31-07-2012, 12:03 PM
a solar ammonia absorption icemarker
6599651-How-to-Build-a-Solar-Icemaker.pdf (Size: 346.54 KB / Downloads: 31)
Types of Refrigeration
Refrigeration may seem complicated, but it can be
reduced to a simple strategy: By some means, coax a
refrigerant, a material that evaporates and boils at a low
temperature, into a pure liquid state. Then, let’s say you
need some cold (thermodynamics would say you need
to absorb some heat). Letting the refrigerant evaporate
absorbs heat, just as your evaporating sweat absorbs
body heat on a hot summer day. Since refrigerants boil
at a low temperature, they continue to evaporate
profusely — thus refrigerating — even when the milk or
vaccines or whatever is already cool. That’s all there is
to it. The rest is details.
Refrigeration
The generator is a three-inch non-galvanized steel pipe
positioned at the focus of a parabolic trough collector.
The generator is oriented east-west, so that only
seasonal and not daily tracking of the collector is
required. During construction, calcium chloride is
placed in the generator, which is then capped closed.
Pure (anhydrous) ammonia obtained in a pressurized
tank is allowed to evaporate through a valve into the
generator and is absorbed by the salt molecules,
forming a calcium chloride-ammonia solution (CaCl2 -
8NH3).
The generator is connected to a condenser made from
a coiled 21 foot length of non-galvanized, quarter-inch
pipe (rated at 2000 psi). The coil is immersed in a water
bath for cooling. The condenser pipe descends to the
evaporator/collecting tank, situated in an insulated box
where ice is produced.
Operation
The icemaker operates in a day/night cycle, generating
distilled ammonia during the daytime and reabsorbing it
at night. Ammonia boils out of the generator as a hot
gas at about 200 psi pressure. The gas condenses in
the condenser coil and drips down into the storage tank
where, ideally, 3/4 of the absorbed ammonia collects by
the end of the day (at 250 degrees Fahrenheit, six of
the eight ammonia molecules bound to each salt
molecule are available).
As the generator cools, the night cycle begins. The
calcium chloride reabsorbs ammonia gas, pulling it
back through the condenser coil as it evaporates out of
the tank in the insulated box. The evaporation of the
ammonia removes large quantities of heat from the
collector tank and the water surrounding it. How much
heat a given refrigerant will absorb depends on its “heat
of vaporization,” — the amount of energy required to
evaporate a certain amount of that refrigerant.
Future Design
A refrigerator, which is able to absorb heat at any time
from its contents, is more convenient than our current
intermittent icemaker. To enable constant operation, a
future design will include several generator pipes in
staggered operation as well as a reservoir for distilled
ammonia. Staggered operation will allow the
refrigerator to always have one or more of the
generators “thirsty” and ready to absorb ammonia, even
during the day when generation is simultaneously
happening. Generation will constantly replenish the
supply of ammonia in the storage reservoir. We are
currently in the first stages of making these
modifications to the icemaker.
Conclusion
The S.T.E.V.E.N. icemaker has both advantages and
disadvantages. On the down side, it’s somewhat bulky
and non-portable, and requires some special plumbing
parts. It requires a poisonous gas, albeit one which is
eco- and ozone- friendly in low concentrations, so
precautions must be taken. In its favor, it has few
moving parts to wear out and is simple to operate. It
takes advantage of the natural day/night cycle of solar
energy, and eliminates the need for batteries, storing
“solar cold” in the form of ice.