الرحيم الرحمن الله بسم

Lecture 3

ASHEM M. ASHAAL

Mean annual irradiance on horizontal surface across the world:

irradiance (w/m2): the rate at which radiant energyIs incident on surface per unit area of surface

The mean annual irradiance on surface is calledThe irradiation.The irradiation (J/m2): The incident energy per unit area on a surface , found by Integration of irradiance over the time usually in day or hour, For example the average annual irradiation in

sahara is 2200KWh/m2 KWh=36X105.(7920MJ/m2)

The extraterrestrial radiation incident on the plane :normal to the radiation on the nth day of the yearVariation extraterrestrial radiation in time of year is shown in figure:

The extraterrestrial radiation on time of year is given by a simple equation (1.4.1a) by spencer 1971 Or provide

more accurate equation (1.4.1b) by Iqbal(1983)

Where, Gsc is the solar constant : is the energy from the sun per unit received on a unit area of surface perpendicular to the direction of propagation of radiation at mean earth-sun distance outside the atmosphere.Gon is the extraterrestrial radiation incident on the plane normal to the radiation on nth day of the year and B is given by:

Example:Calculate the extraterrestrial radiation Gon on February 13 if the solar constant Gsc=1367W/m2 (4.921 MJ/m2 h)

Solution: n=31+13=44Using equation(1.4.1a)

Using equation (1.4.1b)

Gon=1403.119W/m2

The angles and consistent sign convections:

Some of the angle are indicated in figure :

Radiation on an inclined surface:inclined surface: is the surface makes an angle with the horizontal this angle is called the slope(β).Direct radiation: the solar radiation received from the sun without having been scattered by the atmosphere.Reflected radiation: the solar radiation reflected by a surface when the solar radiation incident on this surface. The law of reflection is the angle of the incident ray(i) equal the angle of the reflected ray(i\)where, the angle of incident ray is the angle between the incident ray and the normal to the surface , and the angle of the reflected ray is the angle between the reflected ray and the normal to

the surface as shown in the figure .

Figure. 1. Light reflected from a metal surface with angle of incidence i, equal to the angle of reflection. The dashed line (normal) is perpendicular to the surface

Diffuse Radiation:The solar radiation received from the sun after its direction has been changed by scattering by the atmosphere.direction of beam radiation:

The geometric relationships between a plane of any particular orientation relative to the earth at any time and the incoming beam solar radiation that the position of the sun relative to that planeCan described in term of several angles :β), surface azimuth angle )latitude(φ), declination(), slope

( ,)hour angle(), angle of incidence (), zenith angle(z)

.Solar altitude angle(s), solar azimuth angle(s)

radiation on inclined surfaces :The angle of incident beam of solar radiation on inclined surface is called the incidence angle () as shown in the fig .

calculation of angles of incidence:the equations relating the angle of the incidence of beam radiation on inclined surface () to the other angles are:

For vertical surfaces, β= 90◦ and the equation becomes:

For horizontal surfaces(β=0) as shown in the fig., the angle of incidence is the zenith angle of the sun (z) the equation 1.6.2 becomes:

the equation relating the solar azimuth angle is:

Calculation of sunset hour angle ωs:Equation 1.6.5 can be solved for sunset :hour angle ωs , when z=90o

The profile angle(p)of beam radiation on receiver plane R that has a surface azimuth angle of :The profile angle is the projection of the solar altitude angle on a vertical plane perpendicular to the plane R ( the angle between a vertical plane perpendicular to the plane R and the horizontal as shown in the fig.)Not that the solar altitude and profile angle are the same when the sun is in a plane perpendicular to the surface R. The profile angle p can be determined from:

Examplein example 1.6.3 calculate the number of daylight hour. solution:using equation:

N= 11.7hour

Thank You