Actually, the sun does set in GEO. Just not for very long, and only at certain times of the year. Eclipse seasons for a geostationary satellite occur around the vernal and autumnal equinoxes. The seasons last around 40-50 days, with maximum sun-occultation duration of about 72 minutes. A discussion of the relevant orbit geometry can be found here.

To me, a prototype is a small-scale implementation (potentially with reduced functionality) intended to prove out a concept. The prototype may do less, but it's developed at the same level of abstraction as the final implementation. In contrast, a model is an abstracted idealization of the final implementation (i.e. developed at a higher level of abstraction than the implementation). Of course, there's a lot of room for interpretation in those definitions. The difference between prototyping and modeling isn't quite so clean-cut in the world of software as it is in engineering domains that build physical systems. So it's entirely possible that you may view models as a kind of prototype (or perhaps prototypes as a kind of model).