For most of North America, freeze protection for solar hot water (SHW) systems is essential. Even though a freeze may not be a regular occurrence, over the 15- to 25-year expected life of a SHW system chances are high that freezing conditions will be encountered. It only takes a single freeze to damage pipes, collectors, and other system components; and damage to the building structure can also occur from burst pipes and leaking heat transfer fluid (HTF) or water.
Using an antifreeze solution as the HTF is a common freeze-protection strategy. The chemistry of the fluid keeps it from freezing, down to a certain, known temperature, based on a given level of dilution.
Closed-loop systems are very versatile, and are commonly used for domestic and commercial water heating, for space heating, and for dual-use systems, such as in a snow melt and pool-heating combination.
Systems using antifreeze HTF have a separate plumbed “loop” that circulates through the solar collectors and transfers the heat collected to end-use water in a storage tank through a heat exchanger. This loop is pressurized, meaning that it’s sealed and filled with fluid to a certain pressure at installation, and circulated by a pump operated by a differential controller. When the controller detects that the collector temperature exceeds the stored water temperature by a preset amount, the pump is activated. When the temperature difference decreases to a set amount, the pump shuts off. Whether the system is being pumped or not, the collector loop is always full of HTF.
The pressure of the HTF loop should be less than that of the domestic water. Fifteen to 25 PSI is adequate for most domestic systems. In the event of leakage occurring between the two loops, the higher-pressure domestic water would leak into the HTF, rather than the HTF contaminating the potable water. It is also important to set the HTF loop pressure so that the pressure does not get too high when the fluid is hot. There can be a difference of 20 PSI or more between unheated and hot HTF.
Water can be used as the HTF—compared to other, denser fluids, it takes less energy to pump. Its specific heat is 1, meaning it takes 1 Btu to raise 1 pound of water 1°F; this is higher than other common HTFs, and means that water has a higher heat content per volume and is thus more effective at gathering heat. However, water freezes at a too-high temperature (32°F), offering little or no freeze protection. Plus, it expands as it freezes, increasing the likelihood of damaging its container—the solar collectors or the piping.
Water also boils at a relatively low temperature (212°F), though as with all fluids, when contained in a pressurized loop, the temperature at which it boils increases. This is important, especially if stagnation (meaning that the system stops pumping, even though heat is available) occurs. Stagnation can result from pump failure or if the controller deactivates the pump for “high-limit” temperature in the end-use water. (In poorly sized or underutilized SHW systems, this can be a common occurrence during hot months.) During stagnation, the collector HTF temperature can be far in excess of the boiling point of water, resulting in steam being produced, and fluid loss through pressure-relief valves. While water is an efficient HTF, its narrow operating range precludes it from use in all but the mildest climates.
Using the right mix of antifreeze and water increases freeze protection, and maintains as much of water’s heat transfer capability as possible. Several different chemicals have been used as antifreeze HTF in SHW systems. The most common is propylene glycol, which is used in products ranging from deodorant to toothpaste; as an emulsifier in processed foods; for de-icing airplane wings; and in many other applications. It is closely related to ethylene glycol, which is most commonly used as antifreeze in radiators for engine-cooling systems. Both are more viscous than water and typically are dyed glowing-green, but ethylene glycol is much more toxic and can be fatal if ingested. Ethylene glycol must not be used in SHW systems to avoid contamination of end-use water. (See “Fundamentals of Solar Heat Exchangers” in HP128 for more information.)
Pure, or 100%, propylene glycol has a much wider range of operation than water, with a freezing point of -74°F, and a boiling point of 310°F. It is more dense and more difficult to pump than water, though this is not usually an issue in most residential closed-loop applications utilizing AC pumps. Systems with long pipe runs, and especially with multiple collectors, require careful pump selection. The HTF in many SHW systems is a mix of propylene glycol with distilled water to avoid minerals or compounds that could corrode copper piping and other components. However, depending on the mineral content of the local water supply, distilled water may be unnecessary. A 50/50 or 60/40 glycol-to-water ratio is most common. The ratio should be tailored for local temperatures (see the “Glycol Mixtures” table).
With a much lower specific heat (0.60 Btu/lb./°F), propylene glycol is less efficient at transferring heat than water. Mixing it with water increases its efficiency, so it is best to use the lowest concentration of glycol that will provide freeze protection. The downside is that greater dilution lowers the boiling point of the HTF, potentially subjecting it to high pressures, vaporization, and subsequent fluid loss in the event of stagnation.
High temperatures—though they may be below the boiling point due to the pressurization of the HTF loop—also cause propylene glycol to break down. This causes acids to form, leading to corrosion. This corrosion can affect pipes, fittings, valves, seals, pump impellers, and other components, potentially causing them to fail. Particulates can start to build up in the HTF, which also can lead to clogs or further damage to the pump (see “HTF Breakdown” sidebar).
In the past, other fluids were used in place of glycol, including ethanol (Copper Cricket), bray oil (Novan), silicones, and hydrocarbons. Some HTFs may no longer be available when the system needs to be refilled/recharged. Some of these systems, such as those using bray oil, can be converted to use propylene glycol, but the HTF loop must be very thoroughly flushed with a detergent.