Are you in a freezing climate? If yes, how do plan to keep such a small volume
(1/2") from freezing?
If no, are you just capturing the heat with the collector and "exchanging" it?
What type of heat exchanger? Single or double wall? If you're using a glycol
heat transfer fluid you can't use a single wall exchanger (code).
You want low-iron, tempered glass with a high transmittance. Are you insulating