Heat a Swimming Pool with a Large Fresnel Lens.
I receive viewer mail weekly asking, "With all that heat produced by a Fresnel Lens, can I point a lens at my swimming pool a few hours a day and heat it?" The simple answer is no. Water is clear and any light collected from the Fresnel lens will simply pass the focal point and continue into a wide pattern exiting as reflective light bouncing off the bottom of the pool. The big misconception regarding any magnifying glass or Fresnel lens it the belief that they "magnify or amplify the sunlight" when all they really do is concentrate it.
"What if I focused the lens on a dark object just below the water surface?"
While that would be much better, you would transfer more heat to your pool if you simply got a black object the size of your lens and sunk it to the bottom, no lens needed. Square footage or square meters are what counts when it comes to sunlight. A one meter by one meter (39" x39") Fresnel lens concentrates sunlight to produce roughly 1000 watts of heat energy to a very small point. A sheet of heavy black plastic absorbs the same amount of solar energy but over a large area. Since plastic or almost any material transfers heat effectively when submerged in water, the 1M x 1M black plastic sheet would add 1000 watts of solar heating. The sheet is heating water at the bottom of the pool so you are taking advantage of the thermal layer above. Several black sheets can add a lot of heat assuming sunlight reaches the bottom of your pool. Many pools do not get direct sunlight to the bottom especially in the winter months due to the angle of the Sun.
I know of a person who incorporated 300 Fresnel page magnifiers into a solar blanket for a pool and tested it against a normal clear solar blanket. He was thinking of applying for a patent but was confused with the negative results he was generating. The non-Fresnel blanket was better insulated so it kept the heat in. The Fresnel Lenses basically did nothing other than provide an escape route for heat. I am not sure what came of his project.
If you are set on using a Fresnel Lens, the best way is to create a small copper loop and circulate the pool water through it with either a small solar pump or tapping the pool's pump system. The advantage is that the lens is not blocking light that falls on the pool so you are adding 1 square meter off ADDITIONAL LIGHT to the side at a high transfer efficiency. You will still need to track the Sun with a heliostat.
This is true for large water tanks also. Unless the Fresnel lens is larger than the water tank, focusing light does nothing assuming the entire tank is painted black. Mirrors are a different story. Because mirrors redirect sunlight, they do not impede the direct light falling on the tank so each additional mirror adds sunlight based on whatever size it is. Again, a Fresnel lens would be best suited away from the tank circulating fluid, in this case, as a passive system.
The easiest way to heat your pool is described in the three videos below:
Heating a Swimming Pool Solar Hot Water Part 1
Solar Hot Water Part 2
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