In an indirect active system, also known as “closed loop,” a simple circulation pump moves the non-toxic and food safe antifreeze solution (propylene glycol) through a closed copper pipe loop into the solar collector, through the collector’s absorber pipes, and out of the solar collector. Then, the sun-warmed heat transfer fluid flows into a double walled heat exchanger where it warms the cold water heading into a conventional hot water tank. The antifreeze solution then returns to the pump and again flows into the solar collector without ever mixing with the building’s water. Indirect systems are encouraged in climates with extended periods of below-freezing temperatures for year round solar heating.
In reality, indirect systems are somewhat more complicated than presented here. Differential thermostats, pumps, sensors, and controls are used so the simple systems illustrated work effectively and safely. However solar thermal technology is mature and proven with few maintenance requirements from the installed systems.
Collecting the Sun
Solar thermal systems also differ by the type of collector used to gather and store the sun’s energy. Flat plate collectors are the simplest and most common type. Copper pipes wind back and forth through the flat plate collector, which is painted black to absorb heat and covered with glass, or “glazing,” to prevent heat from escaping. Often the pipes are painted black and bonded to the material of the flat plate collector to maximize heat absorption.
A standard two panel solar water heating system (solar thermal system) will cover 60 to 75% of their hot water heating requirements as shown in the professional output report posted below. In addition, a solar water heating system can be expanded at any time to support radiant heating and pool heating. This will allow even more use of free energy generated from the sun.