A couple of questions: If lead-acid batteries need to be adequately ventilated, how should this be done in super airtight structures? Can the batteries be placed outside the envelop? Is there an optimal temperature they should be stored at?
Also, what is the best way to keep debris from collecting under the modules. This seems like it could be a big problem.
In the NEC, it just says that battery banks need "sufficient ventilation" and does not give instructions. It is something that you could ask the battery manufacturer what they recommend, since they will have a good idea about how much hydrogen their specific battery can be expected to gas off.
If you are putting lead-acid batteries in "super airtight structures", then that does not sound like sufficient ventilation, unless that super airtight structure was big. I would say that you could have ducting and a small computer fan. If you are in a cold location and ventilating from the outside, then you would have to consider the cold temperatures effects on your battery bank. Cold temperatures can decrease the capacity of the battery and super-cold temperatures can cause the electrolyte to freeze.
Batteries can be placed in a location outside the structure, but it could get cold outside. We have batteries in an outside building in the California mountains and the building does not have a door. It can snow here once in a while, but it is not that big of a deal for us.
Here is an air-change for battery ventilation calculator I found:
To answer your question about debris under the PV. It depends on where you are. We like to leave air space under the PV and not to put the PV near trees, where there is shading. Animals, such as pigeons and squirrels are famous for getting debris under the PV or being the debris. Some manufacturers make screens that clip on the sides of the PV. Here is an image of one of those screens: http://www.homepower.com/sites/default/files/styles/article_gallery_active/public/articles/images/15__SNR_EdgeScreenProtector2.fxd_.jpg?itok=DkI-KHQU
The problem with the screen is that the animals are smart and good at finding a work around. Also, the debris can build up by the screen. Best bet, don't put PV near trees. Just like you don't want to put me downwind of a coal plant.