Where can I find solar insolation for windows (tilt=90) for November to April (when solar heat is helpful and when trees are bare)? Ideally considering clouds.
Melville2
| Posted in Expert Exchange Q&A on
[Edit: maybe solved — except for CLOUD EFFECTS (can’t find data / info for my location). Calculations in comment.]
Latitude: 44 degrees north. (80 west)
Most sites about solar insolation focus on photovoltaics, not windows / vertical surfaces.
Most sites assume a tilt of 0 (horizontal) or a continually optimized tilt. They calculate the daily average by dividing the annual by 365, but solar heat in the summer won’t help me in winter so I need the insolation for … let’s say around 180 days from Nov 1 to April 30.
(The full heating season is longer, but if the outside temp is 3 degrees colder than the desired inside temp, and windows heat my house by 6 degrees, only 3 of those are helpful. Also, I have big trees outside my windows so when there are leaves I get near-zero direct insolation.)
Where can I find solar insolation for windows for November to April (when solar heat is helpful and when trees are bare)? Ideally considering clouds.
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Hack answer maybe:
Use pvwatts and just set up a 1kw vertical PV system facing the direction of your windows. Not perfect, but easier than transforming horizontal beam radiation with trig (and then add diffuse with view factor)
Thanks.
Reading the information popup next to the system size, I set the system to 0.16 kw to attempt to approximate 1 square metre. Do you know if I was wrong to do that? Calculations at the end of this comment.
With tilt 90, azimuth 170, system losses 0, array size 0.16 kw (attempting 1 m2*), I get an average 3.22 kwh/m2/day over the whole year, and 3.45 for november to april.
I changed system losses to the default 14% and as expected that doesn't affect the insolation calculations.
I have not been able to find estimates of cloud effects for my location.
Using 3.45:
621.6 over 180 days - per sq m
$0.20 cost of 1 kWh
$124.32 per year solar $ gains - per metre squared
15.6 metres squared
$1,939.39 per year solar $ gains
Now need to find cloud effects.
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*The default PV system size is 4 kW. For a system with 16% efficient PV modules, this corresponds to an array area of approximately 25 m² (269 ft²)
Using the rooftop size estimator, the smallest it would let me make was 3 m2: System Capacity: 0.5 kWdc
4/25 = 0.16. 0.5/3 = 0.16.
The PVWatts data should include clouds from TMY3, I think. But don't quote me on that exact data source. I think it depends on your site.
The national solar radiation database: https://nsrdb.nrel.gov/
google's AI reply:
"Yes, PVWatts uses typical meteorological year (TMY) data, including TMY3 data, to estimate the energy production of photovoltaic (PV) systems"
So cloud effects are already factored in - good!
Thanks
Susdesign.com has a window heat gain calculator
Thanks. https://susdesign.com/windowheatgain/index.php. It does alas require that I set what percent cloud cover to use for each month, and my previous attempts to find that kind of info lead nowhere. .