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Solar Cover...color And Thickness


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#1 pgouellette

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Posted 27 March 2009 - 06:15 AM

I am looking for a new solar cover and interested how much difference thickness and color make.

Clear seems best choice to allow more sunlight through but also may affect water chemistry, so maybe BLUE is the best choice?

Also thickness is 16mil better then 12 mil...seems a thicker cover would minimize loss of heat.

Please give me your thoughts

Paul

#2 Dr. Spa

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Posted 27 March 2009 - 06:33 AM

It's not about letting in light, but absorbing heat. A darker, less reflective surface is better.

Thicker isn't going to be more insulating, but it will last longer as it will take the UV longer to break it down.
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#3 chem geek

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Posted 27 March 2009 - 08:32 AM

I disagree. You absolutely want the cover to be clear to let in infrared and visible light. If you want to cut down chlorine loss, then the cover should be opaque to ultraviolet (UV) light which is often true of many plastic covers with UV stabilizers. If you were to shine light through a cover and the light had more of a yellowish tinge to it, then it is likely absorbing some UV as well as blue.

A dark cover will absorb the sun's energy and heat up, but if the cover is to transfer this heat to the pool water then by definition it must be good at heat transfer which makes it bad at heat insulation. So an optimal black cover that was thin enough to transfer heat efficiently would also lose more heat at night from conduction (though it would at least cut down heat loss from evaporation).

A clear cover will pass light through and most of this will be absorbed by the pool water. Even in a white plaster pool (without a cover) 60% of the sun's energy is absorbed by the water (see technical details in this post). The light energy absorbed by the water is converted to heat energy and this is then trapped by a well-insulating bubble-type cover since still air (inside the bubbles) is a decent insulator of heat. This is essentially the greenhouse effect where light passes through but glass reflects back (holds in) heat.

If you wanted to maximize heating of the pool, then have the pool surface be as dark as possible. That way, the light passes through the cover, gets absorbed by the water and any remaining light is nearly completely absorbed by the bottom (and sides) and this heat is then trapped by the cover.

I have an opaque electric safety cover which unfortunately is somewhat thin and it loses heat at night about twice as fast as a bubble-type cover. So with no cover I'd lose around 4-6F overnight while with my cover I lose around 2-3F but with a bubble-type cover I'd lose around 1.5-2F. I also do not get heating unless I either open the pool during the day or have the solar system turned on. However, my cover is not black (it's tan), but even if it were the amount of benefit from heating during the day would be lost by the heat loss at night. With my cover already being opaque, black would have been a better choice (though not look nice which is why we didn't do that), but a clear bubble-type cover would be even better. Unfortunately, I haven't seen well-insulating covers in automatic systems, at least not when we put in our pool 7 years ago.

If one does decide to use a dark cover, then you need to have the returns force good circulation of water near the surface or else you will mostly have a gradient of heat where it is hot at the surface but the deeper water does not heat up efficiently. If the water near the surface is hotter, it just transfers this heat back through the poorly insulating cover back into the air. If the black cover is a well-insulating bubble-type, then the top of the cover gets hot, but does not efficiently transfer this heat to the underside of the cover touching the pool. [EDIT] An interesting combination I haven't seen in person, but is described on the web, would be where the underside of the bubble-type cover were black but the bubble and top layer were clear. That might work OK though you still need good circulation to prevent just heating the top layer of water. Even better would be a bubble-type cover with a silvery one-way mirror bottom that would let sunlight pass through to the water, but would then reflect light coming from the pool back into the water -- this would appear black from the outside. [END-EDIT]

As for cover thickness, after the physical barrier eliminating evaporation, the next most important aspect retaining heat are the air bubbles, not the thickness of the plastic material. Most of the heat is blocked by the layer of still air in the cover's bubbles. The main reason for having a thicker plastic, as Dr. Spa said, is that it will last longer, but it's also more expensive. So it's a tradeoff with how often you have to replace your cover.

Richard

#4 pgouellette

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Posted 27 March 2009 - 09:28 AM

Richard, thanks for your insight. So if I went for a light blue, UV Stablized, 10or 12ML thick I should be ok.

Any recommendations on manufacturer?

Paul

#5 chem geek

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Posted 27 March 2009 - 10:00 AM

I can't recommend any manufacturers, but note my edits to my post above. If you can get a bubble-type cover that is black on the bubble side touching the water but is clear (or light blue) on the top side, then that is an OK alternative though you need to make sure circulation moves water away from the surface. I have also seen some metallic covers, but they do not appear to be one-way mirror-like.

There is also this cover but though it absorbs light and transmits it to the water, it is so thin that it will lose heat at night and their graphs "cheat" about this with conditions that aren't particularly cool or windy and they were not using clear bubble-type covers for comparison (a more opaque bubble-type cover would not have the water absorb light to create heat). This deception is obvious when one looks at the graphs and sees that the temperature with a blue bubble-type cover is the same as with no cover at all. It just shows that an opaque bubble-type cover just retains heat (say from a solar system), but does not add any because it doesn't let any light through. In fact, the tanks were very shallow at 5 cm so ensured that there would not be very much absorption of light by the water. One really has to be careful when looking at manufacturers claims.

The bottom line is that the ideal cover maximizes the greenhouse effect by letting light into the pool but keeping heat in. If the pool surface can absorb more by being black, then that's better; if the pool cover can reflect light back into the pool (i.e. one-way mirror effect), then that's another way to accomplish the same effect. The bubble-type with a black layer between the pool water and the air insulating layer would be the next best thing assuming circulation was really good. I've seen reports of people with these types of covers that say the surface of their pool gets hot but the bulk pool water does not, but their circulation may not be very good (i.e. above-ground pools with no floor drain, for example).

Richard

#6 littlecloudflower

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Posted 15 June 2012 - 10:55 AM

I am a bit confused. Why does the opaque cover reduce chlorine loss more than the clear covers? Is it because of the sunlight or the heat?

#7 waterbear

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Posted 15 June 2012 - 01:48 PM

It is because of the blocking of UV light. However....

We are forgetting that the MAIN purpose of a solar cover (the floating bubble type) is to PREVENT OVERNIGHT HEAT LOSS FROM EVAPORATION and not heat the water during the day. If you want maximum heating during the day then UNCOVER THE POOL. You are talking about heat transfer through what is essentially bubble wrap which is an insulator! The color of the cover is moot. It is two sheets of plastic with air bubbles

If you are trying to prevent chlorine loss from UV light then increase your CYA and your FC level. Also, pool chemistry tends to be less problematic in pools that are uncovered than in covered pools.


Richard,is comparing his experience with a solid electric automatic pool cover on his pool with a bubble type of floating solar cover which is comparing apples and oranges.

As far as Solar covers go. the thicker the cover the higher the price and the longer the (prorated) warranty so you have to look at cost vs. longevity. In actual practice the 12 ml covers seem to give the best cost/performance ratio. How long a cover lasts depends on many factors, the biggest being how much exposure to sunlight they get (which is wny they are supposed to be covered when removed from the pool to protect them from UV light, which causes them to break down any why many also come with a cover to put over them when rolled up.
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#8 cabowabo

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Posted 19 June 2012 - 03:34 AM

Just my 2 cents to consider also. I have a 12ml bubble type cover that is heavy. I couldnt imagine having a 16ml or anything higher than 12 due to the weight. Heavy covers will indeed last longer but you better have a comercial type solor cover reel, or cut it in very managable sections to remove and put back on if your getting one larger than 12ml.

I have an irregular shaped pool so a managable cover is more important for me, a standard shape will be easier to manage with a heavier cover. Just something to consider.

My last cover was a 12ml clear bubble and it did warm the water better than the previouse blue cover I had. It was the best combination of heat transfer, retainment and durability of any bubble type I had.




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