Unit 3 Solar Energy – Energy Science and Engineering AKTU (B.tech)

Providing Unit 3 Solar Energy – Energy Science and Engineering AKTU (B.tech) Important Questions with answers.
Topic : Introduction to solar energy, fundamentals of solar radiation & its measurement aspects; Basic physics of semiconductors, Carrier transport, generation & recombination in semiconductors, Semiconductor junctions: metal-semiconductor junction & p-n junction, Essential characteristics of solar photovoltaic devices

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Important Questions For Energy science and engineering : 
*Unit-01     *Unit-02    
*Unit-03    *Unit-04 
*Unit-05    *Short-Q/Ans
*Question-Paper with solution 21-22 

Q1. Explain with a neat sketch, the working of a solar cell.

Ans.

• 1. When light enters the p-n junction, photons of light can easily enter the junction due to the very thin p-type layer.
• 2. Light energy, in the form of photons, provides enough energy to the junction to build a number of electron-hole pairs.
• 3. The incoming light disrupts the junction’s thermal balance.
• 4. The free electrons in the depletion zone might swiftly reach the junction’s n-type side. Similarly, the depletion holes can soon reach the p-type side of the junction.
• 5. Because of the barrier potential of the junction, newly produced free electrons cannot cross the junction once they reach the n-type side. Similarly, newly formed holes cannot cross the junction once they reach the p-type side due to the junction’s similar barrier potential.
• 6. The p-n junction will function like a miniature battery cell when the concentration of electrons increases on one side, i.e., the n-type side of the junction, and the concentration of holes increases on the other side, i.e., the p-type side of the junction.
• 7. A voltage known as photovoltage is created. A minor current will flow across the junction if we attach a small load across it.

Q2. Write a short note on solar radiation.

Ans.

• 1. The electromagnetic radiation emitted by the sun is known as solar radiation. Various types of technology can transform this radiation into usable forms of energy such as heat and electricity.
• 2. The solar radiations received by the earth’s surface differ depending on location.
• 3. However, the radiation received outside the earth’s atmosphere differs from that received on the earth’s surface due to absorption, reflection, scattering, and attenuation by atmospheric particles and clouds.
• 4. solar radiation is grouped into the following two categories:

a. Extraterrestrial Solar Radiation :

1. Extraterrestrial radiation is the amount of solar radiation that would be received if the atmosphere did not exist.

b. Terrestrial Solar Radiation :

• 1. Terrestrial radiation is the radiation we get on the earth’s surface and accounts for about 70% of extraterrestrial radiation.
• 2. Solar radiation is scattered and absorbed by the earth’s atmosphere as it passes through it, and some of the scattered energy is reflected back into space.

Q3. Write a short note on:
a. Solar radiance, and
b. Solar insolation.

Ans. a. Solar Radiance:

• 1. The solar radiance is the instantaneous power density in kW/m².
• 2. The sun brightness is heavily influenced by location and local weather.
• 3. Solar radiance measurements are made up of global and/or direct radiation measurements performed at regular intervals throughout the day.
• 4. A pyranometer or a pyrheliometer is used to take the measurements.

b. Solar Insolation :

• 1. solar insolation is the total quantity of solar energy received at a specific location over a given time period, generally expressed in kWh/m² (m² day).
• 2. Solar insolation data is often utilized in the design of simple photovoltaic (PV) systems, whereas solar radiance data is employed in the design of more complex PV systems.
• 3. We can predict the size of a solar collector and how much energy it can produce by knowing the insolation levels of a specific place.
• 4. Sunlight recorders can be used to measure solar insolation. These sunshine recorders count the number of hours in a day when the sun is above a specified level.
• 5. The data acquired in this manner can be used to calculate solar insolation by comparing the measured number of sunshine hours to those calculated using numerous correction factors.
• 6. Cloud cover data using existing satellite photos is the last option for estimating solar insolation.

Q4. Give the classification of semiconductors.

Ans. The semiconductors can be divided into the following two types:

a. Intrinsic Semiconductors:

• 1. A pure semiconductor with no substantial dopant species is known as an intrinsic semiconductor, sometimes known as an undoped semiconductor or I-type semiconductor.
• 2. The quantity of charge carriers is thus dictated by the material’s characteristics rather than the number of impurities.
• 3. Inherent semiconductor conductivity can be caused by crystal flaws or thermal excitation.
• 4. The number of electrons in the conduction band in an intrinsic semiconductor is equal to the number of holes in the valence band.

b. Extrinsic Semiconductors:

• 1. An extrinsic semiconductor is one that has been doped, that is, one that has had a doping agent put into it, giving it different electrical properties than an intrinsic (pure) semiconductor.
• 2. Doping is the addition of dopant atoms to an inherent semiconductor, which modifies the electron and hole carrier concentrations at thermal equilibrium.
• 3. Extrinsic semiconductors’ electrical characteristics make them crucial components of many electron devices.
• 4. Dominant carrier concentrations in an extrinsic semiconductor classify it as either:
  • i. n-type semiconductor, and
  • ii. p-type semiconductor.

Q5. Discuss p-n junction in forward bias and reverse bias conditions.

Ans. A. p-n junction in Forward Bias:

• 1. For the forward bias of a p-n junction, the positive terminal of the battery is linked to the p-type, while the negative terminal is connected to the then-type.
• 2. The potential can be adjusted using a potential divider. At a certain forward voltage (0.3 V for Ge and 0.7 V for Si), the potential barrier is completely removed and current begins to flow. This voltage is referred to as the threshold or knee voltage (V_K).
• 3. As the forward applied voltage exceeds the threshold voltage, the forward current increases exponentially, as illustrated in Fig.
• 4. It creates an extraordinarily huge current when it exceeds a specified safe value, which may ruin the junction due to overheating.

B. p-n Junction in Reverse Bias:

• 1. The p-type is connected to the negative terminal of a battery, whereas the n-type is attached to the positive terminal.
• 2. In this scenario, the junction resistance is extremely high, and almost no current flows through the circuit.
• 3. Minority carriers cause a little current of the order of A to flow in the circuit. This is referred to as reverse current. Fig. depicts the reverse current.
• 4. The reverse current quickly rises to its maximum or saturation value as the reverse bias is increased from zero. The tiny rise is caused by impurities on the surface, which act as a resistor and hence follow Ohm’s law. This generates a current known as surface leakage current.
• 5. If the reverse voltage is increased further, the electrons’ kinetic energy becomes so high that they knock out of the semiconductor atoms. At this point, the connection fails and there is a dramatic increase in reverse current. The junction is now completely demolished.
• 6. Thus, a p-n junction is a one-way device with low resistance when forward biassed and an insulator when reverse biassed.

Q6. Define third-generation solar cells. Explain their various types.

Ans. A. Third Generation Solar Cells:

• 1. They are proposed to be fundamentally different from prior semiconductor devices in that they do not use a standard p-n junction to segregate photogenerated charge carriers.
• 2. Quantum well devices (quantum dots, quantum ropes, etc.) and devices integrating carbon nanotubes are being researched for space applications, with the potential for up to 45% production efficiency.
• 3. These innovative devices, which are still in the research phase, include photoelectrochemical cells, polymer solar cells, nanocrystal solar cells, and dye-sensitised solar cells for terrestrial applications.

B. Types of Third-Generation Solar Cells:

a. Organic PV Cell:

• 1. Organic semiconductor-based solar cells may give a low-cost option for solar PV.
• 2. The active layer of organic solar cells is just 100 nm thick, which is around 1000 times thinner than the active layer of crystalline silicon solar cells and about 10 times thinner than the active layer of contemporary inorganic thin film solar cells.
• 3. Organic semiconductors have a tremendous promise for low cost large area solar cells due to their low material consumption per solar cell and relatively easy cell production.
• 4. As a result, there is a great deal of interest in organic photovoltaic devices.
• 5. Their main advantage is that, unlike Si, they are flexible and can bend without breaking.
• 6. They are also very light and cheap.
• 7. They can still be used after being folded or chopped into required sizes.

b. Dye-Sensitized Solar Cell (DSC):

• 1. The DSC is similar to a thin-film solar cell technology. This technology has not yet been commercialized but is on the approach of being so.
• 2. DSC solar cells can be manufactured to be flexible. It has a high promise as a low-cost solar cell technology.
• 3. This is mostly owing to the wide availability and low cost of the ingredient material, as well as the low processing temperatures.
• 4. The DSC is a photoelectrochemical instrument. It works by using a photon, an electron, and a chemical reaction.
• 5. The operation of DSC is thought to be comparable to that of photosynthesis.

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