14 June 2002
design decisions regarding freeze protection, corrosion resistance, control strategies,
and fluid toxicity issues will be made with respect to this part of the solar energy
system. The transport subsystem consists of all fluid piping on the interior of the
building, a heat transfer fluid, heat exchanger, expansion tank, pumps, and various
types of valves and fittings.
b. Freeze Protection
(1) Purpose. Freeze protection is required in any climate that can
experience temperatures less than 32 degrees F (0 degrees C). However, collectors
may be subjected to sub-freezing temperatures (due to radiant heat transfer to the sky
on a clear night) even when ambient temperatures are as high as 38 degrees F (3
(2) Strategies. Common freeze protection strategies include the use of an
antifreeze fluid in the collector loop, or to drain all exposed piping when freezing
c. Stagnation and Overheat Protection. Stagnation is a condition that may
occur when the system is deactivated while fluid is contained in the collectors during
periods of solar insolation. For example, on a sunny day stagnation temperatures in a
flat-plate collector can exceed 350 degrees F (177 degrees C), leading to vaporization
of the transport fluid within the collector and excessive pressure build up in the system
piping. In the case of a closed-loop system, it is important to ensure that all
components in the collector loop can withstand these temperatures and pressures. A
pressure relief valve and an expansion tank should also be used to protect the system
components and control pressures.
d. Heat Transfer Fluids
(1) Definition. The heat transfer fluid is contained in the collector loop.
Selection of the proper fluid is critical, since certain fluid properties can cause serious
corrosion problems or degrade performance. Only water and propylene-glycol/water
solutions are considered.
Types of Fluids
(a) Water. As a heat transfer fluid, good quality water offers many
advantages. It is safe, non-toxic, chemically stable, inexpensive, and a good heat
transfer medium. Two drawbacks include a relatively high freezing point and a low
boiling point. Excessive scaling may occur if poor quality water is used.
(b) Glycols. Propylene or ethylene glycol is often mixed with water to
form an antifreeze solution. Propylene glycol has the distinct advantage of being non-
toxic, whereas ethylene glycol is toxic and extreme caution must be used to ensure that
it is isolated from any potable water. For this reason, uninhibited USP/food-grade
propylene glycol and water solution will be specified for any solar preheat system that
requires an antifreeze solution.