Friday, May 10, 2013
Above: Steven Vanek with his machine which uses solar thermal energy to make ice.
Everywhere in our world,
refrigeration is a major energy
user. In poor areas,
“off- grid” refrigeration is a critically important need. Both of these considerations point the way toward refrigeration using renewable energy, as part of a point the way toward refrigeration using renewable energy, as part of a sustainable way of life. Solar-powered refrigeration is a real and exciting
possibility.
Working with the S.T.E.V.E.N.
Foundation (Solar Technology and Energy for Vital
Economic Needs), we developed
a simple ice making system using
ammonia as a refrigerant. A prototype of this system is currently
operating at SIFAT (Servants
in Faith and Technology), a leadership and technology training center in Lineville,
Alabama. An
icemaker like this
could be used to
refrigerate vaccines, meat,
dairy products, or vegetables.
We hope
this refrigeration system will be
a cost-effective way to
address the worldwide need for
refrigeration. This icemaker
uses free solar energy, few moving parts, and no batteries!
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.
One of these
details is how the liquid refrigerant is produced. Mechanically driven
refrigerators, such as typical electric kitchen fridges, use a compressor to
force the refrigerant freon into a liquid state.
the way grain alcohol is purified
through distillation. After the generation
process, the liquefied refrigerant evaporates as it is
re-absorbed by an absorbent material. Absorbent materials are materials which have a strong chemical attraction for the refrigerant.
This process can be
clarified using an analogy: it
is like
squeezing out a sponge
(the absorbent material) soaked with the refrigerant. Instead of
actually squeezing the sponge, heat is used.
Then, when the sponge cools and becomes “thirsty” again, it reabsorbs the refrigerant in gas form.
As it is
absorbed, the refrigerant evaporates and absorbs heat: refrigeration!
In an ammonia absorption refrigerator, ammonia is the refrigerant. Continuously cycling ammonia refrigerators, such as commercial propane-fueled systems, generally use water as the absorbent, and provide continuous cooling action.
The S.T.E.V.E.N. Solar Icemaker We call our
current design
an icemaker. It’s not a true refrigerator
because the refrigeration happens
in intermittent cycles, which fit the cycle of available solar energy from day to night. Intermittent absorption
systems can use a salt instead of water as the absorbent material.
This has distinct advantages in that the salt doesn’t evaporate
with the water during heating,
a problem encountered with water as the absorber.
Our intermittent absorption solar icemaker uses calcium
chloride salt as the
absorber and pure ammonia as
the refrigerant. These materials
are comparatively easy to obtain. Ammonia is available on order from gas suppliers
and calcium
chloride can be bought in
the winter as an ice melter.
The plumbing of the
icemaker can be divided into
three parts: a generator for heating the salt-ammonia
mixture, a condenser coil, and an
evaporator, where distilled ammonia collects during generation.
Ammonia flows back and forth between
the generator and evaporator.
Above: Detail of the condenser bath, containing the condenser coil, and the icemaker box below.
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.
Above: About ten pounds of ice are created in one cycle of ammonia evaporation / condensation.
Few materials come close to the heat of
vaporization of water. We lucky humans get to use water as our evaporative
refrigerant in sweat. Ammonia comes close with a heat of vaporization 3/5 that
of water.
During the night cycle, all of the liquefied
ammonia evaporates from the tank. Water in bags around the tank turns to ice.
In the morning the ice is removed and replaced with new water for the next
cycle. The ice harvesting and water replacement are the only tasks of the
operator. The ice can either be sold as a commercial product, or used in a
cooler or old-style ice- box refrigerator.
Under good sun, the collector gathers enough
energy to complete a generating cycle in far less than a day, about three hours.
This allows the icemaker to work well on hazy or partly cloudy days. Once
generating has finished, the collector can be covered from the sun. The
generator will cool enough to induce the night cycle and start the ice making
process during the day.
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.
Caution: Safety First!
Working with
pure ammonia can be dangerous if safety precautions are not taken. Pure ammonia
is poisonous if inhaled in high enough concentrations, causing burning eyes,
nose, and throat, blindness, and worse. Since water combines readily with
ammonia, a supply of water (garden hose or other) should always be on hand in
the event of a large leak. Our current unit is a prototype. We will not place
it inside a dwelling until certain of its safety. Unlike some poisonous gases,
ammonia has the advantage that the tiniest amount is readily detectable by its
strong odor. It doesn’t sneak up on you!
For the
longevity of the system, materials in contact with ammonia in the icemaker must
resist corrosion. Our unit is built with nongalvanized steel plumbing and
stainless steel valves, since these two metals are not corroded by ammonia. In
addition, during operation the pressure in the system can go over 200 psi. All
the plumbing must be able to withstand these pressures without leaks or
ruptures.
Would be solar
icemaker builders are cautioned to seek technical assistance when experimenting
with ammonia absorption systems.
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, an eliminates the need for batteries, storing
“solar cold” in the form of ice.
Access
Authors: c/o S.T.E.V.E.N. Foundation, 414 Triphammer
Rd. Ithaca, NY 14850
SIFAT,
Route 1, Box D-14 Lineville, AL 36266