Green house (maximum) effect

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1. Jun 6, 2015

pforeman

Hi,
I have some thoughts about making a much better pool solar cover. My thoughts are to maximize the temperature increase.
I plan to suspend a plastic film 6 inches above the surface of the pool water creating a green house effect.
Normally, very small plastic are pockets are embedded in the plastic sheet that floats on the waters surface.
My question is which would provide more effect black, which would heat up in the sun more, and radiate it to the air and then the water. Or, clear which allows the sun rays to penetrate through the depth of the water.
Thanks,
Paul

2. Jun 6, 2015

DaveC426913

What advantage does a 6 inch layer of air confer over directing all that energy straight into the water itself?

3. Jun 7, 2015

moatilliatta

Ideally you would have a black film sitting on the water that has a high thermal conductivity, which is then covered by a suspension of highly transparent film of low thermal conductivity.

The black film would absorb all the light that makes it through the transparent layer and conduct it as heat into the water. The the transparent layer would act as a greenhouse and insulate the pool.

4. Jun 7, 2015

DrStupid

A transparent film shouldn't have a significant effect and a black film above the water would be less efficient than no film at all. Place the black film on the ground of the pool.

5. Jun 7, 2015

bigfooted

It's exactly like in a small greenhouse: you prevent convective surface cooling. I saw a commercial on tv last week where they advertised tent-like structures for warming your pool, so it must be true! They were transparent, by the way. The radiation directly heats the water and the plastic prevents the water from cooling to the air by convection. You could maybe just buy a roll of "plastic with air bags" that you can add to a box as protection when you want to mail something.

6. Jun 7, 2015

moatilliatta

Water reflects UV light, so by placing the black film on the top of the water your preventing evaporation (which cools) and absorbing more EM radiation.

7. Jun 7, 2015

DaveC426913

Not sure you read my question correctly. I am asking why a 6" layer of air under the film is better than no layer of air under the film.

8. Jun 7, 2015

DaveC426913

This is true. Evaporative cooling is by far the greatest source of heat loss from a pool. Anything that prevents evaporative cooling (including a physical barrier, but also reducing cross-winds) will go a long way towards keeping the heat in.

The key to making a practical and economical pool cover is to get the most bang for your buck (tackle the biggest culprits) while keeping costs low (by taking advantage of passive, inexpensive techniques and technologies). (Remember, you go all out and just put walls and a roof around it to keep all that heat in, but it's not cost effective.)

9. Jun 8, 2015

pforeman

I didn't calculate anything to come up with the 6" depth. It just happens that in my design, 4 - 6 inches will be easy and cheap to attain.
But thanks,DaveC42..., This is one of the reasons I am asking here. Wheather the air "pillow" is 1 mm, or 6 inches wont affect either the insulation effect of heat loss at night, nor the green house effect during day?
Thanks again,
Paul

10. Jun 8, 2015

DaveC426913

No, I understand that. I was asking the broader question of whether a significant air gap (of any depth) suits your purpose.

I suppose it does, in that air is an excellent insulator. If the film were directly on the water, then heat would be lost straight from water, through film to air, via conduction. An air gap will tend to lessen this.

My general point though, is whether it lessens it enough to make the extra engineering and materials worth it.

Say a simply-engineered contraption costs $100 and solves 80% of the heat loss problem. Does a$200 contraption that solves 90% of the problem make business sense? Only if people will but it.

Don't forget that deploying, retracting and storing the device has got to be trivially easy, or it won't sell.

(In case it isn't obvious, I've got a pool that I've spent a lot of time and effort trying to keep warm with various techniques.)

Last edited: Jun 8, 2015
11. Jun 8, 2015

pforeman

Hi DaveC,
Yes, solar covers cost big bucks.. A good one for my pool is ~120 bucks, and they wear out in 3 years. My plan would cost about \$60 bucks, last longer, be much easier to get into and out of a pool, be much (much) easier to clean, will fold up smaller, last longer, and take a couple of hours to make. But, I my questions pertain to maximizing heat retention at night, and heat capture during the day. I have heard in this thread opinions that mirrored my own concerning clear vs. plastic, but I am still not exactly straight on weather a thick air pillow is better than a thin pillow.
Thanks
Paul

12. Jun 9, 2015

DaveC426913

OK. I'm sold. Let me know when I can send you my measurements for a pforeman Solar Cover Mark I.