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Heat-transfer fluids carry heat through solar collectors and a heat exchanger to the heat storage tanks in solar water heating systems. When selecting a heat-transfer fluid, you and your solar heating contractor should consider the following criteria:

  • Coefficient of expansion – the fractional change in length (or sometimes in volume, when specified) of a material for a unit change in temperature
  • Viscosity – resistance of a liquid to sheer forces (and hence to flow)
  • Thermal capacity – the ability of matter to store heat
  • Freezing point – the temperature below which a liquid turns into a solid
  • Boiling point – the temperature at which a liquid boils
  • Flash point – the lowest temperature at which the vapor above a liquid can be ignited in air.
  • Corrosivity – compatibility with other materials and additives to reduce corrosion
  • Toxicity- only non-toxic fluids can be used in a potable water system.

For example, in a cold climate, solar water heating systems require fluids with low freezing points. Fluids exposed to high temperatures, and should have a high boiling point. Viscosity and thermal capacity determine the amount of pumping energy required. A fluid with low viscosity and high specific heat is easier to pump, because it is less resistant to flow and transfers more heat. Other properties that help determine the effectiveness of a fluid are stability and replacement lifetime.

Illustration of a solar water heater.

Types of Heat-Transfer Fluids

The following are some of the most commonly used heat-transfer fluids and their properties. Consult a solar heating professional or the local authority having jurisdiction to determine the requirements for heat transfer fluid in solar water heating systems in your area.

  • Air
    Air will not freeze or boil, and is non-corrosive. However, it has a very low heat capacity, requires a large heat exchanger to heat the water, and tends to leak out of collectors, ducts, and dampers.
  • Water
    Water is nontoxic and inexpensive. With a high specific heat, and a very low viscosity, it's easy to pump. Unfortunately, water has a relatively low boiling point and no protection against freezing. It can also be corrosive if the pH (acidity/alkalinity level) is not maintained at a neutral level. Water with a high mineral content (i.e., "hard" water) can cause mineral deposits to form in collector tubing and system plumbing.
  • Propylene glycol/water mixtures
    Propylene glycol/water mixtures have a glycol-to-water ratio of 50%/50%, and higher or lower as indicated by the freeze hazard. Ethylene Glycol must not be used due to toxicity, so non-toxic Propylene Glycol is used. These mixtures provide effective freeze protection as long as the proper antifreeze concentration is maintained. Antifreeze fluids degrade over time and normally should be changed every 3–5 years. These types of systems are pressurized, and should only be serviced by a qualified solar heating professional.  Corrosion inhibitors are added to prevent corrosion by providing some reserve alkalinity to counter corrosive acids. 
  • Silicon Fluids
    Silicone fluids have a very low freezing point, and a very high boiling point. They are noncorrosive and long-lasting. Because silicones have a high viscosity and low heat capacities, they require more energy to pump. Silicones also leak easily, even through microscopic holes in a solar loop.

Other types of heat transfer fluids include synthetic, mineral, or aromatic hydrocarbon fluids; refrigerants such as found in heat-pump systems; methyl alcohol; and Ammonia.  Many of these are toxic, flammable, are highly regulated, or entail environmental impacts. While perhaps having industrial uses, these heat transfer fluids would not be found in a household solar water heating system.

See solar water heating system maintenance and repair for more information about liquid heat-transfer fluids.