Finally the fluid must have a low viscosity. This means the fluid used as the heat transfermedium must provide little resistance to gradual deformation by shear stress or tensile stress(DGS, DLR, Ecofys, 2009).6.1 Types of Heat Transfer MediumThere are many common heat transfer mediums used in solar thermal systems today such aswater, air, Hydrocarbon oils, refrigerants and a water/ glycol mixture. The most common heattransfer medium in use is the water/ glycol mixture. The mixture has a high thermal capacity,a high thermal conductivity and a low viscosity rating. The mixture is also non-combustibleso the pipe system can be pressurised. Water is non-toxic to the environment meaning theenvironmental impact of a leak is less severe, it causes less damage to our O-Zone layer andis easy to dispose of after use, as it is biologically degradable. Water is cheap and widelyavailable making it the more economical choice. Glycol is added to the mixture for two mainreasons. Firstly, it reduces the freezing point and secondly, it increases the boiling point ofwater. Adding 40% propylene glycol provides frost protection by reducing the freezing pointof water from 0° C to -23°C. The main operating temperatures of collectors can be as low as-15°C in the winter, meaning it is necessary for the heat transfer medium not to freeze atthese temperatures. If water was used and it operated at these temperatures the water wouldfreeze, causing damage to the entire solar thermal system and the house and may cause thepipes to burst causing severe damage to the house itself. Adding the glycol also increases theboiling temperature from 100°C to 150°C depending on the pressure in the system. Thismeans the water can reach much higher temperatures before vaporising thus carrying moreheat energy (DGS, DLR, Ecofys, 2009).