Never in a hundred years would I have imagined that no one would know where to purchase exhaust fans and intake vents for a greenhouse that would close when not in use, so that all the heat built-up passively during the day would not be exhausted during the cold months and run on DC power from solar panels and batteries.
We have a pretty large window in the roof that will open with a Gigavent (automatically opens by temperature). We got it at Peaceful Valley Farm Supply in Grass Valley CA.
Here is a picture of a panel like one of ours. I took it at the Real Goods Solar Living Center. Of course ours will not be set-up on a portable frame like this one in the picture. They will be on the roof of the studio which is attached to the greenhouse.