Table of Contents
Introduction
Solar radiation is the starting point for all solar energy technologies. To understand any solar system, it is essential to know what kind of energy arrives from the Sun, how it varies in time and space, and how we describe it with simple quantities and units. In this chapter, the focus is on the physical resource itself, not yet on how panels or collectors convert it into useful energy.
Origin Of Solar Radiation
Energy from the Sun is produced in its core through nuclear fusion. Hydrogen atoms combine to form helium and a tiny amount of mass is converted into energy according to Einstein’s relation $E = mc^2$. This energy travels outward and is eventually emitted from the Sun’s surface in the form of electromagnetic radiation.
Electromagnetic radiation includes many types of waves. Solar radiation that reaches Earth is mainly visible light, with significant portions of infrared and smaller amounts of ultraviolet. This mix of wavelengths is what solar technologies use, whether for electricity or heat.
The Solar Constant And Top-Of-Atmosphere Irradiance
Outside the atmosphere, at the average distance between Earth and the Sun, the incoming solar power per unit area on a surface perpendicular to the rays is nearly constant. This is called the solar constant. Its value is approximately $1360$ to $1370 \,\text{W/m}^2$.
This number does not mean that the ground on Earth receives this amount everywhere. It represents the maximum power available at the top of the atmosphere on a unit area that faces the Sun directly. As solar radiation passes through the atmosphere, it is reduced by various processes before reaching the surface.
Interaction Of Solar Radiation With The Atmosphere
When solar radiation enters the atmosphere, it is modified in several ways. Part of it is absorbed by gases such as ozone, water vapor, and carbon dioxide. Part of it is scattered in different directions by molecules, aerosols, dust, and clouds. Some of it is reflected back to space, especially by clouds and bright surfaces like ice.
Only the portion that is neither absorbed nor reflected can reach the ground. On a clear day at sea level with the Sun high in the sky, the power on a surface perpendicular to the Sun’s rays is typically around $1000 \,\text{W/m}^2$. This is already less than the solar constant because of atmospheric losses. Cloudy conditions can reduce the amount dramatically.
Direct, Diffuse, And Global Solar Radiation
Because of atmospheric scattering and absorption, solar radiation at the surface has two main components. Direct, or beam, radiation travels in a straight path from the Sun and casts sharp shadows. Diffuse radiation is scattered and arrives from many directions in the sky, even if the Sun is hidden by a cloud or a building.
On a horizontal surface, the total solar radiation received is the sum of the direct component (after projection on the horizontal plane) and the diffuse component. This total is called global radiation on the horizontal plane. For tilted surfaces, such as solar panels, the same idea applies, but the contribution of direct and diffuse radiation depends on the surface orientation.
Key relation for a horizontal surface:
Global horizontal irradiance (GHI) = Direct horizontal irradiance (DHI\_dir) + Diffuse horizontal irradiance (DHI\_diff)
Where GHI is the total power per unit area on a horizontal surface from both direct and diffuse sunlight.
Power Versus Energy: Irradiance And Irradiation
Solar resource descriptions use two related but different ideas. One is power at a given moment, the other is energy over a period of time. It is important not to confuse them.
The instantaneous power of solar radiation per unit area is called irradiance. It is usually measured in watts per square meter, written as $\text{W/m}^2$. For example, $800 \,\text{W/m}^2$ at noon means that, at that moment, each square meter of surface receives $800$ joules of solar energy per second.
When this power is integrated over time, the result is energy per unit area, called irradiation. It is usually expressed in kilowatt hours per square meter, $\text{kWh/m}^2$, for daily, monthly, or yearly values.
Fundamental relation between irradiance and irradiation:
If $G(t)$ is the irradiance in $\text{W/m}^2$ as a function of time $t$, then the irradiation $H$ over a time period is
$$
H = \int G(t)\,dt
$$
For practical purposes:
Energy per unit area (kWh/m²) = Average irradiance (kW/m²) × Time (hours)
This distinction matches the general difference between power and energy, but here it is applied per unit area.
Spectral Distribution Of Solar Radiation
Solar radiation covers a wide range of wavelengths. For solar energy applications, the most relevant part is from about $0.3$ micrometers (ultraviolet) to about $2.5$ micrometers (near infrared). The intensity is highest in the visible range, roughly from $0.4$ to $0.7$ micrometers, which coincidentally aligns with human vision.
The atmosphere affects different wavelengths differently. For example, ozone strongly absorbs ultraviolet, and water vapor absorbs parts of the infrared. This shapes the spectrum that finally reaches the ground. Different solar technologies respond differently to various parts of the spectrum, but the common reference is the standard solar spectrum at the surface, such as AM1.5, which represents average conditions with the Sun at a particular angle in a clear sky.
Geometric Factors: Angle Of Incidence And Position Of The Sun
The Sun’s position in the sky is not fixed. It changes throughout the day due to Earth’s rotation and throughout the year due to Earth’s orbit and axial tilt. As a result, the angle between the incoming rays and any given surface, called the angle of incidence, changes constantly.
The irradiance on a surface is highest when the Sun’s rays hit it perpendicularly. When the Sun is lower in the sky, or when the surface is not oriented toward the Sun, the same beam of sunlight spreads over a larger area, which reduces the power per unit area.
On a horizontal surface, the effective direct irradiance at ground level can be approximated as
Projection of direct irradiance on a horizontal surface:
$$
G_\text{h} = G_\perp \cos(\theta_z)
$$
where:
$G_\text{h}$ is direct irradiance on the horizontal plane,
$G_\perp$ is direct irradiance on a surface perpendicular to the Sun’s rays,
$\theta_z$ is the solar zenith angle, the angle between the Sun and the vertical.
This simple geometric effect is one of the main reasons why solar energy varies with time of day and season, even if the atmospheric conditions stay the same.
Daily And Seasonal Variations
Even at a fixed location with clear skies, solar radiation is not constant. During the day, it rises from zero at sunrise to a maximum when the Sun is highest, then falls back to zero at sunset. Over the year, the path of the Sun in the sky changes. In summer, the Sun climbs higher and stays above the horizon longer. In winter, its maximum height is lower and daytime is shorter.
These variations affect both the instantaneous irradiance and the total daily irradiation. For example, a clear summer day typically yields a higher daily energy per square meter than a clear winter day at the same place. This pattern is especially pronounced at higher latitudes.
In addition to geometric effects, weather introduces further variability. Clouds can briefly cut irradiance to very low levels, then allow it to rise again, leading to fluctuations that solar technologies must handle.
Geographic Differences In The Sun’s Resource
The solar resource is not equal across the globe. Near the equator, the Sun is generally higher in the sky throughout the year, and days are of similar length. Many subtropical regions combine high Sun angles with relatively low cloud cover, which leads to high annual solar irradiation.
At higher latitudes, the seasonal difference becomes stronger. Summers can have long days and reasonably high irradiance, while winters may be dark and cloudy. Local climate strongly influences the resource as well. Desert regions typically have high clear sky irradiance and very high annual totals, while cloudy coastal or high latitude regions have lower values.
Despite these differences, almost all inhabited places receive enough solar radiation to make some solar applications useful. The key distinction is not between places with and without sun, but between places with more or less favorable solar conditions.
Typical Values Of Solar Resource
For a sense of scale, consider horizontal surfaces over one year. Very sunny desert regions can receive on the order of $2200$ to $2600 \,\text{kWh/m}^2$ per year of global horizontal irradiation. Many temperate regions might receive $1000$ to $1600 \,\text{kWh/m}^2$ per year. Cloudy, high latitude locations may have annual values below $1000 \,\text{kWh/m}^2$.
On a clear day around midday, instantaneous global horizontal irradiance can be around $800$ to $1000 \,\text{W/m}^2$ in many places. These numbers are typical, not exact limits, and actual conditions vary continuously in response to geography, season, time of day, and weather.
Temporal Patterns And Intermittency
Because solar radiation depends on day and night cycles and on clouds, it is inherently intermittent. Short term fluctuations come from passing clouds, while predictable daily and seasonal patterns come from Earth’s motion. These temporal patterns are fundamental to how solar power behaves in energy systems.
From the point of view of resource description, it is common to characterize solar radiation using hourly values, daily totals, monthly averages, and long term statistics. These summary measures capture both the average resource and its variability, which is essential later when designing systems and estimating their output.
Summary
Solar radiation originates from nuclear fusion in the Sun and reaches Earth as a stream of electromagnetic energy. At the top of the atmosphere, the available power per unit area is about $1360$ to $1370 \,\text{W/m}^2$. By the time it reaches the surface, it is reduced and split into direct and diffuse components, which together form the global radiation on a surface.
The solar resource is described using irradiance, a power per unit area in $\text{W/m}^2$, and irradiation, an energy per unit area in $\text{kWh/m}^2$. Geometry, atmospheric conditions, latitude, and season all shape how much solar energy reaches a given location and surface. These characteristics define the Sun’s resource that solar technologies seek to convert into useful energy.