---
title: "Solar radiation, solar cell and plant"
subject: "Energy Environment and Society"
chapter: 3
key: "semester_7/energy_environment_and_society:solar-radiation-cell-plant"
source: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/solar-radiation-cell-plant
site: BCT Engineering
---

# Solar radiation, solar cell and plant

Energy Environment and Society · Chapter 3: Renewable Energy Sources

- Asked in: 2074 Chaitra, 2+4 marks
- Page: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/solar-radiation-cell-plant
- Whole chapter: https://www.bctengineer.com/md/semester_7/energy_environment_and_society/chapter-3

## Question

Write solar radiation as source of energy with solar cell and solar plant function with appropriate diagrams.

## Short answer

Solar radiation from nuclear fusion delivers about 800–1000 W/m² at noon on a clear day. A solar cell is a p-n junction converting photons directly to DC electricity; each cell produces 0.5–0.6 V. PV plants connect modules, use MPPT and an inverter, and feed the grid; Nepal's plants include Devighat and Butwal.

## Answer

#### Solar radiation as an energy source
The sun is a gaseous sphere where nuclear fusion of hydrogen into helium releases energy as electromagnetic radiation. The sun's surface is at about 5778 K. The solar constant is about 1367 W/m² outside the atmosphere. After absorption and scattering, 800–1000 W/m² reaches the ground at noon on a clear day. Daily radiation in Nepal is about 4–5 kWh/m² (average), among the best in South Asia, with about 300 sunny days a year. The potential is estimated at several thousand MW (commonly cited around 2,100 MW for commercial-scale solar). It is free, clean, abundant and renewable but intermittent and dilute.

#### Solar cell
A solar cell is a p-n junction semiconductor diode (usually silicon) that converts sunlight directly into DC electricity by the **photovoltaic effect**.

1. A silicon wafer has an n-type layer (doped with phosphorus: extra electrons) on top of a p-type layer (doped with boron: extra holes). At the junction, electrons diffuse into the p side and holes into the n side, leaving a **depletion region** with a built-in electric field (about 0.6–0.7 V for silicon).
2. When photons with energy greater than the band gap of silicon (1.1 eV) strike the cell, they create **electron–hole pairs**.
3. The built-in field sweeps electrons towards the n side and holes towards the p side, so charges separate.
4. With metal contacts on the front (grid) and back, and an external load connected, electrons flow through the load, giving DC current. Photons with less energy than the band gap pass through, and extra energy of higher photons becomes heat.

```text
   Sunlight (photons)
     | | | |
  ===========  front grid contact
  |  n-type  |   (electrons)
  |----------|   junction / depletion layer
  |  p-type  |   (holes)
  ===========  back contact
      \___[ Load ]___/
         current flow
```
A typical cell gives about 0.5–0.6 V and, under full sun, about 30–35 mA/cm²; cells are connected in series and parallel to make modules.

#### Solar PV plant (grid-connected)
```text
 Solar array --DC--> Inverter --AC--> Transformer --> Grid
      |                  |
   (modules)      MPPT, protection, meter
```
Optionally, a battery bank with a charge controller serves off-grid systems.

- **Modules** (panels) are connected in series for voltage and in parallel for current to form an array.
- **Maximum power point tracker (MPPT)** adjusts the load to extract maximum power.
- The **inverter** converts DC to AC at 50 Hz.
- A transformer steps up the voltage, and protection and metering connect it to the NEA grid.
- Examples in Nepal: Nepal's utility plants of several MW, for example Devighat, Butwal and Kathmandu valley rooftop solar; the Nepal Electricity Authority also buys power from private solar producers.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which photons create electron-hole pairs in silicon?
   A. Photons with less energy than 1.1 eV
   B. Photons with energy greater than the band gap (1.1 eV)
   C. Only infrared photons
   D. All photons equally
   Answer: B. Lower-energy photons pass through; extra energy of higher photons becomes heat. (Key point: Band gap)

2. What separates the electrons and holes in a solar cell?
   A. The built-in electric field of the depletion region
   B. An external battery
   C. The front grid contact
   D. Cooling of the cell
   Answer: A. The field sweeps electrons to n side and holes to p side. (Key point: Junction field)

3. What is the correct order in a grid-connected PV plant?
   A. Array, transformer, inverter, grid
   B. Inverter, array, grid, transformer
   C. Grid, inverter, array
   D. Array, inverter, transformer, grid
   Answer: D. The inverter converts DC to AC, then a transformer steps up voltage for the grid. (Key point: PV plant chain)

4. What does an MPPT do?
   A. Converts AC to DC
   B. Steps up voltage to grid level
   C. Adjusts the load to extract maximum power from the array
   D. Stores energy in a battery
   Answer: C. MPPT keeps the array at its maximum power point. (Key point: MPPT)

## Related in Energy Environment and Society

- [Solar cell principle and applications](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Asolar-cell-principle-applications) — Energy Environment and Society, chapter 3
- [Working of a solar cell](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Asolar-cell-working) — Energy Environment and Society, chapter 3
- [Characteristics curve of solar cell](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Asolar-cell-characteristics-curve) — Energy Environment and Society, chapter 3
- [Solar thermal energy and application](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Asolar-thermal-application) — Energy Environment and Society, chapter 3
- [Hydropower source and plant types](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Ahydropower-plant-classification) — Energy Environment and Society, chapter 3
- [Solar constant, insolation, irradiance, peak sun](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Asolar-constant-insolation-definitions) — Energy Environment and Society, chapter 3, asked 3×

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