Searching for the best secondary coolant

Many people who start building liquid chillers use automotive coolant. Usually this turn out to be a bad choice, but why? The automotive coolant becomes too viscous at low temperatures, and it's heat transfer properties are way worse than those of plain water.
What properties make automotive coolant so bad, and water so good as a heat transfer fluid? Well, here are the most important ones:

  • Thermal conductivity
    Thermal conductivity is the property which indicates how well a liquid conducts heat. This is important since at the metal/liquid boundary the speed of the liquid is essentially zero. The liquid 'sticks' to the metal, and forms a slowly moving boundary layer between moving liquid and metal. By increasing turbulence and speed of the liquid this boundary layer can become thinner, but it can never be entirely eliminated.
    A higher thermal conductivity of the liquid reduces thermal resistance, and therefore the temperature differential between the metal and moving liquid.

  • Heat capacity
    Heat capacity is the property which indicates how much a fixed amount of liquid rises in temperature when a fixed amount of energy is put into it. The higher the heat capacity, the less the liquid will warm up due to the heat generated by the processor.

  • Viscosity
    Viscosity is the property which indicates how 'liquid' a liquid is. Motor oil, for example, is more viscous than water. Peanut butter is more viscous than motor oil.
    For our purposes, a lower viscosity is better. This reduces the friction against hose walls, yielding a higher flowrate. On average (but not always), the slowly moving boundary layer between metal and moving liquid is thinner, yielding less thermal resistance.
    For watercooling a liquid which resembles the viscosity of water would be the best option, since pumps for water can be easily obtained. A pump made for water will lose efficiency when used with a liquid which has a higher or lower viscosity. Imagine pumping (high viscosity) peanut butter with an aquarium pump. Or (low viscosity) air. It won't work.

  • Freezing point
    Evaporator coils are always colder than the liquid. The freezing point of the liquid should be at least 5 °C - 10 °C lower than the projected liquid temperature.

And of course, the liquid used should not be very flammable, not extremely toxic, not terribly expensive, and not extremely agressive.

A few suitable liquids are summarized in the following graph which I found somewhere in a newsgroup. If you are the original author, please let me know so I can add proper credits.

The freezing point of various liquids and mixtures.
(Original source of the image unknown)

Based on this graph alone, all liquids seem to be suitable. But, let's look at the viscosity and thermal conductivity of various liquids

The viscosity of various liquids and mixtures.
(Original source of the image unknown)

Oops! Based on the freezing point alone, isopropyl alcohol, glycol and a glycol/water mixture seemed good candidates. But the viscosity of these liquids is too high to work flawless with a pump made for water.
Let's check the conductivity.

The thermal conductivity of various liquids and mixtures.
(Original source of the image unknown)

Glycol/water and methanol/water seem to perform fine. Please note the logarithmic scale of the graph, which distorts the relationships between the liquids somewhat when you are not used to it.

Based on the freezing point, the viscosity and thermal conductivity, methanol/water seems to be the best liquid to use. How much methanol should we add to the water? The less methanol we add, the better the mixture approximates the good properties of plain water.

The freezing point of varying water-methanol mixtures.
(Original source of the image unknown)

Up to a 35/65 mixture, water/methanol is inflammable and pretty cheap (EUR 7,00 for a 5 liter can) and not very agressive. However, methanol is a very toxic substance which can cause blindness and death. Because methanol molecules are very small, the liquid will diffuse right through the skin when you spill it over your fingers. Looks like a no-go? Well, further research showed that the MAC (Maximal Allowable Concentration) values for methanol in air and methanol per kg of body mass are quite high. Normal precautions such as working in a well ventilated area, using latex gloves and protective eyewear when working with the liquid are sufficient to keep you out of the danger zone.
The second disadvantage of methanol is that the pure liquid is extremely flammable. Even worse: it burns with an almost invisible flame. You can have a large fire without seeing a single flame.

If you consider using methanol in your own system:

YES! It is toxic.
YES! It CAN and WILL kill or at least harm you when not handled carefully.
YES! Pure methanol is extremely flammable, and burns with an invisible flame.
NEVER! work with the liquid without proper precautions.

Never say I did not warn you. If you feel uncomfortable, use ethanol (the alcohol found in alcoholic beverages) instead of methanol. You need to add more of it to water to reach the same freezing point, and it will reduce the efficiency of your coolant, but in case of a severe accident you only end up drunk instead of dead.

Except for some extremely expensive heat transfer liquids such as fluorinated hydrocarbons or chlorinated fluorocarbons (HFC/CFC's. 3M's Fluorinert is an example of a HFC) methanol/water is about the best you can get. And it is pretty affordable and reasonably safe too.