---
title: "Short note: Solar water heater"
subject: "Energy Environment and Society"
chapter: 3
key: "semester_7/energy_environment_and_society:solar-water-heater"
source: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/solar-water-heater
site: BCT Engineering
---

# Short note: Solar water heater

Energy Environment and Society · Chapter 3: Renewable Energy Sources

- Asked in: 2069 Chaitra, 2 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/solar-water-heater
- Whole chapter: https://www.bctengineer.com/md/semester_7/energy_environment_and_society/chapter-3

## Question

Write a short note on solar water heater.

## Short answer

A solar water heater uses a collector and insulated tank. Water heated in the collector rises by thermosiphon to the tank, providing water at 50-70°C. It saves electricity/fuel and is widely used in Nepalese homes and hotels. An electric backup may be added.

## Answer

A **solar water heater (SWH)** uses solar radiation to heat water for bathing, washing and industrial use, which saves electricity, LPG or firewood.

#### Main parts
- **Collector:** flat-plate collector (black absorber plate with copper tubes, glass cover, insulation) or evacuated tube collector.
- **Insulated storage tank** for hot water.
- Pipes, a cold-water inlet, and a hot-water outlet.
- Optional electric back-up heater for cloudy days.

#### Working
Sunlight passes through the glass and is absorbed by the black plate, heating the water in the tubes. Hot water, being lighter, rises to the tank and cold water from the bottom of the tank enters the collector. This **thermosiphon** circulation needs no pump (the tank must be higher than the collector). Large systems use a pump (forced circulation).

```text
   Hot water out
      ^   [ Tank ]
      |    ^    |
   rising | hot  | cold falls
      |   [Collector]<--sun
   Cold water in
```

Water temperature reaches 50–70 °C. It is widely used in Nepal (urban houses, hotels, hospitals). Advantages: free energy, low running cost, no pollution. Limits: lower output on cloudy days, scaling in hard water, and freezing in very cold regions unless an antifreeze loop is used.

## Check yourself

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

1. How does thermosiphon circulation work?
   A. A pump forces water through the collector
   B. Water flows because the tank is below the collector
   C. Steam pushes water out
   D. Hot water rises to the tank and cold water enters the collector, with no pump
   Answer: D. Density difference drives flow; the tank must be above the collector. (Key point: Thermosiphon)

2. Why is the absorber plate black?
   A. To reflect sunlight
   B. To stop water from freezing
   C. To absorb more solar radiation as heat
   D. To produce electricity
   Answer: C. A black surface absorbs most of the incident radiation. (Key point: Absorber)

3. Which is a limitation of solar water heaters?
   A. High running cost
   B. Lower output on cloudy days and freezing in very cold regions
   C. Pollution from fuel
   D. Cannot be used in Nepal
   Answer: B. They depend on sunshine and need antifreeze in cold places; running cost is low. (Key point: Limits)

## Related in other subjects

- [Flat plate collector and solar water heater](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Aflat-plate-collector-construction-water-heater) — Energy Resources, chapter 2
- [Regenerative scheme with open feed water heater](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aregenerative-open-heater-layout) — Power Plant Equipment, chapter 7
- [Regenerative cycle: 4 MPa, 400°C, open heater](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aregenerative-cycle-4mpa) — Power Plant Equipment, chapter 7, asked 2×