Is that what is lowering the values so much?? Are you aware of any research on atmospheric transmissivity variation based on surface relative humidity? If that’s the case, shouldn’t the model for ‘expected maximum’ take into account the current humidity in the calculations?
Thanks for any insight on this!
Best regards,
Ken
The calculation has one value plugged in for atmospheric attenuation, that’s the number you can change in WD, and really that needs to be customized for the location. I found a lot of papers this morning regarding this when I searched but nothing that I had time to read. I don’t believe it takes altitude into effect.
None of the models I looked at when I wrote this routine had any customisation for the actual location, either altitude or anything else. The attenuation factor used as default is the one I found at the time to be used in the calculations. As I said I’ll have a play and see what I get.
Stuart
I would assume that adjusting that factor would be better than just reducing the calculated result, because it would take into account the length of the path through the atmosphere at varying elevations.
I have changed my private copy of the maxsolar script to allow different attenuation factors to be tried. The actual calculation for the reduction in max radiation possible is
$atten = $maxradposs * $attenuation * (1 - sin($sun_elev));
$maxrad = $maxradposs - $atten;
so you can see that the max radiation is first calculated then multiplied by the attenuation factor and the sun elevation comes into it as well, this is logical since the distance travelled through the atmosphere will alter as the sun traverses the sky. Now the confusion factor here when comparing the attenuation factor used with the one displayed on the solar control panel is that Brian displays the digits after the decimal point, so a value of 5 is actually 0.5, at least it should be - Brian will have to confirm that.
I seem to remember when I was writing this code that I had some discussions about elevation and concluded that you would have to be very high for it to have any great extent, that is you would need to be on top of a mountain before you would need to alter anything. I believe Brian externalised the attenuation factor to allow those (and there were one or two) living at altitude who found their actuals were higher than expected to adjust the expected values.
Stuart
I’m on the east coast and have the same issues in the summer (~75% of expected). 950~1000 W/m2 is a typical clear sky value in June with low dewpoints and I have found in comparison to other sensors at similar latitudes this is pretty much the normal. We get really heavy haze at times (75 F dewpoints) and I find that it really only attenuates the solar about 8~12% of the clear sky value (UV is not effected very much at all).
The only way to get to the 1000+ range is with large white cumulonimbus clouds as this adds the extra couple 100 w/m2 as diffuse radiation (no clouds - nothing to reflect off of).
What I can’t figure is that once we get into Fall my values approach 100% for clear sky values throughout the entire day. It seems to me that the expected values are not correct as the sun elevation changes from season to season. Expected is low in the Fall and high in the Summer with the rest in between. Someone with more experience with solar readings pointed this out to me last year and I found this to be true.
As I have said before the calculation is based on the stuff used in various scientific websites in the USA and elsewhere. Until we can identify some changes to these equations which have been verified by the experts I am loathed to suggest we change the way it works. Take a look at the values for your location given here and compare with what my script or WD gives (using Extraterrestrial Global Horizontal Solar Irradiance (W/m2)). Let us know what you find. This may give some clues as to what is happening.
Stuart
Stuart,
Thanks - good site! Just by looking at the numbers I would say the site, your calculator, and WD closely agree with the values. I check my readings against this site: http://www.whoi.edu/mvco/description/SolRad.html. The site is a few degrees north of me but they use an Eppley Model PSP (Precision Spectral Pyranometer) which I believe is a step up from our VP2 sensors. As long as our weather is similar my readings match fairly well.
I don’t have any suggestions at this time on why we don’t match the experts. I just wanted to mention I’m seeing the same values as others.
So today in Christchurch, NZ, it’s a perfect cloudless blue sky day, beautiful clear atmosphere (as far as I can tell), yet my solar sensor is sitting at a pretty constant 88.6%. I can’t imagine solar conditions ever get much better than this so I guess this means I should apply an adjustment factor of about 89% which should get me very close to achieving an adjusted 100% reading?
Cheers,
Simon
Oops :oops: I think I got this the wrong way around. I’ve re-adjusted the sensor to 110% and now my solar % is reading approx 98%. I’ll keep an eye on this to see if it ever goes above 100% with this revised setting.
OK, so now I’m confused :?. I went to Stuart’s link http://www.nrel.gov/midc/solpos/solpos.html and punched in my lat and long, today’s date etc. I checked this twice to be sure I wasn’t screwing up the coordinates. The data that comes back states that at 3:20pm the max theoretical solar gain is 539 W/M2. Yet my logs show the actual solar gain recorded was 411 W/M2. From memory (because I can’t find where this is logged - does anyone know if it is logged?) the % of maximum was showing around 90%, whereas from the figures above it should have been more like 76%.
I’ll try to monitor this over the next few days, to see if this was just an anomaly.
Simon
Simon to log all this solar data you really need a custom log file. See this thread for details of how to set it up. You can then review all the solar data, expected, actual percentage and hours of sunshine. This will allow you to do comparisons with that site I mentioned as well as my maxsolar.php script.
Stuart