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  • FishGun
    replied
    Sensij:
    Yeah and to be honest I didn't pore over his numbers...didn't watch the vid...just took a quick swag at the delta and your point about the .4-.5% loss. It fits. I probably shouldn't have commented.

    But beyond the exact details of his experiment, I think there are reasonable gains waiting to be squeezed-out by a good idea.

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  • russ
    replied
    Originally posted by sensij View Post
    Fair enough, but water used in this way will never pull the temp down by 20 deg C. Here is an example of a misting system using a whole lot more water that achieved 2 deg of cooling. Any water that recirculates will rise in temperature without an additional heat exchanger to cool it back down, further defeating the cooling power of the system.
    Not nice to confuse them with facts - conjecture is so much more fun!

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  • sensij
    replied
    Originally posted by Ian S View Post
    I believe the O.P. is talking ambient temperature of 35C not panel temperature. Panel temperature can easily run 20 C deg higher under full mid day sun and calm wind conditions.
    Fair enough, but water used in this way will never pull the temp down by 20 deg C. Here is an example of a misting system using a whole lot more water that achieved 2 deg of cooling. Any water that recirculates will rise in temperature without an additional heat exchanger to cool it back down, further defeating the cooling power of the system.

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  • Ian S
    replied
    Originally posted by sensij View Post
    20-25 deg C? Really? If the system is operating at 35 deg C as the poster suggested, that would require cooling to 10-15 deg C, or 50-60 deg F. That is not going to happen with the method proposed.
    I believe the O.P. is talking ambient temperature of 35C not panel temperature. Panel temperature can easily run 20 C deg higher under full mid day sun and calm wind conditions.

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  • sensij
    replied
    Originally posted by FishGun View Post
    About 10% gain...so prob a 20-25deg reduction...seems reasonable.
    20-25 deg C? Really? If the system is operating at 35 deg C as the poster suggested, that would require cooling to 10-15 deg C, or 50-60 deg F. That is not going to happen with the method proposed.

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  • FishGun
    replied
    Slick. Thanks for sharing. I think the on-off graph is more of a seller than the 2 diff days overlayed.
    About 10% gain...so prob a 20-25deg reduction...seems reasonable. It pretty much always comes down to cost to purch/install/run/maintain. If you can do it for less than the cost of 5-15% increased output than it's a winner. If not, it's still fun to mess with!

    I've seen about 15% increase in output washing/spraying my panels in midsummer. Also have seen the highest output of the year in March...suns at pretty good angle and temps are cold, but some of the gains are from the sun reflecting off of snow too.

    I've been toying with something similar for my array as well...or at least trying to figure a way to cool cheaply. Alum fins, alum duct, alum fins with circulating fluid, misting the back of panel, dripping the back panel? Can't decide which to do. Or if any.

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  • sensij
    replied
    Originally posted by bouncintigger View Post
    I have more data than I know what to do with, and believe me it all points to massive power gains when the cooling system is on. Here's what happens when I pulse the system on / off in short intervals:

    [ATTACH=CONFIG]5744[/ATTACH]
    These data would be more compelling if the time that the water was turned on and off was overlaid on the chart. The idea that "massive" power gains are available is misleading... the temperature coefficient of the panels is published on most data sheets, and is something around -0.43% / deg C for many panels currently in production. A (substantial) 10 deg C reduction in panel temperature would be expected to result in an improvement of a 4.3% in power.

    The method of cooling used here is hard to understand. By running liquid water over top of the panels, some amount of sunlight will be reflected or adsorbed by the water, reducing the transmitted irradiance. I would guess that there is some phase shift in the pulsed data shown here, where the "peak" shown occurs after the water has turned off, letting the full solar irradiance hit the panel, but generating higher power from the cooler panels. The magnitude of the peak may be bigger than what would be predicted from panel temperature coefficient alone because of the fact that the power in the "trough" of the pulse may be artificially depressed by the blocking effects of the liquid water.

    A more effective way to cool the panels, I think, would be to mist them from the back side. Let nothing come between the sun and the panel, and by misting, it will push the cooling mode more towards evaporation than conduction. With a pan underneath, any excess liquid water could still be collected and reused. I am still doubtful that even that improved cooling scheme would be a net benefit.

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  • russ
    replied
    Analyze BS and you get more BS. The gains are vaporware

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  • DanS26
    replied
    Commercial company already gone down this path....although in a little different manner.....

    http://www.szna-usa.com/pv-thermal.html

    Leave a comment:


  • J.P.M.
    replied
    Enjoy your learning experience. Good luck.

    Leave a comment:


  • bouncintigger
    replied
    Originally posted by J.P.M. View Post
    There are a lot of environmental variables that affect system output. Maybe a fundamental blunder: While I believe your numbers, I'd wager a guess you do not have sufficient instrumentation to justify or explain the diff. in output you describe. Maybe the 25th was windier and/or from a diff. direction for example. Or, perhaps the ambient temp. profiles as f(time) were different. I can get 5% diff. in output on consecutive seemingly very close, cloudless insolation days on what seems to be wind vector diff. alone. Also, know that what your eye may be telling you are identically "sunny" days may, and commonly do have insolation levels that are different by a couple % or more.
    I have more data than I know what to do with, and believe me it all points to massive power gains when the cooling system is on. Here's what happens when I pulse the system on / off in short intervals:

    Peaks.jpg


    Originally posted by J.P.M. View Post
    Also, no way to tell, but as a 1st approx., depending on wind, dew point and a few other things, probably something like half any additional cooling that is achieved by your scheme is probably achieved is through evaporation.

    FWIW, I'd put the reservoir on a scale and keep track of the tare weight.
    You're missing the point. Evaporation is the aim of the system, not a problem to overcome! The biggest problem is losing water onto the tiles. The latent heat of evaporation required to balance the steady state sensible heat we're trying to remove amounts to a relatively small amount of water. I have tracked my water consumption (at the Watercorp meter... scales on the tank obviously won't work as it's always kept full with the supply float valve) and as I said, it's ~ 500 mL/min when the ambient temperature is ~ 35°C. Most of the time, the net water use is much less than that.

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  • J.P.M.
    replied
    Originally posted by bouncintigger View Post
    One more thing. You actually want evaporation rather than collection and recycle. The perfect system would not have any water collected and recycled, with all of it evaporating. The latent heat of evaporation far exceeds the heat capacity of the water. Dry air is better than humid air at any given temperature.
    Have fun with this system, but I'm of the opinion you're not gaining much, if anything in terms of a net increase in overall efficiency - As Sunking already noted, more than likely the parasitic energy far outweighs any increased panel efficiency from any lower panel temps achieved.

    There are a lot of environmental variables that affect system output. Maybe a fundamental blunder: While I believe your numbers, I'd wager a guess you do not have sufficient instrumentation to justify or explain the diff. in output you describe. Maybe the 25th was windier and/or from a diff. direction for example. Or, perhaps the ambient temp. profiles as f(time) were different. I can get 5% diff. in output on consecutive seemingly very close, cloudless insolation days on what seems to be wind vector diff. alone. Also, know that what your eye may be telling you are identically "sunny" days may, and commonly do have insolation levels that are different by a couple % or more.

    Also, no way to tell, but as a 1st approx., depending on wind, dew point and a few other things, probably something like half any additional cooling that is achieved by your scheme is probably achieved is through evaporation.

    FWIW, I'd put the reservoir on a scale and keep track of the tare weight.

    Leave a comment:


  • bouncintigger
    replied
    One more thing. You actually want evaporation rather than collection and recycle. The perfect system would not have any water collected and recycled, with all of it evaporating. The latent heat of evaporation far exceeds the heat capacity of the water. Dry air is better than humid air at any given temperature.

    Leave a comment:


  • bouncintigger
    replied
    Originally posted by Sunking View Post
    Couple of issues of simple physics.

    1. The energy used to cool the panels is more than the extra power produced so you have net negative gain.
    2. I understand why you are using a water softener to prevent scale build up, but the trade off anyone that lives near a sea is salt water is hell on panels. You are trading Calcium and Magnesium for Sodium Chloride (salt).

    Unfortunately you are not the first to go down this road. Many have gone down this road before you and discovered the gain does not offset the energy used to run the pumps. That is why you don't see any systems for sale. Then you got the environmental whackos would eat you alive because you are wasting considerable amounts of water, especially in Perth where you have extremely low humidity means most of the water evaporates before you can collect it and recycle.
    Thanks for the feedback, but I have to disagree with you:

    1: A like for like comparison of hot day with and without cooling enabled, without any cloud cover whatsoever:
    32.8 kWh (12th January - 35°C max temperature) without cooling system running
    36.8 kWh (25th December - 32°C max temperature) with the cooling system running

    Comparison.jpg

    I use two 4L/min diaphragm pumps for this system, that have a maximum rated power of 31.2W each. The two temperature switches are ~ 1W each so you can ignore them. The pumps typically only run 50% of the time in the blistering sun and much less before 10am and after 2pm. Even if they were both running flat out, non stop for 12 hours in the day, they would only consume 0.75 kWh, but I've measured the load with a portable power meter and the typical daily consumption for the whole system is ~ 0.1 kWh.... negligible really.

    As for the water, you make a valid point that the water consumption is high. Currently I'm using ~ 500 mL/min net during the blistering hot part of the day. The vast majority of this though is not evaporative, but drift loss wasting going onto the tiles. I expect I'll be able to at least halve this consumption with some tweaks to the setup. Even if we assume 500 mL/min over 6 hours, this is only 120 L/day or ~ 15c. As I said, I expect to be able to get this down by half with some tweaks to the setup. When comparing this against the 4 kWh gain in PV output, the cost of the water is trivial (although yes yes... water is precious).

    Regarding the water filter, you're mistaken about the chloride and how the filter works. The resin is a cation exchange resin and doesn't load chloride at all. The regeneration process in NaCl or HCl only exchanges Mg2+ and Ca2+ with either Na+ or H+. The chloride remains in the backwashing tank. Provided the filter is suitably flushed prior to using the water on the roof again, there won't be any more chloride in the water than there is in the scheme supply (my house is ~ 200 ppm from a Mohr titration test). The only ions you'll get on your roof if you run the filter correctly, are Na+ (or H+), and sulphate / carbonate / bicarbonate. In the case of H+ regeneration, we're only talking ~ 150 ppm, and there's more than enough basic carbonate to neutralise the acidity. The water will be exceptionally pure and you'll have no chloride corrosion troubles.

    If I've missed something, please put me straight, but as far as I can tell, the number stack up....

    Thanks for the input though I really do want to know if there's a fundamental blunder I've made so please don't let this post dissuade you from trying to convince me further!

    Leave a comment:


  • inetdog
    replied
    Another point of simple physics affects those who want not only to cool their PV panels, but to make use of the heat removed from the panels at the same time.
    Sort of "Can I attach a solar thermal panel to the back of my PV panel and use it for heating?"
    The point is that for heating you want to get the thermal part of the panel as hot as possible, while you want that same fluid loop to cool the panel to the lowest practical temperature. For any application except pool heating, the two optimal temperatures are so far apart that any practical compromise will not in fact be practical.
    For pool heating, you have the problem of the corrosive effects of pool water, and I have still not heard from anyone who has made this application work.

    Leave a comment:

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