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Solar Data and "Max" Values

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whitenemesis
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Solar Data and "Max" Values

Postby whitenemesis » Fri Jan 28, 2011 11:52 pm

Curious how is the Max value calculated? When plotting solar data and solar max the max doesn't match the highs from the solar data. Presumably its averaged?

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steve
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Re: Solar Data and "Max" Values

Postby steve » Sat Jan 29, 2011 9:51 am

The theoretical max is calculated according to the Ryan-Stolzenbach formula. I used a spreadsheet I found here: http://www.ecy.wa.gov/programs/eap/models.html.

You will need to experiment with the settings to get good results, but I am told (see a thread elsewhere) that you should expect a few "spikes".
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whitenemesis
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Re: Solar Data and "Max" Values

Postby whitenemesis » Sat Jan 29, 2011 5:53 pm

Wow! Some things to experiment with ;)

Thanks Steve!

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steve
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Re: Solar Data and "Max" Values

Postby steve » Sat Jan 29, 2011 6:13 pm

I still haven't found good settings for me. I decided there was little point experimenting in the middle of winter, given how little sun we get here!
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whitenemesis
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Re: Solar Data and "Max" Values

Postby whitenemesis » Sun Jan 30, 2011 9:18 pm

Any mileage in adding a Solar Energy output ( in BTU or Langley)? Perhaps a bit too specialist ..

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steve
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Re: Solar Data and "Max" Values

Postby steve » Sun Jan 30, 2011 9:41 pm

If you can point me at some formulae, I'd be happy to give it a go.
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Gordon-Loomberah
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Re: Solar Data and "Max" Values

Postby Gordon-Loomberah » Mon Jan 31, 2011 1:40 am

Just dont use whatever Davis use for their calculation, it's got errors in it. It makes the plot of Solar Energy in Ly quite useless, with some days showing up to 2X other identical sunny days.
Loomberah Weather: http://gunagulla.com

whitenemesis
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Re: Solar Data and "Max" Values

Postby whitenemesis » Tue Feb 01, 2011 10:09 am

Mmm,, not sure how Davis do their calculations but the plots for Solar Energy, Solar Max and Solar Radiation are almost indistinguishable!

I think I'll import data into Excel and do my own conversions. I'd like to include data from my PV system anyway so may as well do it all together.

Thanks again for those links Steve Image

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Gordon-Loomberah
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Re: Solar Data and "Max" Values

Postby Gordon-Loomberah » Tue Feb 01, 2011 10:44 am

My solar energy plots are generally very similar to solar radiation, but sometimes there are very large extended peaks where the energy curve jumps up very high, sometimes for an hour or more at a time. I just ignore solar energy these days, given how flaky it is. I'm away at work and my home internet seems to be down the past 3 days, so cant post an example ATM.
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Re: Solar Data and "Max" Values

Postby paulsintes » Tue Oct 02, 2012 3:11 am

Solar readings.png
I have a similar problem. The Davis solar sensor on my station (http://www.lrl.co.nz/weather) consistently reads about 40% higher than the theoretical max for Christchurch NZ. Tweaking the trans factor does not and cannot solve the problem as the t/max for this area and time is quite correct according to a number of local met department solar sensors. The sensor on my station is over reading, rather than the trans factor under reading. So the Davis sensor on my station (and others I believe) is completely out of calibration. Davis don't seem to want to know about the problem and are reluctant to provide any useful data regarding their calibration source or whether indeed the sensor can be recalibrated. Their solution is to refer me to the local agents who know even less about measuring solar radiation. Apparently another sensor is on its way to replace the supposedly "faulty" one but I'm not holding my breath that this will solve the problem. Perhaps an attenuating or polarising filter might be the answer. I do believe however that if Davis are going to offer expensive solar add-on sensors they ought to be calibrated properly before they are sold.
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floggitt
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Re: Solar Data and "Max" Values

Postby floggitt » Tue Oct 02, 2012 7:13 am

Glad to see this thread as my readings are identical to everybody else's. Would it be possible to add a calibration factor in the setup options in cumulus. I have not had a chance to look yet (only had the station a week) and we dont see much sun here in Wales! but how does the data look in weather link?
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steve
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Re: Solar Data and "Max" Values

Postby steve » Tue Oct 02, 2012 7:54 am

paulsintes wrote:The Davis solar sensor on my station (http://www.lrl.co.nz/weather) consistently reads about 40% higher than the theoretical max for Christchurch NZ. Tweaking the trans factor does not and cannot solve the problem as the t/max for this area and time is quite correct according to a number of local met department solar sensors.
I have to admit I just changed my settings until my theoretical max curve looked reasonable compared to my actual readings on a clear sunny day, and the result stays pretty good throughout the year. I only use the readings for sunshine hours, and I get good results for that. The absolute solar radiation values mean nothing to me.

solar.png


Unfortunately I have nothing to compare my readings to; I assumed they were reasonably correct as my other Davis sensors give good results. Sorry If I'm teaching you to suck eggs (you probably know more about this than I do), but you are aware that the Davis sensors measure 'global' irradiance? What do the sensors that you are comparing your readings to measure?
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steve
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Re: Solar Data and "Max" Values

Postby steve » Tue Oct 02, 2012 7:55 am

floggitt wrote:Glad to see this thread as my readings are identical to everybody else's. Would it be possible to add a calibration factor in the setup options in cumulus.
How do you know that your solar readings are wrong? What are you comparing them to?
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paulsintes
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Re: Solar Data and "Max" Values

Postby paulsintes » Tue Oct 02, 2012 10:22 am

The solar constant or the maximum amount of solar radiation that reaches the top of our atmosphere at 90 degrees to the sun is about 1369 watts per square metre. The Ryan-Stolzenbach formula calculates the amount of radiation that can reach the surface of the earth at a given time and date due to various losses in passing through the atmosphere. This is the theoretical maximum possible radiation. The sensor can report less than t/max due to losses through cloud, fog, pollution etc. but cannot report more. Therefore if a sensor does read and report more than the theoretical possible maximum, by definition it must be either faulty or out of calibration. Altering the trans factor doesn't help when you are trying to establish the true amount of solar radiation falling on solar collectors under test.

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steve
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Re: Solar Data and "Max" Values

Postby steve » Tue Oct 02, 2012 11:04 am

paulsintes wrote:Altering the trans factor doesn't help when you are trying to establish the true amount of solar radiation falling on solar collectors under test.
I agree; to do that, as with any sensor, you need to compare with a calibrated sensor which measures the same thing, do you not?
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