Thermal Freeze Solar
Protection 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 appli- cations 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 heating have fluid circulating through them and are typically installed out- side. 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 con- sideration. 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 tem- peratures? 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 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.
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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 fail- ure 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 man- ufacturer 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 pro- vide 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 to handle cold conditions or removing the fluid from the ele- ments through the use of a drainback design. Anti-freeze is commonly mixed with water to lower the
freeze point and protect systems from damage associated with cold conditions. Propylene glycol is a standard prod- uct which is mixed with water and used in closed loop, pressurized solar thermal systems. The ratio of water and glycol can be adjusted to provide different levels of freeze protection (a 50/50 mix is standard). The glycol/water mix is separated from the potable water through a heat exchanger. The introduction of anti-freeze and a heat exchanger to
the system may have a negative impact on the efficiency. If a heat exchanger is not large enough or is not “solar friendly” this impact can be magnified. Since solar ther- mal systems operate best at low differential temperatures, the heat exchanger needs to be sized to work with rela- tively small differences in approach temperatures, some- times as little as 10°F, and low flow rates when compared to boiler operations. One benefit of using closed, pressurized systems is that
there are no substantial piping requirements other than avoiding excessive fluid velocity and increased pressure
Continued on page 36 April 2011
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