MIL-HDBK-1003/13A
2.4 Thermosyphon, batch, and integral storage collector systems. A
variation of the DHW system is the thermosyphon system which uses the
principle of natural convection of fluid between a collector and an elevated
storage tank. As water is heated in the collector it rises naturally to the
tank above. The bottom of the tank should be mounted about 2 feet higher
than the highest point of the collector. This is the main disadvantage in
that structural requirements will often prohibit the weight of a water tank
on a high point of the structure. Also, since the thermosyphon system is
connected directly to the potable water supply it is difficult to protect
from freezing. However, new models are coming on the market that use freon
as the heat transfer fluid, solving the freezing problem. The advantages of
thermosyphon units are that they do not require pumps or electronic control
systems. Hence the costs to purchase and operate these components are
eliminated. Also these systems save by virtue of eliminating these
components as a source of reliability or maintenance problems. A last
advantage is that they are completely independent of electrical grid power.
Batch and integral storage collector (ISC) systems are similar in that they
also do not have pumps or controllers. Batch systems (often called
"breadbox" also) are simply a black painted storage tank (or several)
installed in a weathertight box and glazed with glass or plastic. They
depend on their heat transfer by flow of water through the system initiated
whenever there is demand for water by the occupants.
Integral storage collectors put the tank and collector together to form a
large mass of fluid to be heated by the sun. The intent is to have a large
enough mass of water that freezing will not be a problem except in the
severest of climate. Surprisingly only about 30-40 gallons of water are
needed to accomplish this over most of the United States. ISC systems also
depend on system demand for their flow, but some models have also been
configured to use the thermosyphon principle.
The testing of these units is different than regular solar collectors since
the ASHRAE 93-77 standard explained in Section 2.1.9 does not apply. These
units must be tested as "whole systems". The method is given in ASHRAE
Standard 95-1981 (Section 1.2). At the time of this edition of the handbook
much of this data is just becoming available. The trend seems to be that
thermosyphon systems are probably the most efficient followed by ISC and
batch systems in that order.
NCEL has installed and tested a thermosyphon system and compared its
performance to a pumped system that uses a differential (on/off) controller
and one with a proportional (continuous) controller. As shown in figure
2-11a, the tank temperature rise for a one-day test was very similar for all
three systems (Durlak 1982). Although the performance of the thermosyphon
unit was very slightly lower it is not enough to rule out the use of these
systems especially when their advantages of improved reliability and
maintenance are considered. The important conclusion of these tests is that
the performance is similar enough that the choice of which to use can be made
by considering other pertinent factors of the installation.
The results of system tests on these models are reported in the Directory of
SRCC Ratings (Section 1.2).
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