The effect of dirt buildup

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drees
drees Solar Expert Posts: 482 ✭✭✭
Last week it was abnormally cool and we had a bit of light drizzle in the mornings for a couple days along with heavy marine layer.

When the sun started coming out again, I noticed that there was abnormally high variance in panel-to-panel output - over 15% when normally it's within 5%.

I climbed up on the roof and found that the bottom edge of the panels had collected a good deal of dirt and thanks to the very light rain was not enough to wash the dirt over the lip of the panels leaving a 1-4" heavy dust/dirt film. The nearly flat panel mounting of about 4* no doubt contributed to this - if the panels were mounted at even a 10* angle I imagine that this wouldn't be an issue.

With an old bath towel I was able to wipe the panels pretty clean in 15-20 minutes except for a few specs here and there. So at least the resolution was quick and easy. In the picture of the panels, I had already wiped the row on the right.

Don't read too much into the difference in power numbers before/after - time of day was different by some amount and the dirty day still had some lingering high clouds.

I estimate that the array as a whole was down about 8-9% comparing the pre-cleaning output to the same time today. That's actually better than I expected considering how ugly they looked!

Pre/post cleaning, the dirtiest panels dropped in voltage and jumped in current. The panels with the lowest output dropped 3-4 V (37-33 V for example) and current jumped almost 1.5 A (3.1-4.4 A for example). The panels with the highest output dropped about 2 V (35.5-33 V for example) and current jumped about 0.5 A (3.9-4.4A for example) - seemingly nearly unaffected by the dirt - down a couple percent. I wonder how a central inverter would have handled this big disparity in panel output?

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: The effect of dirt buildup

    For panels to be "self cleaning", that is why we recommend that panels should be mounted with at least a 5 degree tilt. Otherwise, you will have issues like you have seen.

    Vmp/Imp/Pmp curves are fairly flat and you probably would not see see the entire string output dropped by 10-15% with just one panel per string with dirt on it. But--with a central inverter, I would not have seen the variation to notice that some panels where dirty.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • drees
    drees Solar Expert Posts: 482 ✭✭✭
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    Re: The effect of dirt buildup
    BB. wrote: »
    For panels to be "self cleaning", that is why we recommend that panels should be mounted with at least a 5 degree tilt. Otherwise, you will have issues like you have seen.
    Yeah - currently they are mounted at 3-4* or so. Unfortunately limited by height restrictions because of my nearly flat roof - had to keep the panels under the height of the gable you can see to the left (picture was taken facing nearly south) to avoid permitting issues. If it becomes a big problem I'll have to see how I can get the panels tilted.

    But it's probably easier/cheaper to just add another 600W of panels and wait for heavy rains in the winter to wash things off rather than modify the mounts and space the panels out more!
    BB. wrote: »
    Vmp/Imp/Pmp curves are fairly flat and you probably would not see see the entire string output dropped by 10-15% with just one panel per string with dirt on it. But--with a central inverter, I would not have seen the variation to notice that some panels where dirty.
    Right - but if you can see from the pics the amount of dirty varied a good deal on each panel and affected the panels much differently - some it seems to be only a couple percent - others it was nearly 20%! Average seemed to be around 10%.
  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
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    Re: The effect of dirt buildup
    drees wrote: »
    Yeah - currently they are mounted at 3-4* or so. Unfortunately limited by height restrictions because of my nearly flat roof - had to keep the panels under the height of the gable you can see to the left (picture was taken facing nearly south) to avoid permitting issues. If it becomes a big problem I'll have to see how I can get the panels tilted.

    I think here there is laws that prevent anything from impacting the efficient use of the solar resource.

    here is snippet of my HOA letter:
    Below are the Arizona State Statutes for your reference:
    33-439. Restrictions on installation or use of solar energy devices invalid; exception
    A. Any covenant, restriction or condition contained in any deed, contract, security agreement or other instrument affecting the transfer or sale of, or any interest in, real property which effectively prohibits the installation or use of a solar energy device as defined in section 44-1761 is void and unenforceable.
    B. A deed, contract, security agreement or other instrument affecting the transfer or sale of, or any interest in, real property entered into before April 17, 1980 shall not be subject to the provisions of this section.
    http://www.azleg.gov/ars/33/00439.htm
    33-1816. Solar energy devices; reasonable restrictions; fees and costs
    A. Notwithstanding any provision in the community documents, an association shall not prohibit the installation or use of a solar energy device as defined in section 44-1761.
    B. An association may adopt reasonable rules regarding the placement of a solar energy device if those rules do not prevent the installation, impair the functioning of the device or restrict its use or adversely affect the cost or efficiency of the device.
    C. Notwithstanding any provision of the community documents, the court shall award reasonable attorney fees and costs to any party who substantially prevails in an action against the board of directors of the association for a violation of this section.
    http://www.azleg.gov/ars/33/01816.htm

    44-1761. Definitions
    In this article, unless the context otherwise requires:
    1. "Collector" means a component of a solar energy device that is used to absorb solar radiation, convert it to heat or electricity and transfer the heat to a heat transfer fluid or to storage.
    2. "Heat exchanger" means a component of a solar energy device that is used to transfer heat from one fluid to another.
    3. "Solar daylighting" means a device specifically designed to capture and redirect the visible portion of the solar beam spectrum, while controlling the infrared portion, for use in illuminating interior building spaces in lieu of artificial lighting.
    4. "Solar energy device" means a system or series of mechanisms designed primarily to provide heating, to provide cooling, to produce electrical power, to produce mechanical power, to provide solar daylighting or to provide any combination of the foregoing by means of collecting and transferring solar generated energy into such uses either by active or passive means. Such systems may also have the capability of storing such energy for future utilization. Passive systems shall clearly be designed as a solar energy device such as a trombe wall and not merely a part of a normal structure such as a window.
    5. "Storage unit" means a component of a solar energy device that is used to store solar generated electricity or heat for later use.
    http://www.azleg.gov/ars/44/01761.htm
  • drees
    drees Solar Expert Posts: 482 ✭✭✭
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    Re: The effect of dirt buildup

    It's not the HOA that will have the issue, it's the city who will throw up additional permitting requirements if I exceed the current height of the permanent structure.

    Perhaps I should have just dealt with it then - oh well, live and learn! Not really a big deal to have to climb up on the roof once or twice a year to wipe the off - as I said, it only took 15-20 minutes. Takes longer to drink my morning coffee. :)