Solar Radiation

boisblancboyboisblancboy Solar Expert Posts: 131 ✭✭✭✭✭
Obviously solar panels need unobstructed sunlight to produce their highest output. This may be a stupid question and I may be wording it incorrectly as well but I will give it a try.

I understand that clouds/shade are bad and full sun/sunny day is optimal. I am wondering if there are certain atmospheric conditions that would cause your output to be lower even on a bright sunny day? Are there any sites that would be able to give you a forecast of what is coming or is currently happening?

Im not looking for the average full sun hours a certain area recieves. Anyway, hope i asked that right and didnt confuse anyone.:confused:

Thanks

Comments

  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Solar Radiation

    Hot day, (high PV temperatures) haze, smoke, light cloud, partial shading will all diminish output. High relative humidity will diminish output as well. Low solar angles,, near sunrise/sunset, or high latitude exacerbate most of these issues as the the sun has to shine through more atmosphere because of the angles.

    Crystal clear cold give you the best yield all things being equal. Edge of cloud events can dramatically improve output momentarily. Reflection off of snow can have a considerable positive effect as well.

    Hope this answers you question,

    Tony
  • boisblancboyboisblancboy Solar Expert Posts: 131 ✭✭✭✭✭
    Re: Solar Radiation

    Yeah that answers some questions. How about a weather forecast site that is directed toward us solar power consumers? If something even exsists like that, besides getting a weather outlook of mostly clear and sunny. Just curious.

    Another random question. During solar storms is there any small time of increased output?
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Solar Radiation

    You might try Wunderground and use the UV index as an analog for solar intensity.

    Wunderground also has (at least for my area) some private sites that have solar irradiation measurements (next to the search box, pull-down "personal weather stations").

    Here is one example near me.

    It is nice because it includes both Solar Irradiation and UV index numbers... You watch/sample over time and see how well this works for you.

    Here is a UV forecast site from the EPA.

    You might have to play with it--For example, use x/5 * your system clear day output where x=<5... Where x>5, set it to 5... This would help adjust for the differences in UV blocking by the atmosphere due to lower sun angle.

    After a couple of months, you might have a pretty good model to fit your system.

    Or, just look at the cloud forecast and guess if your system is clear=x,xxx watt hours per day, and if it is less, then 1/2 for cloudy weather and 1/10 for stormy weather (just guesses from what I have seen with my system).

    I am not sure how accurate you can get with this... In the end, watching closely your battery's state of charge (a battery monitor is the way to go here) is going to be most important. If the state of charge falls below ~75%, then you should charge in the next day (wait for sun, or fire up the generator).

    If you get to 50% state of charge, then fire up the genset and recharge to at least 80% and let the sun finish charging or run the generator up to ~90% SOC if no sun will be avialable in the "near" future (more than 24 hours to clear weather, for example).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Solar Radiation
    Another random question. During solar storms is there any small time of increased output?

    Not so that you can accurately measure it on the ground. Around 0.1% average variation.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • boisblancboyboisblancboy Solar Expert Posts: 131 ✭✭✭✭✭
    Re: Solar Radiation

    Well it should be interesting to record some information and see what correlation it might have.
  • mike95490mike95490 Solar Expert Posts: 9,361 ✭✭✭✭✭
    Re: Solar Radiation

    And there are certain cloud conditions that can focus light too.

    After seeing this in person, in a plane, and taking my own photos with my cell phone cam (yep, my photos are as bad as you think they could be) I found this one on the web, great photo of the effect:
    http://www.coasttocoastam.com/gen/page2781.html?theme=light
    (dang site re-arrangement, pic is gone)
    Note the intense center part of the lighting, and how it fades to "average" outside the rainbow area.

    Apparently, a common name for this effect is "Cloud Glory" which explains why I look up lensing or whatever, I got nothing.
    Full page, tech data, more links, start here:
    http://www.atoptics.co.uk/droplets/gloim1.htm

    A bit more research leads me to another name: Heiligenschein
    http://www.atoptics.co.uk/droplets/heilig.htm

    Opposition Effect appears on the airless moon, as the photo in the
    link below shows dramatically.
    http://www.atoptics.co.uk/atoptics/oppos1.htm
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • boisblancboyboisblancboy Solar Expert Posts: 131 ✭✭✭✭✭
    Re: Solar Radiation

    You know I have seen that a few times, but of course didnt know anything about it!
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Solar Radiation

    You probably have several effects going on there...

    One is the edge diffraction effect--light being bent around the outline of the aircraft (sort of like a lens) and focusing into the shadow area (not very visible in this photo)... In theory, I guess, the changes in angles of sun could cause more rainbow rings--usually I would only expect one bright and one dimmer rainbow effect).

    The other has to do with the index of refraction of water. Multiple rainbows can be seen if conditions are correct.

    A glory seems to be a combination of multiple effects:
    A glory is an optical phenomenon appearing much like an iconic Saint's halo about the head of the observer which is produced by light backscattered (a combination of diffraction, reflection and refraction) towards its source by a cloud of uniformly-sized water droplets. The association with a halo is not coincidental, but derivative, though a real glory has multiple colored rings. Most people see only one ring. The glory, however, can show many rings when the cloud is made of uniform water droplets. Sometimes the rings fluctuate wildly in size. This happens when a plane, for instance, skirts a canyon of clouds and its glory shadow comes and goes.[1]

    The angular size is much smaller than a rainbow, about 5° to 20°, depending on the size of the droplets. Since it is seen in the direction opposite the sun, it is most commonly observed while airborne, with the glory surrounding the airplane's shadow on clouds (this is often called The Glory of the Pilot). The phenomenon is also known as an anthelion.
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Solar Radiation

    hey, i once saw 7 rainbows at one time.:D and no i wasn't smoking anything.:p i did see this for real and it was roughly looking to the north that i had seen them. i never did find those leprechauns.:grr
  • audredgeraudredger Solar Expert Posts: 272 ✭✭
    Re: Solar Radiation

    Mike, The effect is called " UFO " effect. LOL
  • mike95490mike95490 Solar Expert Posts: 9,361 ✭✭✭✭✭
    Re: Solar Radiation

    NASA today, lists a photo of one of their planes, with a Cloud Glory too.

    http://earthobservatory.nasa.gov/IOTD/view.php?id=46980&src=eoa-iotd
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • RCinFLARCinFLA Solar Expert Posts: 1,394 ✭✭✭✭
    Re: Solar Radiation

    Just to put the perspective bounds on the levels, just outside the atmosphere there is about 1400 watts per square meter. Best at sea level runs 1000-1200 watts per square meter.

    Silicon PV cells like orange-red to high IR wavelengths so blue to UV radiation doesn't do much other then perhaps contribute to panel heating.

    High altitude, cool, clear skys would be nice.
  • SlimDieselSlimDiesel Registered Users Posts: 17
    Re: Solar Radiation

    Back to the OP...

    While this site is used mostly by astronomers to forecast coming night-time conditions, it would also work for daylight because the underlying supercomputer prediction maps its based on don't distinguish day from night.

    Will work for all North American locations.
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