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 Closed-loop systems require several special components. (Figure 2-11) One of
these is an expansion tank in the collector loop, to compensate for the expansion
and contraction of the solar fluid as it is heated and cooled.
Another special component is a check valve. When the fluid in the collectors is cold
(usually on winter nights), it becomes more dense and drops to the bottom of the
collector loop. Warmer solar fluid from inside the building rises to the top of the
collectors.
Eventually, all the solar fluid is slowly flowing in reverse through the system. This
chilling process can start another similar flow in the water loop between the heat
exchanger and the storage tank. The final result is that heat from the storage tank
escapes through the solar collectors to the outside. The check valve prevents this
"reverse thermosiphoning."
FIGURE 2-11
A Closed-Loop Solar
Heating System
OPERATION
2.2 CLOSED LOOP SYSTEMS
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