Solar covers

[allanb]

I've been reading the sales brochure offered with a solar cover (the plastic "bubble" type - bache à bulles) and I'd like to know whether I can believe everything in it.

I can understand that the cover will reduce evaporation, and retain heat during the night that would otherwise be lost.  Maybe that's enough to justify it.  But the brochure goes further than that and claims that in full sunlight the cover actually makes the pool warmer than it would otherwise be (a gain of 8ºC is mentioned).  Is that believable?

I think that the amount of heat radiated by the sun and falling on a given area (the pool surface) is fixed.  A few people have suggested that each little bubble acts like a lens and concentrates the sun's energy; that may be true, but I don't think it's a plausible explanation.  I'm not a physicist, but it seems to me that if you focus radiation on a small point, you will make that point very hot, but the rest of the area will be slightly cooler - you aren't generating any additional heat.

On the other hand, it's in the sales brochure, so it must be true - right?

[halfblind]

By operating as an insulating cover at night it will keep the pool warmer. During the day I fail to see how an insulator will suddenly turn into something that will allow the suns energy to pass through. Sales blurb I think !

[Araucaria]

I imagine the physics behind the claim is based on the cover being a dark colour which reflects less of the light/heat falling on the surface of the pool, so that more heat is transmitted to the water.

Think how pools sparkle in the sunlight - that is all reflected light (or, putting it another way, wasted heat).

I have no connection with any commercial operation and I don't have one of these myself.

[chem geek]

Actually, the covers that heat the most during the day are clear covers since they let most of the sunlight through to the pool.  An uncovered white plaster pool of average 4.5 foot depth will absorb 60% of the sunlight which will heat the water.  If you have colored plaster or a dark vinyl surface, then even more is absorbed, probably around 80-85% (see this post for more info).  The darker covers can absorb sunlight and covert it to heat, but a lot of that heat is radiated/convected back to the air and only some gets transferred to the water and that assumes that only the flat back surface touching the water is colored while the bubbles themselves are clear (if the bubbles are colored, then there is minimal heat transfer to the pool water).  Also, if the circulation of water isn't good, then the surface water is mostly what gets heated and this also tends to make the heat conduct back out through the cover to the air.

The downside with a clear cover is that is lets the UV through so doesn't help with reducing chlorine loss from breakdown from the UV in sunlight.  With a more opaque cover, the chlorine demand drops, but so does the amount of heating.

In the case of an uncovered white plaster pool with 60% sunlight absorption and 4.5 foot average pool depth, the water is heated by about 0.7ºF (0.4ºC) per hour at peak sun directly overhead (1000 Watts per square meter).  However, with the pool uncovered, there can also be evaporation unless the air is very humid.  If one normally loses 1/4" of water to evaporation per day, then that is a loss in temperature for this same depth pool of 4.9ºF (2.7ºC).  Basically, you lose almost as much heat from evaporation as you gain from sunlight absorption, unless the air is more humid so evaporation is less.  So a cover virtually eliminates the evaporation not only allowing heat to be retained, but some additional heat to be absorbed (especially with a clear cover).

[Théière]

Thanks Chemgeek, that saved me a lot of typing.

We have a clear cover on test at the moment. I really believe that the marketeers have streched a point here, either they tested the cover in Death valley or 8 deg is the accumilated fig of the 4 deg loss without a cover + the 4 deg you get with the clear cover. We will wait and see.

[chem geek]

I don't think they are talking about an 8 degree rise in temperature each day (and I'm talking Farenheit, not Celsius) -- that definitely seems like a stretch -- but a good mostly clear bubble-type cover should be able to get a pool's water temp to at least 15ºF (8.3ºC) warmer than the average day/night air temperature after about a week, at least during the middle of summer.  At other times of year, a 10ºF (5.6ºC) should be achievable which might be the best a darker cover could do in the middle of summer.  With a solar panel system that is around 80% of the pool's surface area plus even an opaque cover to prevent evaporation, one can easily get 20-25ºF (11.1-13.9ºC) in the outer months and 25-30ºF (13.9-16.7ºC) in the middle of summer.  With a clear bubble-type cover the solar panel area doesn't need to be as high.  Of course this all varies somewhat at the more extreme latitudes and assumes sunny days every day.

[steve]

I reckon a pool with a cover used properly will give +4 c on a pool that is not covered.

Steve

[Gardian]

We've had one of these plastic bubble covers for about 3 years and would say that it definitely reduces / arrests overnight temperature loss. It gets removed during the day, so I've no idea about the claims, but accept entirely the other poster's comments.

They're worth having, I'd say.

One word of warning though - after a couple of years the bubbles have started to crack up and break off. I mentioned this to the supplier the other day and he said that the cause was a combination of heat and chlorine and that it was essential to put the supplied plastic cover over the rolled up bache during the day. Don't know if he's right, but it sounds plausible.

[steve]

Yes he is :) have you felt the heat these covers build up when on the roller ? amazing ....

Steve

[Repulse]

And if anyone is looking for one I have a summer with roller and winter one for sale :-)

[Dog]

[quote user="allanb"]I've been reading the sales brochure offered with a solar cover (the plastic "bubble" type - bache à bulles) and I'd like to know whether I can believe everything in it.

I can understand that the cover will reduce evaporation, and retain heat during the night that would otherwise be lost.  Maybe that's enough to justify it.  But the brochure goes further than that and claims that in full sunlight the cover actually makes the pool warmer than it would otherwise be (a gain of 8ºC is mentioned).  Is that believable?

I think that the amount of heat radiated by the sun and falling on a given area (the pool surface) is fixed.  A few people have suggested that each little bubble acts like a lens and concentrates the sun's energy; that may be true, but I don't think it's a plausible explanation.  I'm not a physicist, but it seems to me that if you focus radiation on a small point, you will make that point very hot, but the rest of the area will be slightly cooler - you aren't generating any additional heat.

On the other hand, it's in the sales brochure, so it must be true - right?

[/quote]

Even with night time losses you wouldn't want to leave the cover on for more than a few weeks - the pool would be boiling!

I reckon they get the figures from a pool that's a few inches deep.