FRESNEL LENS SOLAR HOT WATER, Heat Solar Pool Heater Solar Swimming Pool Heater This is a large Fresnel Lens focused on a copper tube with a 12 V pump (powered by two 6 watt solar panels) with water circulating through the unit. Water, seven gallons, was raised from 63 F to 106 F in one hour. It is not super efficient as heat is lost at the open top of the glass tank. This is designed to illustrate one method of solar heating a swilling pool with a Large Fresnel Lens. TESTING HDD AQUA BLOCK.