passive solar heating of pools

 THIS IS THE DESCRIPTION OF AN EXPERIMENT THAT SHOWS HOW IT IS THE SOLID WALLS OF A POOL THOR ROCKS IN A POOL THAT HEAT IN THE SUN. USING A FRESNAEL LENS IS DANGEROUSE IT SHOULD ONLY BE USED BY ADULTS AND ADULTS THAT HAVE THOUGHT A BIT ABOUT THE DANGERS AS IT IS NOT A OBJECT THAT OUR CULTURE TEACHES US HOW TO USE WITHOUT DANGER AS IT TEACHES US HOW TO USE FIRE.
I LIKE THE EXPERIMENT BECAUSE OF HOW IT TEACHES US THAT IT IS THE BOTTOM OF A POOL OR ROCKS IN IT THAT WILL HEAT NOT BECAUE I THINK THA TA FRESNEL LENS IS A GOOD WAY TO HEAT A POOL.

  Using stones in pools as a way of catching heat has been mentioned here, it has been mentioned as atactic of Sepp Holzers.
  The couple Dan and Denise Rojas who do lot of solar energy videos on you tube, did an experiment that demonstrates how this work that i have been wanting to write about for some time.
      At last i have tried to draw on the internet and can offer an inperfect, badly spelt  illustrated account of their experiment to encourage everyone who can get you tube to ols better. look at their experiment to understand passive solar heating of ponds and po 
      It is Dan who is the scientist, handy man  and Denise is the great woman behind him or maybe, if we follow Sarah Palins new version of this saying, the surprised woman behind him. It is a good version, a lot of women are surprised when the OK, but full of faults, guy they married actually gets things off the ground. I dont approve of her, i don't think she has the good of the least fortunate, underprivileged, at heart, i know the rich say that by looking after the rich the poor will get on but that has never worked historically only a mixture between looking after the poor awithout completely doing for the rich seems to really work, but she is funny.

     Fresnel Lens Sun Collector. experiment of the Rojas's to show a scientific fact.             
       Their exper¡ment consists of passing  the rays of the sun, that have passed through a Fresnel lens, that concentrates them, a fresnel lens is a sort of magnifying glass that they use in the windows of lighthouses, through a glass aquarium with water in it.
    Dan Rojas says that, as the water is transparent the concentrated ray of sunlight go straight through it without heating it much. He proves this because the light goes through the water and glass of the aquarium and burns a hole in the table under the aquarium, barely affecting the water in the aquarium. Certainly the table burnt before the water in the aquarium got hot.
    I have labeled things in the drawing in Spanish because i was not thinking hard enough. 
  aquarium, is the same in both languages and
     fuego - fire
    mesa - table.

I don't have much to add to the topic, but I really love that illustration, rose.

are fresnel lenses easy to come by or jury-rig at home?

The second part of the experiment is to put a thick piece of iron in the bottom of the aquarium were the concentrated ray of sunlight hits is it. With this object in the pool we soon see a disturbance in the water above the piece of metal that shows us that the water is getting hot.
  A stone though it would not be so efficient as a big piece of metal would have the same effect. As would sides to the pool that were made of tiles and so weren't transparent and even more so if they were black and so reflected little light back. agri rose macaskie

I am not sure what is trying to be accomplished, here,

If you want hot water put in a tank and paint the out side of the tank black in a summer day it will get very warm,

If you want a pool of water heated, (pool as in swimming) I would say there is two ways,
some darker color in the pool that will absorb heat, or a panel that is dark that water can be pumped up and let it slowly drain over the collector,

yes the Fresnel lens, will work, or one could take a bunch of mirrors and focus the light on to an object, the dark object, will collect the heat,  (the same as the magnify glass to light a fire),  the lens concentrates the lights (power) into a focused point).

Bird man I suppose the experiment only means that the sun heats solid objects more than it heats water.
 Water is however said to be a good retainer of heat once it has got hot.
 Sep Holzer probably wants to grow tropical fish on the mountains of Austria without having to use electricity, while the Rojas’s want to reduce energy consumption in swimming pools.
   If the Rojas’s had had the sun without a Fresnel lens heating the object in the water they would not have been able to show that the light of the sun in a transparent container does not heat water much, there could hardly have been such visually evident manifestation of this, without a lens that worked like a magnifying glass to concentrate the sunlight they would not have been able to produce a fire under the tank when the light went straight t through the water and glass before they put a metal object in the pool. You would ot have been able to see the turbulence in the water when the metal object got hot, the object would have heated more slowly without leaving a visible commotion in the aquarium to show how much more the light affected a solid object than a transparent one.
     The Rojas’s are  suggesting that you could use a Fresnel lens for heating pools but their experiment also works to prove to us why a rock in the water will help warm it.
     Their experiment makes me understand that if I had a pool lined with thick dark tiles, the earthen ware of tiles, terra cota, is a heat accumulating material,  the sun shining all day would heat up your pool a lot. It makes me believe what Sepp Holzer says about putting rocks in water. As the Rojas’s are interested in us using less energy my deeper understanding of the subject would probably make them happy.

   I wonder if the sun leaves more heat in deep pools because they are darker, as they have more water in them, this is not something we would not notice much as the heat would spread out in a greater volume of water if the pool were deeper. I suppose deep pools are often only darker because their taller sides caste a deep shade in them.

    I suppose there are a lot of factors in this theme.
1)    That the sun will not heat a transparent object like clean water much, it is the bottom and sides that will or won’t be affected by the sun.
2)    That the solid bits in the water that will heat will heat more if they are dark i don’t think that the Rojas’s mention this in their experiment, though I have not checked through it for details before writing this. I mention it as I have learnt it from solar cooking, because dark holds more light, it retains light energy instead of reflecting it, light colours white sand (sand heats up a lot in sun) reflect the suns energy, I suppose volcanic sand that is black would heat up more.  
3)    Another factor would be that if the object placed to catch the sun is made of a heat accumulating object it will heat more. Materials like dead leaves pond more likely to insulate things than to accumulate heat at the bottom of a it will not warm up much however black it is. This is not said in their experiment but it is implied they have a bit of metal in the pool metals accumulate heat.  
agri rose macaskie.

OK I understand what your wanting or trying to do now,

one thing I would caution, if one wanted to heat a pool of water with living creatures, in it, and use the lens Idea, any thing that would swim near the focal point could get severely burnt,  as they neared the focal point, as if you took the same lens and placed it focal point to some thing flammable it can and will start a fire in short order.

when we were kids the windmill had a supply tank, that feed the stock tank across the road from it, and we would swim in the supply tank, about 16 foot square and about 6 foot deep when full,  the sun would warm it very nice most of the summer, it would grow a nice moss coating on the walls of the tank and it absorbed the heat from the sun in a few days, and would be very warm  and was tolerable even when the mill had been pumping fresh water in. 

I would think that some darker tile would do nice to absorb the heat from the sun,


and yes I would think if one use a solar water collector and wanted some additional mass to store the heat rocks would make a good heat mass,
as in the old solar home, they would use bins of rocks to hold the heat from air systems, fro the most part when water was the medium just a large tank of water was used,  and there are some benefits as the heat rises and the water will separate the hot and cold if it is unto stirred up much, just like a hot water heater, that takes the hot water off the top of the tank, and the cold ender the bottom through a drop tube,  and one can get most all the tank of hot water out of the tank before cold come out if one uses the water before ti can reheat the new water, 

I have never read of using the combination of rocks and water but the idea of thermo mass is proven and I do not know why it would not work.

(in reality I do not know if it would help that much over just more water storage),  I would guess that rock would hold more heat than water as rock is denser.

Here are the problems:

First, you'd need to make sure the volume of water is not too much or the water will dissipate the heat too quickly.  Second, not only will the water surface will reflect a lot of the light, the refraction in the water will have the focus point dancing about.

Birdman says the fresnel lens could damage fish and he is probably right, I imagine fresnel lenses are really dangerouse, but scientific experiments are often dagerouse mildly dangerous they are designed  to prove a principal or to demonstrate it and this one demonstrates a principle, dots the i's about how the sun will heat things in water  but not the water alone so much. So you may understand how to heat your pool .
 I suppose that the videos should have warnings to explain that sunlight and fresnel lenses are dangerouse. You may think that it is clear as daylight that this is a strong instrument thqt you should use with care it burns a hole in the table top in a fews minutes but still if you don't say th¡s is dangerous then you may get people using it without taking proper precautions.
     FIre is dangerouse so are solar cookers the box ones and severla others don't concentrate the light, they only reflect it pretty diffusely into the oven or onto the cooking pot  but magnifying glasses and parabolic lenses do concentrate sunlight and Dan  Rojas shows how they can be used and if you want people to use alternative engergies it is important that they should know the power of the sun, lots people have businesses so the instruments for solar enery could have a professional use.
      Dan Rojas sets concrete alight in one video, with the sun and a fresnel lens. A person wrote a comment on that video to  warn him that concrete can explode if yiou heat it up that much, he wrote back explaining why this concrete was iunlikely to explode, if i remember right but stil,l there is one danger with fresnal lenses for you, exploding concrete. Fresnel lenses are used in th film industry according to my brother in law during shooting . Some people  may be smiths or some such who work with high temperatures and a full realisation of what the sun can do will extend the use of this clean but also dangerous energy. It could be used on sunny days and dirty and expensive energies used on other days.  
      The french have a giant reflector made to melt metal in the South of France, Obviously it would be in a place children dont have access to . The videos of the Rojas  show how strong solar energy can be, that solar energry can be effective but the safe ways to use them have not been discussed. While we, most of us, know what is safe and what is dangerouse with a fire, we know not to build up the fire to high for example, the rules for using reflectors are not yet part of our society. Agri rose macaskie.

on the concrete exploding, most likely it will just pop the top surface off, and the reason it does that is there is trapped water and it pops like pop corn does, and it can or will pop off the surface at times, (it can be dangerous tho),  the same thing will happen with a cutting torch or some welding processes. (I have had small sections pop at times when molten metal has got on it or the torch flame heats it),

I know when I was a kid I would play a lot with a magnifying glass and it was only about 3" across,  and burnt my self a few times and burnt a lot of wood, but if you would take a large lens I can not imagine the heat on / at the focal point,

I think it is an interesting experiment, and I know of some test experiments,  solar  heat collector, using mirrors and a sun trackers, usaly heating oil and then using the very hot oil to make steam or other.
http://www.nrel.gov/docs/fy01osti/28751.pdf

http://www.scienceclarified.com/scitech/Energy-Alternatives/Solar-Power.html

http://nenmore.blogspot.com/2010/03/best-solar-power-plant.html

here is a simple collector that would be simple and cheap to build and if done right  I would think that it may even flow with out a pump by convection currents the heat rising, if set correctly.
http://ffaat.pointclark.net/blog/archives/107-DIY-Solar-Pool-Heater.html

tel, i am not sure i read your message the first time, thanks a lot for saying you like the drawing, i have always been a messy drawer but paining was what i studied  so it is not as if i don't have a bit of experience in painting that helps you mess through, i liked painting.
  I don't know where you get a fresnel lenns but i heard that they are used i film making, shile filming hevea only knows how. I thnk they can be pretty dangerouse as can parabolc reflectors.There is a video of a big parabolic reflector cooker burning food called the death ray in you tube, it has a water melon spitting put steam about half a minute or something after being put in the path of the focal point of the reflected sunlight. It was impressive. agri rose macaskie.

The largest I can currently find, 17" x 16" http://www.surplusshed.com/pages/item/l3656.html

at times surplus ones come up to 49-1/8" x 37-1/8" I think from projection TVs, at one time they were offered by, http://www.sciplus.com/

Do some Google or Internet searches.

I thought your drawings were good illustrations as well.

found some bigger, the places I have little knowledge about, just posting for information purposes,  http://www.greenpowerscience.com/SHOPFRESNELHOME.html

bird man gosh you found some fresnel lenses. i thought they would be nearly impossible to buy.
  I did not know that you could have popping concrete, i was really amazed when i saw the fresnel lens heating concrete. The mans world is so much more exciting than the traditional womans one, we might get pricked with a needle you have popping concrete,  we cook a stew and you manage a blow torch and melt metal. rose.

bird man, gosh you found some fresnel lenses. I thought they would be nearly impossible to buy.
 I did not know that you could have popping concrete, i was really amazed when i saw the fresnel lens heating concrete. The mans world is so much more exciting than the traditional womans one, we might get pricked with a needle you have popping concrete,  we cook a stew and you manage a blow torch and melt metal. Suppose everything can get boring after a while though, rose.

For a very simple pool heater check out this:

http://sugarmtnfarm.com/hot-cool-natural-pool/

I just installed another 2,000' of pipe, water line from a spring, and during the day the water heats up quite a bit. The 1" black plastic pipe lays on the ground through our fields. The spring is so cold that if I dip my hands in it they become painfully cold in seconds. Yet, at the other end of that 2,000' of pipe the water is quite tepid, pleasant.

With a fresnel lens you will have to ajust it with the sun moving. Another thing Dan did was make a parabolic trough mirror that would run, unadjusted, from 9am to 4pm heating water the equivilant of a 3kw heater running for 7 hours per day. You can have the pool covered because the parabolic trough would run outside the pool and the water would flow because of convection [hot things are less dense, so they rise] Install the inlet at the bottom of the pool, output somewhere up top. The pool will heat itself with the sun.

Also you can get fresnel lenses chea or free on craigslist pulling them out of the front screen of projection screen tvs 40"+

I have a 50" one sitting in my back yard that you can have if you pay for shipping.

I like that you can concentrate the sun with it, but my money is going to parabolic trough mirrors.

I would have a solar pump circulating water through a metal pipe, focus the fresnel lens on the pipe, not the water. Eliminates the chance of burning things in the pool.