Harry Another surprising source of cold air would be from the Ranque-Hilsch vortex tube. The problem is that the vortex tube is not very efficient since the hot side flow is usually wasted.
It might be possible to combine a parabolic reflector with a vortex tube in series if one needed colder air than otherwise available. HLV wrote: Here is a similar investigation using parabolic and elliptical reflectors and also a simple reflector with flat sides sloping at 45 degrees. With the parabolic reflector they managed to a cool an emitter 20 degrees below ambient temperature at night. The elliptical reflector was almost as good. https://youtu.be/7qZodSfFQCM Harry Jones Beene wrote: Of interest: https://www.researchgate.net/publication/262806145_Blue_Sky_Cooling_for_Parabolic_Trough_Plants
