Power Shift: Charting the Sun Path

By , March 27, 2015

Our biggest debate concerning our new photovoltaic array may be placement.

We mounted our current array on a southerly rock face on the promontory we refer to as our Power Point. Its orientation to the sun and natural angle both fall within the recommended parameters for mounting solar panels in our region. We could eke out some extra power were we to tinker with angle an orientation. We could maximize our power by mounting them on a sun tracking frame, but these are very expensive, would greatly increase our workload to install, and, in our severe weather, could easily prove disastrous. A sturdy timber frame on the rock face offers an excellent compromise in average sun intake (called insolation) and maximum security against property damage.

Even so, I can’t help fiddling with the concept, nor can I seem to stop worrying over making the most of the new system.

The composite skyline photo with various heights of the sun (Photos: Mark A. Zeiger).

The composite skyline photo with various heights of the sun (Photos: Mark A. Zeiger).

A sun path or solar azimuth helps in situating solar panels. We made sun path charts for our latitude and longitude before we installed the current array, but never took the further step of placing our local horizon and shade silhouettes on it. I have always wanted to do that, not just for the solar panels, but for the gardens, and every other aspect of the “homestead” that depends on sunlight (or in some cases, lack thereof).

So, while we waited for the new panels to arrive by barge, I set to work creating our own sun path.

I started by going on line to a sun path calculator/plotter. There are many to choose from; I like the University of Oregon’s calculator best. I plugged in our latitude, longitude, and selected other features, such as deciding whether to use standard or daylight savings time, and, a bit annoyingly, ink color (why do they default the chart title to magenta?). Then I selected whether I wanted the output in PDF or PNG. I made both for different uses.

We got a nice period of sunny weather just after the standard spring equinox. We chose a sunny weekend day and made a one day chart for it (also an option at U of O’s site) then planted our camera on a tripod at the base of the solar array. Each hour on the hour, we went out and snapped two photos of the skyline, one with the sun on the left side of the image, and one with the sun on the right.

The next day I chose the images to use, and stitched them together into one long image using software. I then had a photograph of our entire southern skyline.

After this, things got tricky. If you look at a sun path chart, you soon realize that it is spread flat, like a global map, which distorts the view somewhat. A better way to look at the chart is to curl the page so that you’re looking into a U-shaped piece of paper. To be entirely accurate, you should actually roll it into a tube so that the 0° and 360° marks (which both indicate north) touch. The trick then is to look at your date in a meaningful way!

My task, then, to get the most accurate representation of our skyline, was to distort my photograph in the same manner as the chart.

I’m sure there’s a way to do this mathematically, but I did it with software. I won’t detail it here. If you’d like to know the whole process, I’ll gladly go into minute, pedantic detail at another time.

In short, I figured out how to size the photo to match the scale of the chart, then compress the photo to match the distortion of the chart’s graphic data. I then overlaid the photo on the chart, traced the skyline onto the chart, and saved it all as a PDF.

The compressed skyline image overlaid on the sun chart (Screen Shot: Mark A. Zeiger).

The compressed skyline image overlaid on the sun chart (Screen Shot: Mark A. Zeiger).

It’s rather startling to compare the image on paper to the reality. On the left side of the chart, The Mountain With No Name, by far the most imposing presence in our view shed, shrinks down to seeming insignificance. That short space in the graph represents the majority of our daylight. That’s hard to reconcile with what the graph seems to be telling us!

The completed "homestead" azimuth chart (Mark A. Zeiger)

The completed “homestead” azimuth chart (Mark A. Zeiger)

The finished chart confirmed what we already knew: most of our sun comes earlier in the day. Our east facing property shades in the afternoon even on the summer solstice. The cliff on which our panels sit faces east of true south 12°, which allows us to take a bit better advantage of our sun path.

Once I had the chart to consult, I went right back to mulling over whether to install my panels securely but unmoveable, or try to work out a way to make them adjustable through the seasons. I suppose that’s the sort of thing that keeps me off the streets at night . . . .

If you’re coming late to this conversation, catch up starting with the post Power Shift: A New/Old Battery Bank.

2 Responses to “Power Shift: Charting the Sun Path”

  1. Angie says:

    “To be entirely accurate, you should actually roll it into a tube …. The trick then is to look at your date in a meaningful way!”

    Did you mean “data”? If not, I just love the image of you looking at Michelle meaningfully through a paper tube. Ah, romance.

  2. Mark Zeiger says:

    Funny, Angie! And thanks for catching that–being friends with a professional proofreader has its advantages!

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