The sun’s supply of clean, renewable energy can be used for solar generation of heat.  For solar thermal systems, a solar collector converts the light that penetrates its glass panes (short wave radiation) into heat.  The heat is created by the absorption of the sun’s rays through a dark-coated, usually metal (copper or aluminum), plate called the absorber.  In the absorber, there is a system of pipes filled with a heat transfer medium (usually a water-glycol mixture).  The liquid heated by the absorber flows around the closed circuit using a pump to a heat exchanger that transfers heat to the water to be used for commercial or industrial use.  The controller will only start the solar circuit pump when the temperature in the collector is a few degrees above the temperature in the lower area of the solar water storage.
Pool systems pump the water directly through the solar collectors.  Since the water in the pool is usually kept at least 80°F, collectors for pool heating systems may not have glazing or insulation.