This condition is called night sky
radiation.
Mh: Incidently, there are two rea-
sons why water that freezes will burst
its plastic, metal, glass, or stone con-
tainer. Actually, they
are simply properties of
water. One, water is
virtually incompress-
ible. Two,
water
expands slightly as it
changes from a liquid
to a solid. Water immo-
bile inside a small tube or
pipe and exposed to a freeze,
then, will begin to expand as it
becomes ice. Unable to compress
itself, it makes a bigger volume by
breaking whatever contains it.
Rob: True. It's actually the differ-
ent strategies used to combat the
potential of freezing that define the
major types of systems and their rela-
tive complexity. I've categorized
existing systems into seven types:
integral collector/storage, ther-
mosiphon, three-season, drain-back,
drain-down, re-circulation, and active
closed-loop.
Mh: Will you describe them all,
first generally and then assess their
merits and liabilities from your own
experience?
Rob: I would be glad to. I must say
first that my experience with solar hot
water is limited to my service area
(coastal northern California) which is
a fairly benign climate with occasion-
al light freezes. I ask your readers to
keep this in mind as I speak of vari-
ous systems.
1. The integral collector/storage
is the simplest and historically oldest
type of solar water heating system.
Paint a tank black, put it in a big
crate, insulate it on all sides except
the one covered by glass or plastic,
and point it at the sun. Water in the
tank is heated directly by the sun and
stored in the same unit. In the trade,
this is also know as a breadbox-type
system. An example of a manufac-
tured unit of this type is the
ServamaticTM. Produced in the 1970s,
many are still operational today. The
same principle can be seen in today's
ProgressiveTubeTM unit (Fig. 5).
These are also in-line units, posi-
tioned between the well and the
shower. You get as much hot water as
they collect and store.
Mh: This is a popular design in
homebuilt units, too. Simple, cheap,
and often made with recycled materi-
als. I once took a shower at a ranch I
was visiting from water heated in a
long thin 20-gallon tank inside an
old, big refrigerator with a transpar-
ent cover pointed south. I had a long,
hot shower in the cold night air. Good
experience.
Rob: I have very rarely had to serv-
ice an integral collector/storage type
system, which is a testament of their
durability.
2. The thermosiphon system is
another solar water heating method
(Fig. 6). Sunlight strikes tubes and
fins inside a collector box through
which water or glycol is
circulating. The inlet and
outlet of the collector are
plumbed, respectively, to
the inlet and outlet of the
storage tank. If we were
talking about electricity
and polarity, we'd say
the collector is in parallel
with the storage tank. Still, it
forms a loop. The heated fluid moves
from the collector to the storage tank
and back to the collector through a
process called thermosiphon. This is
a natural convective action. If you
plumbed this as an open system, the
storage tank could be your own water
heater.
Mh: I'd like to elaborate on a few
things you've said. Thermosiphon
results when water heated in the col-
lector expands and rises, pushing
cooler water in the rest of the loop
September/October 2000 Backwoods Home Magazine
46
Fig. 5: (above) An integral
collector/storage unit
Fig. 6: (above) Position the stor-
age tank above the collector to
prevent nighttime reverse flow.
Fig. 7: (left) A check valve pre-
vents reverse flow when tank is
even with collector.
Drawing: Thermal
Conversion Technology
GLAZING GASKETS
CASE
ABSORBER/
STORAGE TANK
FLUID CONNECTION
INSULATION
GLAZING