Dear Solar Guy,

        Your musings on active solar water heating in the last issue
prompts me to ask, "What are passive solar water heaters"?  Since you
might comment on them, just thought I'd remind you.

- G. Placid,  Palacios.

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Dear Placid,

        Was afraid no one would ask !  Passive solar water heaters are
those which don't have any active components, such as a pump or control
element and sort of "work on their own".

        There are several designs for passive systems, but there is
generally a common feature: where the warm water is stored.  Since warm
water is lighter than cool water, warm water tends to rise and in passive
systems is stored in or above the collector.  The simplest passive system
is the simple "batch" concept.  An example would be a shallow insulated
vessel with a top glazing. Here the warmed water is stored in the vessel
until drawn off and replenished, through float control valve.  This
concept isn't very practical because, the collector must be more or less
horizontal, condensation on the inside of the glazing reduces efficiency,
the need for atmospheric pressure (glass can't take any pressure),  and
there are large losses at night through the glazing.

        The second is the so-called "thermosiphon" concept.  In this
design a storage tank is elevated above the collector.  The water warmed
in the collector rises and then flows through a tube to the top of the
storage tank, as cool water from the bottom of storage flows through a
tube to the bottom of the collector.  Under sunny conditions this "natural
circulation" persists, but under overcast conditions or at night then
circulation stops, as the warm water wishes to stay "upstairs" in the
insulated storage tank. This design has been very popular in the past in
climates where there is little chance of freezing, such as Hawaii, Israel
and northern Australia.

        The other common design has a storage vessel that is integrated
with the collector, usually an insulated horizontal cylindrical vessel
connected to the collector at its top.  The warmed water rises to the
vessel under solar conditions and stays there otherwise.  In the last two
concepts the potable water is in the collector and tank which are at
supply water pressure.  As warm water is drawn off the tank, make-up water
flows to the bottom of the tank.

        There is also a clever "passive" design which has a separate low
boiling temperature fluid in the collector and separated from the potable
water by a heat exchanger.   In this design the storage vessel is allowed
to be below the collector.  This design has the advantage of being
passive, yet immune to freezing.  This design, though not available
commercially, will likely be more expensive that the simpler passive designs.

        Although passive designs are very appealing, with the exception of
the last example, they are generally not accepted in most climates of
North America due to the possibility of freezing.  One can modify them to get
around the freeze problem, such as go to a second fluid in the collector
and include a heat exchanger, and other design features, but then the
inherent advantage (simplicity) of the passive design is lost.

        So unless you live in southern Florida or Hawaii, or other very
temperate climate, its probably best to stick with an active design.

The Solar Guy