De ionised water in cooling circuit

[thinwater]

...it should perhaps be called "antifreeze and summer coolant"

There are some downsides. Notably an increase in viscosity requires more pumping power and a reduction in specific heat capacity requires a greater flow rate ( a double whammy there)

Once upon a time (when we were young) the summer/winter designation was normal. Now the standard (ASTM, meaning US and UK) nomenclature is engine coolant.

Yes, the glycol has some negatives. Higher concentrations can cause overheating. For this reason it is VERY seldom recommended to exceed 50% (up to 70% in true arctic areas).

 

[Rocksteadee]

We used to use WFI. This is water for injection.... into people so is pretty pure
It is demineralised, then distilled, then deionised.
It is that pure it is unstable and will make 316 stainless rust as it leaches out anything it comes into contact with, does not do you much good if you drink it.
Sadly company closed, management dumped 240,000 litres down the drain as they wanted to scrap the vessels for a few thousand and were not happy when us engineers told them that the stuff was worth £4 per l.

If you have a dehumidifier the water it produces is distilled enough to use for battery top up and antifreeze mixing