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Solar Thermal Freeze Protection Continued from page 36 T1

enough storage and is not oversized, closed loop systems can work well and run at high efficiency for years. However, it may sometimes be difficult to avoid such con- ditions and sometimes an oversized system is required. An example would be a system that is contributing to a heat- ing load as well as hot water, or a commercial system that has high loads during the week and almost none over the weekend. For such systems it may be prudent to utilize a drainback method of freeze protection to avoid the issues associated with pressurized anti-freeze systems. A drainback system runs at low or atmospheric pressure

and uses water as the heat transfer fluid. This can help improve the system efficiency since water’s thermal prop- erties are more ideal for use in transferring heat. A heat exchanger is still required since the pressurized potable water system must be separated from the lower pressure solar system, so it is important that this heat exchanger be properly sized and designed. When the system is not operating (no available solar

energy and the tank is at maximum temperature) the fluid in the collector, supply and return lines drain empty of water. The water that drains from the system is stored in a drainback tank that is located in a freeze protected area. If the collector is sufficiently hotter than the bottom of the main storage tank, the solar pump will turn on and lift the water up the supply piping pushing the air out of the col- lector and return line into the drainback tank. As this hap- pens the piping begins to fill with water and eventually will operate similar to a closed loop system once all the air is


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Solar Pump Station

Sol ar Pump


Cold Wa er WI

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Preheated Water Ou


Externa Heat

Exchanger Exchange T2

An example of a solar thermal system using an external heat exchanger. The preheated water leaving the tank is then distributed through a conventional water heating sys- tem. Certain valves and small components are not shown.

removed from the piping. In order for a drainback system to function properly as a

freeze protection mechanism, it must drain completely. All the piping has to slope back to the drainback tank at a suffi- cient pitch (¼” per foot) to ensure that fluid does not get “hung up” in a section of pipe exposed to freezing condi- tions. This can be difficult on certain jobs such as ground mounted systems where there may not be enough of a height difference between the collector and the drainback tank. One important element to the operation of the system is that air must be able to reenter the system otherwise it will not drain- back. If this is not able to take place, a vacuum will hold fluid in the exposed piping and freezing can occur. It should also be noted that due to the design of the internal flow paths, certain types of collectors simply will not drain com- pletely and therefore cannot be used in these systems. A drainback system will also require slightly more ener-

gy since the pump has to lift the water up and through the collectors. This energy cost can be minimized through cer- tain piping designs so that once the piping is flooded with water, the system operates with similar pumping require- ments as a closed system. All types of freeze protection have important considera-

tions that must be addressed when installing a solar ther- mal system. Keeping a system from freezing is only one part of the design. Sufficient research of system types should be carried out before a final method is chosen so that the best method for that installation is carried out. Each application is unique and differing methods of freeze protection have been used in many installations, some of which have been in service for over 20 years. Proper design, installation and equipment selection will help ensure that the system runs safely and efficiently. n

Eric Skiba is a technical engineer at Apricus Inc. April 2011

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