To ensure that your installation works at its peak efficiency Contemporary Energy recommends a special heat transfer fluid, Fernox S1, which is blend of non-toxic Propylene Glycol and high temperature corrosion and scale inhibitors.
These inhibitors contain no silicates, which can form a gel at high temperatures and precipitate on to the tube walls etc. They also contain no nitrites, phosphates or amines, and are bio-degradable (so can be disposed of safely through the normal drains).
This fluid is supplied as a concentrate or ready to use. It is available from any good plumbers' merchant. 

This fluid when used at 40% dilution provides protection against freezing to minus 20 degrees C.  At higher concentrations it can provides protection against freezing to minus 50 degrees C.

Fernox S1 Solar Fluid has been especially formulated for use in solar panels. It is based on propylene glycol blended with corrosion inhibitors. This Solar Fluid has a very low order of oral toxicity and can be classified as “Relatively Harmless” (aka non-toxic) with an LD50 rating in excess of 20,000 mg/kg of body weight. The specialist inhibitors contained in the fluid are specifically formulated to withstand high temperatures and have been successfully tested to +300C for short periods – e.g. during stagnation.