See related: 1st Floor Air Conditioning

I'm devising a solar-powered fan workshop at CAMP on Cherokee St. (stlcamp.org) sometime this summer. Since the nature of such a workshop benefits pretty much everyone (excepting Ameren UE, perhaps), I'd like to first trade some knowledge and tips with the Interwebs, especially if this helps others do similar projects in town.

Ready-made Solar-powered attic fans exist, although they're not cheap ($300 typical on Ebay, compared $60 for non-solar powered one from Home Depot). These fans are often bundled with additional features (temperature control electronics, overpriced solar panels, over-engineering) that price them well outside the range of usefulness, especially considering their theoretical benefit of relieving strain on urban power grids during the hot season.

Pragmatically, a solar-powered exhaust fan is one of the simplest and most useful devices to run on free energy. It will naturally run on demand (when the sun shines!), and best of all, it can be readily cobbled together out of spare parts. That last bit is the most important, since the folks selling the $300 gizmo would probably prefer you not know that, and since that makes a homemade solar fan an excellent example of "green building," as it can be made from parts that might otherwise be thrown away.

In particular, the basic components to the fan are...

• 12V DC 20W fan, ideally a venturi fan, but any metal fan can do
• Used solar panels, enough to provide 10Watts minimum
• Spare plywood, table saw & jigsaw, elbow grease

Important details about building a useful exhaust fan, and not just a solar-powered gimmick

• Whatever opening used for the exhaust fan (e.g. a window) must be sealed outside the fan shroud. Otherwise, the fan will only generate a vortex around its immediate vicinity and not move any significant air in our out. (Reference)
• The fan should have a CFM (cubic feet per minute) rating in the 100's. 500 is good, 800 is better, 1000CFM is great. How to determine CFM? Best way is to look at how much power the fan requires. This ready-made solar unit advertises "up to" 815CFM from a 10W 12VDC fan, for example. So, you can probably expect average airflow greater than 500CFM if your homemade unit uses a 12VDC 20W fan with a 20W solar panel. (Remember these are used solar panels; they probably won't work at full capacity anymore.)
• As implied above, expect your used solar panels to have reduced capacity. So what may have originally been a 30W panel will probably only offer 20W max by the time you put it to use.
• Can't find a solar panel large enough? Just connect smaller panels in series. That is, connect the (-) negative terminal from one panel to the (+) positive terminal of the other panel to combine their power capacity.
• Use heavy gauge wire. Standard 115V electrical wire (e.g. Romex) is more than adequate. Suggest minimum 16awg wire otherwise.
• No cheap plastic parts (excepting wire nuts and elements of the solar panel). Your fan will receive direct sunlight, and extreme heat if it goes in your attic. Cheap plastic (i.e. PVC) will turn brittle and fail under these conditions, so unless you really enjoy climbing onto a hot roof, don't use it.

Used solar panels are actually pretty commonplace these days, since advances in solar power efficiency has created legions of older-generation products that are basically too inefficient to be fashionable anymore. But theses things will work great for powering a homemade window fan, and people are literally selling them like they're going of style.

Questions outstanding:
Leads on local sources for used (or even broken) solar panels?
Leads on local sources for 12V DC ventilation fans?
Others' experience with similar homemade solar power projects?

Tips on buying used solar panels:
http://www.otherpower.com/otherpower_solar_used.html
Caveats on buying used solar panels:
http://www.clean-energy-ideas.com/articles/used_solar_panels.html
Repairing solar panels:
http://www.otherpower.com/otherpower_solar_repair.html

Sources for used solar panels (not local):
http://www.sunelec.com/ (big panels!)
http://www.73.com/a/0108.shtml
Ebay search for "solar panel"