In this active, closed-loop system, incoming potable water is routed tothe solar storage tank, but never into the collectors. A water andantifreeze mixture circulates from the collectors through a coil ofpipe in the solar tank, and then is pumped back through the collectors.(In most climates, a 50/50 propylene glycol and water mixture will keepcollectors from freezing.) The potable water is warmed by heat transferthrough contact with the pipe.
These systems require an expansiontank and a few other auxiliary components for filling, venting, andmaintaining the system. A definite advantage to antifreeze systems isthat the collectors can be mounted anywhere. These systems are prettymuch the only choice in very cold climates.
The following illustration includes the primary components of any pressurized glycol system.