Page 305 - Applied Photovoltaics

Step 2—Select battery capacity. For the above load and allowing for 15 days of
battery storage, we require a battery capacity of
4.17 A × 24 h × 15 days = 1500 Ah.
Step 3—First approximation of tilt angle. This is based on site information and
usually involves selecting a tilt angle 20° greater than the latitude. For example, for
Melbourne, which is at latitude 37.8°S, the first approximation for tilt angle is latitude
+ 20° = 57.8°.
Step 4—Insolation. From available site insolation data, calculate insolation falling
on the array at the calculated tilt angle.

Table G.1. Average monthly readings of direct and diffuse radiation falling on a
horizontal plane in Melbourne.
month direct (S) diffuse (D)
mWh/cm 2 mWh/cm 2
January 629 210
February 559 144
March 396 166
April 309 127
May 199 98
June 167 79
July 195 82
August 254 120
September 368 148
October 500 197
November 491 241
December 676 214

Example 1—From Table G.1, in January:
S = 629 mWh/cm 2
2
D = 210 mWh/cm .
Therefore, on a plane tilted at 57.8°, the direct component, from Eqn. 1.21, is
sin 8 . Į 57 q
S 629 (G.1)
57 8 .
sin Į
where
Į = 90° – 37.8° – į (from Eqn. 1.6)
į = 23.45° × sin[(15 – 81) × 360 / 365]
= –21.3°

given that the day of the year d = 15. Therefore
Į = 73.5°
2
S 57.8 = 493 mWh/cm .
The total insolation (direct + diffuse) falling on the array is therefore
R ȕ = S 57.8 + D
2
= 703 mWh/cm .

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