Not a spacecraft engineer, merely somebody who has studied physics a long time ago, so take my answer with a grain of salt.
Basically, there are two ways you can design a cooling system: one that involves phase change, and one that simply transports heat from A to B by moving a heat carrier, usually a liquid.
Phase change systems typically have a higher efficiency, because the phase change has larger relative energy than heat capacity. But it also has two disadvantages: it only works near the boiling point of the medium, and you need to deal with all the pressure changes that come with a phase change.
The Parker probe seems to use a "mere" transport, and there the heat capacity and the working range of the medium is very important, and water is a pretty good choice on both of these criteria.
Oh, and you also don't want something very corrosive to destroy your expensive space craft from the inside :-)
IIRC the ISS uses two cooling systems, one based on water that is used throughout the station, and then an ammonium-based system that takes the heat off the water and transports it to the heat radiators. But on the ISS, maintenance is possible.
Basically, there are two ways you can design a cooling system: one that involves phase change, and one that simply transports heat from A to B by moving a heat carrier, usually a liquid.
Phase change systems typically have a higher efficiency, because the phase change has larger relative energy than heat capacity. But it also has two disadvantages: it only works near the boiling point of the medium, and you need to deal with all the pressure changes that come with a phase change.
The Parker probe seems to use a "mere" transport, and there the heat capacity and the working range of the medium is very important, and water is a pretty good choice on both of these criteria.
Oh, and you also don't want something very corrosive to destroy your expensive space craft from the inside :-)
IIRC the ISS uses two cooling systems, one based on water that is used throughout the station, and then an ammonium-based system that takes the heat off the water and transports it to the heat radiators. But on the ISS, maintenance is possible.