wind powered solar cooling

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christian09
christian09 Registered Users Posts: 22 ✭
in Solar Beginners Corner #1
I am reposting the same message, because it may apply to 2 separate topics.
Hi,
I live in South Florida, where our temps rarely go below 80 F and in the summer(5-6 mos.) its 92 F with a near 100% humidity.
I am trying to find a way to effectively cool my roof mounted solar panels. I have noticed on cooler and windier days(this January), my solar output has increased at least 10%-15%. I have a suspicion that when the real heat comes, I will have a loss nearing 20% due to heat.
I have read that our wet bulb temp is way to high for the water based cooling systems used in the deserts. I have been looking at the venturi effect to increase wind speeds by 3-4x, so the "cooler" 90 F wind blows harder underneath my panels, and constantly eliminates the 120+ air under the panels. One way is to create some kind of set of tapered pipes to fit under each panel. Theoretically, this should increase any "cooler" incoming wind into the underside of the panels, thus increasing air pressure, and moving the hot air out quicker. Maybe have them direct the air towards the top of the array.
My other idea is to make some solar powered fans, place them at the bottom of the array(pointed toward the apex of the roof). I would then place some kind of venturi "tube" in in front of the fans, to exponentially increase the wind speed. Obviously, I would make these removable, since in FL we have tropical storms, and I don't want 200 mph winds taking my panels for a ride. I would appreciate any input, and/or ideas on cooling my system, in such a unique climate.
Thanks
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: wind powered solar cooling

    Although the concept has merit (especially if the air movement is driven by fans so that it can be of a known velocity) it does have some possible faults. The first being if you will get enough temperature difference to increase thermal transfer to a rate that is actually beneficial. The second is that you would need more panel to power the fans. If you take this power from the existing array you'll run in to the old "takes out more power than it gains" problem. If you power the fans from a separate array you may still be gaining less than you get. It would be a matter of how efficiently you can harvest the solar and apply it to moving enough air to reduce the panel temp.

    All in all I'd say it would make an interesting experiment, but it's doubtful there would really be any net gain. The air movement to be had from thermal syphon will be minimal, and probably not enough to effectively cool the panels (which operate much above ambient temperature).

    I'm sure Inetdog can give us some insight to this as well.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #3
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    Re: wind powered solar cooling

    If the panels are >6" off the roof--There is nothing else really for you to do that is cost effective or practical (that I have seen so far).

    If your panels are <3" off the roof, lifting the rack could help significantly due to improved air flow/cooling.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #4
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    Re: wind powered solar cooling
    Cariboocoot wrote: »
    All in all I'd say it would make an interesting experiment, but it's doubtful there would really be any net gain. The air movement to be had from thermal syphon will be minimal, and probably not enough to effectively cool the panels (which operate much above ambient temperature).

    I'm sure Inetdog can give us some insight to this as well.
    Why, thank you Coot, I will give it a try. First number your paragraphs. :-)

    1. To analyze this in detail, you need to know more information than we have so far.
    2. The ambient temperature is 90 F. OK. The air temp between the panels and the roof is 120 F. OK. Now, what is the temperature of the free air right above the panels and what is the temperature of the panels themselves?
    3. Without knowing the panel temperature (top surface and bottom surface) and the temperature coefficient of power for these panels, it is hard to evaluate the benefits of increasing air flow, etc. If the air above the panels is close to 90F (with no captive layer of warm air) the best you could do without flow effects is reduce the difference between panel temp and air temp by 1/2. What is that worth in terms of power?
    4. If the limiting factor is the transmission of heat across the interface from the panel to the air, rather than the temperature of the air, then increasing air velocity at either top or bottom surface could help a lot more than just a factor of two.
    5. Putting up more panels to meet the input capacity of the rest of the system is likely to be the most cost effective measure you can take. As Bill noted, without other measures increasing the panel to roof spacing becomes less effective after 6".
    6. What is the slope of the roof? For very shallow slopes, more distance may help proportionally more, as well as open space between rows of panels.
    SMA SB 3000, old BP panels.
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  • christian09
    christian09 Registered Users Posts: 22 ✭
    #5
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    Re: wind powered solar cooling
    inetdog wrote: »
    Why, thank you Coot, I will give it a try. First number your paragraphs. :-)

    1. To analyze this in detail, you need to know more information than we have so far.
    2. The ambient temperature is 90 F. OK. The air temp between the panels and the roof is 120 F. OK. Now, what is the temperature of the free air right above the panels and what is the temperature of the panels themselves?
    3. Without knowing the panel temperature (top surface and bottom surface) and the temperature coefficient of power for these panels, it is hard to evaluate the benefits of increasing air flow, etc. If the air above the panels is close to 90F (with no captive layer of warm air) the best you could do without flow effects is reduce the difference between panel temp and air temp by 1/2. What is that worth in terms of power?
    4. If the limiting factor is the transmission of heat across the interface from the panel to the air, rather than the temperature of the air, then increasing air velocity at either top or bottom surface could help a lot more than just a factor of two.
    5. Putting up more panels to meet the input capacity of the rest of the system is likely to be the most cost effective measure you can take. As Bill noted, without other measures increasing the panel to roof spacing becomes less effective after 6".
    6. What is the slope of the roof? For very shallow slopes, more distance may help proportionally more, as well as open space between rows of panels.
    I will get the rest of the info for you...thank you. I think the temp. coefficient is this?
    NOCT 46 °C
    TC Isc 0.034 %/K
    TC Voc -0.34 %/K
    TC Pmpp -0.48 %/K
    Operating range -40°C to 90°C
    thanks again?
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  • christian09
    christian09 Registered Users Posts: 22 ✭
    #6
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    Re: wind powered solar cooling
    BB. wrote: »
    If the panels are >6" off the roof--There is nothing else really for you to do that is cost effective or practical (that I have seen so far).

    If your panels are <3" off the roof, lifting the rack could help significantly due to improved air flow/cooling.

    -Bill

    Thanks BB, my panels are around 4", supposedly due to hurricane code.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
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    Re: wind powered solar cooling

    So what you're looking at is trying to reduce the actual operating temp (the NOCT 46C/115F rating may be low in fact) down to the STC rating of 25C/77F (or lower than that). It is doubtful this can be accomplished with air movement alone, as panels really aren't very good at conducting or radiating heat. You need quite a thermal difference to have any noticeable effect. Winter temps, for example, can cause panels to super-conduct and increase Voltage significantly. Sometimes too much.

    If you could gain that further 2" of standoff it would go a long ways to improve things because it will greatly decrease the amount of heat being reflected back to the panels by the roof.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #8
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    Re: wind powered solar cooling

    Remember, practically speaking, we are probably talking about a 5-10% maximum recovery in power by cooling the panels--At best. Just at the boundry of even measuring the difference without better instrumentation and a reference cell to get the differences due to cooling vs all the other effects (amount of sun light, ambient temperature, dirt on panels, aging, humidity, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • christian09
    christian09 Registered Users Posts: 22 ✭
    #9
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    Re: wind powered solar cooling
    Cariboocoot wrote: »
    So what you're looking at is trying to reduce the actual operating temp (the NOCT 46C/115F rating may be low in fact) down to the STC rating of 25C/77F (or lower than that). It is doubtful this can be accomplished with air movement alone, as panels really aren't very good at conducting or radiating heat. You need quite a thermal difference to have any noticeable effect. Winter temps, for example, can cause panels to super-conduct and increase Voltage significantly. Sometimes too much.

    If you could gain that further 2" of standoff it would go a long ways to improve things because it will greatly decrease the amount of heat being reflected back to the panels by the roof.

    I cannot do this due to hurricane codes. Yes, yesterday we had a bone chilling day time temp in the 50's and my panels were 20% higher than any day this month. I just wish I could find a way to cool. I have free well water on my irrigation system, so maybe a little misting may help me in the lower humidity months. Temp is still in the 80's, but humidity is way down, until about May/June-Sept. What do you think?

    thank you.
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  • christian09
    christian09 Registered Users Posts: 22 ✭
    #10
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    Re: wind powered solar cooling

    I will get these data points. What is the best way to obtain them? I have a wonderful harbor freight laser temp guage?
    thanks
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
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    Re: wind powered solar cooling

    Misting has been tried. Two problems: one being the energy needed to pump the water up to the roof and on to the panels exceeds the power gained, two being the water on the panels itself can "lens" causing hot spots - and it leaves mineral residue behind.

    Anything likely to drop temperatures enough to be effective is also likely to consumer more power than it produces as panels themselves simply aren't very efficient.

    No doubt someone will suggest:

    1). IR blocking film on the panels;
    2). Peltier-Seebeck powered cooling fans;
    3). Thermal refrigeration system;
    4). Other scheme that basically ignores physics. :p

    What seems to work in theory too often falls apart when put into practice due to lack of insight into the ramifications of the whole. Some of it might work under the right circumstances, but most of the time would not so you'd have whatever mechanism sitting around doing nothing (even if operating).

    Maybe a large install with 20% efficient panels in a relatively stable environment that could be predicted and compensated for ...

    Given the cost of panels these days its cheaper to accept the losses and add 10% to the array size.
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