Solar Radiation Sun-Earth relationships Array Orientation Solar Radiation Data Sheets US Energy Consumption A false color image of the sun enhances the turbulent nature of the sun’s photosphere, including a roiling surface, sunspots, and giant flares. Distance from Sun to Earth Even over the vast distance, an enormous amount of energy reaches Earth from the sun. Harnessing the Sun Solar irradiance is solar radiation power per unit area. Inverse Square Law The inverse square law states that radiation energy is reduced in proportion to the inverse square of the distance from the source. Solar Irradiation Solar irradiation equals to solar irradiance over time. Electromagnetic Spectrum The electromagnetic spectrum is composed of electromagnetic radiation with varying with wavelengths. Solar Spectrum The wavelength distribution of the sun from UV to infrared reaching earth. Atmospheric Effects Solar radiation entering Earth’s atmosphere becomes direct, diffuse, or albedo radiation. Air Mass Air mass is a representation of the amount of atmosphere radiation that must pass through to reach Earth’s surface. Peak Sun Hours Peak sun hours is an equivalent measure of total solar irradiation in a day. Average Annual Isolation Map Insolation maps rate locations by their average daily peak sun hours. Terrestrial Solar Spectrum The atmosphere absorbs extraterrestrial radiation at certain wavelengths, resulting in an altered spectral distribution for terrestrial radiation. Spectral PV Device Response PV devices are designed to respond to the wavelengths corresponding to the greatest amount of solar energy. Pyranometers A pyranometer measures total solar irradiance from the whole sky. Handheld Pyranometers Handheld pyranometers use less precise sensors than precision pyranometers but are more suitable for field measurements. Some use separate sensors that are connected to a meter, while some have the sensor and meter integrated in one unit. Pyrheliometers A pyrheliometer measures the direct component of solar irradiance, which is important when installing concentrating collectors. Reference Cells Reference cells output a certain electrical current for each unit of solar irradiance received. Earth’s Orbit Around the Sun The ecliptic plane is formed by Earth’s elliptical orbit around the sun. Solar Declination The equatorial plane is tipped 23.5° from the ecliptic plane. Over a year, this orientation produces a varying solar declination. Solstices The summer solstice occurs when the Northern Hemisphere is tipped towards the sun. The winter solstice occurs when the Northern Hemisphere is tipped away from the sun. Equinoxes The fall and spring equinoxes occur when the sun is directly in line with the equator. Solar Azimuth and Altitude Angles Solar azimuth and altitude angles are used to describe the sun’s location in the sky. Sun Paths The sun’s path across the sky at various times of the year can be illustrated on a diagram. The diagrams change for different latitudes. Solar Window The solar window is the area of sky containing all possible locations of the sun throughout the year for a particular location. Array Orientation Array orientation can be described using azimuth and tilt angles. Optimum Array Tilt Angles Energy production at certain times of the year can be optimized by adjusting the array tilt angle. Seasonal Declination The average seasonal declinations define the optimal tilt angles for those periods. NREL Solar Radiation Data The National Renewable Energy Laboratory (NREL) provides solar radiation data for various locations, times of the year, and array orientations for use in planning PV installations.