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How to Make a Passive Solar Water Heater. Every morning, billions of photons are sent streaming from the sun toward Earth. When they strike a surface, the atoms get excited & begin to move faster, generating heat, or warming up. This is called thermal conduction & is the scientific principle behind the design of passive solar water heaters, which uses no moving parts or fuel. Following these directions you can build an inexpensive water heater that will heat water to 150 degrees.