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Photovoltaics

PV Module

Angles

Richard Perez and Sam Coleman

©1993 Home Power

Photovoltaic (PV) modules work by converting sunshine directly into electricity. Sunlight is the essential

ingredient. PV modules work best when their cells are perpendicular to the Sun’s incoming rays. Adjustment of static mounted PV modules can result in from 10% to 40% more power output yearly. Here’s the angle.

Getting Perpendicular

Keeping the module perpendicular to the incoming sunlight means that the module intercepts the maximum amount of sunlight. If you have trouble visualizing this concept, take this magazine outside and hold it up to the sun while observing its shadow. If the magazine (or module) is edge on to the sunlight, then it casts a small shadow. If the magazine’s cover (or module’s face) is perpendicular to the sunlight, then the shadow is as big as it will ever be. The size of the shadow shows us exactly how much sunlight is being intercepted. In the case of a PV module, maximum shadow means maximum power.

Solar Azimuth

S

θ

EW

N

θ

The problem is that the Sun constantly moves in relation to the stationary PV module. Actually, the apparent motion of the Sun is due to the Earth’s motion, but for our purpose here this celestial fact is mere trivia. Even if we place a module so that is perpendicular to the Sun at solar noon, it is not even close to perpendicular in the morning and evening. This daily east to west solar motion is called solar azimuth. Also consider that the Sun’s apparent height in the sky changes from winter to summer. This yearly north to south solar motion is called solar declination. And you thought solar power was simple. Well, it really is...

Actually you can face a PV module south, tilt it so the included angle between its face and the ground is your latitude, and you’re done. It will work and it will work well. What we are talking about here is squeezing anywhere from 10% to 40% more power from PV modules by keeping them as perpendicular as possible to the incoming sunlight.

An Angular Matter

It’s matter of angles. If the module is to be kept perpendicular to the sun’s daily east to west motion (azimuth), then a device called a tracker is used. A tracker follows the sun’s daily motion and provides anywhere from 25% to 35% more power from the PVs hitchhiking on its back.

If you keep up with the sun’s seasonal north to south migration, then manual adjustment boosts PV power production by up to 10%. The chart on the next page has all the data necessary to accomplish this seasonal, north/south, adjustment.

Cosine Stuff

While using PV modules is very simple, the mathematics describing their angular relationship to the sun are very difficult. I sought help from Sam Coleman who is adept at ritual trigonometry. After covering

Solar Declination

up

NS

Left: a bird's eye view of solar azimuth, the sun's apparent east to west daily motion.

Above: a ground level view of solar declination, the sun's seasonal north to south motion.

14 Home Power #36 • August / September 1993

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