Fundamentals of Solar Energy: Radiation, Geometry, and Time

Solar Energy Fundamentals

Solar Radiation Components

Solar radiation consists of different spectral components:

• UV (Ultraviolet): Approximately 7%
• Visible Light: Approximately 47%
• Infrared: Approximately 40%

The Solar Constant

The solar constant is the amount of solar energy per unit time incident on a unit area surface outside the Earth's atmosphere, at a distance from the Sun equal to the average Sun-Earth distance. This constant varies slightly because the distance between the Sun and Earth is not constant.

Atmospheric Influence on Solar Radiation

Not all solar radiation intercepted by our planet reaches the surface due to the atmospheric layer. On days with clear skies, the maximum solar irradiance can reach 1100 W/m².

Types of Solar Irradiation

There are three main components of solar irradiation:

• Direct Radiation: This is received directly from the Sun without deviating in its passage through the atmosphere.
• Diffuse Radiation: This changes its direction, mainly due to reflection and diffusion in the atmosphere.
• Albedo Radiation: This is direct and diffuse radiation received by reflection from the ground or other nearby surfaces.

Direct radiation is generally the largest and most important component for solar energy applications. The proportions of direct radiation, diffuse radiation, and surface albedo received depend on meteorological conditions, the surface slope relative to the horizontal plane, and the presence of reflective surfaces.

Measuring Solar Energy Magnitudes

• Irradiation: This is the total solar energy that falls on a particular area over a period of time.
• Irradiance: This is the radiation incident on a given area at a specific moment, also known as radiant intensity, and is measured in W/m².

Irradiation on a Surface

The amount of energy due to direct radiation depends on the inclination of the surface; its value is maximum when the surface is perpendicular to the incoming radiation. Total radiation is measured with a device called a pyranometer.

Solar Geometry Principles

The Sun's position varies with the season, causing shadows to differ across locations and times of year.

Key Angles in Solar Geometry

• Solar Height (Altitude Angle): This is the angle between the apparent position of the Sun in the sky and the horizontal plane at a given location.
• Solar Azimuth: This is the angle formed by the horizontal projection of the Sun's position and the true south direction.

Both solar height and azimuth, at any given instant, will differ across various locations on the planet.

Solar Time and Angle Calculations

The number of hours of sunshine is the length of time between sunrise and sunset. This duration depends on geographical location and time of year.

Essential Definitions and Concepts

• Latitude: The angle between the normal to a geographic point and the plane of the Equator.
• Longitude: The angular distance of a place east or west of the prime meridian. In some solar calculations, conventions may assign negative values to east longitudes and positive to west longitudes.
• True Solar Time: This is derived from local civil time, adjusted by factors such as the equation of time and longitude correction.
• Hour Angle: This is the angular displacement of the Sun east or west of the local meridian, calculated as 15 degrees per hour (360 degrees / 24 hours).
• Angle of Incidence: This is the angle between the direct solar radiation striking a solar collector and the perpendicular to the collector's surface.