Solar Heat Transfer Fluid

How Heat Transfer Fluids Work

A heat transfer fluid plays a vital role in the operation of solar water heating systems. It carries heat energy from the solar collectors through the heat exchanger and finally into the tanks that store the heat.

Solar Heat Transfer FluidImage: RESOL GmbHA heat transfer fluid comes in many different varieties, and the various qualities of each variety can significantly affect how well a solar water heating system works. Different environmental and climatic conditions demand the use of a specific variety of heat transfer fluid.

For example, a heat transfer fluid with a low freezing point works best in a place that experiences cold weather, while a heat transfer fluid with a high boiling point will work well in a solar water heating system in a hot environment.

Common types of heating transfer fluids include:


Air – Air is beneficial because it does not freeze, boil, or corrode.  However, it does have a low heat capacity and leaks easily (see also: Air Collectors).

Water – Water is cheap, non-toxic, and easy to pump because of its low viscosity and high specific heat. Downsides of water as a heat transfer fluid include its high freezing point, low boiling point, and corrosiveness if the pH is not kept neutral. (Used in Drainback Solar Water Heating Systems).

Hydrocarbon oils – Divided into aromatic refined mineral oils, synthetic hydrocarbons, and paraffin hydrocarbons, hydrocarbon oils have a lower freezing point, lower specific heat, and a higher viscosity. Hydrocarbon oils need more energy to be pumped, but they are also comparatively inexpensive.

Silicones – The benefits of silicone as a heat transfer fluid is that it lasts for a long time, is noncorrosive, and has an extremely high boiling point and low freezing point. However, they leak easily and take more energy to be pumped since they have low heat capacities and a high viscosity.

Glycol/water mixtures – Glycol and water are kept at a 50/50 or 60/40 ratio. Examples of these solar antifreezes include ethylene and propylene. Glycol/water mixtures are common in indirect / closed-loop systems.

Refrigerants – This heat transfer fluid is one of the best performers on cloudy days because it responds quickly to sunlight. With a high heat capacity and low boiling point, it only takes a little refrigerant to transmit a large amount of heat. Refrigerants are commonly used in air conditions, heat pumps, and, of course, refrigerators. Because of the negative environmental effects of CFC refrigerants such as Freon, solar systems that use them are no longer manufactured. Older systems that use refrigerants need to be serviced by a professional technician who knows how to comply with the regulations governing their use.