GREEN SYSTEMS
| SOLAR DOMESTIC HOT WATER | BY ERIC SKIBA
A
ccording to the Solar Energy Industries Association more than 30,000 solar water
heating systems were installed in 2010. These types of systems convert energy from the sun into thermal (heat) energy and transfer that energy to a fluid. A warm fluid has endless potential in residential and commercial applications and can be used to heat hot water for domestic consumption, heat homes through hydronics and keep swimming pools warm well into the shoulder months. Solar thermal collectors for water
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heating have fluid circulating through them and are typically installed outside. Since a large portion of the country sees freezing temperatures during the year, protecting the fluid inside the collectors from freezing is an important design consideration. Freeze protection is a common
topic of discussion among the solar thermal industry and arguments over the “best” method of protecting a system have been going on for years. Perhaps this is due to the fact that there is no best method. There are many ways to achieve the same end result, but the benefits and downsides of different methods of freeze protection should be considered so that the system operates efficiently and safely.
Mild climates Some areas see minor periods of
freezing throughout the year. In these areas, where the temperature does not drop below 30°F for more than a few days a year, it is still possible to circulate potable water directly through the collector (direct flow system). If the goal of the system is to heat hot water, circulating potable water directly through the collector is highly efficient. Any time that a heat exchange can be avoided is ideal, but how can this type of system be protected from unforeseen freezing temperatures? Most modern solar controllers
have incorporated freeze protection mechanisms into their programming. These functions operate by monitoring the temperature at the collector and circulate fluid through the collector if the conditions reach a point where freezing could occur. This function uses energy that was
An example of a drainback system utilizing a hot water storage tank with an internal coil heat exchanger. When the system is not operating, fluid sits inside the freeze protected area. Certain valves and small components are not shown.
previously gained during the day through solar collection or in the worst case, uses traditional fuel sources to provide freeze protection. For mild climates where freezing conditions occasionally occur, the electrical and heat losses of this method are negligible in terms of total annual operation. During a power outage, however,
the combination of no electricity and freezing temperatures can cause damage to a system. One simple solution is to use a valve that opens and causes water to drain from the system. By creating movement through the collector freezing is less likely to occur. These valves must be of good quality since a failure can cause damage to the system and potentially the building.
It should also be noted that certain
types of collectors are more susceptible to freezing than others and also may not be approved for contact with potable water. The manufacturer should always be consulted before installing a direct flow system.
Cold climates Controller based freeze protection
can be a useful method if freezing rarely occurs, but most regions have weather that warrants a more robust method of protecting a solar thermal system. The goal of the design is to provide simple, reliable freeze protection while minimizing costs and reduction in system efficiency. This is achieved by either using anti- freeze to increase the fluid’s ability
Solar Thermal Freeze Protection
phc april 2011
www.phcnews.com
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