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MIL-HDBK-1003/13A
Silicones have the advantage of lasting the life of the system with little
maintenance.  While this helps minimize operating expenses, the initial cost
of silicones is markedly higher than that of other available heat transfer
fluids.  However, the high initial cost of silicone heat transfer fluid may
be less than the savings that result from minimum maintenance and no
replacement of collector fluid.  The use of silicone fluid allows absorbers
with aluminum fluid passages to be used without fear of corrosion.  The
savings gained from the use of aluminum absorbers as opposed to copper
absorbers could be substantial.
2.1.7.2 Hydrocarbons.  Hydrocarbon oils, like silicones, also give a long
service life, but cost less.  They are relatively noncorrosive, nonvolatile,
environmentally safe, and most are nontoxic.  They are designed for use in
systems with lower operating temperatures, since some brands break down at
higher temperatures to form sludge and corrosive organic acids.  Typical
closed-cup flashpoints run from 300 deg. F to 420 deg. F, but the fluids with
higher flashpoints have a higher viscosity.  The HUD bulletin on minimum
property standards for solar heating systems recommends a closed-cup
flashpoint 100 deg. F higher than maximum expected collector temperatures.
Unsaturated hydrocarbons are also subject to rapid oxidation if exposed to
air, necessitating the use of oxygen scavengers.  Some hydrocarbons thicken
at low temperatures and the resultant higher viscosity can cause pumping
problems.
Newer hydrocarbons are being developed which do not harm rubber or materials
of construction, since this has been a problem with hydrocarbons.  In
general, they cannot be used with copper, as it serves as a catalyst to fluid
decomposition.  The thermal conductivity of hydrocarbons is lower than that
of water, although the performance of some brands is much better than others.
The cost of typical hydrocarbon and other synthetic heat transfer oils vary
from about $6/gal to $20/gal.  A typical liquid collector of 500 ft2 plus
the piping to and from storage will require from 20 to 30 gallons of
collector fluid.  The lower heat capacity and higher viscosity of these oils
will also require larger diameter pipe, increasing materials costs further.
If hydrocarbon fluids are used, the additional capital cost should be
compared with expected savings due to lower maintenance costs.  The use of
aluminum absorbers rather than copper absorbers will also result in
substantial savings.
2.1.7.3 Distilled water.  Distilled water has been suggested for use in
solar collectors since it avoids some of the problems of untreated potable
water.  First, since the distillation process removes contaminants such as
chlorides and heavy metal ions, the problem of galvanic corrosion, though not
completely eliminated, should be alleviated.  However, distilled water is
still subject to freezing and boiling.  For this reason, an anti-freeze/anti-
boil agent such as ethylene glycol is often added.
2.1.7.4 Water-anti-freeze.  Nonfreezing liquids can also be used to provide
freeze protection.  These fluids are circulated in a closed loop with a
double wall heat exchanger between the collector loop and the storage tank
(see Figure 2-5).
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