Solar Hot Water Systems
Antifreeze is one word that Solar Thermal installers and owners should understand. For the Solar Thermal contractor, this antifreeze is more accurately described as the heat transfer fluid (HTF) necessary for the proper functioning of solar thermal systems in different regions of the country. Primarily referred to as glycol, the product comes in different formats, however SunEarth recommends usage of the Dow Chemical Dowfrost HD propylene glycol heat transfer fluid (HTF).
Solar water heating systems have the unique characteristic of producing very high fluid temperatures during summer stagnation conditions. In a forced circulation system, a mechanical pump is utilized to efficiently circulate the Dow Chemical Dowfrost HD propylene glycol heat transfer fluid (HTF) throughout the system. Thus specific glycol requirements utilizing special high temperature corrosion inhibitors to protect the fluid from breaking down rapidly, such as the Dow Chemical HD Propylene Glycol. The HTF protects the collector piping from freezing and inhibits scaling deposits that can reduce performance in indirect systems. If the wrong glycol is used in a solar water heating system, the fluid can break down rapidly. This can result in plugged collectors, blocked pumps, and in extreme situations systems that must be abandoned entirely.. Proper application and maintenance of the HTF can protect your water heating system to minus 60° Fahrenheit.
Operating a solar thermal system without proper propylene glycol levels can cause permanent damage to the system itself, testing propylene glycol is an important component of proper system maintenance. Fortunately, there are several reliable methods available to evaluate this.
Use test strips to test a sample of the propylene glycol. Dip the strip in the test substance to indicate the reserve alkalinity, boiling point, freezing point, and PH.
Increase accuracy by using a refractometer. This is a small device that can determine the freezing point of propylene glycol solutions.
When the system is operational, periodic testing of the glycol must be done to ensure that it is still suitable for use and providing the correct level of freeze protection. If a leak is to occur the system should be repair and recharged with glycol mixture. Diluted glycol concentration from simply topping off charge with water leads to a lack of freeze protection. Burst pipes or frozen solar collectors can often be the result of this problem. An annual check-up will include verification of the glycol concentration with a refractometer, and a check of the glycol PH level using litmus test strips. Glycol that has gone bad typically will have a low PH level, which if left unchecked, will start to corrode the metal components in the system. When the PH level is too low the glycol must be drained, the system flushed and recharged with new glycol.