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# Solar pool cover/heater

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A pool cover for concrete pools made from an array of Fresnel lenses designed to focus sunlight on the bottom of the pools surface. Preferably black porcelain tiles and grout.
The lenses would be suspended above thin, clear, air filled bladders to insulate against heat loss.

Heat generated should be absorbed by the water faster than the concrete can slag much the same way a paper plate full of water will no burn up in a fire.

When optimum temperature has been achieved, bi-metallic fasteners holding the individual lens bladders together expand to allow excess heat to escape, and remain contracted when the sun is not shining.

 — 2 fries shy of a happy meal, Jan 23 2016

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Hang on. You do realize that lenses covering the surface of the pool will harvest no more energy than a lensless pool of the same size?
 — MaxwellBuchanan, Jan 23 2016

 Yea, there's the same amount of energy per square foot. When I was a kid my dad even recited some weird engineering limerick to me about the concept that I don't remember. Picture pouring gas on a fire via a a sprinkler or through a series of funnels. Same thing.

I always wondered if a black bottom pool gave more heat to the water by absorbing the light that got to the bottom or if you were better off with a light bottom that reflected the light back up through the water again. I'm guessing the black bottom is better.
 — doctorremulac3, Jan 23 2016

Yes.
 — MaxwellBuchanan, Jan 23 2016

 //You do realize that lenses covering the surface of the pool will harvest no more energy than a lensless pool of the same size?//

I honestly did not. I... guess I missed that class.
 — 2 fries shy of a happy meal, Jan 23 2016

Suppose a black material were floated on the water and elevated lenses were focused on it. Then would the resulting heat be transferred down to the water?
 — whatrock, Jan 24 2016

 Yes, with the same efficiency as if you painted the bottom of the pool black and got rid of the lenses.

C'mon. I mean, really. There's a finite amount of solar energy hitting the pool. A negligible proportion is reflected by the surface of the water, unless the sun is very low. As long as everything else gets absorbed, that's it. So it doesn't matter whether the pool has a black membrane on it, or the bottom of the pool is black, or the water is just really murky. And lenses schmenses.
 — MaxwellBuchanan, Jan 24 2016

 // and elevated lenses were focused on it //

Well, if the lenses are elevated, then they can be much larger than the surface of the pool. For example if you use a Sherlock Standard magnifying glass to heat a pool the size of an ant, you will get some nice warm water. Scale it up a little if you'd like, but be sure to close the blinds before getting in the pool.
 — scad mientist, Jan 25 2016

 Problem with that is you're better off just having solar collectors which is what they do. Rather than having an ungainly array of lenses or mirrors arrayed around the pool, which by the way would need to track the sun to keep focused on the water, you just pipe the water through a solar collector aimed generally south and placed somewhere convenient and out of the way like a roof.

Plus, picture floating around your little pool with lots of lenses focusing the sunlight at little points all around the water. One moment you're enjoying batting beach balls around with friends the next moment your balls are on fire because they got under one of these focused points of light.
 — doctorremulac3, Jan 25 2016

 Been done, though not WKTE.

Pool covered in a layer of bubble wrap.
 — normzone, Jan 25 2016

 We used to have one of those blue bubble-wrap type solar pool covers.I don't know why I didn't take the umbra into account. I guess that after watching the videos on just how hot concentrated light can get, I didn't figure on just how much energy it takes to change the temperature of water.

 So, technically... this concept could be used to focus light, onto something able to withstand the heat, and create a passive heat-pump as long as the Fresnel lenses were arrayed so as not to shade the pool with their umbra.

 I can use this.

 Y'know, I don't thank you folks often enough for teaching me things, even if involuntarily, which I probably would never have otherwise clued into.

Thanks eh. You all rock.
 — 2 fries shy of a happy meal, Jan 26 2016

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