cooling solar panels

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lotek
lotek Solar Expert Posts: 65 ✭✭✭✭
in Solar Beginners Corner #1
Has anyone tried alternative methods for keeping PV panels cooler? Maybe data to support or negate methods? I was thinking of a soaker hose along roof to keep it wet in hotter weather.
I was told that water on the top of the panels(IE: sprinkler) reflected light and made them less affective. Im wondering if the trade off in temp for light might be worth it?

I understand that if you set your panels off the roof 4" it allows air flow to help cool panels, and that light colored roofing keeps roof temp down. '
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,461 admin
    #2
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    Re: cooling solar panels

    I tried spraying a hose on my roof panels on a hot day (95F+)--And I could not see a difference (I don't think I can even reliably see 10% difference on my GT inverter's output).

    I think Solar Guppy tried this on his array in Florida (and he as a real lab for measuring solar array performance)--and he did not find it worth the waste of water or pumping it.

    I believe the "rule of thumb" is 6" or greater air gap over roof surface for optimum panel cooling.

    Just wash your panels when they get dirty--although--I again, am hard press to see any gross improvements in power output with clean panels (using 10% as my detection level).

    The government's solar calculator uses 95% as the "dirty panel" average derating.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #3
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    Re: cooling solar panels

    People often forget that water, and the energy to move it is not "free" I can't imagine that the energy required to pump water up to say 30' on a roof would be more than recaptured by increased efficiency of the panels. As has been suggested in a similar thread,, if you incorporated cooling of the panels with heating of the water for a purpose you MIGHT see some gain,, but as has been pointed out,,the temperatures desired for either domestic hot water or space heat is greater than say +120f,, getting into a range where you might be reducing the efficiency of any PV.

    For all the trouble,, build a simple flat plate collector for hot water,, and get plenty of air under your PV.

    Tony
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,461 admin
    #4
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    Re: cooling solar panels

    And solar PV panels will still work when warmer (less than optimum mounting)--just 5-10% less power. Not ideal, but if an issue, get 10% more panels.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • AntronX
    AntronX Solar Expert Posts: 462 ✭✭
    #5
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    Re: cooling solar panels

    I thought about sealing the back of my panels with big piece of plexiglass and circulating water through that cavity to try and cool PV cells. I would pump cold water from the canal and let exaust water flow back. If extra power generated with cooler panels will be greater than power taken by the water pump, then it is worth it.

    What got me thinking about PV panel cooling is my observation on one very cold partly cloudy February day. As a cloud blocked the sun, panels cooled down close to ambient temperature. Then when that cloud passed and unobstucted sun got through, the charge controller reported 390 Watts out of 340W (rated) PV system. On a hot but clear day, I only get up to 270W at solar noon, due to high panel temperature.

    Now, what needs to be calculated, is how many gallons per minute will I need to effectively remove waste heat and how much power will that pump require.

    My two panels side by side occupy an area of 2.983 square meters. At 1000W/sq. m. that equals to 2.983 KW of solar radiation hitting the panels. Assuming i can get 390W of power out of them while cooling them, that leaves 2.593 KW of waste heat to be absorbed by cold incoming water.

    I do not know what temperature my incoming water will be, and I don't know how many GPM I can get with only 120W of extra power panels will generate while being cooled. Can someone find out how many GPH 120W can pump? Knowing that GPM rate, what temperature incoming water needs to be to remove 2.6KW of heat and for this experiment to break even?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,461 admin
    #6
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    Re: cooling solar panels

    AntronX,

    My back of the envelope calculations (1 digit accuracy):


    2,593 W * 3.41 BTU/Hr = 8,842 BTU/Hr.

    1 gallon of water weighs ~8lbs, 1 BTU is 1 lb of water changing 1 degree F...

    Estimate that you want a 10F rise in water temperature maximum.

    8,842 BTU/Hr. * 1/10F temp rise * 1/8 lbs per gallon = 110 GPH or ~2 GPM

    From this chart, assuming a 30' head and 5 GPM flow rate for low pressure circulation pump, solar powered. #35 was a 311 watt pump that required 400 watts of solar panels to run it...

    wind-sun_2055_5528020Conergy 7446-HT Suncentric Pump
    7446 With High Temperature Option
    Regular (MSRP) price: $1,165.47

    Now, we don't need high temp option, and perhaps can get away with a pump 1/2 this size...

    So--150 watts of power / 200 watts of solar panels to get a 120 watt gain--if all things are equal... Which they probably are not.

    There may be a limit to how much you can "cool" the solar cells from back side cooling due to relatively poor thermal transfer characteristics off the rear. There may need to be a relatively large temperature difference between the cells and through the rear to move much heat.

    From another thread, rear mounted solar thermal collectors are only 40% instead of 80% efficient for a "normal solar thermal collector".

    Assuming that a panel is 15% electrically efficient and 40% efficient cooling through the rear (this is for 120F+ water--so it cooling efficiency through the rear may be greater)--that would indicate that about 45% of the heat is being radiated out through the front of the panel. So--a wild guess would be you can reduce the solar cell temperature rise by roughly 1/2 with rear side cooling.

    An improvement, but not what you saw with a sun from behind cloud event.

    Anything I left out or where I made a major assumptional error? :roll:

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #7
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    Re: cooling solar panels

    Isn't the back of a PV panel where the J-box is or at the very least where the feed wires come out of the array? Some how trying to seal the back of the panel, keeping the water out of the systems,, sealing against thermal expansion and contraction just seems to be WAY more trouble than it is worth.

    Perhaps a different approach would be to do a trickle/drip system flowing down the roof into a circ tank,, drawing some of the heat off the panels,,

    For my money,, I would spend my money on more conservation first,,, and then another panel.

    Tony
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  • AntronX
    AntronX Solar Expert Posts: 462 ✭✭
    #8
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    Re: cooling solar panels

    Better to just add more panels for the price of that pump, hoses, plexiglass, time...
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #9
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    Re: cooling solar panels

    Mounting style will help manage PV array temperature rise. Figure on ~+35 degrees C if roof mounted, ~+30 C if rack-mounted over a flat roof, and ~+25 C if pole mounted. The difference between +35 C and +25 C is a power gain of ~4%.

    HTH,
    Jim / crewzer
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