here is a idea, know what a solar chimney is?
http://en.wikipedia.org/wiki/Solar_chimney
maybe you could incorporate darkmatters ideas also, paint the lower part, Just need to do the lower half white (6' or so) the top black separate the two sections with a ceiling.
which would also help add some structural stability ( grain bins gain strength from being filled and pressure being equally distributed,at least that's my understanding.)
put a vent port in the center of the ceiling and the top section with a cap (like they do with chimneys to keep they rain out.) should act as a solar chimney. you could also include a fan there to help if necessary though I think you could reverse flow in the winter to help warm the bottom space just hook it to a thermostat in the upper half so it only pulls air when its warmed.
you will need to provide a place for the cool air to come in, windows and door perhaps, but I just keep having an idea of plastic culvert pipe buried a few feet below ground as the fresh air input. if you could best option would be to coil it under the foot print of the bin the ground should remain cooler there so you would not have to go as deep.
I believe permies(permaculture folks) would call these earth tubes? basically a simple geothermal cooling system.
where there's a will there's a way. I may be over thinking it but these are sound scientific principles that would require no power input (unless you wanted to use the fan idea)
more the sun shines or hotter it get the more it works.
