Hydrocarbon oils have a higher viscosity and lower specific heat than water. They require more energy to pump. These oils are relatively inexpensive and have a low freezing point. The basic categories of hydrocarbon oils are synthetic hydrocarbons, paraffin hydrocarbons, and aromatic refined mineral oils. Synthetic hydrocarbons are relatively non-toxic and require little maintenance.
Aromatic oils are the least viscous of the hydrocarbon oils.
Silicones have a very low freezing point, and a very high boiling point. 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. Because CFC s have been found to damage the atmospheric ozone layer, their production is being phased out in the United States. Alternative refrigerants that are less harmful to the environment are being developed to replace them.