Trying to keep up w/ Chem Geek

[ShelleyAnn]

I've been reading Chem Geek's thread on pump efficiency. I see his lips moving, but can't understand what he's saying being only a mortal.

I installed and AG 15 x 30 last spring. I was unsure the size of pump/filter after reading the posts here that pumps are too large so I called the manufacturer (Waterford) to confirm what size I needed. I told them I was to receive a 1.5hp pump and a 21" high rate sand filter and their response was that those two units were designed to work together. Okay, good enough for me. Heck I even called the manufacturer and they said that was what works...right?

After being on this fabulous web site for a year now, I'm getting that ole feeling again that I am way overboard on the pump. I don't understand head, curve or any of the other terms regarding pumps, but I've just received a new 4 x 20 solar panel and planned to add it to my 2 4 x 10s I used last year. I plan to go from flexable hose to a more permanent pvc rig and even make a rack to angle them toward the sun. Am I going to cause a shift in the quantum flux and cause havoc for the world and all it's inhabitants with this pump? What I really mean is am I going to blow the lid off of all my stuff?

Shelley

[CarlD]

Chem_Geek loves to get technical and has been a WONDERFUL contributer to all our knowledge. Sometimes we have to filter it down to practical usage rules for the non-tech pool owner. But, even when I can't follow it, I value his input enormously. When I can, I sometimes even can add an intelligent comment!:rolleyes:

But that's OK. As posters we have some limits on what we can say, but analysis of the chemistry and physics of pools is pretty open. The REALLY high-level stuff goes into the China Shop.

The SIMPLEST solution I can offer to your solar panel question is really simple--just have an adjustable valve to control the water flow to the solar panels and valve it down so the pump doesn't over-stress them and cause them to fail. That's ALL you need to do.

[Poconos]

I think Carl just left out the solar panel bypass line. You really don't want to restrict the total flow so the easiest way is to add valves so you can divert only part of the total flow to the solar panels. That's what I do. I basically split the filter output 3 ways. Straight to the returns and one each to the two solar collectors with ball valves in each line. The solar collectors just dump into the deep end from a couple of hoses under the diving board but you could just as well plumb them back into the return line.
Al

[CarlD]

Of course, Al is right. I was simply saying that the pipe to the panels you restrict with the valve. I didn't mean the flow to the returns--you do NOT want to mess with that!

[ShelleyAnn]

So that post was not the really high level stuff huh? I guess I'm glad I can swim! :)

Last year I had ALL the water going thru the 3x9s tandem. Was I just lucky they didn't leak? It did seem like a LOT of pressure going thru, but this was the way the pool guy set them up. They worked like a charm in the noon time sun.

Don't tell me...is this Waterford pump going to last 25 years with proper maintenance? I'm already wanting a different refrigerator with our 10-year old one is working just fine. I don't want to push the husband over the edge asking for a new pump too.

[CarlD]

So that post was not the really high level stuff huh? I guess I'm glad I can swim! :)

Last year I had ALL the water going thru the 3x9s tandem. Was I just lucky they didn't leak? It did seem like a LOT of pressure going thru, but this was the way the pool guy set them up. They worked like a charm in the noon time sun.



Most pool guys have no idea how to set up solar panels.

The EASIEST way is to have a Y in the return line. One branch of the Y goes to the return(s) with nothing in between. The other goes to the solar and you can have 1 valve or 2 or 18, depending on your setup. I have 3 valves. The first is just an on/off cuttoff for all the solar panels, then there's ANOTHER Y off of that one that each has a valve for each set of panels (I have 2 sets of solar panels).

Can't get you a picture of it right now, though.

[ShelleyAnn]

Thanks everyone for all your input.

Carl, I will wait for the ice to melt in your neck of the woods so you can send me that pic! We are up to our eyeballs in snow here so it's not like I'm being held up or anything.

From what I gleened from the Hot Sun Industries web site, my panels were upside down all last year, meaning the pipe leaving the panel was at the bottom, not the top so air could have been trapped in the upper corner. Live and learn. In another 100 years, I'll have EVERYTHING figured out.

[CarlD]

Ok,
I've attached a pic of my return setup. To the left of the red ball-valve handle, you'll see a Tee--the part going up is the return to the pool, the part going right to the ball valve is for the solar panels. To the right of that you'll see another tee and on each leg is a sluice valve, with the blue knobs.

Each sluice valve is for a solar panel group (my solar panels are 2'x4' and I have about 32 of them). I can open each group just a crack, depending on whether my pump is on low speed or full speed. At full speed I open each group 1 and 1/4 turn. At low speed I open them all the way.

Each group of solar panels has its own return at the far end of the pool.

Very, very simple. Hope this helps!

[ShelleyAnn]

Yep, very simple, but beautiful.

You've got about 256sf of panels. I will have about 134sf for my 15 x 30 AG. I remember reading somewhere here that it's quantity of water thru the panels that gives the most hot water. I sure did that last year with forcing all 1.5hp thru 2 panels that measure 3 x 9 each!

I do see some flexible pipe in the right side of the pic. Is that connected any way or is that just some hose for vaccuming? I currently have everything connected with white flexable hose, but would like to go to pvc. How much $$ do you have invested in the materials (pipes and knobs) would you guess?

[chem geek]

ShellyAnn,

It is not true that simply increasing the rate of flow through the solar panels increases the amount of heating to your pool. There is a point of diminishing returns. Though every manufacturer's solar panel is different, take a look at this link (http://www.fafco.com/WarmWater/06359G_SunSaver_Spec.pdf) which is a common solar panel from FAFCO and look at the graph that says "EFFICIENCY vs. FLOW". You will notice that you get to 80% efficiency at a rate through the panel of 4 GPM, but that the efficiency increase is slowing way down. The maximum flow rate recommended for this panel is 8 GPM, but the efficiency won't be very much higher (it won't be more than 90%).

When water flows faster through your panel, it spends less time in the panel so gets heated up less. So while you push more water through the panel at higher flow rates, it gets heated up less and the net result is roughly the same amount of temperature rise in your pool. You should generally stay near the recommended flow rate for your panel and not exceed its maximum rate.

I don't know enough about your system to know if you need a 1.5 HP pump, but that seems extraordinarily high. I have exceptionally long runs for my solar setup (due to many roof segments and hips) and am already over-pumped at 1.0 HP and could easily use 0.75 HP. It is true that sand and DE filters have higher pressure (at the same flow rate) compared to cartridge filters, but even so the pump seems over-specified. Your pool builder is recommending a 21" high rate sand filter and yet your pool is an above-ground 15x30 which is not a very large pool (it's not small, but it's not so big as to require "high-rate" and a large pump for fast turnover). This just doesn't seem right to me.

At any rate, you most certainly want to have a bypass for your solar panels so that only some of the water goes through them since there is no way you can run high rates like 50 GPM through just 3 panels (for 17 GPM each) unless your panels are of a design very different than the FAFCO one in the link above. The setups that Al and Carl described in previous posts sound good where you can split the output of your pump to go partly through the solar panels and the rest into your pool. My only concern is that your PB is designing a high flow-rate system, including a filter designed for such flow rates, without good reason (except more expense for a larger pump and high-flow rate filter). Even if your pool were 4 feet deep throughout, that would be about 13,500 gallons so to get one turnover in even 4 hours (so two in 8 hours) would be 56 GPM, but it sounds like your PB is designing your system for even higher flow rates. Maybe when he is quoting 1.5 HP he is talking about an "up-rated" pump so that this is roughly equivalent to a 1.0 HP "full-rated" pump. That would make more sense.

Richard

[CarlD]

When water flows faster through your panel, it spends less time in the panel so gets heated up less. So while you push more water through the panel at higher flow rates, it gets heated up less and the net result is roughly the same amount of temperature rise in your pool. You should generally stay near the recommended flow rate for your panel and not exceed its maximum rate.

Richard, I am amazed at this. You are usually SO good at the science, but this is just flat-out wrong. It SEEMS to make sense that the water needs to heat up, and if it goes too fast it won't, but it's still wrong. It simply violates the laws of themodynamics. It's all about fluids.

Think about it: If your statement is true, then on a windy day, the BEST way to stay warm would be to open your coat...the air whistling past you wouldn't have time to get warm from your body and bleed off that heat.

But obviously, that is NOT true--the faster the wind moves past you, the more heat it sucks out--it's called "Wind Chill Factor" and you freeze your ka-loonies. This is JUST as true of the fluid in your solar panels--the faster it moves through the system the more heat it sucks out. Ideally, your panels on a hot, hot summer day should not be any hotter than your pool water.

Remember: Your solar panels are mechanically IDENTICAL to a car's radiator. In fact, if your pool is too hot, you run the panels at night to cool it. In a radiator, if you are idling in a traffic jam on a hot summer's day, your car overheats. You get moving, it cools down. The MORE fluid (air) passes accross the radiator, the more it cools down--faster is better. Do not mistake temperature for BTUs. It's BTU quantity that heats your pool. The more BTUs you can get into the water in a given amount of time, the faster the water heats up!

Why? Well a British Thermal Unit is the amount of heat energy necessary to heat one pound of water one degree farenheit. That's the WHOLE secret to why more flow is better. More heat energy, not more temperature. I'd MUCH rather have 10 gallons/minute of water 1 degree warmer than my pool than 1 gallon every 10 minutes that's 10 degrees warmer--my pool will heat up 10 times faster because it's moving 10x the BTUs.

My system DEPENDS on that--my panels are my deck and the water running through the deck simultaneously warms the pool while keeping the deck cool to walk on barefoot on the hottest days.

(BTW, in Metric it's called a KiloCalorie, or Calorie for short--though really a calorie is 1/1000 of a KiloCalorie. A Calorie is the amount of heat energy necessary to raise one kilogram of water one degree celsius--exactly the same as a BTU but with different measuring units.)

But there ARE limiting factors--I cannot run that high a pressure through my system or the panels start to spring leaks--that's the really, really big one. Too much pressure can damage the system...and I have some leaking panels to repair this winter...:( (anybody good at plastic welding?)

The other limiting factor is cavitation--bubbles in the system. When the water bubbles it cannot make complete contact with the panel's surface and cannot conduct heat away from the panel. This is VERY hard to achieve unless you have a lot of air in your system. Usually, it will start leaking first.

Both of these factors are result of the friction/resistance of the system. Lowering that resistance can increase efficiency immensely.

I increased the efficiency of my system by, I estimate, at least 40% by splitting my system into the two halves. I have 30 panels (I miscounted before) and originally they were all in sequence, from the first to the second, etc. The water had to pump through ALL 30 panels before it came out the return, nice and warm, but slowly. The resistance of the system meant any more water pressure and the early panels would leak.

So I split the system into 2 halves of 15 panels each. You can see the Tee in the picture. Each 15 panel half can flow FAR more water than the 30 panel whole because its resistance is half of what it was. And, with two halves, that higher flow is DOUBLED for the same 30 panels (now 15 and 15 in parallel to each other). The result? The last two seasons my pool heats up far, far faster than it used to.

Of course, since each 15 panel half has half the surface area it's maximum BTU capability is half. But I've doubled the flow rate for the same surface area and the result is my pool heats up far faster. 3 or 4 degrees a day is now 6 to 8 degrees. I'm getting more BTUs into my water.

Heat energy (BTUs) not temperature is the key to warming your pool.

The efficiency ratings of solar panels require a little different analysis--the loss of efficiency is due to back-pressure (friction), not the water moving "too fast to heat up".

As I was thinking about this, I remembered that I first came across Chem_Geek's natural mis-perception in a completely different context. Kevin Cameron is an utterly brilliant columnist for Cycle World Magazine. He specializes in technical issues. One article, several years ago, he took on this issue. The question he addressed was the same: If the water flowed too fast through the engine, wouldn't it not have time to heat up and bleed off the heat from metal? He explained why this perception was just plain wrong.

Well, the answer to this is FAR more critical to internal combustion engines, particularly performance-based engines like sports-cars and most motorcycles that are both highly stressed. An over-heating race engine, at speed, can seize and launch a poor rider as a human cannonball at 150mph or more! DEFINITELY far worse than a little loss of heating efficiency in our water. Plus, a leaking pressurized engine cooling system is deadly dangerous--that water can scald and cut like a knife--water at pressure has a FAR higher boiling point than 212 Deg! With motorcycle engines the penalty for being wrong is far more catastrophic!

Cameron's explanation was basically what I have offered above--the more water moves through the block's water jacket, the cooler the engine. Only cavitation sets it back. Otherwise, the more the better. It's exactly the same system, just in a different context with different constant values.


I remember reading somewhere here that it's quantity of water thru the panels that gives the most hot water. I sure did that last year with forcing all 1.5hp thru 2 panels that measure 3 x 9 each!

So, ShellyAnn, you keep doing what you were doing--that is, in fact correct! You want as much flow as you can without damaging your system.

The only other reason I had to lower my flow rate was on a different pool with a different set up. Too high a flow rate on too small a pump kept my returns from circulating my water properly--but that has NOTHING to do with the heat transfer issue!

[ShelleyAnn]

My filter is a Waterford 21" sand filter and the manual says its max flow rate is 43.6GPM and the Max Operating Pressure is 50 PSI. It also states the turnover is 15,700 gal in 6 hours. The pump is a 1.5 hp Waterford and right on the cover of the manual it is matched to the filter I have. So the pump and filter are rated for one another, that's been established, but why is this not matched for my pool?

My solar panels are the kind that roll up in the winter and are right along side of the pool. I plan to put them up on some sort of rack this year, but I'm not sure if they will be at ground level or 3 feet up. They won't be on the roof. The two I have from last year are " Midwest Saturn Solar" and no where in the manual does it state max pressure, but it does say to keep the chlorine between 1-2 PPM. :) They have a lever built in the middle to adjust flow down one side of the mat and up the other. If the lever is in the open position, water just passes thru the pipe without being forced down the mat. I just purchased a sun heater brand from Leslies to add to them, but that manual doesn't state max pressure either.

[CarlD]

ShellyAnn,
These are a very typical type of solar panel. They can work EXTREMELY well if set up correctly. But I think 3'x9' is a tad small--you would need about 3 or 4 to work nicely on your pool. You should be able to figure out a rough'n'ready safe pressure. You want the max flow you can get, but you don't want to spring leaks.

The side valve does work well. With it, if you plumb correctly, you won't need valves like mine--but I WOULD put in a shutoff--like my ball valve--what you couldn't see in the picture, behind the support, there's a quick release. I can take down the solar panels in the fall and still cirulation the water that way.

I would totally ignore the 1-2ppm chlorine max. It's worthless. They are covering their back bumpers, but if you follow it, you will probably have a TERRIBLE time preventing algae. You see, if you have enough sun to run the panels, you will need stabilizer in your water. At even 20ppm of stabilizer, you need to keep your chlorine between 2 and 5 ppm.

Trust me, if you have enough sun to make your panels effective, you'll need 30-40ppm of stabilizer--and for that you need to run 3-6ppm of chlorine. Here, Chem_Geek can give you much more exact numbers than I can--he's done some great research on stabilizer levels and effective chlorine levels.

But my point is you won't be able to keep chlorine between 1 and 2 ppm realistically...unless you spring for salt-water generation of chlorine.

But given your pump and your pool, you should have no problems running your panels, keeping your pool clean and having a great summer!

[chem geek]

Carl,

My comments below in blue in some of your quoted text. I think we have a mutual misunderstanding and are both right and wrong simultaneously depending on which point is being made. I think the bottom line is that you are right that with an object at a certain higher temperature than a fluid, then the faster the fluid flow, the faster the heat transfer. However, my point was that in the solar panel at recommended flow rates, you are already getting 80% of the amount of energy you can possibly get (i.e. what the sun delivers) and at maximum flow rates stated for the panel you get 90% so it makes little sense to go beyond that and the difference between the 80% at 4 GPM and 90% at 8 GPM is an increase in heat transfer of only 12.5% (10/80) for a doubling of flow rate.


Richard, I am amazed at this. You are usually SO good at the science, but this is just flat-out wrong. It SEEMS to make sense that the water needs to heat up, and if it goes too fast it won't, but it's still wrong. It simply violates the laws of themodynamics. It's all about fluids.

Think about it: If your statement is true, then on a windy day, the BEST way to stay warm would be to open your coat...the air whistling past you wouldn't have time to get warm from your body and bleed off that heat.
You are right that more heat will transfer from your fixed body to a moving fluid (air) if the fluid is moving faster (and is at lower temperature). However, the effect in a solar panel is not the same as with your body because the heat removed from the solar panel by the moving water lowers the panel's temperature, which lowers the amount of this increase of heat transfer (more flow DOES give more heat transfer, but this increase is less and less as you go to higher flow rates -- i.e. diminishing returns). This is why the efficiency chart in this link (http://www.fafco.com/WarmWater/06359G_SunSaver_Spec.pdf) is the shape that it is (essentially a log curve). Your body is different because it attempts to maintain its temperature by speeding up metabolism, but if your body's temperature dropped to the same as the wind temperature, then all the wind in the world won't lower your temperature any more.

But obviously, that is NOT true--the faster the wind moves past you, the more heat it sucks out--it's called "Wind Chill Factor" and you freeze your ka-loonies. This is JUST as true of the fluid in your solar panels--the faster it moves through the system the more heat it sucks out. Ideally, your panels on a hot, hot summer day should not be any hotter than your pool water.
Refer to this chart (http://www.weather.gov/os/windchill/index.shtml) showing the Wind Chill Factor. This shows the point you are making that higher wind transfers heat at a faster rate. Again, this is true IF the temperature of the object you are getting heat from does not change. This is not true with a solar panel. The faster water movement transfers heat faster which lowers the temperature of the panel so that limits the improvement of heat transfer. The net effect is the log curve that never quite gets to 100%. So while it is true that pumping more water will get more heat out of the panel, the amount of incremental increase quickly becomes small and increasingly smaller at higher flow rates. At 4 GPM you already get 80% of the total amount of possible heat you could get. At 8 GPM you get around 90% (not shown on the graph, but extrapolated). It would take an infinite flow rate to achieve 100% efficiency, but who cares? You can't get more energy out of the panel than is being given to it by the sun. If you get 90% of it at 8 GPM, then that is plenty. It's not worth all the problems of higher flow rates with their extra losses in friction through the pipes and cost of electricity for pumps to try and get much more than that.

Remember: Your solar panels are mechanically IDENTICAL to a car's radiator. In fact, if your pool is too hot, you run the panels at night to cool it. In a radiator, if you are idling in a traffic jam on a hot summer's day, your car overheats. You get moving, it cools down. The MORE fluid (air) passes accross the radiator, the more it cools down--faster is better. Do not mistake temperature for BTUs. It's BTU quantity that heats your pool. The more BTUs you can get into the water in a given amount of time, the faster the water heats up!
Faster is better, but the returns are diminishing. At some point, going faster doesn't cool that much faster because the water in the radiator can't get cooled lower than the air temperature. Or put another way, if the air temperature were the same as your engine, then all the wind in the world isn't going to cool your car.

Why? Well a British Thermal Unit is the amount of heat energy necessary to heat one pound of water one degree farenheit. That's the WHOLE secret to why more flow is better. More heat energy, not more temperature. I'd MUCH rather have 10 gallons/minute of water 1 degree warmer than my pool than 1 gallon every 10 minutes that's 10 degrees warmer--my pool will heat up 10 times faster because it's moving 10x the BTUs.
Again, you are talking about the efficiency curve and yes it is much better to have faster flow, especially initially since it increases the heat transfer rate so quickly. But again, the returns are diminishing as you approach 100% efficiency. At 4 GPM you are already extracting out 80% of the heat that there is to extract (i.e. the sun's energy that hits the panel) and at 8 GPM you are extracting out 90%. I don't disagree that 16 GPM through the panel might extract an extra 5%, but the pressure required to do so would exceed the recommendation for the panel (you mention that below) and be more costly throughout the system.

So I split the system into 2 halves of 15 panels each. You can see the Tee in the picture. Each 15 panel half can flow FAR more water than the 30 panel whole because its resistance is half of what it was. And, with two halves, that higher flow is DOUBLED for the same 30 panels (now 15 and 15 in parallel to each other). The result? The last two seasons my pool heats up far, far faster than it used to.
Most solar systems are plumbed to be in parallel, not in series. This is consistent with what you are saying in terms of optimizing water flow. With a solar panel that has a maximum of 8 GPM, you clearly must go in parallel to get to higher overall flow rates (for the system, not per panel). Why is your solar system in series at all? Why aren't ALL of the panels in parallel? Well, maybe not all because then you would need 30x4=120 GPM in your system, but MOST of the panels should be in parallel -- two banks of 15 where the 15 are in parallel and the two banks are in series (60 GPM for system, 4 GPM per panel). To equalize flow (in a bank of parallel panels), you have to make the path length for the water equal for all panels, but that is easy to do by routing connecting all panel bottoms and tops to each other (i.e. everything is in parallel), and then have the input go into the first panel and take the water coming out of the panels from the very last panel (which has a long pipe run to get back to where the first panel is placed).

Here's a diagram to show you what I mean (and similar to how my solar system is hooked up, except the roof isn't so straight). You can see how the total path length for water going through any of the panels is the same (this makes the flow rate equal in all panels). All water comes from the lower left and moves to the right, then goes up one of the panels, then continues to move right to the end, then turns around and comes all the way back.


<--- __________________________________________________ _____.
______.______.______.______.______.______.______._ __|
| | | | | | | |
| | | | | | | |
| ^ | ^ | ^ | ^ | ^ | ^ | ^ |
---> __.______.______.______.______.______.______._____ _.


Of course, since each 15 panel half has half the surface area it's maximum BTU capability is half. But I've doubled the flow rate for the same surface area and the result is my pool heats up far faster. 3 or 4 degrees a day is now 6 to 8 degrees. I'm getting more BTUs into my water.
I agree that you want more flow rate through each panel, but again, the returns are diminishing. If your panels were truly in series, then the flow rate could have been so slow that you were in the lower left area of the efficiency curve so it's not surprising that your efficiency went way up splitting into two.

The efficiency ratings of solar panels require a little different analysis--the loss of efficiency is due to back-pressure (friction), not the water moving "too fast to heat up".
No, it's the exact opposite. If you look at the link for the FAFCO panels, you will notice the expected increase in pressure loss at higher GPM (it varies as the square of the GPM), but higher GPM results in higher efficiency. If you are saying that the slowdown in efficiency is due to this back-pressure, that is only partially true. Even if there were no friction at all, you would still end up with an efficiency curve that looked similar because faster and faster speeds simply cannot pull more heat from the sun than the sun is delivering to the panel. Again, though the percentage efficiency increases, the returns are diminishing so that in absolute terms of amount of heat gained through faster flow, it's vanishingly small at higher rates.

[CarlD]

Richard:

We ARE getting esoteric--but the fundamental statement I disagreed with was that the water needed time to be heated by slowing down the panel. In practical terms, day to day use of solar panels or motorcycle engines this is at best misleading, at worst, just wrong.

but if your body's temperature dropped to the same as the wind temperature, then all the wind in the world won't lower your temperature any more.

I accept this as true only if you accept that the body's metabolism is irrelevant. Because, as EVERY kid who ever got out of a pool wet knows, the wind can chill you even THOUGH the air temp is higher than your skin...More importantly, an engine (which doesn't sweat) won't be cooled if the air is as hot as the engine. Evaporation of course causes cooling.

You can't get more energy out of the panel than is being given to it by the sun. If you get 90% of it at 8 GPM, then that is plenty. It's not worth all the problems of higher flow rates with their extra losses in friction through the pipes and cost of electricity for pumps to try and get much more than that.

This is absolutely a true statement--I have discussed friction as a limiting factor.

An interesting question about why my panels are in serial, rather than parallel. It's a practical matter. The FantaSea pool system uses the deck as the solar panels. They are 2x4 panels that surround the pool, and you walk on them...they therefore can only practically can be connected serially. In parallel would involve SO much piping as to be both impractical, and highly ugly. My better half made clear a condition of the pool was that it must be nice to look at

Here's the deck:
http://home.earthlink.net/~dashmanc/pool/offdeck.jpg

You can see the deck, but it's also the solar panels, so they are hidden in plain sight!

Here you can see that if there were a lot of hoses hanging down and joining into a bus hose or manifold, it would look hideous. Even the one hose to the controls on the left is unappealing.

http://home.earthlink.net/~dashmanc/pool/wholepool.jpg



I stand corrected on the increase of efficiency curve.

However, in practical terms, you do want to maximize flow through your system. My rules of thumb still hold:
1) If the water coming from the panels is warmer than than the pool, it's working, even if it's only one degree.
2) Practically speaking, on the hottest day, if your panel(s) are very hot, you aren't using them efficiently. They should be moving enough water to keep them almost as cool as the pool.
3) More flow is better than less, because...
4) It's BTUs that matter, not temperature. You want heat energy to heat your water. Usually, the more water you flow, the more BTUs you get, even if the water from the panels is a little cooler.
5) If your panels are leaking, you are pushing them too hard! (experience talking)
6) This may be beautiful but it has nothing to do with pools!:p

http://home.earthlink.net/~dashmanc/pool/LeaningTower1.jpg

[chem geek]

Great pictures! And it helps me understand your solar system -- thanks. The statement I made that caused all this ruckus was:

When water flows faster through your panel, it spends less time in the panel so gets heated up less. So while you push more water through the panel at higher flow rates, it gets heated up less and the net result is roughly the same amount of temperature rise in your pool. You should generally stay near the recommended flow rate for your panel and not exceed its maximum rate.

I assumed that the flow rate was already in the efficient range (i.e. 80%). Instead of my saying "it spends less time in the panel so gets heated up less" I should have phrased this as "it spends less time in the panel so has a lower temperature rise. The total amount of heat transferred is more, but not proportional to the amount of flow rate. Doubling the flow rate does not double the amount of heat transferred if you are already in the efficient range." That's what I should have said. The part about "the net result is roughly the same amount of temperature rise in your pool" is true if "roughly" doesn't mean "exactly" but "approximately".

For your system in series there is no question that the resistance is so high such that your flow rates would be too low for what your pump can deliver (plus you need a bypass for the rest of the flow). So splitting your system makes absolute sense. In your case, you were not near the efficient range for your system so increasing the flow rate made a big difference. For the roof-mounted or rack-mounted solar panels like the FAFCO one I was referring to, you always want to have most of the panels put in parallel so that you get the flow rate into the efficient range.

Well, I'm glad we got that all figured out. Thanks for helping me phrase things better.

Thanks,
Richard

[CarlD]

Not a problem.

Generally, it's better to plumb in parallel. And, most solar systems should be done that way. But you do run into situations where it is simply not practical, like my pool.

I've spoken to FantaSea that they should re-consider how they plumb the solar deck. I don't know if they will--it's not their prime concern...right now they are having troubles replacing the fabricator for the solar panels, and it's hard to find plants that can handle the large blow-molds they use. It's a pity because they create a really unique pool choice. I get most of the benefits of an in-ground pool--it's 16x40, rectangular with a deep end, but few of the hassles--like the tax assessment increase they hit you with for I/Gs!

My conversion to a split system required minimal expense. You see the second Tee in the pic I posted earlier, the sluice valves and a few common fittings, and some extra hose. At the other end of the pool, I added two standard returns, and had to buy a hole-saw to cut them in. Each "side" of the system has its own return at the far end of the pool. The original return, at the near end, can be plugged. But I converted it to an overflow drain.

I simply routed a piece of hose from the return up the maximum height of the water that I want, then down to a drain. It's shaped like an upside-down sink trap. If I get too much rain, I don't have to worry about it overflowing and doing water damage, it's self-leveling.

So, for maybe $50-$100 in tools and fittings I vastly increased the efficiency of my system.

[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/sunheater/MANUALS/S220%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 (http://www.poolforum.com/pf2/showpost.php?p=41763&postcount=14) 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 (http://www.fafco.com/WarmWater/06963B_Solar_Bear_AGP_Spec.pdf) 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.

[ShelleyAnn]

Believe it or not...I actually understood what Daggit was saying. I wonder, though, if you plum in a series, this non-turbulent flow is non-existant as the water flows down one side of solar tubes to a 2" pipe, hangs a right, and then flows up the other side... 3 times.

Hate to ask it, but since we're all looking out our windows at feet and feet of snow and have nothing else to do but TALK about pools, which will get me the most hot water, turbulence (series) or flow rate (parallel)? Actually, I'm going to word it like this...which will make my pool warmer faster, as that is ultimately what I'm after.

Shelley

[CarlD]

Move your pool to Puerto Rico! That will get it warmer faster!:D

The issue of turbulence is unrelated to series or parallel. Plumb in parallel if you can.

[msm859]

Since we are talking about solar heating, has anyone tried heating their pool with a solar system that is smaller, but is enclosed with glazing and insulation to make hotter water?

[chem geek]

Enclosing a solar panel that is already at 80-90% efficiency in a "greenhouse" of glass will not improve the efficiency or generate more heat. Essentially, the water flowing through the panel pulls nearly all of the energy hitting it from the sun and the panel as well as the air near it in the "greenhouse" will remain cool. Yes, I know that Carl doesn't think that my FAFCO panels are that efficient and he may be right, but more on that later -- just hang with me on this assuming that the panels are operating efficiently.

There will be a benefit with regard to cutting down heat LOSSES if the air temperature is cooler than the water temperature, especially if there is wind.

The reason that solar hot water heaters are enclosed in the "greenhouse" style of glass is precisely for the reason of preventing heat LOSS, not for capturing more energy. Hot water is hotter than typical air temperatures so it is important to keep the temperature around the panels as hot as the water. Insulated multi-pane glass would ideally allow most of the sun's energy through but would prevent heat loss by keeping the temperature of the air around the panels to be the same as the water flowing in the panels.

[EDIT] It might make sense to use the "greenhouse" style for heating spa water since that is so much hotter than air (usually). I've never heard of solar used for spa heating and the raw panels are only specified to go up to around 90F though I can get my pool to around 92F if I don't tell the thermostat not to do that. I don't see why a hot-water solar "greenhouse" style panel couldn't be used for spa water though multiple panels would likely be needed since the spa has more volume (around 300-600 gallons) than a hot water heater (around 50-100 gallons) [END-EDIT]

Essentially, you need to look at this as if there is a limited amount of energy from the sun hitting a square area on the Earth so the best way to capture more of that energy is to have a larger area of panels. This assumes that you are already at high efficiency (80%-90%), which the flat "tube" panels have if you are at their recommended flow rates of 4-8 GPM. The "floor tile" types of solar heating, like the one that Carl has, are apparently not operating near peak efficiency since increasing flow rates in his system (by his splitting into two parallel systems) increased the heat dramatically (and probably cooled the tiles down as well).

Carl, I agree with you that the panels in my solar system should be cool to the touch even on a blistering hot day. If they are not, then they are not at 80%-90% efficiency. This summer, I'll go up to the roof to see if the panels are cool. They are connected in parallel, not in series, and right now have a flow rate of around 5.5 GPM so theoretically should be above 80% efficiency. I've never seen "steam" or "hot air wigglies" coming from the panels, but I'll go up and feel them on a hot day just to see how efficient they truly are. Though I wouldn't normally trust a manufacturer's specs, I worked with the FAFCO folks when I was getting an MBA in college and they seemed to have integrity, but then again that was over 23 years ago so there may be new players.

By the way, my diagram was supposed to show PARALLEL panels with all bottoms of the panel piped together and all tops piped together. I'm sorry if my drawing didn't make that clear. The water (as if you are sitting on a water molecule) flows through the bottom pipe, up ONE of the panels, then through the top pipe.

Just for the heck of it, let's calculate the maximum amount of possible solar heating capability based on the energy from the sun. The amount of solar radiation reaching the Earth's surface with the sun directly overhead (so noontime summer in northern latitudes) is around 1000 Watts per square meter of area on the Earth which is about 93 Watts per square foot (or 317 BTU/hour per square foot). You simply won't get more energy than that no matter what you do. Interestingly, the FAFCO documentation at this link (http://www.fafco.com/SolarPoolHeater/06359G_SunSaver_Spec.pdf) claims over 1000 BTU per square foot, but that isn't BTU/hour so is equivalent to around 3-4 equivalent "peak" hours (in Florida, by the way) so they are quoting a daily BTU rate. Let's see what happens if we assume the manufacturer FAFCO is right about 80% efficiency at 4 GPM. They essentially are claiming that they are heating my pool water at 800 Watts per square meter. My 12 panels have a mixture of sizes, but the total area is 36.74 square meters (effective area). So in theory, they claim that I am getting, at noontime peak during the summer, 29,400 Watts (almost 30 killowatts) which is about 100,000 BTU/hour (about half of the 200,000 BTU/hour of my gas heater's output) and is also equivalent to 25.3 million calories per hour. 1 calorie raises 1 ml of water 1 degree Celsius. My pool is 16,000 gallons or 60.6 million millilters so that means I should expect a peak temperature increase of 0.4 degrees Celsius per hour or 0.75 degrees Fahrenheit per hour.

I have measured the temperature rise in my pool with the solar on and I would say that APPROXIMATELY this peak temperature rise is about right. It is not, of course, a super-accurate measurement, but I would usually see about a 3 degree increase over 4 hours near noontime in June. The pool loses about 2 degrees overnight in the summer (remember that we try to keep it at 88F and have an opaque cover) and the solar typically clicks on around 10 AM (the pump starts at 9 so the solar turns on when the solar panels are warmer than the pool water) and by 1 PM or 2 PM at the latest, the pool is back to its lovely 88F. So I'm pretty certain that the panels are at least over 50% efficiency if not close to their 80%. My actual flow is more than the 4 GPM (it's closer to 5.5 GPM) so my efficiency should be higher (about 84%), but I've got the piping problem I mentioned in an earlier post that is likely causing "low flow" to three of the panels. And you are right that I should shoot for 8 GPM and 90% efficiency if I want to heat my pool a little faster -- I just need to balance that with the tradeoff of pump electricity costs.

Richard

[CarlD]

You may want to check with the manufacturer to see if this is recommended or will void the warranty. It seems to me that most enclosed systems I've seen are in permanent roof-mount systems for household hot water--the "greenhouse" may well prevent freezing. But that's just a guess.

Usually, since it's for summer swimming, it's not an issue. Even if my panels can keep my water warm when it's 50 degrees and windy out, I'm not going in! (unless I have to repair a leak--been there, done that...brrrrr! THAT will convince you that a wetsuit is great to have around!)

[chem geek]

Regarding the laminar vs. turbulant flow discussion, I did some calculations on that a while ago when I started looking at my piping situation and I thought it strange that the FAFCO solar panels specification showed a parabolic curve for the head loss vs. flow rate since that implies turbulant flow. I counted around 200 tubes per panels with spacing around 1/4" so at the desired 4 GPM that would be 4/200=0.02 GPM per tube. Even if the inner diameter of each tube were as low as 0.1", the Reynolds number would be 632 which should be laminar flow (which has a Reynolds number less than 2000).

I also ran some calculations to predict head loss and could not get their results. I wrote to them about this asking them if they actually measured the head loss or if they calculated it. They wrote back (which gives them points in my book -- many vendors don't even respond to questions) and said that

The head loss was physically measured on a sample number of collectors matched against theoretical calculations. The contributors to the overall head loss include the header pipe, small tubes, and metering plenum that evenly distributes flow to the small tubes. It appears the metering plenum is the missing component in your calculations.

They were absolutely right. I had not accounted for the metering plenum -- I didn't even know it was there. So their tubes probably aren't that narrow in inner diameter so the flow is most certainly laminar inside those tubes, but getting from the main pipe into these tubes goes through constrictions designed to restrict overall flow rate and to evenly distribute the water and THAT is where the turbulant flow exists and is probably where the bulk of the head loss comes from (thus resulting in the parabolic curve).

Now, interestingly, FAFCO has a dimpled version of their solar panel (see this link) (http://www.fafco.com/SolarPoolHeater/FAFCO-Revolution.html) called "Revolution" that causes the water in the small tubes to spiral (slowly). This makes the panel output about 5% more energy due to its higher efficiency at transferring heat from the sun to the water because the water is better "mixed" in the tubes so that it all gets heated thus having a lower temperature difference between the water and the panel itself (remember, as Carl pointed out, that the highest efficiency is achieved when there is a minimal temperature difference since that keeps the panel cooler and minimizes the radiative losses -- a cool, or at least "air temperature" panel is an efficient panel).

Richard

[CarlD]

The nature of my panels makes the turbulence FAR more important than it typical Fafco roll-up panels. Fafco uses the manifold-to-tubule design to maximize the surface area exposed to water.

I don't have the physics and fluid dynamics understanding that these other guys do, but I suspect the tubules reduce the laminar/turbulence effect.

My panels, on the other hand, have a series of relatively large chambers, each about 2' across and 4" long by 1" high that go across the panel. The water flows like a snake from one chamber to the next and out the far end. With such relatively large passages, I suspect the laminar effect is more evident so anything that breaks that down makes them work better.

Somewhere or other I have the FantaSea documentation about heat energy transfers of the panels.

Mainly, much of solar heating can be handled by rules of thumb and common sense. While Richard's and Daggit's analysis will let your squeeze every BTU available out of your system, it's VERY easy to get excellent results you are DELIGHTED with using a simpler approach.

The great thing about the roll-up panels is it is VERY easy to add another if you aren't happy with the amount of hot water you are generating. If you start by having your panels having a surface area equal to 1/3 your pool's surface area (a MUCH lower number than normally recommended--you can go down as far as 1/4 and still get good results) and set it up so you can easily add more panels (in parallel) you should be able to easily find the optimal set-up for your pool. Ah, the joys of PVC glue-on fittings, PVC pipe, and TigerFlex! I DO recommend a good solar cover when you are heating it and not swimming. It will add its own good measure by both insulating the water and by acting as a greenhouse. I've been happiest with heavier weight clear plastic covers. I don't think blue or opaque work as well.

My pool got to 98 F last summer--I found it too hot to enjoy, but my wife believes there's ice cubes in the water (and that she can feel them!) if it's below 92 deg.

Meanwhile my plastic welder arrived this week and I have to learn how to use it...

[drogers]

Dragging up an old thread to add a new wrinkle... I was talking to a solar installer today who relayed some interesting theories regarding the parallel vs serial, and flow rate debate.

Much of California is expected to eventually move to time of day billing for electrical usage, which sicks for solar, because the highest electrical bills will exactly match up with the most efficient time to run your solar panels.

There are experiments going on surrounding running panels in series at VERY low flow rates to get a much higher delta-T on a given quantity of water while using less electricity.

The theory is that dropping from the 90+% efficiency level on the panels to the 70s or 80s is a workable tradeoff if you can cut electric use by 50-80%. There are caveats however:

* a higher delta-T on water leaving the panels means that heat loss in the pipes returning to the pool may become an issue in some installs (insulated pipes would then be required)

* you may lose a few days of your swimming season if you were relying on that extra 10-20% efficiency on each end of the warm season

* nobody's run all the numbers yet (added run time, etc)...

thoughts???

Frankly, I'm concerned about paying for solar at this point looking at the future cost of running my pump during peak hours. makes the heat pump look closer and closer in overall TCO. If my climate were a little more amendable to heat pumps, the balance might tilt at some point in the coming years.

[chem geek]

Take a look at this PDF file (http://www.fafco.com/SolarPoolHeater/06359G_SunSaver_Spec.pdf) and look at the EFFICIENCY vs. FLOW chart. Though one can debate whether this is accurate, it's at least a starting point for discussion. According to this chart, the solar panels are 80% efficient at the recommended flow rate of 4 GPM per panel. The maximum flow rate of the panel is 8 GPM and the minimum is 3 GPM. The max and min are recommendations. The panels themselves can handle an awful lot of pressure so the maximum can probably be exceeded without a problem, but there might be issues with the "header" that distributes water evenly if the flow rate is too low, BUT I DON'T REALLY KNOW.

At 2 GPM the efficiency is about 70% while at 1 GPM it's about 60%. So let's do an analysis at 2 GPM as an example, comparing against 4 GPM. My own system is rather large with 12 panels so at the normal 4+ GPM that is 48+ GPM total (my panels are connected in parallel) while 2 GPM per panel would be 24 GPM total. For the Inteliflow (and 4x160) pump, I determined the pump curves to be determined with a pretty good fit by the following equation:

Head (in feet) = (RPM/350)^2 - (GPM^2)/470

and the output power is given by

Output Power (in watts) = Head (in feet) * RPM * 0.188165

Near the peak efficiency point, the pump is about 50% efficient and this point occurs roughly where the specific speed of the pump is at 1320 per the following formula:

Specific Speed = RPM * sqrt(GPM) / (Head^0.75)

Now things get tricky to calculate since we need to determine the system curve. The panels don't add much to the Head and have the following head loss formula (remember that the panels are connected in parallel so the loss for one panel is the loss for the system of panels):

Solar Panel Head Loss (in feet) = (GPM^2) / 8

The cartridge filter I have has a head loss curve as follows:

Cartridge Head Loss (in feet) = (GPM / 62)^2

Though the actual head loss calculation for a 2" (nominal) pipe (the size of all of my piping except for the suction side) is complex, I can see that it is approximated by the following formula:

2" Pipe Head Loss (in feet per 100 feet) = (GPM^1.8) / 295

and the suction side with TWO 1.5" pipes is approximated by the following formula (for each pipe):

1.5" Pipe Head Loss (in feet per 100 feet) = (GPM^1.8) / 90

so the actual GPM to use for the 1.5" pipes is half of the system GPM since there are two suction pipes right up to the pump.

Let's assume 100 feet for each 1.5" suction pipe and 400 feet for the 2" pipe on the output side of the pump. Then our expected head loss at 48 GPM and at 24 GPM will be as follows:

48 GPM: 4*(48^1.8)/295 + ((48/2)^1.8)/90 + (48/62)^2 + ((48/12)^2)/8 = 21.7 feet

24 GPM: 4*(24^1.8)/295 + ((24/2)^1.8)/90 + (24/62)^2 + ((24/12)^2)/8 = 5.8 feet

Using the Inteliflow equation and solving for RPM (which we don't really need, but I want to cross check against the Intelliflo curves) I get:

RPM = 350 * sqrt(Head + (GPM^2)/470)

48 GPM: RPM = 350*sqrt(21.7 + (48^2)/470) = 1805
24 GPM: RPM = 350*sqrt(5.8 + (24^2)/470) = 928

The output power is:

48 GPM: Output Power = 21.7 * 48 * 0.188165 = 196 Watts
24 GPM: Output Power = 5.8 * 24 * 0.188165 = 26 Watts

Assuming that we are roughly hitting the 50% efficiency spot on our pump power curves, then this means that whereas before we were at around 400 Watts, with half the GPM rate we are now at around 55 Watts. However, there is an overhead of power lost in the pump windings and this appears to be around 80 Watts so in reality our output power is probably something closer to comparing 450 Watts vs. 130 Watts or a savings of around 70%. We only dropped in efficiency of the solar panel from 80% to 70% so clearly this is a large savings overall in electricity costs even if you run the pump 14% longer to make up for the loss in solar panel efficiency.

Note that to make all of the above work out well, you have to have a variable speed pump. You COULD try just using slower flows and downsizing a single speed pump, but it's harder to hit the sweet spot. With my current pump and piping situation, I actually see nearly 30 PSI at my filter gauge so including suction loss it's probably 32 PSI or so actual total head loss (that's about 75 feet!) with a 65 GPM flow rate (based on my current pump curves). This implies an effective output pipe length of closer to 1000 feet so something is very very strange about my system that I haven't figured out yet (perhaps all those twists and turns in the pipes are adding up to much more than I think).

Note also that cutting the GPM rate in half means that it takes longer to have a full turnover of your pool. In reality you need to run your pump twice as long, not just 14% longer (to make up for the solar efficiency). So when dropping the GPM rate in half from 48 GPM to 24 GPM you are really comparing 450 Watts against 260 Watts for equivalent run times so the savings is really around 40%. Due to the fixed power consumption "winding losses" (and other losses) of the pump regardless of RPM, it doesn't make much sense to go much slower.

Richard

[ShelleyAnn]

Okay...I've purchased two additional 3.5 x 9 solar panels for a total of 4. I was going to plum them in series, however, I don't think the water would be in each panel long enough to heat it up enough. (Last year the water was snaked down one half of the panel and up the other, then to the second panel, down one half of it and up the other and to the pool, the water felt down right hot coming out of the return!) Over the winter I realized the panels are only rated at 8 gpm and I'm forcing the entire 43 thru them! I plan on giving each panel it's own feed to lower the psi, but if it only goes in one end, travels 9 ft. and out, instead of down one half and up the other and out for 18, feet will it heat the water up enough?

I'm also tossing the 1.5" corregated flex pipe the PB installed for some 2" flex PVC with the smooth inner surface hoping to decrease drag. This seems to make sence except the solar panel's opening is only 1". I don't know if I should use 2" if it's opening is only 1".

Any suggestions?

Shelley

[CarlD]

I was going to plum them in series, however, I don't think the water would be in each panel long enough to heat it up enough.
....

Any suggestions?

Shelley

Yes. Abandon the assumption that the water needs to heat up in the panels and come out "hot". There ARE limiting factors to solar panels, and C'Geek can (and has) explained them. But they are limits to a fundamental truth: the more water you move through the panels the more heat you'll get in your pool.

Think of your panels as a giant car radiator working in reverse. Do you think that SLOWING DOWN the rate of water flow through your engine will cool it MORE???? Of course not--the faster the water moves the more heat is pulled out of the block to be distributed by the radiator. Sure there are limits, but that's why when your water pump isn't moving as much water, your engine overheats. The panels work the same way.

Remember how I ALWAYS say: In direct sun your panels should be cool to the touch, or, at most, mildly warm. Imagine: it's 95 degrees and the sun is blistering, downright cruel. But your solar panels are COOL! What is going on? Turn 'em off and they get firey in minutes. Clearly the water is cooling them. EXACTLY!

And all that lovely heat energy is dumping into your now-comfortable pool!

Sure, there are maximum efficiency points on the panels, and too much pressure can cause cavitation, not to mention LEAKS, and there's fancy issues with the fluid in the center moving faster than the fluid on the wall of a chamber--but Chem_Geek can explain that to get MAXIMUM EFFICIENCY from a panel.

Meanwhile, parallel plumbed panels each can function as a separate entity, at max efficiency.

In general (with some caveats and exceptions) the more flow you get the faster your pool heats up. You don't "wait for the water to warm up" any more than your car engine waits for the water to warm up in the block.

It "seems" to make sense, but it doesn't really.

I keep hammering this: You want BTUs, not temperature to warm your pool. Pool heaters are rated in BTUs--because that is how heat energy is transferred.

Remember: A BTU (British Thermal Unit) is the amount of heat energy needed to raise 1 pound of water 1 degree Farenheit. It takes 10x more BTUs to raise 6800 pounds of water 1 degree that it takes to raise 68 pounds of water 10 degrees. (that is, 100 cubic feet vs 1 cubic foot)

[ShelleyAnn]

CarlD - Let me ask it this way...Do you think the water will be as warm as last season when I forced all 48 GPM thru 32' of solar panel?

[CarlD]

CarlD - Let me ask it this way...Do you think the water will be as warm as last season when I forced all 48 GPM thru 32' of solar panel?

I think if you plumb all your panels in parallel rather than serial you will see a SIGNIFICANT boost in effectiveness, especially if you add the two new panels that way.

Remember: It's BTUs that matter, not temperature.

Once I get my panels hooked up, with this nice weather I should see temp gains of 8-10degrees per day. When all the panels were in serial, I'd see 4-6 degrees per day. (my panels are 2'x4' but I have about 30 of them--split into 2 parallel plumbed groups).

[ShelleyAnn]

Carl D - First off, thanks so much for hanging in there with me. :)

Question - Won't I have the same amount of water going thru as last year, just without the too-high pressure?

Last question - Do you think I should bag the corregated hose I had last year and spend the $$ on flex PVC with the smooth lining? Will there be that much difference?

Thanks again.

[chem geek]

ShelleyAnn,

I'll take a stab at this one, at least for your first question. When connected in parallel, the total flow rate will be much higher because there will be far less resistance in the solar panel piping compared to being connected in series. Think of it this way -- your pump is spinning at a certain fixed rate (RPM) so trying to force water through a longer but narrower pipe (i.e. solar panels in series) is much harder so will result in higher pressure and lower flow rates compared to forcing the water through a shorter but much wider pipe (i.e. solar panels in parallel) which will result in lower pressure and higher flow rates. When I refer to flow rates, I mean through the entire system into your pool, not "per panel".

Now it is true that the flow rate per panel when connected in parallel may be less than the flow rate when they are connected in series, but when in series the water heats up as it goes from panel to panel and this higher temperature water radiates its heat back into the air via a hot panel (the last panels in series, especially). When connected in parallel, the panels stay cool and absorb heat from the sun most efficiently. The much larger total flow rate more than makes up for the smaller temperature increase on output into your pool -- this is what Carl means when he says it is the BTUs that matter, namely the total heat that is transferred to your pool.

Imagine it this way. If you heated one cup of water to 100F higher temperature and added it to your pool, it would have the same effect if you took 100 cups of water to 1F higher temperature and added it to your pool (ignoring the small change in total water volume). It's the same amount of added heat in either case. The reason it is better to add more water volume at smaller temperature increase is that this is more efficient. You can imagine that having a cup of water at 100F higher temperature than your pool water won't stay that way very long sitting in a cup -- it will lose heat rapidly since it is so much hotter than the surrounding air. In the same way, water flowing through solar panels that is much hotter won't stay that way if the air around the panel is cooler (which is typically the case -- the air is cooler, but the sun's energy makes the panel slightly warmer and transfers this heat to the water flowing in the panel).

Richard

[CarlD]

Carl D - First off, thanks so much for hanging in there with me. :)

Question - Won't I have the same amount of water going thru as last year, just without the too-high pressure?

Last question - Do you think I should bag the corregated hose I had last year and spend the $$ on flex PVC with the smooth lining? Will there be that much difference?

Thanks again.

You know, I really don't know the answer to this. It SEEMS to make sense, but there will be water trapped along the edge while other water moves past it, but I'd be lying if I said I could give you a definitive answer.

I personally prefer TigerFlex because I can use glued-on fittings, which I consider far more reliable than barbed fittings and hose clamps. Yet corregated hose is VERY easy to work with and will bend to fit FAR tighter corners than T-Flex. The connections between my myriad little panels are all corregated and T-Flex would be impossible to use.