How to Choose the Ideal Heat-transfer Fluid for Solar Water Heating System ?

When deciding on which heat-transfer fluid best suits your needs, there are some crucial factors to take into account. In solar water heating systems, these fluids are responsible for transporting heat through heat exchangers and solar collectors to the heat storage tanks. Needless to say, the decision on which fluid to use is a crucial one.

For starters, you should consider:

  • Thermal capacity: The ability to store heat.
  • Boiling point and freezing point: The temperature at which it boils or at which liquid transforms into a solid, respectively.
  • Flash point: The minimum temperature at which the vapor above a liquid is able to be ignited in air.
  • Viscosity: The liquid’s resistance to sheer forces.
  • Coefficient of expansion: A material’s small changes in length or volume due to minimal temperature changes.

Different Heat-transfer Fluids

Air does not freeze, boil, or corrode. The drawback is that it has an extremely low heat capacity and is prone to leaking out of collectors, ducts, and other areas of the unit.

Water doesn’t cost much, is nontoxic, and, due to a high specific heat and extremely low viscosity, it’s also easy to pump. The problem with water is that it has a high freezing point, low boiling point, and can be corrosive if neutral pH levels are not maintained. If you elect to use water, use soft water, as hard water tends to cause the formation of mineral deposits in your system.

Glycol/water mixtures typically have around a 50/50 glycol-to-water ratio. Ethylene and propylene glycol are considered antifreezes, and will provide protection against freezing if the proper concentration is maintained. These fluids degrade over time and should be replaced every 3 years or so.

Hydrocarbon oils have a lower specific heat and higher viscosity than water, meaning they require more energy to pump. Advantages of these oils are that they are relatively inexpensive and feature a low freezing point, making them ideal for cold temperatures.

Refrigerants and phase change fluids are often used in refrigerators, air conditioners, heat pumps, and other similarly common appliances. These fluids have a high heat capacity and low boiling point, which allows a small amount of the refrigerant to transfer a relatively large amount of heat efficiently. They also respond to solar heat rapidly, making them less dependent on sunlight than other sources of solar power.

Silicones have an incredibly high boiling point and low freezing point. Similar to hydrocarbon oils, these fluids have low heat capacities and a high viscosity and, thus, require more energy to pump. Unfortunately, silicones leak easily through even the smallest of holes.

What to Choose

There are many factors to consider when choosing a heat-transfer fluid. When making your decision, think about the temperatures the fluid will be exposed to. Naturally, fluids that will be exposed to high temperatures should have a high boiling point, while those that will face more frigid temperatures should have a lower freezing point. The good news is that, with so many to choose, you can easily find one that fits your purposes.


Mahan’s Thermal Products are known as the #1 source for heat exchangers. Having been in business since 1969, serving the industrial market with the design and sale of heat transfer equipment, Mahan’s continues to address all your heat transfer needs During that time Mahan’s has grown into a complete provider of heat transfer components, systems, parts and services. Mahan’s also has the capability to clean, repair or build all brands and types of heat transfer products. Mahan’s has developed many long-term relationships with our clients, earned by a commitment to provide the best product at a competitive price, backed by almost 45 years of service. Go online to mahans.com to learn more.

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