Solar Thermal Information

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in solar combisystems. Most systems using fin-tube
baseboard are designed around relatively high water
temperatures (180ºF or higher). While it is possible to
lower the supply water temperature by adding fin-tube
baseboard to the system, lowering it from 180ºF to 120ºF
typically requires about 3.5 times as much fin-tube length
for the same heat output. Few rooms can physically
accommodate this, and few occupants would accept the
associated aesthetics.
Some radiant floor panels require water temperatures
well above 120ºF at design load conditions. Such panels
will not allow a solar thermal subsystem to perform as
well as the previously discussed radiant panels. Careful
analysis using solar simulation software can quantify the
differences in expected solar subsystem performance.
Cast iron radiators sized for steam heating but converted
for use with higher temperature water are also unlikely to
be suitable. The possible exception would be a building
that has undergone extensive weatherization since the
steam radiators were installed. In some cases, the
significant reduction in heating load relative to the surface
area of the original radiators might allow operation
at water temperatures within the range that can be
consistently supplied by solar collectors. In such cases,
the original radiator system should also be internally
cleaned and flushed to remove any accumulated residue
associated with steam heating.
4. ANTIFREEZE-BASED SOLAR COMBISYSTEMS
The use of antifreeze fluids to protect solar collectors and
exposed piping against freezing is very similar to its use
in other hydronic systems. The piping circuits containing
the antifreeze solution are closed loops. When the system
is commissioned, these loops are purged of air and
slightly pressurized. In a closed, fluid-filled piping circuit,
the weight of the upward flowing fluid is exactly balanced
by that of the downward flowing fluid.
ADVANTAGES OF
ANTIFREEZE-BASED SYSTEMS:
In a properly purged closed piping circuit, the circulator
is only responsible for overcoming the head loss due to
friction between the piping components and the fluid. It is
not responsible for “lifting” fluid into unfilled piping. That
task is handled by a separate filling/purging pump. Thus,
the circulator in a closed, fluid-filled piping loop can be
relatively small. In residential combisystems, the collector
loop circulator might only require 25 watts or less of
electrical input energy. This keeps the operating cost of
the solar collection subsystem low.
Another advantage of antifreeze-based systems is that
tubing to and from the collector array can be installed in
any orientation. This allows flexible stainless steel or other
types of coiled metal tubing to be installed in spaces where
it would be difficult or impossible to install rigid tubing, or
to maintain a minimum slope on that tubing.
DISADVANTAGE OF
ANTIFREEZE-BASED SYSTEMS:
One disadvantage of antifreeze-based systems is that a
heat exchanger is required between the antifreeze solution
in the collector circuit and the water in the remainder of
the system. Any heat exchanger imposes a performance
penalty on the system because it forces the collector
circuit to operate at a temperature higher than that of the
water near the bottom of the storage tank. The magnitude
of this temperature difference depends on the sizing of
the heat exchanger. The greater the internal surface of the
heat exchanger is relative to the rate of heat transfer, the
less the performance penalty. The performance of heat
exchangers and the penalty factor they impose on solar
combisystems are discussed in Appendix B.
Another disadvantage of antifreeze-based systems is
that glycol-based antifreeze will chemically degrade
and become acidic over time. In such a state, the
fluid can cause internal corrosion of both piping and
collectors, eventually leading to failure. This degradation
is accelerated at higher temperatures. It is not good for
glycol-based fluids to be maintained at high temperatures
over long periods of time. This can happen in solar
combisystems that generate far more heat in summer
than is required by the load. It also implies that annual
testing of the pH and reserve alkalinity of glycol-based
antifreeze solutions is imperative.
HEAT DUMP PROVISIONS:
The potential for thermal degradation of glycol-based
antifreeze solutions justifies the need for a “heat dump”
provision on antifreeze-based solar combisystems.
The heat dump provides a way for the system to shed
excess heat when the solar storage tank has reached
a user-set upper temperature limit and there is no
immediate need for further heat in the system. Heat
dumps can take the form of:
• Additional storage tanks.
• Passive or fan-forced convectors through which the
hot antifreeze solution is diverted when necessary. These
convectors are typically located outside the building and
dissipate heat directly to outdoor air.