In this document
Solar irradiance encompasses various components that play a role in different aspects of solar energy analysis. After obtaining initial data from satellite-based models, further modeling and calculations are needed to obtain direct and diffuse components and aggregate solar irradiance values into meaningful metrics.
Overview
In solar energy applications, there are two main parameters that are commonly used:
GHI is the acronym for Global Horizontal Irradiation/Irradiance, which measures the available solar resource for site comparisons. GHI is calculated as the sum of direct and diffuse radiation received on a horizontal plane, and it is the most influential site parameter when studying the available solar resource for photovoltaic power plants.
DNI refers to Direct Normal Irradiation/Irradiance, which is essential for technologies like concentrating solar power (CSP) and concentrated photovoltaic (CPV) systems. As the name suggests, DNI is calculated based on the solar radiation hitting a surface that is perpendicular to the sun's rays.
Since satellite-based models primarily provide GHI, it is necessary to separate this into direct and diffuse components to gain a more detailed understanding of solar resources. This component separation is often achieved using models like the Perez model, which applies empirical data related to solar position and atmospheric conditions.
Additionally, understanding solar irradiation—the total energy received over time, which is sometimes confused with solar irradiance—is essential for accurate energy output calculations. Factors such as the sun’s position (elevation and azimuth angles) and the surface’s tilt and orientation are key to determining accurate solar irradiation values.
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Representation of solar irradiance components
Component splitting model
Global irradiance is the primary output of semi-empirical irradiance models, representing the total power per unit area received from the sun on a given surface. It consists of direct and diffuse irradiance components.
This component reaches the surface without being reflected or scattered. It originates directly from the sun’s direction, producing shadows and providing higher energy intensity. Direct irradiance decreases with increased cloud cover.
This component is the part of the irradiance that is scattered by the atmosphere before reaching the ground. It arrives from all directions and has a lower energy intensity, increasing as cloud cover grows.
Understanding the direct and diffuse components is essential for a thorough analysis of solar resources. The Perez model (see the paper) is widely used to separate global irradiance into these components. This model, along with similar ones, relies on empirical relationships that consider solar position, atmospheric conditions, and cloud cover. It applies empirical coefficients to estimate the diffuse fraction of GHI based on parameters such as the clear-sky index and solar zenith angle, using predefined tables or equations for various sky conditions.
Recent advancements in these models have improved the accuracy of cloud-related radiative effects. This involves using empirical submodels tailored to each sky classification to more precisely separate GHI into its direct and diffuse components.
Aggregation of irradiance into irradiation
Solar irradiance and solar irradiation are two terms that are often used in the context of solar energy, but they have distinct meanings:
Solar irradiance is the power per unit area received from the Sun in the form of electromagnetic radiation. It is a measure of how much solar power is hitting a surface at any given moment. It is typically measured in watts per square meter (W/m²). It is an instantaneous measurement, meaning it represents the solar power received at a specific point in time.
Solar irradiation is the total amount of solar energy received per unit area over a specific period of time. It represents the cumulative energy from solar irradiance. It is typically measured in kilowatt-hours per square meter (kWh/m²). It is an integrated measurement over a set period, such as an hour, day, or year.
To calculate solar irradiation from irradiance values, it is essential to know the sun’s relative position to the surface, and the sun and surface position angles describe the respective positions. By considering these factors, solar irradiation calculations can be accurately determined.
Sun Position Angles
Sun Elevation Angle: This angle, also known as solar altitude, measures the sun's height in the sky above the horizon, ranging from 0° (horizon) to 90° (directly overhead at zenith). The sun's elevation angle varies throughout the day, peaking at solar noon, and also changes seasonally due to Earth's axial tilt. Higher elevation angles yield more direct sunlight and, consequently, higher solar irradiance.
Sun Azimuth Angle: This angle denotes the compass direction of incoming sunlight, measured in degrees from true north in a clockwise direction. It ranges from 0° (north) to 360°, with 90° representing east, 180° south, and 270° west. The azimuth angle changes throughout the day as the sun traverses the sky from east to west.
Surface Position Angles
Orientation Angle: This angle describes the compass direction the surface faces, relative to true north, and ranges from 0° (north) to 360°.
Tilt Angle: This angle measures the inclination of the surface relative to the horizontal ground, ranging from 0° (flat surface) to 90° (vertical surface).
Further reading
Dynamic global-to-direct irradiance conversion models. Perez, R., Ineichen, P., Maxwell, E., Seals, R., and Zelenka, A., ASHRAE Transactions-Research Series, 1992.
GISPLIT: High-performance global solar irradiance component-separation model dynamically constrained by 1-min sky conditions. José A. Ruiz-Arias, Christian A. Gueymard, 2024, Solar Energy, vol. 269, p. 112363, 2024.