H 0 is the total daily amount of extraterrestrial radiation on a plane perpendicular to the Sun’s rays.Irradiation is given the symbol H, so that: Just to be confusing the intensity of solar radiation is called irradiance and is measures in the units of power per unit area (W/m 2 or kW/m 2) however, the total amount of solar radiation energy is called irradiation and is measures in the units of energy per unit area (J/m 2). Note that since cosine values fall between 1 and –1, I 0h will never be greater than I 0, and I 0h = I 0 at point P’ where cos θ Z = 1 (θ Z = 0°). It can be seen from figure 2.2 that θ Z is also the angle of incidence of the Sun’s rays on a horizontal plane.
Where θ Z is the solar zenith angle described in section 1.2 and I 0h is the extraterrestrial irradiance intensity on a horizontal plane. as the distance between the Sun and Earth changes thought the year): The solar constant can be used to calculate the irradiance incident on a surface perpendicular to the Sun’s rays outside and the Earth’s atmosphere (figure 2.1) on any day of the year (i.e. The solar radiation intensity falling on a surface is called irradiance or insolation and is measured in W/m 2 or kW/m 2. Even this value is inaccurate since the output of the sun changes by about ☐.25% due to Sun spot cycles. A yearly average value is thus taken and the solar constant equals 1367 W/m 2. The word ‘constant’ is a little misleading since, because of the Earth’s elliptical orbit the intensity of the solar radiation falling on the Earth changes by about 7% between January 1 st, when the Earth is nearest the Sun, and July 3 rd, when the Earth is furthest from the Sun (figure 1.2). The solar constant (I SC) is the average radiation intensity falling on an imaginary surface, perpendicular to the Sun’s rays and at the edge of the Earth’s atmosphere (figure 2.1). I0 = irradiance on a plane perpendicular to the Sun's rays Figure 2.1: The Sun's rays incident on the Earth.
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