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SOLAR WATER HEATING

by Bob Fairchild

    As with space heating, establishing general efficiency is
vital before installing solar water heating equipment.  Low flow
showerheads and faucet aerators will significantly reduce water
heating costs.  Adding extra insulation to water tanks and piping
will also reduce the amount of energy used.  Switching from
electric to gas, propane or wood can also reduce costs.   Solar
energy can be used to economically heat moderate amounts of water
for domestic use.  Design, cost, and performance solar water
heating units very widely.  As a general rule, one can expect 1-2
gallons of hot water per square foot of collector on a moderately
sunny day.

     There are two general types of solar water heating systems:
active and passive.  Active systems use pumps; passive systems do
not.  Active systems are more complex and expensive that passive
systems, but generally provide more hot water faster.
Additionally, water heating systems can be divided into batch
types and circulating types.  With a batch system the collector
serves as the storage tank.  In a circulating system there is a
collection panel and a separate storage tank.  Water or
antifreeze solution (with a heat exchanger coil at the tank)  is
circulated by a pump or by passive natural circulation, either
thermosiphon (natural convection) or percolation (in a new type
of commercially available system).  Circulating systems are more
complex and expensive than batch systems.

    A solar showerbag is about the simplest solar water heater.
It is a passive batch system consisting of a vinyl bag with clear
front and black back, a filling port, and a tube with a shower
nozzle for emptying.  The bag is filled and placed in the sun and
then hung up to allow gravity flow when used.  It will work on
sunny days from late spring to early fall.

    A breadbox solar water heater is the next step up.  It is
also a passive batch design.  It generally consists of one or
more black tanks in a glazed and insulated box.  The box can be
placed on the roof, on the ground, or integrated into the roof
with the tanks actually in the attic.  The tanks act as
collectors as well as storage units.  Multiple tanks are
generally used with pumped water systems to reduce the effect of
mixing cold water into the heated water.  It is possible to use a
one tank system with pump fill and gravity empty.  If integrated
into the roof, a breadbox system can work year round.  Separate
box systems, because of their greater exposed surface, may
experience freezing problems and may need to be drained during
sub-freezing weather.

    A breadbox water heater should be well insulated and double
glazed for winter use.  The glazing angle should be 30-60 degrees
from the horizontal and should roughly face south. Second hand
water heater tanks are commonly used.  They can be mounted with
the long axis horizontal or angled. Tanks in series should be
connected so that the cooler water comes in at the bottom and
hotter water flows out at the top.

    A panel system is even more complicated, having a separate
panel and storage tank.  It is a circulating system.  For winter
operation the panel must be protected from freezing unless it
uses anti-freeze and a heat exchanger.

    A panel system can be passive, using natural convection for
circulation.  In this case the panel must lie below the tank and
the piping between panel and tank must run continuously uphill
with no places for air bubbles to collect and stop the flow.  For
effective thermosiphoning, the collector panel needs at least 1/2
inch pipe for vertical sections and 3/4 inch pipe for horizontal
sections.   An active panel system can either employ thermal
(sensing temperature differences) or photoelectric (sensing
light) controls to activate the pump or a solar electric powered
pump can be used which automatically regulates flow by the amount
of sunshine it receives.

    Most solar water heater panels consist of flat plate
collectors inside glazed and insulated boxes.  Flat plate
collectors consist of a set of pipes with sheet metal absorbers
attached.  Copper pipe and sheet is best, though steel can be
used.  Aluminum sheet is often used for an absorber.  Generally
the sheet is attached to the pipes with solder, welding, or
special heat conducting cement.  Plastic pipe does not make a
good collector as it conducts heat poorly, and cannot withstand
high temperatures.

    Flat plate collectors come in two basic types:  serpentine
or "ladder".  Serpentine uses one continuous pipe that snakes
back and forth up the panel.  Water flows in one end and out the
other.  Ladder types use a larger pipe for "rails" at top and
bottom, with numerous smaller pipes running up and down between
them as rungs.  Water flows in at a lower corner and out the
opposite upper corner.

    One commercially available panel system is the Copper
Cricket.  It uses percolation to circulate heated fluid down to a
heat exchanger in a water tank.  It is a unique and elegant
system, but costs  $2000 for the equipment alone, making it
uneconomical in most situations.  Concentrating collectors are
not generally used for domestic hot water.  They are used to
produce high temperature, high pressure water or steam for
mechanical power or commercial/industrial processes.  They
require tracking mechanisms and very clear days.  Evacuated tube
collectors are another high performance system too expensive for
domestic applications.

    Good designs and materials are important in solar water
heaters.  Weatherproof materials, coatings, hardware, and details
are essential.  Glass, aluminum, and silicone caulk are more
expensive than some of the alternatives, but make up the
difference in durability and performance.

    Use pattern also affects the performance of a solar water
heater. The water is hottest late on sunny days.  If showering,
dishwashing, and clothes washing can be concentrated at this
time, a solar water heating system can be used to maximum
effectiveness, and cost for other water heating fuels minimized.

    Costs:  solar shower bag $10-$15.  Batch solar water heater
$250 (scrounge and scrape) to $1000 ("gold-plated").  Panel
systems $200 and up depending upon size and complexity.  Do-it-
yourself makes it much more economical and educational.
Commercially available systems at $2000+ are not worth it.

              ------------------------
REFERENCES

Breadbox Solar Hot Water Heaters.  Butte, MT: NCAT,  1981.  (P.O.
Box 4000,  Butte, MT  59702).

Campbell, Stu.  Build Your Own Solar Water Heater.  Garden Way
Publishing,  1978.