| Unlike open loop systems, closed loop systems do
not directly heat the water in the storage tank. By
this I mean the water in the tank does not circulate
directly through the solar collectors. There are two
types of closed loop systems – the glycol indirect
system and the drainback system. In this type of system
a separate fluid is circulated through the collectors
and a heat exchanger is used to transfer this heat
to the water in the storage tank. This heat exchanger
can either be built into the storage tank, or separate
from it. If separate an additional pump is required
to circulate water from the storage tank through the
exchanger. The heat exchanger fluid is typically propylene
glycol. Additional items such as an expansion tank,
pressure gages, and fill valves are also required. |
Glycol Indirect System
Like the open loop system, the glycol
closed loop system has been around a very long time.
It also alleviates two of the three problems encountered
in an open loop system. By using glycol as the heat
transfer fluid the chance of freeze damage is eliminated.
Additionally it eliminates the problem of fouling
the collectors in areas that encounter hard water.
There still remains the problem of high limit protection
and the potential of failure of the additional components
added (expansion tank, pressure gages, etc). Figure
2 depicts a typical indirect glycol closed loop system. |
