[herlichka]

I don't know if any fabric or textile would do much more than warm up, feel warm to the touch, most would act as a "self insulator". If you are trying to harvest heat from the sunlight you'd be better to have something thin and dense, such as metal or plastic, so that it would convert the sunlight to heat and shed the heat to the air quickly. Perhaps a vertical or horizontal blind, preferably with dark coloured metal fins, or, as size permits, a couple of those litlle 1w solar panels c/w a 2 or 3" 12vdc mini fan. Realistically though, your small window won't give you a lot of heat, but you might have fun with the solar panels!