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Old 09-27-15, 10:34 AM   #1
Piwoslaw
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Default DIY insolation meter (almost)

Poland's government has finally matured (or was forced by the European Union) to introducing renewable energy incentives which actually work - in the last year I've noticed a lot of new PV systems on private houses. We've wanted solar too, but always thought we have too much shade from the tall trees around our house. Now I want to confirm this.

I'd like to check exactly how long the roof of our house gets direct sunlight during the day, at different times of the year. Yes, there are complicated and/or expensive gizmos which will do the job, but can I do it low tech? I'm not really interested in quantifying how many watts of energy hit the roof, minute by minute, a yes/no is enough.

My idea is to make a primitive insolation sensor - 2 photo- or thermoresistors on the north and south sides of a "wall". When in the shade, both sensors give the same value, when in the sun, the sensor on the southern side of the "wall" will show more light/heat than the one on the shady north side. In fact, I there would be 5 "south" sensors (1 on each corner of the roof plus 1 in the middle), while the "north" need only be one (could be anywhere, as long as it is always shaded). There could also be a "control" sensor in a spot which always gets sunlight (when available), then comparing each "south" sensor to the "north" and "control" will additionally tell if there is partial shading. Collecting data for 1 cloudless day once a month should be enough?

What do you think? Am I overdoing this? Yes, getting pictures of the roof during the day would also work...

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Old 09-27-15, 11:47 PM   #2
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Get a cheap camera that supports time lapse and manual (fixed) exposure, then have it take a picture of the roof every minute or so during the day. Repeat for different sides.
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Old 09-28-15, 07:20 AM   #3
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Mike has a great idea (time lapse photos).

Also, the following site has great information throughout the world. It can't take into account trees, but it will tell you all else.

PVWatts Calculator

As for trees, I would go up on the roof and map out the parahelion (path of the sun) on the shortest day of the year, the respective equinoxes and on the longest day. Deciduous trees have leaves that fall in winter and they should not shade a lot.

How close are the trees to the roof? And how tall?

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Old 09-28-15, 12:15 PM   #4
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Doing photos takes the fun out of it

One tree (deciduous) is close enough to shade the roof even in the summer. This tree can be shortened, as it is on our property. The neighbors have trees which are farther away but taller, some of them evergreens (pine).
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Old 09-28-15, 07:35 PM   #5
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If you think doing time lapse takes the fun out of it, I'd suggest finding a simple video camera and an old working VCR. One video tape for each season you want to cover. Start recording around 9AM, and stop about 3PM. Use recording timers if you want coverage outside the "peak" sun hours.

Fast Forward through the tape to watch your shadows. I accidentally stumbled upon this method while trying to figure out which neighborhood child was messing with things in my front yard.
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Old 09-28-15, 08:57 PM   #6
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What's a vcr? Is it like a DVR or something?
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Old 09-29-15, 08:15 PM   #7
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Quote:
Originally Posted by jeff5may View Post
What's a vcr? Is it like a DVR or something?
It is a primitive video recording device utilizing sequential access magnetic media, rather than direct access media. Video Cassette Recorder = VCR.

Technically, I could build a similar setup using my DVD recorder and a video camera, the issue is recording length of a blank DVD.
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Old 10-26-15, 02:17 PM   #8
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Any updates on this Piwoslaw?
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Old 10-27-15, 01:12 PM   #9
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None

I think I'll go with just taking pics of the roof every so often.
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Old 12-06-15, 02:47 PM   #10
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OK, you were right: No need to fool around with insolation sensors when a picture will tell you more. In my case it told me that solar may not be a good choice.

It was sunny today, and since the winter solstice is only 2-3 weeks away, I'd get a good idea of the situation today. So around solar noon (about 11:30 local time) I crawled up the ladder and saw that the whole roof is in the shade of the leafless trees Looking North:


Just to get an idea of how deep into the shade it is, I took a shot from the north corner into the sun:

You can see that the sun is not even near the tips of the branches. To the west is a window which would give at most ~1 hour of direct sunlight before the sun hides behind the pine trees. In the summer I might get good solar, but I doubt that the amount of energy coming through between the tree branches would help our heating system in the winter. And next year those trees aren't going to be any shorter

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