Sounds solid—just break it into two loops: a PV‑driven fan with a 12 V DC motor, 300 mA at 12 V gives about 3.6 W, enough for a 200 CFM fan if you keep the static pressure low. Then capture the exhaust with a small heat exchanger, piping the warm air through a small copper shell to pre‑heat incoming air from the PV panel. That way the panel sees a higher temp and its output is a bit higher. Keep the fans color‑coded and log the temps every 10 min; that data will let you tweak the fan curve until you hit the sweet spot. And remember to size the solar array for the peak load, not the average—over‑size by 30 % to cover cloudy days.

Good point about the panel timing—use a 12 V, 50 W panel so you can pull 4 A during peak sun. For the backup, a 12 V 5 Ah Li‑Ion battery gives 60 Wh, enough to run a 3.6 W fan for 16 h of shade, so you’ll stay above 30 °C. Switch the fan to a 12 V, 300 mA brushless motor; its torque curve is flat up to 2 kHz, so you can map the fan speed with a simple PWM loop. If you switch to aluminum, go 6061‑T6; its thermal conductivity is 167 W/m‑K, only 40 % of copper, but you cut weight by 60 %. Use a 1‑inch OD tube for the heat exchanger; that keeps the pressure drop under 0.2 inH₂O. That should keep the system tight and efficient.

Sounds good, keep the battery specs in a spreadsheet so you can track when to swap it out. I’ll log the heat‑exchanger temps and tweak the airflow once we hit the real world. Will let you know how the first run goes. Good luck!