Plastic solar panel considerations

kitamimanJanuary 9, 2011

First I would like to thank all members who have replied to my posts in the past. I really appreciate their help.

Pool is infinity edge, approx 20,000 gallons, surface area 525 sq ft. Pump is Intelliflo, with Pentair 520-sq ft Clean & Clear Plus Cartridge filter (oversized to reduce resistance to flow).

I am installing a set of 10 typical FAFCO-type plastic solar panels, each 20' x 2'. The 1.5-inch inlets/outlets of the panels will be connected in parallel between 3-inch flow and return pipes.

I am at the same latitude as Miami and subject to hurricanes, so the panels will be ground mounted, which means that they will be below the water level of the pool and thus always full of water.

In case a panel springs a leak I am thinking it would be wise to put a Hartford loop in the flow pipe just before the panel feeds (there is another one in the main flow circuit that would incidentally protect the return pipe from the panels). Based on the above setup, I have a couple of questions.

1) My main need for heating is between November and March. I was going to build low racks for the panels or perhaps retaining walls to allow me to slope the sand (all terrain is sand here) beneath each panel and achieve a slope of 25 degrees or so. However, the more I look at it the more I am realizing how much work and expense is involved. Would it make more sense to simply lay them flat on the ground and take the hit on efficiency?

I was thinking that an alternative might be to simply add an extra panel or two to recoup the loss in efficiency of laying flat, but how about pressure/pump load - is adding extra panels a problem? I am unsure of the effect, in that extra panels presumably mean more resistance, but there again the additional channels linking the large flow and return pipes might result in less resistance...?

2) There will be times in the summer (90F+ here) when the power is off and the pool is unattended (will use mosquito dunks). Even without that, we are subject to fairly frequent power cuts that can last several hours. What about overheating if the water is not circulating? Are there any precautions I should take?

All input is very much appreciated - flying solo here as no PBs available...

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Solar below grade has issues, especially in your geo.. Without circulation, water will stagnate in the panels. Water will turn to steam and burst the panel to relieve pressure on those hot days. While the Hartford loop will prevent losing much water when the system is off, it won't do anything when it gets turned on. Gusher!

Covering the pool with a solid safety cover will keep the mosquitoes out when you are not there. Dunk pollute the water. No light, less algae, no larvae.

Is a heat pump a reasonable idea?


    Bookmark   January 9, 2011 at 12:29PM
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I would agree with you that it might be more cost effective and certainly easier to just add panels to a flat install rather than build a rack. A larger flat install would probably be my choice. However, it might be a little harder keeping the panels clean with them on the ground. Adding the panels in parallel will reduce the total head loss so as long as you plumb them correctly, it shouldn't be a problem.

As for leaving water in the panels, the panels themselves are much more resistant to heat than is the PVC piping feeding the panels. If the water gets really hot, the PVC can soften some but since they are not installed on a roof, there will be no suction to collapse the pipes so you should be ok. CPVC is also an option although I am not sure you really need it in this case. If the panels were in the desert on top of a two story house, I might recommend that.

Also, if you are really concerned about this, I would use isolation valves for the supply and return of the panels to isolate the panels from the rest of the plumbing and have a drain valve near the panels so that you can drain them if necessary.

    Bookmark   January 9, 2011 at 12:32PM
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Thank you both for quick response.

poolguynj - Rats! I already bought the panels so would like to use them. Would pressure really build to the point of blowing the panels - would it not simply push water into the pool? How about using PRV (one? one per panel?)...or could I possibly add small solar pump to provide minimal circulation even when power is out...?? Finally, if I drain the panels when I am away in the summer, are they OK empty?

Cover would be nice but difficult to secure well enough in hurricane country with my pool (52'long L-shaped infinity lap pool). I read that you could bring water back OK after using dunks; any other readers have experience of using them?

mas985 - Will take your advice re valves/drain - thank you.

    Bookmark   January 9, 2011 at 1:10PM
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In order for the water to turn to steam, it needs to get to 212 degrees F. As the temperature rises in the panels, the heat transfer rate due to radiation, convection and conduction will increase to a point where it equals the heat gain of the sun so water temperature will remain high but no where near the boiling point of water. I estimate that heat loss and heat gain equilibrium will be reached at a water temperature of around 140 degrees. Again enough to soften the PVC but no where near boiling point.

    Bookmark   January 9, 2011 at 2:50PM
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Steam forms well below boiling. Boiling is just submerged evaporation and can form higher pressures. Fresh coffee anyone? The boiling point is relative to atmospheric pressure.

If the entire panel is flooded and stagnant (not moving), and, pressure may build but the temperature of the water vapor and air will rise. Isolation valves will make this pressure build more. Eventually, a leak will erupt, usually in a rib, not in the main plumbing. This is why panels need to drain when not in use.



    Bookmark   January 10, 2011 at 8:50AM
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Technically evaporation and boiling are two different mechanisms driven by the same thing, vapor pressure. Evaporation occurs at a water/air boundary while boiling occurs within the liquid itself. Also, by definition, steam can only be caused by boiling and not evaporation. So evaporation cannot occur in the panels but only at the pool surface and boiling will not occur until the water reaches 212 deg F which is not possible because of heat loss.

However, as the water heats up in the panels, the water density decreases and it will start to expand. In a closed system, this will cause an increase in pressure but what we are talking about here is not a closed system. The panels are connected to the pool and water will migrate towards the pool equalizing the pressure. It is not possible for pressure to build up in an open system.

But even in a completely closed system, the panels would still not burst because the change in volume of the water is very small and can be absorbed by the panelâÂÂs elasticity. Water going from 85 degrees to 212 degrees will only expand by 4% (see link) which is easily absorbed by the panels.

Also, I was not proposing to close the valves with water in the panels. The valves should only be closed when the panels are EMPTY (i.e. full of air). This can be accomplished with a vent valve and a drain valve and with the isolation valves closed. But again, I don't think this is necessary and there are also a lot of examples to illustrate this point.

There are many pools operating today that allow water to sit in the panels without any ill effects. One thing to keep in mind is that when an automatic solar control shuts off a solar system, the panels do not drain automatically UNTIL the pump is shut off so many installed systems today have water that sits in the panels with hot sun beating down on it, including mine. In addition, there are many panels installed that do not have vacuum release valves with the panels sitting on the ground and so never drain unless manually drained. Most AG pools are plumbed this way. If this was a problem at all, there would be a lot of people reporting burst panels. But again, the physics simply does not support bursting panels.

Here is a link that might be useful: Properties of Water

    Bookmark   January 10, 2011 at 12:39PM
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I would expect a roof mounted solar panel with a release to drain via a slight venturi action. The intake side is typically what gets valved off. The vac release has no more water pressure and can open. The water wants to seek it's own level. The pool return flow would draw in the water from the panel(s). Any water left after the pump shuts would then drain normally.

The OP still has ideas about a below grade system. I don't see it being a good idea in the long run. Mice chew everything. Birds peck. Landscapers err. Kids play ball and trip. etc... Murphy sees all.

I still don't like the idea of dunks polluting the water, though I do understand your motivation. Big pool. Safety covers are usually good to Cat II unless hit by sharp, flying objects



    Bookmark   January 10, 2011 at 2:33PM
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On your first point, the weight of the water must overcome the pressure of the pump before any water will drain out of the panels. For example, in my setup, the pump pressure with the solar off is about 11 PSI. Therefore, the water column that the pump can hold up is 11 * 2.31 or 25.41' which is higher than my panels so the panels will not drain and in fact the VRV doesn't open until the pump is shut off so the panels can't drain. Anyway, I have confirmed this by removing one of the EPDM tubes close to the VRV and a very slow stream of water drips out of the header indicating positive pressure. However, because the pressure is somewhat balanced between the supply and return sides of the panel, no water will actually flow through the panel during this time.

Your second point, I don't total disagree with you but both birds and mice would also create holes on top of a roof as well if they really wanted to but I suspect there is no benefit in doing so which is why they won't bother. Panels are certainly more vulnerable to the other things but again, solar panels on the ground is not a new concept and many others have been sucessful doing so. Also, I think the OP was describing panels below the water line not necessarily below grade.

    Bookmark   January 10, 2011 at 3:09PM
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Thank you both again for your input.

Just to be clear, I am talking about putting the panels on the ground surface, which owing to the local lie of the land is maybe 3 feet below the water level in the pool. Even if I do build racks, they will be the minimum height required to get the tilt needed, or possibly just banked sand within retaining walls, which is still ground mounted. The problem is hurricanes; my concern is not stuff flying around and hitting the panels or pool covers, it is the panels (+racks if wooden) and pool covers themselves becoming airborne. House is beachfront facing the open Atlantic, and 100 mph + winds within the lifetime of the panels is pretty much a given.

I must say that Scott's mention of mice does give me pause for thought; I know that they will chew plastic, because a rodent - I'd like to think it was a mouse but suspect it was a larger relation - chewed a hole in the plastic pipe in the bottom of my dishwasher, presumably because it contained water, as the panels will too of course. However, I can't see how racks or any other mounting would help - the critters get everywhere, even up to the roof (how is a mystery). I'd like to think that outside it would be a lot easier to find water than it was when trapped in my kitchen. The large area of solar panels and rodents' ability to burrow makes it pretty much impossible to make them 'mouse' proof. All I can think of is caging the lot and rounding up the local cats. If anyone reading this forum has actually had panels chewed then please post...

Thanks again for all input.

    Bookmark   January 11, 2011 at 12:55AM
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Flying panels! Broken plumbing. Beach erosion. I can see that.

Even if the panels were strapped down, any anchorings for the straps could shift with the sand.

I think you'd be better served with a heat pump. At least it won't blow away.


    Bookmark   January 11, 2011 at 5:50AM
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