Trying to keep up w/ Chem Geek

[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 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.