Greetings and welcome Emily, thanks for sharing your
project and question. Same to you Curt, would love to see pics if they are out there. There is probably no easy solution here. I would advise to be very careful about using grid-tied circulation as the fan's
energy consumption often exceeds the heating energy saved with most systems like these. PV powered fans are a natural fit for projects like that as the free, power is used in a passive fashion when you need it most. Curts fans suggestions look good for AC current and there are many exciting, ultra efficient fans coming onto the market. Anybody know of some similar DC powered fans that could be hooked right up to a small PV panel?
This passive solar design is very unconventional by today's standards. I
think the problems you describe are typical for homes that ignore modern passive solar design in favor of
greenhouse or sunroom integrated homes. They gain a tremendous amount of heat in the day and then quickly lose it as the sun goes down and can also have the effect of bringing the rest of the house down with it. Modern passive solar design keeps a more modest south glazing ratio to square footage and focuses on air sealing and insulation to keep the heat in as long as possible.
I suspect that your house is ripe for some very cost-effective weatherization efforts. Namely, air-sealing everywhere and adding insulation in the attic. Folks looking to build new
should see Emily's problems as an opportunity to improve on. Blower
door directed air sealing could have made a world of difference on the performance of this home. Its not too late of course but its much harder and less effective once things are built.
I like Curts suggestion but be careful about the energy loads of the circulation devices. I suspect that going through the trouble to try to actively circulate the air above 65 will not result in measurable savings. I suppose tearing off the sunroom/greenhouse and doing a more modern south facing wall is out of the question?