Solar radiation Basics
Extraterrestrial Radiation
Solar radiation incident outside the earth's atmosphere is called extraterreastrial radiation. On average the extraterrestrial irradiance is 1367 watts/meter^2(W/m^2). This value varies by +/- 3% as the earth orbits the Sun. The earth's closest approach to the sun occures around January 4th and it is farthest from the sun around July 5th. The extraterrestrial radiation is:
Io = 1367*(Rav/R)^2 W/m^2 ……………………………………(1)
Where Rav is the mean sun-earth diastance and R is the actual sun-earth distance depending on the day of the year. An approximate equation for the effect of the earth-sun distance is:
(Rav/R)^2 = 1.00011 + 0.034221*cos(B) +0.001280*sin(B) +0.000719*cos(2B) + 0.000077*sin(2B)………………………………………(2)
Where : B = 2Ωn/365 radians , n = day of year, i.e 15th January means 15, 21st March means(31+28+21=80). There are 365 or 366 days in year.
Putting the value of 21st March= 80 in equation (2)
(Rav/R)^2 =1.0147 ………………………………….(3)
Putting the value of equation (3) in equation (1)
Io = 1367*1.0147
= 1387.1 W/m^2
Solar Declination angle: It is angle between the plane perpendicular to incoming solar and the rotational axis of earth. The earths’ axis is tilted 23.5°. On the 21st June the declination angle is +23.5°, when the northern hemisphere is pointed towards the sun. In 21st December the earth is on the other side of the sun and the earths’ axis in the northern hemisphere is -23.5° pointing away from the sun. During the spring & fall equinoxes (21st March & 21st September) the earth’s axis is perpendicular to an imaginary line drawn between earth and sun and the declination angle is 0°. The declination angle D for any day can be calculated by this formula.
D = 23.45Ω/180*sin(2Ω*(284+n)/365)…………………………(4)
If we pt the value n =80 (21st March), than D = 0°
Zenith Angle:
