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  • Sunking
    replied
    Originally posted by J.P.M. View Post

    Yea, pretty much. As I wrote, if it was practical at this time, it would have been done already. Maybe in the future. Hope springs eternal.

    I'd suggest a possible slightly different take based on the definition of what "works". Given enough resources, I can make most any energy scavenging or energy improvement work in the sense of "will it function ?" Whether it's cost effective, practical or serviceable and thus fit for purpose, and above all safe, is a different set of considerations in my book. To differentiate theoretically possible and every day practical often takes knowledge and experience.
    Know what this reminds me of? I think you will agree and get a laugh out of it. [B]Misting your AC condenser coils. [/B]

    Leave a comment:


  • ButchDeal
    replied
    Originally posted by solarix View Post
    I know the OP is having a fun time "improving" his solar system and it is worth it to him, but I guarantee you the return on investment is horrible to do this on a typical customer's install.

    yep much better return in simply adding more modules, and more production too.

    Leave a comment:


  • solarix
    replied
    I know the OP is having a fun time "improving" his solar system and it is worth it to him, but I guarantee you the return on investment is horrible to do this on a typical customer's install.

    Leave a comment:


  • J.P.M.
    replied
    Originally posted by Sunking View Post
    You would also need a control. Couple that with the equipment needed, is impossible for any DIY to determine. It is just way beyond their means and knowledge.

    Besides it has already been done a hundred times by peole who have the means and knowledge. Thus why there is no product out there for it. Does not work.
    Yea, pretty much. As I wrote, if it was practical at this time, it would have been done already. Maybe in the future. Hope springs eternal.

    I'd suggest a possible slightly different take based on the definition of what "works". Given enough resources, I can make most any energy scavenging or energy improvement work in the sense of "will it function ?" Whether it's cost effective, practical or serviceable and thus fit for purpose, and above all safe, is a different set of considerations in my book. To differentiate theoretically possible and every day practical often takes knowledge and experience.

    Leave a comment:


  • Sunking
    replied
    Originally posted by J.P.M. View Post
    I'm not knocking your efforts, but unless you've got a lot of test equipment to monitor before/after, side/side performance, I'd be careful about inferring too much.
    You would also need a control. Couple that with the equipment needed, is impossible for any DIY to determine. It is just way beyond their means and knowledge.

    Besides it has already been done a hundred times by peole who have the means and knowledge. Thus why there is no product out there for it. Does not work.

    Leave a comment:


  • J.P.M.
    replied
    Originally posted by ncs55 View Post

    As far as modifying the module frame, that would be inexpensive, and would make for a lighter weight module. And the modified module that I am experimenting with does run cooler and has a better production value than the un modified module. Natural upward convection would be the most cost effective way to cool as opposed to all of your other statements about forced cooling etc. The idea here is to let as much heat out from under the module without using anything that draws more electricity to operate it. As far as any wind under the array, that would only help to cool the modules. If you have been here and done this can you share your data with me? It would be helpful in my experiment.
    I'd suggest any frame modification might not be worth the effort. Structural considerations dealing with panel rigidity are more important than heat transfer considerations from improved air flow under a panel. Having at least 6" of clearance under a panel is probably more important, easier to achieve and more practical.

    I'm not knocking your efforts, but unless you've got a lot of test equipment to monitor before/after, side/side performance, I'd be careful about inferring too much. There's not a whole lot that's new in the heat transfer business, with most any practical and workable improvements to lower PV operating temps. and thus increase efficiency having been tried. Most large arrays and all residential ones I know of or have heard about do not use aux. cooling or radically modified designs to lower panels temps.

    My experience is that most schemes to improve efficiency via lower temps. come mostly from people who know little about solar energy engineering and even less about heat transfer.

    While not discouraging your efforts or enthusiasm, if there was a better, more practical and most importantly cost effective way to do it, it would have happened by now.

    As you and others note, the trick is to do it effectively, meaning safe, workable and cheap. We ain't there yet.

    As for natural convection, while important both above and below a panel mounted parallel to a roof, it will be quickly overcome by any forced convection of even a moderate wind, and is usually close to or about an order of magnitude less than the cooling that even a light breeze of about 1 m/sec. might cause. The obstructions under an array, standoffs, flashings, cabling, etc., may even enhance the heat transfer rate under an array by increasing turbulence, depending on the application and dimensional particulars.

    I'd suggest checking the open literature for information about natural (gravity induced) and forced (wind driven) convection and how it affects panel and array temps. Since my data for my location and array is more voluminous than can be transmitted here, and is mostly a confirmation of what's already in the open literature, but somewhat specific to my particular application, and also more extensive than what can be transmitted here, the answer to your last question is a respectful decline. See a decent undergraduate text on heat transfer, then see Duffie & Beckman. You'll get more out of the time spent.

    Without such information, and the background it takes to understand what's contained in those sources you're mostly spinning your wheels and/or covering ground already travelled anyway.

    Leave a comment:


  • SunEagle
    replied
    Originally posted by ncs55 View Post

    As far as modifying the module frame, that would be inexpensive, and would make for a lighter weight module. And the modified module that I am experimenting with does run cooler and has a better production value than the un modified module. Natural upward convection would be the most cost effective way to cool as opposed to all of your other statements about forced cooling etc. The idea here is to let as much heat out from under the module without using anything that draws more electricity to operate it. As far as any wind under the array, that would only help to cool the modules. If you have been here and done this can you share your data with me? It would be helpful in my experiment.
    Certainly any type of heat "radiator" on or part of the panel should help reduce losses from high temps and show some type of efficiency improvement.

    The problem is how much does it cost to manufacture that "radiator" as compared to how much energy you gain due to lower temps at the cells.

    There is always a balance between making a product the performs the best it can yet still keeping the costs down to meet the net profit goal for the company.

    Hopefully you may find a solution to getting rid of the heat on a solar panel without increasing the cost to make it.

    Leave a comment:


  • Dink
    commented on 's reply
    Thanks, I just left them a similar question via their website. I am not opitimistic that a PV system (by itself) could make up for the solar pool heater. I live in FL and panels get REALLY hot and thus lose their efficiency. Since I don't have room to do both, I thought this type of solution would improve the PV efficiency while still heating my pool (though probably at a degraded level from a straight-up solor pool heater). These are my assumptions, not engineering calculations....though I have some experiance with each system independantly. I had an 18kw PV system at my last house and have the solar pool heater on my present home. Are you aware of any other companies other than FAFCO that have done this (married the PV to Solar hot water/pool)?
    Thanks again, I really appreciate your input!

  • ncs55
    commented on 's reply
    Therma hexx are a newer product. I called them and asked about doing this. They have been working with a company overseas but have no data as of yet. It would be better to use your roof for PV rather than pool heating. You may be able to install enough PV to offset heating the pool. Although this is not the most optimal way.

  • Dink
    commented on 's reply
    Thanks for the info on Therma Hexx - I am unfamiliar with them but I will reasearch them now! For my situation, I already have a 13 year old solar pool heater that is starting to fail. This system is locatated on my only suitable southern oriented roof. I need to the keep the solar pool heater since the pool itself is mostly shaded by the house; however, I want to add photovoltolic system to cut back on my $600.00+/mo electric bills. Have you seen or heard of Therma Hexx installed on a pitched roof with photovoltolic sytem installed on top? ....after a quick scan of their website, I am not seeing it.

  • ncs55
    replied
    Originally posted by J.P.M. View Post

    The short answer to your first statement: It's because water absorbs solar radiation.

    Water or any translucent or semi transparent material, be it solid or fluid is never 100 % transparent to solar radiation. Water, just like air, reflects and absorbs solar radiation. Taking air for a minute, if it did not reflect and absorb solar radiation, the normal (in the context of perpendicular to the direction of propagation) intensity would be the same at sea level as above the atmosphere - currently defined as 1,367 W/m^, +/- ~ 3% to account for the earth's elliptical orbit. Also, the irradiance increases seen at higher elevations above sea level would be absent.. The same effect can be seen in water. At some ocean depth, the environment is dark. That's because water reflects solar radiation at the surface and absorbs solar radiation as the sun traverses the water. At some depth all the solar radiation has been absorbed and it's lights out. How much decrease is a function of the path length through the water, but the point is, the radiation does not increase.

    The medium through which the solar radiation passes absorbs some of the solar radiation. That's the mechanism responsible for the heating the medium. If solar radiation were not absorbed, water sitting in the sun would not heat up.Entropy at work for you and me.

    I'm not a botanist, but I'd guess the discoloration you note may have something to do with chemistry or photosynthesis or some other mechanism. Now, it may be that water droplets, because of the roundness induced by surface tension could act as a lens of sorts. The light hitting a tiny part of the drop surface has it's angle for refraction changed by the water. Because the drop is (hemi)spherical, the surface geometry (roundness) will tend to focus the light to some degree on a small(er) portion of the surface under the drop, the same way surface ripples can be seen on the bottom of a swimming pool, and for the same reasons, and also similar to what allows something called a Campbell Stokes sunshine recorder to work (see the net). However, the total surface under the drop will "see" less overall sunlight. The (small) area of enhancement that's brighter will benefit at the expense of the rest of the area under the drop, which will see less light, with the overall effect being less total sunlight hitting the surface under the drop as entropy dictates and a heat (energy balance) will confirm, by knowing that the lost surface irradiance will show up as increased droplet temp. That's the simple version. It does not consider evaporative cooling effects - some other time, OK ?

    As for cooling solar devices. The way to determine how energy is lost form a solar device, be it solar thermal or PV, or anything else is to do an energy balance on it.

    Take a solar panel, sitting in the sun and happily producing electricity for its owner. Say the normal (again perpendicular) irradiance is 1,000 Watts/m^2 , it's solar noon and the panel is perpendicular to the sun's direct rays. Also assume that for about 10 min. on either side of solar noon, the rest of the environmental conditions don't change enough to warrant worrying about them - clear skies, constant air temp. and a steady 5 m/sec. wind blowing, say from the west, the panel temp. is constant as is electricity flow. Everything is in what's called steady state and constant - an idealization, but sometimes real world conditions get close either by luck or by lab environment.

    Now, imagine a "surface" around the panel, top, bottom and sides, much like the concept of a surface from solid geometry, and do some energy accounting. Because things are steady state, the energy that crosses the boundary in one direction - the solar going in - must equal the energy crossing the boundary in the other direction (going out), either as electricity or as heat for this simple example. The waste heat must, can and will be eliminated as dictated by the energy balance.

    Say the panel is 1 M^2 and generates 175 Watts at STC. And say you've contracted me (having done such things several hundred time) to measure your panel temp. at the above conditions and I come up with 60 deg. C as the panel temp. Because of temp. effects the panel eff. has dropped from 17.5% to (.175)* (1- 0.005(60-25) = .144 = 14.4%. So, 1,000-144 = 856 Watts will be rejected one way or the other - that's what a heat balance dictates. The natural cooling methods - forced convective heat transfer via the wind or natural convection via gravity, or thermal radiation heat transfer, from the top, bottom and sides of the panel to the surroundings, and some small amount of conduction heat transfer through the racking. All those paths must add up to 856 Watts of heat transfer.

    The amount of heat transferred = The sum of all: (the rate of heat transfer for each mechanism) * ( temperature difference each mechanism "sees").

    Since the temp. of the panel is the same for each mechanism, the more heat transfer mechanisms there are, AND/OR the more efficient the heat transfer mechanisms are at removing heat per deg. of temp. diff., the cooler the panel will run.

    A gross oversimplification to make a point: Say [U]all[/U] the heat transfer is from wind blowing across the top and bottom of the array (only) and the air temp = 30 C. That means that each degree of temp. diff. removes heat from the panel at a rate of (856)/(60-30) = 28.5 Watts. Now change (drop) the air temp. to 20 C. What that means is that while the rate of 28.5 Watts/m^2 per deg. C. stays about the same, panel temp. must drop because it can only reject 856 Watts (for this simple example). So the panel temp. will drop to (856)/ 28.5 = (panel temp. - 20) == -->> panel temp. = 50.0 C. (Which is also an example of why panels run cooler when it's cold.)

    Now, apply some super doper closed loop cooling device or modification across the back of the panel that uses water as the cooling medium at, say, 20 deg. C. that will produce a heat transfer rate across the back of the panel of 200 W/m^2 per deg. C of temp. diff., water to panel, and forget about the front side convection loss for now. Since 856 Watts must will be removed, the temp. diff. between the panel and the cooling water will be: (856)/200 = 4.3 C, giving a panel temp. of 20+ 4.3 = 24.3 C. Kind of cool huh ?

    Because of the added cooling at the back, and now going back to the top side convection loss, a simplified and not strictly correct way to do it is to say the front side loss per degree of temp. diff., panel to air, will be about half the front + back loss, or 28.5/2 = 14.2 W/m^2 per deg. C. So, (and using the lower 20 C. air temp.) the front side heat transfer ~ 14.3*( (24.3-20) = ~61.5 Watts.

    What will happen in this case is that the panel temp. will float somewhere between 24.3 C. and a bit more or less but always be such that between the heat loss rates and temp. differences of all the heat transfer mechanisms running in parallel, and all the electricity produced, they will all fluctuate very slightly minute to minute (or second to second) in such a way as to always, between all energy transfer mechanisms, equal the 1,000 Watt input all for the purpose of maintaining that energy balance.

    Now, the backside aux. cooling, as shown above, can be a very effective way to cool things. My numbers were cherry picked to be easy to follow, but they are not too far off reality. Forced convection heat transfer coefficients using water can easily be 1 or 2 orders of magnitude more than coefficients using air. The pumping costs are low compared to gas heat transfer, but the construction and maint. costs are higher. Overall, its a bunch of tradeoffs, but water cooling takes the price in terms of effective shock value cooling ability.

    However, note that while the panel temp. dropped quite a bit, and the efficiency improved to essentially STC eff. (with an ~ 24.3 C. operating temp.), the added output amounted to about 21 Watts or 12.5% or so. A cost analysis might show that aux. forced cooling might be effective for large arrays, but I suspect the level and type of design necessary to achieve the #'s I've shown with either water or air would be rather costly and impractical for industrial or municipal size plants, and out of the question for residential type large installs. I cite the dearth of such schemes at this time as support for that statement.

    Lastly (Whew !), I'd suggest that modifications to the racking or the backside of panels or similar methods probably won't do much to increase the heat transfer from the backside of an array. First off, any meaningful flow is mostly forced convection wind driven. Second, the back (or underside) flow is more likely turbulent or transition zone (laminar to turbulent) flow, with most of the turbulence being due to the obstructions - the racking, posts or supports. That's good for heat transfer because turbulence is a better flow regime for heat transfer than laminar or streamline. And so, "smoothing out" or streamlining the array structure may do little good since turbulent flow is usually better for heat transfer. In any case, I'd not put a whole lot of faith in reliance on enhancements to natural (gravity driven) convection, which is usually so weak that it's easily overwhelmed by a even a slight breeze. I suppose such attempted enhancements can't hurt, but they're usually pretty unreliable and pretty costly for the benefit realized. It's been my experience that such benefits and their usual paucity are not well understood by those who try to make such things work. Been there, done that.

    As usual, take what you want. Scrap the rest.
    As far as modifying the module frame, that would be inexpensive, and would make for a lighter weight module. And the modified module that I am experimenting with does run cooler and has a better production value than the un modified module. Natural upward convection would be the most cost effective way to cool as opposed to all of your other statements about forced cooling etc. The idea here is to let as much heat out from under the module without using anything that draws more electricity to operate it. As far as any wind under the array, that would only help to cool the modules. If you have been here and done this can you share your data with me? It would be helpful in my experiment.

    Leave a comment:


  • ncs55
    commented on 's reply
    Yes FAFCO has them but FAFCO products are not the best product for pool heating. I see more FAFCO failures than any other type of pool heating system. Therma hexx would be my choice if I were going that route. You have to ask yourself the age old question, is the money spent on extracting the heat going to be less than the production gained from the PV. Usually it is not.

  • Dink
    commented on 's reply
    Looks like the hybrid panels (Photovoltaic cooled by water - warm water goes back to the pool) are now commercially available. They are produced by FAFCO. <<http://coolpvsolar.com/>>

  • ButchDeal
    commented on 's reply
    and certainly do not lead to &quot;SIGNIFICANT output boost&quot;
    For the cost to do the install and maintain, you could more cheaply add more modules

  • J.P.M.
    replied
    Practical and cost effective trumps the laws of nature when human endeavor is concerned. Economizer schemes such as these, while possible, are neither practical or cost effective at this time.

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