Trying to keep up w/ Chem Geek

[ShelleyAnn]

Guys. GUys. GUYS! OVER HERE.

Yes 3 x 9 mat was too small. I paid for a 4 x 20 and it came 3 x 9 out of the box so they gave me another one. I'm not sure if all mats actually measured smaller than advertised, wouldn't surprise me. This year I'm adding the new 4 x 20, but the manual does not suggest parallel for some reason as you can see from the link. What's up with that?

http://smartpool.com/website/sunheat...%20English.pdf

Also, why is a 1.5hp too much pump for my pool?

Shelley

[CarlD]

ShellyAnn,
We didn't forget you!

No, they are not in parallel. Though, curiously, when they are off, you can turn on any one panel.

It is simply the easiest way to plumb them. There is NO reason not to plumb them in parallel. However, the piping may get a little complicated. You will have to set up two pipes or

If you go back to my rules of thumb (Just over the Leaning Tower of Pisa), they will work for you...nothing either Richard or I have said really contradicts that.

[ShelleyAnn]

I've been thinking... (crowd goes quiet with anticipation)...If I plum 3 panels, one after the other as the manual suggests, I will only be able to move so much water as it winds thru the panels one after the other. If I plum them in parallel, I will be able to move 3 times the water at a time. 3 times the water, more heating value?

[chem geek]

Yes, putting the panels in parallel will let you move more water through and that will provide more heat, but as Carl mentioned the plumbing is more complicated (the diagram I drew in this post shows a parallel configuration that equalizes the flow to be the same for all panels). It won't be 3 times as much for a variety of reasons, but it will be more. You can never get more heat from these panels than the amount of solar radiation that hits them. There will also be a limit as to how much water you can move since it is recommended not to exceed a flow rate of 10 feet/second in pipe and with 2" pipes that's around 100 GPM, but you probably aren't at that point and any excess can go to a bypass to your pool. I also don't know what sort of pressure these panels can take without leaking, either, but as Carl says you can try configurations to maximize heating until you reach a limit or problems occur. By having a valve to allow for a variable bypass, you can tweak the flow rate through the panels for maximum heating without getting leaking or excessive flow rates.

If your 4x10 and 4x20 panels are similar to the FAFCO ones I linked earlier (or to this link which is for 4x10 and 4x20 panels designed for above ground pools so may be more similar to what you are using), then they have a maximum recommended flow rate of 8 GPM per panel and a maximum pressure of 30 PSI (intermittent up to 45 PSI). With your 3 panels hooked up in parallel, that would mean 24 GPM maximum recommended flow rate. But again, you can try more but may have them leak at some point. Just remember that at 8 GPM per panel you are already getting 90% of the heat you can possibly get out of the panels so at most a faster flow rate will only get you 10% more heating (this is different than Carl's situation because his solar panels are completely different as decking where the flow rates were not even close to the efficient range with so many panels hooked up in series). Depending on the panel, it is possible that too high a pressure (to get higher GPM flow rate) might cause a panel to burst or break which is why I think you should look at the specs for the specific panel you are using and not exceed the maximum flow rate that they specify or certainly not the maximum pressure they specify.

Richard

[CarlD]

Ok,
But let's add this: If you have 3 panels hooked as in your diagram, ShellyAnn, (in series) you will only be getting 8 gpm AT MOST, and probably less, but the water will be hotter.

Actually, I think your max will be LESS than 8gpm, due to tripled resistance. If it was electricity I'd estimate it at 1/3 (less than 3gpm) but this is water and I don't know it as well. But the pressure WILL give you less flow---let's ball-park it at 6gpm.

As Chem_Geek says, hooked in parallel, the panels will give you 24gpm -- or 4 times what I guess-timate the series flow rate to be. Each gallon you flow from the series must be FOUR TIMES HOTTER than each gallon from the parallel to get the same effect.

But what does 4 times hotter mean? Well, that depends on the difference between the pool water temperature and the panel water discharge temperature. If the parallel panels' water is 1 degree warmer than the pool, then the series must be 4 degrees warmer. If the parallels are 10 degrees warmer, the series must be FORTY DEGREES warmer to have the same effect.

Why? Because a BTU is the amount of heat energy needed to raise 1 pound of water 1 degree. So 4 pounds of water 1 degree warmer than the pool is the same as 1 pound of water 4 degrees warmer. Converting pounds to gallons doesn't change anything but the raw amount of BTUs, but not the ratio. (I can't remember if a gallon of water weighs 8.3 lbs or 9 lbs--).

Oh, it's SO much easier to figure in metric and calories...1 liter of water weighs 1 kilo...
24 liters/minute that is 1 degree (Celsius) warmer than the pool adds 24 calories /min. 6 liters/ min 4 deg warmer adds the same 24 calories/min... so if the difference in temp is 10 degrees for the 24-lpm, then it's adding 240 cal/min. The series must be 40 deg warmer at 6-lpm to add the same 240 cal/min.

For reference, 40 degrees celsius is the difference between freezing (0 in Celsius, 32 in Farenheit) and 105 degrees farenheit--a dangerously high fever!

Sorry, you can't bet on gettin' that from solar panels! Not even in Farenheit!

No, it's far more efficient to run them in parallel.

BTW, Richard, I continue to be dubious of manuf calculations: For Fafco to run at their efficiency levels of even 80%, the panels will still be easy to touch even on a blistering day. But you won't get that if you plumb them in series. I STILL go by the rule-of-thumb that if the panel is hot to the touch, or too hot to touch, your flow rate is far too low.

When the panel is hot to the touch, all that good heat is being radiated back into the air. Unless the pool water is already hot, you are nowhere near your maximum transfer efficiency, either your theoretical maximum or your effective maximum.

On a more positive note, manufacturers like Fafco grossly overestimate the number of panels you need to warm your pool. THEORETICALLY, my panels on my deck are far too small in surface area, being only 1/3 the area of my pool. HAH!

My parents had a 13,000 gallon 18x33 oval AG that only got 6 hours of direct sun per day (525 square feet). Even with a solar cover they were lucky on a GOOD day to get 80 degree water--it was usually 78. As they aged that became more and more uncomfortable for them.

I added a 4x20 and a 4x10 panel plumbed in serial--120 square feet with 6 hours of sun per day. All summer the pool ran at 84 degrees, far, FAR more comfortable for them. I had them run the panels whenever the water from them was warmer to the touch than the pool. They lay on the ground and the ground got warm (actually, baked), and after the sun passed would continue to transfer heat BACK to the panels. And this was north of New York City, where swim season is only June thru August.

So, given that, even in series you are likely to get very nice results!

[Daggit]

Ok, I'll toss something in here to go more esoteric. While your discussion on heat transfer and flowrates is in the right direction, its for some of the wrong reasons (or reasoning). Time in the box has a lot less to do with it than turbulent flow.

Water (or any fluid) flowing through tubes very slowly will have a laminar flow pattern, which is to say that the water along the walls stays along the walls and the water in the center stays in the center of the tube. There is no cross mixing. This is a very in-efficient way to transfer heat since only the water near the wall heats up much in the tube...the heat doesn't have time to get to the center.

At very high flow rates the flow is fully turbulent, meaning that the is nearly full mixing within the tube and water near the wall at one point gets moved to the center and vice versa. This allows more heat transfer since more water contacts the tube wall and can get to near wall-temperature.

There is a middle zone between laminar and turbulent called the transition zone where there is partial mixing.

We calculate the amount of turbulence with a dimensionless number called the Reynolds number (I won't include the formula, you can google it). Reynolds numbers below about 2000 are laminar, over about 4000 are fully turbulent.

When we design industrial heat exchangers (one of the things I do) we are carefully to match tube size and flow rate to get Reynolds numbers in the turbulent range to be sure of good mixing within the tubes and good heat transfer coefficient. This is likely the most important factor in determining the overall efficiency in your solar panel, and why published efficiency numbers increase with flowrate. Since pressure drop also increases with flowrate, I would guess that the panel builders design them for only as much pressure as is needed to get to a flowrate that is just above optimum Reynolds numbers for heat transfer.

Oh, BTW, wind chill isn't a very good comparison to what's going on in the solar panel. Wind chill is evaporative cooling determined mostly by the difference between wet-bulb and dry-bulb air temperatures on your damp skin, and not simply the air flow removing heat from you body. The phase change from liquid water to vapor takes roughly 1000 Btu per pound of water, which is a lot of cooling energy.

[CarlD]

Great Post, Daggit!

Water (or any fluid) flowing through tubes very slowly will have a laminar flow pattern, which is to say that the water along the walls stays along the walls and the water in the center stays in the center of the tube. There is no cross mixing. This is a very in-efficient way to transfer heat since only the water near the wall heats up much in the tube...the heat doesn't have time to get to the center.

At very high flow rates the flow is fully turbulent, meaning that the is nearly full mixing within the tube and water near the wall at one point gets moved to the center and vice versa. This allows more heat transfer since more water contacts the tube wall and can get to near wall-temperature.

I JUST saw something exactly like this in the latest Cycle World I got this week. Kevin Cameron in his "Top Dead Center" column, talking about engine cooling says nearly the same thing, but not as clearly. I've been taking the position that the greatest flow rate your system can tolerate will give you the most bang for the buck. I think the real limiting factors are the pressure the panels can tolerate without leaking, and the amount of additional flow the pump can sustain before you lose effective water motion from your regular returns. You seem to reach these limits long before others.

In my Fanta-Sea solar deck panels, leaking is a MAJOR issue--I've just purchased a plastic welder so I can fix a panel with a split seam. As it's a special panel cut to fit around the skimmer, I can't use my other spare panels instead--I must fix it.

BTW, I surprised about wind-chill. I didn't think it included evaporating water--just the flow over the surface. I have ridden a motorcycle at 10 to 20 degrees F for hours on end without a windshield when I was young and foolish and it was one of the most unpleasant painful experiences that did NOT require a trip to a doctor! I still remember spending over an hour and a half in a Burger King in Cheraw, SC in 1980, just thawing out...

My point? I certainly wasn't sweating, but that wind DEFINITELY made it feel like 20 below and it was just sucking the heat out of me, despite multiple layers and wind-proofing.