The averaging is usually done over all days of a

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A solar designer is primarily interested in average values for a location. The averaging is usually done over all days of a month and indicated by a bar over the symbol. Tabulations showing the hourly variation of global and diffuse radiation ( I g and I d ), the amount received per day ( H g and H d ) and sunshine hours per day (S) are available for many locations in India. A general idea of the availability of solar radiation over different regions can be obtained by constructing solar radiation maps. Figure 3.8 shows the distribution of the average daily global radiation, while Fig. 2.9 shows the distribution of the average daily diffuse radiation (refer book). From Fig. 2.8, (not included) it is seen that the annual average daily global radiation received over the whole country is around 450 langleys per day. Peak values are generally measured in April or May, with parts of Rajasthan and Gujarat receiving over 600 langleys per day. In contrast, during the monsoon and winter months, the daily global radiation decreases to about 300-400 langleys per day. From Fig. 3.9, it is observed that the annual average daily diffuse radiation: received over the whole country is around 175 langleys per day. The maximum values measured over the whole country are about 300 langleys per day in Gujarat in July, while minimum values, between 75 and 100 langleys per day, are measured over many parts of the country during November and December as winter sets in. 3.9 Solar Radiation Geometry In order to find the beam energy falling on a surface having any orientation, it is necessary to convert the value of the beam flux coming from the direction of the sun to an equivalent value corresponding to the normal direction to the surface. Relationships for making this conversion will Figure 2.8: Record of global and diffuse radiation flux measured on a clear day
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10 Dec Mar June Sep Dec now be given. Angle of incidence θ It is the angle between an incident beam of flux I bn and the normal to a plane surface . Then the equivalent flux failing normal to the surface is given by I bn cos θ. The angle θ can be related by a general equation to φ the latitude, β the slope, γ the surface azimuth angle, δ the declination, and, ω the hour angle. Each of these will first be defined . Latitude φ The latitude φ of a location is the angle made by the radial line joining the location to the centre of the earth with the projection of the line on the equatorial plane . By convention, the latitude is measured as positive for the northern hemi- sphere. It can vary from -90° + 90°. Slope β : The slope β is the angle made by the plane surface with the horizontal . It can vary from 0 to 180°. Surface azimuth angle γ The surface azimuth angle γ is the angle made in the horizontal plane between the horizontal line due south and the projection of the normal to the surface on the horizontal plane . It can vary from -180° to + 180°. We adopt the convention that the angle is positive if the normal is east of south and negative if west of south.
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