Solar Radiation. Characteristics of the Sun. The sun has layers The middle is called the core – it is 27,000,000 degrees! The outer layer is the chromosphere The layer under the chromosphere is the photosphere, where light and other forms of energy are released
The sun is a star and solar mass is composed predominantly of the two lightest elements:1) hydrogen( H) which makes up about 70% of the mass.2) helium( He) about 27% .3) 3% remaining of solar matter is made up of all the other 90 elements.Itsdiameter is about 1.39×106 km.The core temperature varies between 8×10^6 K and 40×10^6 K .
It is an internal energy generator and distributor for other planets such as the earth.
90% of the energy is generated in the region between 0 and 0.23R which contains 40% of the sun’s mass.
The sun generates an approximate energy of about
(1 x 10 ^(1 8 )kWh/a).
The amount of energy in the sunlight reaching the Earth's surface is equivalent to around 10,000 times the world's energy requirementsand only0.01 per cent of this energy would cover mankind's total energy needs.
Solar radiation Solar radiation is the emission from the sun into every cornerof space and appears in the formof electromagnetic wavesthat carry energy at the speed of light causedby the conversion of H into Hethrough solar fusion.Solar irradianceis the intensity of solar power, usually expressed in Watts per square meter [W/m^2].Electromagnetic Spectrum is the range of all types of electromagnetic radiation, based on wavelength.
The sun’s radiation (solar) energy :
It first interacts with the atmosphere and then reaches
to the earth’s surface and the amount of energy reaching the atmosphere is known as solar constant.
EQ = 1367W/m2
47% of this energy ( as an average) reaches the earth’s surface and 57% is absorbed by the atmosphere.
When light passes through the Earth's atmosphere, the irradiance is reduced as a result of:1)reflection off the atmosphere.2)absorption by molecules in the atmosphere ( O3, H2 O , O 2 , CO2 ) The atmospheric absorption accounts for about two thirds of the incoming irradiation and it is primarily due to water vapor and to a lesser degree by CO2 that exist in the atmospheric composition.3)Rayleigh scattering (molecular scattering)is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light4)Mie scattering (scattering of dust particles and pollutants in the air).
The incoming irradiation at any given point takes different shapes and depends on :1)geometry of the earth.2)its distance from the sun.3)geographical location of any point on the earth.4)astronomical coordinates.5)the composition of the atmosphere.
Sunlight on the Earth’s surface comprises from:1)Direct Radiation radiation from the sun that reaches the earth without scattering
2)Diffuse Radiation: radiation that is scattered by the atmosphere and clouds
It represents how much atmosphere the solar radiation
has to pass through before reaching the Earth’s surface.
AM = 1/ Cos Өz
Air Mass (AM) equals 1.0 when the sun is directly overhead
at sea level.
At high altitudes or in a very clear days, Peak Sun may be more than 1000 W/m^2 but it is a practical value for most locations.
Zenith is the point in the sky directly overhead a particular location –as the Zenith angle Өz increases, the sun approaches the horizon. AM = 1/ Cos Өz
Distribution of solar radiationExact knowledge of the sun's path is important for calculating irradiance values .Solar azimuthThe daily motion of the sun from East to West. Solar declinationThe Sun’s apparent height in the sky changes from winter to summer. This yearly north to south solar motion is called solar declination.
Solar Declination is the angle between the equatorial plane and the ecliptic plane. The solar declination angle varies with the season of the year, andranges between –23.5º and +23.5º.
Summer Solstice is at maximum solar declination (+23.5º) and occurs around June 21st –Sun is at Zenith at solar noon at locations 23.5º N latitude.
Winter Solstice is at minimum solar declination (-23.5º) and occurs around December 21st
Days are significantly shorter than nights.
occur when the solar declination is zero. Spring equinox is around March 21st and the fall equinox occurs around September 21st –Sun is at Zenith at solar noon on the equator.
Around the equinoxes the daily [rate of] change is at maximum as oppose to change of declination during the solstices when it is at its minimum.
Measurement of solar radiation Solar radiation is either measured directly using instruments, or indirectly by analysing satellite images.
References:1)Fundamentals of Photovoltaic Modules and Their Applications by Julian Hunt FRS, University College London, London, UK.2)Planning and Installing Photovoltaic Systems A guide for installers, architects and engineers second edition.3)Solar Energy Fundamentals and Modeling Techniques by Zekai.