Trying to keep up w/ Chem Geek

[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) 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 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)