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The application of Appropriate Technology

Most small PV systems have the panels fixed so that they do not track the sun across the sky throughout the day. For fixed panels the maximum power output can be achieved when their surfaces are perpendicular to the sun at solar noon. Note that due to the vagaries of national time keeping solar noon is unlikely to be 12pm, rather it it the point in time at which the sun is at its daily zenith.

Panel angles for latitudes from 70° to -70°, negative panel angles indicate that panel is inclined to face north.

Figure 8: Panel angles for latitudes from 70° to -70°, negative panel angles indicate that panel is inclined to face north.

Panels should be fixed on the north-south axis since at all times of year, at solar noon, the sun will be directly on this line. If your system is in the northern hemisphere and above the Tropic of Cancer (i.e. has a latitude greater than +23.45°) your panels will always be inclined to face south because the sun’s daily zenith will always be in the southern skies. Similarly, if your system is in the southern hemisphere and below the Tropic of Capricorn (i.e. has a latitude less than -23.45) your panels will always be inclined to face north. Note that outside the tropics the sun is never directly overhead.

If your system is in the tropics (i.e. has a latitude between -23.45° and +23.45°) matters are not so simple. At the equator the sun is directly overhead at solar noon on the equinoxes (21st-23rd March and 22nd-23rd September), but reaches its daily zenith in the northern skies from March to September and the southern skies from September to March. In the northern tropics (i.e. latitudes between 0° and +23.45°) as the latitude increases the sun follows a similar pattern, although it will be directly overhead on days that approach the summer solstice (21st-22nd June). If your site is on the Tropic of Cancer the sun’s daily zenith will always be in the southern skies and will be directly overhead on the summer solstice. The sun in the southern tropics (i.e. latitudes between 0° and -23.45°) is in the southern skies for some of the year between the autumnal equinox and the vernal equinox and will be directly overhead on dates approaching the winter solstice as the latitude decreases, until at the Tropic of Capricorn the sun is always in the northern skies and directly overhead on the winter solstice.

The Arctic Circle is at 66.5° and the Antarctic Circle is at -66.5°, beyond these latitudes the sun will be completely absent for some of the year and ever present at other times. When the sun never sets it circles in the sky, never being directly over head.

The above discussion illustrates that PV panels sited in the tropics will need to be inclined to face south for some of the year and north at other times. Figure 8 shows a diagram that makes choosing panel angles relatively simple. Find find latitude of your site on the y-axis on the right hand side, then follow the line that corresponds most closely to your site and using the numbered days on the x-axis read the panel angle off on the y-axis on the left hand side.

Figure 9: The equinox, summer and winter angles for panels at a latitude of 10°.

Figure 9: The equinox, summer and winter angles for panels at a latitude of 10°.

You could reset your panel angle every week however setting it four times a year will give good results. Firstly in November set a winter angle, then in February set an equinox angle until April when a summer angle is set and finally reset the equinox angle in August. Figure 9 shows how these angles can be calculated, the correct angles for the beginning and end of each period should be bisected to find the average angle for that period: for a latitude of 10° the winter angle is 28°, the summer angle is -7° and the equinox angle is about 10°. A rule of thumb is that the equinox angle will be about equal to your latitude, the summer angle will be about 15° less and the winter angle will be about 15° greater. You may wish to change the panel angle every month around the equinoxes since the recommended angle is changing rapidly from week to week at these times of year.