This large spot Fresnel lens can be used as a direct heater focused on copper coils with active fluid circulation. This is the only reasonable option for heating a swimming pool with a Fresnel Lens. Pointing the lens directly at the pool water does not work. This requires a computer hard drive water cooling kit or regular copper tubing.
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.