Water boils at 10°c when it placed at 27km above ground.why?

All liquids/gasses can be used as refrigerants. Even water. It is not practical to use the water as a refrigerant because the pressures that correspond to the usual air conditioner temperatures are too low (corresponding compressor would have to be very large and it would be difficult to make an efficient one).

Water is used as a refrigerant in heat pipes probably in the heat sink of your laptop. Since there is no need to compress the water vapor in the heat pipe the usual reason to not use the water goes away. Heat pipe is not a heat pump but only an excellent conductor oh heat.

Lookup the pressure/temperature chart for different liquids. Also lookup the heat pipes. They are fascinating.