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| Flat roofs are a poor choice for solar panels whether for heat or photovoltaic in temperate and climates more distant from the equator. Every flat plate collector of solar energy gets the greatest power or heat when its perpendicular to the sun's rays. Fortunately the effectivity falls as the cosine of the error angle, and at 45 degrees error that is 0.7071, or down 29.3%. And when the earth spins at 15 degrees per hour that means the solar collector that is perpendicular to the noonday sun isn't horrible between 9AM and 3 PM solar time. Actually the solar energy collected falls off faster at 9 and 3 because the path through the atmosphere is longer and so more is absorbed in the atmosphere if its clear.
Most commercial installations have been using tilted collectors, and some follow the sun and adjust the tilt angle regularly. There is a detectable benefit to tracking, but usually the cost of the tracking hardware isn't covered by the gain in collected solar energy, its easier (especially with solar panels retailing for under $5 a watt unlike the $10 or !2 from a few years ago) to add area than to deal with the complications of tracking. Then the most common commercial practice is to tilt the panels at an angle very near the local latitude as that keeps the panel averaging closest to optimum year round without changing the tilt angle.
In 1974, I built a solar heat collecting panel, 8' x 24' tilted at 42 degrees located 42 degree 3 minutes north. I had to take off the plastic glazing in the summer to keep it from melting the plastic when I didn't want heat and that let water get into the structure and after a few years it rotted. I learned that I could have made the collecting surface vertical and that would not have hurt too much in the winter and would have helped keep it from overheating in the summer and kept water out better. And that in the winter snow cover on the ground could reflect sunlight to more than make up for the tilt angle error. Or I could have tossed some sheets of foil faced foam out in front of it for times when there wasn't snow to do the reflection.
Solar collectors flat on a flat roof aren't the worst possible, but not the best either. The worst would be tilted so the sun hit the edge, not the face, e.g. tilted to fast the nearest pole of the earth.
Gerald J. | |
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After rooftop solar panels, pavement solar panels
