What system designs should be used to prevent freezing?
Cold-climate solar water heating systems
While most everyone today knows that solar photovoltaic systems turn sunlight into electricity for a wide variety of residential, commercial, industrial and agricultural applications, not as many people are aware that solar heating systems turn sunlight into heat for an equally wide variety of end-use applications — and at a much higher efficiency (20% versus 75% or higher).
Many of the carbon dioxide emission reduction activities these days are focused on reducing electricity consumption through the use of PV systems, despite the fact that water heating and space heating combined make up the single largest component of building energy consumption in the United States, according to the U.S. Energy Information Administration. Solar thermal technologies can play a major role in this effort.
One aspect of solar heating system design selection is based on the geographical location where the system will be installed. It is generally agreed within the solar thermal industry that all locations in the mainland United States are subject to either regular or occasional freezing conditions, including southernmost Florida. Hawaii remains the only state where freezing temperatures do not occur in populated areas (exceptions being: Mauna Kea at 13,796 feet on the island of Hawaii, Mauna Loa at 13,678 feet and Haleakala on Maui at 10,023 feet elevations). So, what system designs should be used to prevent freezing?
In climates around the world where temperatures do not approach freezing, solar water heating system designs typically incorporate operating strategies where potable water is circulated directly through the solar collector(s), and then delivered to the building hot-water supply.
Where freezing temperatures are a possibility, however, this design can lead to the water freezing inside the solar collector, which can inflict permanent damage due to ruptured tubing. In fact, freezing temperatures inside a solar collector can be as much as 5° F to 8° below the surrounding air temperature, due to a phenomenon known as “night sky cooling,” or radiative cooling, where a warm body radiates heat to a cooler body — in this case the dark, nighttime sky.
So, in climates where a system can be expected to be exposed to freezing temperatures, one of two system types are generally used — “drainback” or “closed loop antifreeze.”
Drainback solar water heating system
A drainback system, as pictured in Figure 1, uses a freeze protection strategy where all piping to and from the solar collector on the roof to the water heating system in the building is continuously sloped at a slight angle back toward the water heating system.
When the system is at rest, approximately 1/3 of the “drainback tank” near the water heating system, and all the piping below that level, is filled with heat-transfer fluid (water or a water/propylene glycol antifreeze mix); the balance of the piping and the drainback tank contain unpressurized air.
When solar heat is available in the solar collector, a “differential controller” senses this and energizes a pump to circulate heat-transfer fluid through the solar collector and back to the drainback tank. The fluid continues on to an internal (pictured) or external heat exchanger, which transfers the heat to the solar storage tank. At this point, a “siphon” has been established, significantly reducing the amount of pumping energy required to keep the system in operation.
When the solar collector(s) cool off (passing clouds, or the sun setting), the controller shuts the pump off and the fluid in the solar collectors, and piping “drains back” into the drainback tank. Since all of the fluid has now been evacuated from the parts of the system exposed to freezing temperatures, the system is safe.
Closed-loop antifreeze solar water heating system
In some circumstances, usually due to the configuration of the building or the location of the solar collectors, it is not possible to install solar-system piping in such a way that it can be continuously sloped from the solar collector(s) all the way back to the water heating system, and therefore a drainback system is not feasible for that project.
A pressurized, closed-loop antifreeze system design is the alternative. Many contractors prefer closed-loop antifreeze systems because the routing of the system piping is more forgiving, making system installation faster, however building codes require that the piping must still be installed in such a way as it can drain. On the other hand, antifreeze solutions are subject to degradation after extended use — in some cases requiring replacement after five to ten years.
The efficiency of the two system types are approximately equal, since they can both use the same type of solar collector. By using one of these two system types, a solar water heating system can be successfully operated anywhere in the United States. RJ 2.0