---
title: "Beam, diffuse and global radiation"
subject: "Energy Environment and Society"
chapter: 3
key: "semester_7/energy_environment_and_society:beam-diffuse-global-radiation"
source: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/beam-diffuse-global-radiation
site: BCT Engineering
---

# Beam, diffuse and global radiation

Energy Environment and Society · Chapter 3: Renewable Energy Sources

- Asked in: 2071 Shrawan, 3 marks
- Video: [EMSC1006 W6 Lecture Solar Radiation Part 2](https://www.youtube.com/watch?v=2OoQEoK_zgM) — EMSC1006 Teaching Videos
- 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/beam-diffuse-global-radiation
- Whole chapter: https://www.bctengineer.com/md/semester_7/energy_environment_and_society/chapter-3

## Question

Define beam, diffuse and global radiation and show the relation between them.

## Short answer

Beam radiation is unscatered sunlight; diffuse radiation is scattered from the atmosphere; global radiation is their sum: G = Gb + Gd (horizontal), where Gb = Gbn cos(zenith angle). For tilted surfaces, reflected ground radiation is added.

## Answer

When sunlight passes through the earth's atmosphere, part is absorbed and scattered by air, gases, dust and clouds. The radiation reaching the surface therefore has two components.

- **Beam (direct) radiation, Gb:** solar radiation that reaches the earth's surface without scattering, travelling in a straight line from the sun. It casts a shadow and can be concentrated by mirrors and lenses.
- **Diffuse radiation, Gd:** solar radiation that reaches the surface after being scattered by molecules, clouds, dust and aerosols, coming from all directions of the sky. It casts no shadow. On cloudy days nearly all the radiation is diffuse.
- **Global (total) radiation, G:** the sum of beam and diffuse radiation received on a surface (usually horizontal). It is what a pyranometer measures.

#### Relation
**G = Gb + Gd**   (on a horizontal surface, Gb = Gbn cos θz, where Gbn is the beam normal radiation and θz is the zenith angle)

For a tilted surface, the total also includes the radiation reflected from the ground (albedo): Gt = Gb·Rb + Gd·(1+cos β)/2 + G·ρ·(1−cos β)/2, where β is the tilt angle and ρ the ground reflectance.

The diffuse fraction Gd/G is high on cloudy days (up to 100%) and low (10–20%) on clear days. Flat-plate collectors and PV use both beam and diffuse radiation, but concentrating collectors can use only the beam part.

## Check yourself

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

1. What is the relation between beam, diffuse and global radiation on a horizontal surface?
   A. G = Gb - Gd
   B. G = Gb x Gd
   C. G = Gb + Gd
   D. G = Gd / Gb
   Answer: C. Global radiation is the sum of beam and diffuse components. (Key point: Relation)

2. Which describes diffuse radiation?
   A. Radiation travelling straight from the sun casting a shadow
   B. Scattered by molecules, clouds and dust, arriving from all directions with no shadow
   C. Radiation reflected only from the ground
   D. Radiation outside the atmosphere
   Answer: B. Diffuse radiation is scattered and shadowless; beam radiation is direct. (Key point: Diffuse radiation)

3. Why can concentrating collectors use only the beam part?
   A. Mirrors and lenses can focus only direct, parallel rays
   B. Diffuse radiation has too much energy
   C. Diffuse radiation is absent in all weather
   D. They face the ground
   Answer: A. Scattered radiation comes from all directions and cannot be concentrated. (Key point: Concentrating collectors)

## Related in Energy Environment and Society

- [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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