An indirect circulation system pumps heat transfer fluid (typically antifreeze solution) through solar collectors, transferring captured heat to potable water via a heat exchanger before storage or use. The thermal fluid never contacts drinking water, providing safety and freeze protection in cold climates. Indirect systems are the preferred choice for regions experiencing winter freezing.
Technical Details
Indirect systems circulate propylene glycol or ethylene glycol solutions with freeze protection to negative 50 degrees Fahrenheit. A plate or shell-and-tube heat exchanger transfers heat from the thermal fluid loop to the domestic water loop with efficiency ratings of 85 to 95 percent. Typical system pressure ratings reach 100 to 150 PSI to accommodate both loops and pressure relief valves.
Applications and Practical Significance
These systems are essential for any solar installation in climates with freezing winters, including northern United States and Canada. The heat exchanger adds 15 to 20 percent to initial costs but prevents damage from freeze cycles and eliminates complex freeze protection procedures. Indirect systems maintain reliability across diverse climates while protecting both equipment and water quality through separated fluid circuits.