test 322

Q1. How does the total amount of solar power incident on Earth compare with the power consumption rate of our society?

A: The Sun delivers an enormous amount of power to Earth—around 173,000 TW of solar energy continuously—whereas human society’s total power consumption is only about 18 TW. That means the solar energy incident on our planet is roughly 10,000 times greater than the power we currently use. This enormous surplus illustrates that, in principle, solar energy could more than meet our needs if we could capture and convert it efficiently.

Q3. What is a Solar Constant? What is its value? Does it change with latitude or longitude?

A: The solar constant is defined as the intensity of the Sun’s radiation at the distance of Earth, measured at the top of the atmosphere on a surface perpendicular to the Sun’s rays. Its value is approximately 1360 W/m². Because it refers to the flux just outside the atmosphere, the solar constant is essentially the same everywhere on Earth—it does not vary with latitude or longitude (though Earth’s slightly elliptical orbit causes small seasonal variations in the actual value).

Q4. What is Solar Insolation? Does it change with latitude or longitude?

A: Solar insolation is the power per unit area of sunlight actually reaching Earth’s surface (after passing through the atmosphere). Insolation varies significantly with latitude because of the curvature of Earth: near the equator sunlight arrives more directly (higher insolation), whereas near the poles it arrives at a slant (lower insolation). It also changes with time of day and season as the Sun’s angle in the sky shifts. Longitude by itself does not affect the long-term annual average insolation, though it does determine local time of day.

Q5. How does the global average solar insolation compare with the solar constant?

A: When you average the solar constant over Earth’s entire spherical surface (including night), you divide by four—so the mean solar insolation is about 1360 W/m² ÷ 4 = 340 W/m² at the top of the atmosphere. After accounting for atmospheric absorption and reflection (clouds, aerosols), the long-term average reaching the ground globally is closer to 239 W/m². Thus, the global average insolation at Earth’s surface is roughly one-quarter of the solar constant.

Q6. What factors determine Solar Insolation?

A: Several factors set the insolation at any location: the solar constant itself (the Sun’s output and Earth’s orbital distance), the geometry of Earth (latitude, axial tilt, and whether it is day or night), and atmospheric conditions (cloud cover, dust, water vapor). Together, these determine how much sunlight is available to be absorbed at the surface.

Q7. What factors affect Solar Insolation at a given geographical location?

A: At a specific site, insolation depends on its latitude (which controls the maximum solar elevation), the season (due to Earth’s tilt altering day length and sun angle), the time of day (sunlight peaks at local noon), weather (cloudiness, haze), altitude (higher elevations receive slightly more insolation), and local terrain or shading (mountains, buildings can block sun).

Q9. What is the electromagnetic composition of solar energy reaching the upper atmosphere?

A: Solar radiation at the top of the atmosphere follows a modified blackbody spectrum centered around 5800 K. Roughly 50% of the energy is in the infrared band, about 40% in the visible range, and around 10% in the ultraviolet.

Q11. What are the four essential elements in Passive Solar Heating Systems?

A: A passive solar design relies on:

1. Collection: Large, equator-facing glazing (south-facing in the Northern Hemisphere) to admit sunlight.

2. Insulation: High-R-value materials to retain heat.

3. Distribution: Architectural features (thermal mass flooring, interior layouts) that move heat by natural convection or conduction.

4. Storage: Thermal mass (concrete, stone, water) that absorbs excess heat by day and releases it when temperatures drop.

Q13. What are the essential elements of a Solar Thermal Power System?

A: A solar thermal power plant comprises three main components:

• A concentrator (mirrors or lenses) that focuses sunlight to generate high temperatures,

• A heat engine (e.g., a steam turbine or Stirling engine) that converts thermal energy into mechanical work, and

• A generator that turns that mechanical work into electricity.

Q14. What is concentration ratio? What kind of reflector has the best concentration ratio?

A: The concentration ratio is the ratio of the focused sunlight intensity at the receiver to the incident direct sunlight intensity. The highest concentration ratios are achieved by two-axis parabolic dish reflectors (point-focus systems), which can concentrate sunlight hundreds or even thousands of times over.

Q16. What are Power Towers? What are Heliostats?

A: A power tower system uses a field of heliostats—flat, sun-tracking mirrors—that continuously reflect sunlight onto a central receiver at the top of a tall tower. The receiver absorbs the concentrated solar energy (usually heating a fluid or generating steam) which then drives a turbine-generator.

Q17. What is a semiconductor? Give an example.

A: A semiconductor is a material whose electrical conductivity lies between that of a conductor and an insulator. Its conductivity can be precisely controlled by doping and external fields. A common example is silicon (Si).

Q18. What are the dopants that give rise to N-type semiconductors in silicon?

A: N-type silicon is created by adding pentavalent impurities such as phosphorus (P) or arsenic (As), which donate extra free electrons to the conduction band.

Q19. What are the dopants that give rise to P-type semiconductors in silicon?

A: P-type silicon results from doping with trivalent elements such as boron (B) or gallium (Ga), which create “holes” (positive charge carriers) in the valence band.

Q20. What is a P-type semiconductor? What is an N-type semiconductor?

A: In a P-type semiconductor, the majority carriers are holes (positive charges), whereas in an N-type semiconductor, the majority carriers are electrons (negative charges), due to the respective dopants.

Q24. What is the energy payback time of solar panels?

A: The energy payback time is the length of time a panel must operate to generate the energy used in its manufacture. For modern crystalline silicon panels, this is on the order of 2 years.

Chapter 10: Indirect Solar Energy (Water & Wind)
Q1. What fraction of US indirect solar energy production is in the form of hydropower and wind power?

A: Of the roughly 10% of U.S. energy derived from indirect solar sources, hydropower contributes about 7% of total U.S. energy (≈ 75% of the indirect solar share) and wind power about 2–3% (≈ 25%).

Q2. Which countries generate most of their electricity through hydroelectric power?

A: The two most notable examples are the Democratic Republic of Congo (almost 100% hydro) and Norway (about 97% hydro), which meet nearly all their electricity needs from hydropower installations.

Q5. Is hydropower energy of high quality or low quality? Is it limited by the second law?

A: Hydropower is considered high-quality mechanical energy because it can be converted to electricity with very high efficiency (turbines often exceed 90%). Unlike thermal power (heat engines), hydropower conversion is not constrained by Carnot efficiency limits—its losses are primarily mechanical (friction and turbulence).

Q8. What is the main advantage of hydropower plants and why do grids depend on them?

A: The principal advantage is rapid dispatchability: hydropower output can be ramped up or down almost instantaneously to match demand. This makes hydro plants invaluable for peak load balancing and maintaining grid stability.

Q9. How does wind power change with wind speed?

A: The power available in the wind varies with the cube of the wind speed (P ∝ v³). Thus, a small increase in wind speed yields a large increase in power.

Q11. What are the two kinds of wind turbines? Which is more efficient? Pros/cons?
A:

• Horizontal-Axis Wind Turbines (HAWT): About 45% efficiency, require yaw control and tall towers, widely used for utility-scale generation.

• Vertical-Axis Wind Turbines (VAWT): About 30% efficiency, accept wind from any direction without yaw, simpler maintenance (ground-level gearbox), but lower overall performance.

Q12. What is Betz Power Coefficient? What is its value?

A: The Betz limit is the theoretical maximum fraction of wind energy that a turbine can extract, equal to 59%. No turbine can exceed this limit due to aerodynamic constraints.

Chapter 11: Energy Carriers (Electricity & Hydrogen)

Q1. What is electricity? Why is it not an energy source?

A: Electricity is the flow of electric charge (an energy carrier), not a primary energy source. We must generate electricity from other sources (fossil fuels, nuclear, renewables), because there are no natural “wells” of electricity to tap into.

Q2. What is the primary energy source for most electricity in the US and in France?

A:

• United States: Predominantly fossil fuels (~ 60–65% of generation).

• France: Largely nuclear power (~ 70–75% of generation).

Q4. What is voltage? What is Ampere? What is Ohms?

A:

• Voltage (V): Electrical potential difference (analogous to pressure in a fluid).

• Ampere (A): Rate of electric current (flow of charge, analogous to volumetric flow rate).

• Ohm (Ω): Unit of electrical resistance (analogous to frictional resistance in a pipe).

Q5. What is Ohm’s Law?

A: Ohm’s Law states V = I·R, meaning the voltage across a resistor equals the current through it times its resistance.

Q7. What factors determine transmission power loss?

A: Power loss in lines is Pₗₒₛₛ = I²·R: it increases with the square of the current (I) and is proportional to the line resistance (R).

Q8. To minimize transmission loss, choose high voltage or high current?

A: Use high voltage (which reduces current for a given power level), thereby minimizing I²R losses.

Q9. What is a transformer? What principle does it employ?

A: A transformer is an AC device that steps voltage up or down using electromagnetic induction between primary and secondary coils wound on a magnetic core.

Q11. Why isn’t chemical hydrogen an energy source?

A: There are no natural reservoirs of free H₂ on Earth—hydrogen must be manufactured (from water or hydrocarbons) using other energy, making it an energy carrier rather than a primary source.

Q12. Why is there no hydrogen energy source on Earth?
A: Free hydrogen is light and escapes Earth’s gravity or reacts readily to form compounds (water, hydrocarbons), so it does not accumulate in exploitable form.

Q13. What are the four methods of hydrogen production?
A:

1. Steam Reforming of natural gas (most common).

2. Electrolysis of water (using electricity).

3. Thermal Splitting at very high temperatures.

4. Photolysis (solar-driven splitting via semiconductors or biological systems).

Q16. What is electrolysis? Its efficiency?

A: Electrolysis uses an electric current to split water into hydrogen and oxygen. Current commercial electrolyzers run at about 75% efficiency (electrical energy to chemical energy in H₂).

Q18. What are fuel cells? What is a PEMFC? Efficiency?

A:

• Fuel Cells electrochemically convert a fuel (e.g., H₂) and an oxidizer (O₂) into electricity, water, and heat—like a battery with continuous fuel supply.

• A PEMFC (Proton Exchange Membrane Fuel Cell) uses a polymer membrane to conduct protons from the hydrogen-fed anode to the oxygen-fed cathode.
  PEMFCs typically operate at 50–60% electrical efficiency in practice.

Q19. Theoretical maximum efficiency for hydrogen fuel cells?

A: The thermodynamic upper limit for converting hydrogen’s chemical energy to electricity is about 83%.