O2​+2H2​O+4e−→4OH−

Overall:

2H2+O2→2H2O+Electric Energy2H_2 + O_2 \rightarrow 2H_2O + \text{Electric Energy}2H2​+O2​→2H2​O+Electric Energy 

g) HAWT and VAWT

HAWT (Horizontal Axis Wind Turbine): High efficiency, large power output, requires yaw mechanism.
VAWT (Vertical Axis Wind Turbine): Omni-directional, easier maintenance, lower efficiency.

h) Seebeck and Peltier Effects

Seebeck effect: Temperature difference between two dissimilar conductors generates an emf.
Peltier effect: Heat is absorbed or released at a junction when electric current flows.

i) Recycling

Recycling is the process of converting waste materials into reusable products, conserving resources, reducing energy consumption, and minimizing environmental pollution.

j) Floating Drum vs Fixed Dome Biogas Plant

Floating Drum
Advantages: Constant gas pressure, easy operation
Disadvantages: High cost, corrosion

Fixed Dome
Advantages: Low cost, long life
Disadvantages: Variable pressure, construction complexity

SECTION B – Long Answers (10 Marks Each)

2(a) Renewable & Non-Renewable Energy Sources and Global Warming

Renewable energy sources such as solar, wind, hydro, biomass, and geothermal are naturally replenished and environmentally friendly. Non-renewable sources like coal, oil, and gas are finite and emit greenhouse gases. Non-conventional energy sources reduce CO₂ emissions, mitigate global warming, enhance energy security, and support sustainable development.

2(b) Hydrothermal Resources: Classification and Applications

Hydrothermal resources are classified based on temperature into low, medium, and high-enthalpy resources. Applications include electricity generation, space heating, greenhouse heating, industrial drying, and bathing.

2(c) Closed Cycle MHD Generating System

In a closed-cycle MHD system, an inert gas or seeded gas circulates in a closed loop. The hot conducting gas passes through a magnetic field, inducing emf in electrodes. The gas is cooled, recompressed, reheated, and reused, improving efficiency. (Neat schematic to be drawn in exam.)

2(d) Thermoelectric vs Thermionic Conversion & Thermoelectric Generator

Thermoelectric conversion uses solid-state junctions (Seebeck effect).
Thermionic conversion uses electron emission across a vacuum gap at high temperature.
A thermoelectric generator converts heat directly into electricity using p-type and n-type materials connected electrically in series and thermally in parallel.

2(e) Biogas Availability and Biomass Energy Conversion

Biogas is produced by anaerobic digestion of organic waste. Biomass energy conversion includes combustion, gasification, pyrolysis, and fermentation. Biogas provides clean fuel for cooking, electricity, and heating.

SECTION C – Very Long Answers (10 Marks Each)

3(a) PV Arrays, Charge Controllers & Pros/Cons of PV Conversion

PV arrays consist of interconnected modules to meet voltage and power requirements.
Charge controllers regulate battery charging and prevent overcharging or deep discharge.

Advantages:                                                            Clean and renewable

Low maintenance                                                      Modular and scalable

Disadvantages:                                                        High initial cost

Intermittent supply                                                    Low conversion efficiency

3(b) Direct and Indirect Applications of Solar Energy

Direct applications: Solar cooking, water heating, drying, desalination.
Indirect applications: Wind energy, hydropower, biomass energy (all driven by solar radiation).

4(a) Solar Thermal Collectors & Flat Plate Collector

Solar collectors are classified into non-concentrating (flat plate, evacuated tube) and concentrating (parabolic trough, dish).
A flat plate collector consists of absorber plate, transparent cover, insulation, and fluid tubes.
Advantages: Simple design, low cost, reliable operation.

4(b) Solar Pond Based Power Plant

A solar pond stores thermal energy using salinity gradients. Hot brine at the bottom drives a heat engine to generate electricity, with cooling via a cooling tower.

5(a) Geothermal Power Plants & Developments

Types include dry steam, flash steam, and binary cycle plants. Recent developments focus on enhanced geothermal systems (EGS), improved drilling, and binary cycle efficiency.

5(b) Molten Carbonate Fuel Cell (MCFC)

MCFC operates at high temperature (~650°C). Carbonate ions transfer from cathode to anode.
 

Reactions:
Cathode: O2+CO2+2e−→CO32−O_2 + CO_2 + 2e^- \rightarrow CO_3^{2-}O2​+CO2​+2e−→CO32−​
Anode: H2+CO32−→H2O+CO2+2e−H_2 + CO_3^{2-} \rightarrow H_2O + CO_2 + 2e^-H2​+CO32−​→H2​O+CO2​+2e−

6(a) Wind Energy Conversion & Power Fluctuation Control

Wind turbines convert kinetic energy of wind into mechanical and then electrical energy. Fluctuations are managed using pitch control, yaw control, energy storage, and grid integration.

6(b) Betz Model and Maximum Power Extraction

Power extracted:

P=12ρAV3CpP = \frac{1}{2}\rho A V^3 C_pP=21​ρAV3Cp​

Maximum theoretical efficiency (Betz limit) occurs when Cp=0.593C_p = 0.593Cp​=0.593, i.e., 59.3%.

7(a) Biomass Gasification

Gasification converts solid biomass into producer gas (CO, H₂, CH₄, CO₂). Heating value is about 4–6 MJ/m³. Applications include power generation, heating, and fuel synthesis.

7(b) OTEC Power Plant

OTEC uses temperature difference between warm surface water and cold deep seawater.
Advantages: Continuous power, renewable.
Environmental effects: Thermal pollution, nutrient upwelling impacts marine life.