Trying to keep up w/ Chem Geek

[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 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 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:


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.





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!

[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/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?