Why do "solar covers" have bubbles?

[M T]

I'm picking plastic bits out of my pool as yet another pool cover disintegrates. In our area, there is no pretense that pool covers are recyclable. Pieces of it will be stuffing my trash barrel for at least a month. That's a hidden cost (not to me, but to the environment) in the use of solar covers. (By the way, I noticed that the bubbles within a few inches of the seams failed before the bubbles further away from the seams. Something (heat?) in joining sheets at the seams appears to weaken the bubbles.)

As I go to pick out a new solar cover, I find nothing has changed in the last couple of years. As consumers, I think we all agree that the primary purpose of solar covers is to prevent evaporation, thus saving water and either saving energy or extending the pool season. The advertising verbiage about the bubbles focusing sunlight is just an advertising gimmick..

Saving on chemical costs is a secondary advantage. There is some advantage to solar covers that filter out that part of sunlight that breaks down hypochlorous acid (chem geek indicated it was around 235-240nm in this thread - That's UVC). I don't know whether the thinnest (8 mil) covers block 100% of that wavelength, or whether thicker covers block more than thinner covers.

Meanwhile, the pool cover needs to withstand UV radiation itself as well as be resistant to the effects of pool chemicals. A quick web search indicates there are a number of additives to protect polypropylene (etc.) from UV radiation. I presume these are already in what we buy, but what I saw suggested longer life that we're seeing. Perhaps the pool chemicals interfere with or leach out the additives. I wish the vendors or manufacturers, or some testing entity can identify exactly what is in we get sold.

But, my question today, why do all the solar covers have thin-skinned bubbles?

The covers that I've had over the last 20 years have an upper layer that is flat and relatively thick compared to the skin of the bubbles. The material that is the skin of the bubbles is quite thin. In my experience, it is usually the skin of the bubbles that fails first. That isn't surprising because a thinner material is likely to degrade or be abraded more quickly.

What is the purpose of the bubbles? I suspect they are instrumental in the buoyancy of the the cover. They also implicitly cause users to always orient the covers the same way. Potentially they provide some rigidity of the cover or other structural benefit (consider how Saran Wrap folds & clings to itself).

Or maybe the bubbles are just there to make the consumer think they provide some benefit ("focusing the sunlight", "transferring heat to the water") That way, consumers don't go out and buy cheaper plastic sheets that are not specifically sold for pools (Lots of examples at Amazon).

Why aren't pool covers offered that have a flat surface toward the water? If a 12-mil pool cover was a single sheet of plastic, with no differentiation in cross section, why wouldn't that be a better option? The full 12-mil of plastic would have to fail rather than just some thin (1-mil?) bubble layer.

I'd welcome any manufacturer to tell us why their solar covers are constructed the way they are. (Hmmm... Maybe I should search for patents.)

The one possible answer I can imagine is that the bubbles and the main sheet are different materials. The bubbles are more resistant to pool chemicals and are there to help protect the main sheet, lifting it above the water. The main sheet is more UV resistant and protects the bubble layer. (Even so, why not make the skin of the bubbles thicker?)

Is anyone here knowledgeable about the construction of the solar covers - why the design is pretty uniform, and what UV protection mechanism is used for these solar covers?

 

[Newdude]

Hey M T !!!! The bubbles make it float. Rain, morning dew, sprinklers, etc would sink it if it was otherwise a tarp.

The bubbles need to be thin or they'd add considerable weight to the cover. If you flattened them all out they'd already be as large as the cover. Going thicker would need larger bubbles. Which would add more weight and then need even larger bubbles.

So the thin bubbles are the inherent weak point of any cover, and likely its cause of demise. We reccomend going as thin and cheap as possible and considering them disposable. The thick covers don't stop any more evaporation than the thin ones and for twice the cost, the same thin bubbles will not last twice as long. Thin covers are also much easier to manage so you're less likely to snag it on something before the bubbles pop.

You are correct about reducing heat loss through evaporation. I saw one ad the other day claiming a solar cover 'made' up to 10 degrees of heat and that's just flat wrong. It will retain 5-7 degrees of yesterday's warmth overnight, looking like a 'gain' today.

 

[M T]

Polypropylene has a density of 0.9g/cc, so it will float, but just barely. If that tendency to float isn't enough, one could make a sandwich of material/air/material, where the air is trapped in a waffle layer between two layers that are thicker than the thin skin of the bubbles. You could make the gridding of the waffle smaller or larger, as desired.

The disadvantage of interior bubbles is the trapping of water. We've probably all seen bubbles get water in them and then eventually grow algae.

The current design of covers doesn't include an easy way for water on the surface of the cover to go into the pool, except by removal of the whole cover from the pool. I'm not sure a design to allow draining is possible unless the upper surface were slanted.

I shudder at seeing the word "disposable" for this much bulk plastic. Last time I had to throw a pool cover in the trash, I talked with the local recycling agency to confirm they, nor anyone nearby, could not recycle it.

 

[Bperry]

It’s bubble wrap and made to be cheap. Why not just cancel the cover all together? My families pool as a kid was never covered for 45 years. My parents just got a cover a few months ago because they didn’t want to fix the solar heating.

 

[poolside1]

Bubble covers last a few seasons at best. I am considering placing a clear flat tarp over the pool and pool deck. I'd like to use a reel to roll it up if I can find a reel to fit the tarp. Grommets on tarp can be attached to pool deck so it doesn't fall into the pool. Any thoughts?