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The configuration shown in Figure 2-4 will be from 10%-25% less efficient due
to the temperature penalty associated with the heat exchanger and the low
specific heat of the heat transfer fluid as compared to water.  Note an
additional pump is also required.  If the heat transfer fluid is toxic or
non-potable (such as antifreeze) then a double-walled heat exchanger must be
used for protection.  The different types of heat exchangers are explained in
Figure 2-5 (National Solar Heating and Cooling Info Center, 1979).
It is difficult to estimate the most cost effective freeze protection method.
Some studies have shown that for many areas in the U.S., the recirculation
method is best particularly where freezing days are few in number.  It tends
to have the lowest capital cost and energy use cost.  However, all the
methods except heat transfer fluids rely on the presence of electricity to
operate.  A simultaneous electrical failure and freezing condition would
result in potential failure of the systems.  An exception is that new
thermally actuated draindown valves are becoming available to replace the
sometimes troublesome solenoid valves.  Therefore, the absolute safest system
would be the nonfreezing heat transfer fluids and these might be considered
for the very cold parts of the country (Boston, Chicago, etc.).  Each
potential project should be considered individually using local weather
criteria, freeze protection capital costs, additional energy to run the
system, reliability, maintenance, and type of system as the criteria.  Often
a detailed computer simulation would be required to choose.  However, any of
the methods will provide some degree of protection.  If heat transfer fluids
are selected for corrosion or freeze protection, the following paragraphs
discuss pertinent criteria.

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