You said it was a flat roof, but you also mentioned ridge vents. Is part of the roof flat, and part peaked or ridged? If part of it is peaked and there are gables, gable vents are pretty standard, relatively easy to install, and work well as passive vents. Especially if the layout will allow for them on opposite ends to produce some crossflow. I'm really surprised that you don't already have some venting of some kind.

Stepping up to a powered vent fan will obviously work much, much better, but only you can decide if your situation calls for the added expense of equipment, installation, and electricity. The solar-powered fans are better than passive vents, but many of the "consumer" models are a bit on the anemic side. What kind of ambient temperatures are you dealing with...what part of the country? A more substantial fan will do a much better job, but will require "makeup air" from somewhere to let the fan do its job. Opening windows slightly in the lowest part of the house is the typical method. (This may raise security issues.) Many folks use whole house fans to great effect this way.

No one can tell you much without a bigger picture. These days powered attic vent fans are generally thought to be a big mistake. More often than not, they suck air out of leaks in the attic that lead to the living space.

and there are several threads both here and at
www.hvac-talk.com to explain in depth why NOT
to install PAV's
do a search of both sites.

best of luck,

howdy ionized...made any decisions on your project yet?