I make a electronic design (and i thing to build it )for to work a pc fan in a radiation shield
The day it will turn foul turn with 12 volt dc ,at the night at half turn with 6 volt dc
For low drop the volt from 12 volt to 6 volt i use one resistor 80 ohm at 1 watt from the type :
12-6/0.075 =80 και 6*0.075 = 0,45w
Sollar panell --> charge controller-->battery-->80 ohm resistor to relay for 6 volt dc at fan the night.
Solal panel--> 12 volt at fan the day ,
Solar panel -->i use 162 ohm resistor for drive to relay to cut the contacts to relay at 10 volt dc and down because before without resistor the relay closed the contacts at 7 volt dc (i make experiments with a power suply 0-30 volt).
Fan--> to relay
When it is day with sun solar panel give 12 -16 volt and the relay closed to batery circuit and solar panel it give power to fan ,the night the when drop the power from solar panel ,open to circuit to relay and give power to fan from the battery .
Tha material it is
A sollar panel 12-16 volt 10 watts
A charge controler 12 volt 5 amber
one batery lead acid 12 volt 7,2 Ah,or 17 Ah for better results.
a pc fan for radiation shield 12 volt 8 x 8 cm
one relay 12 volt
a water proof case for to put inside batery,charge controler,pcb with relay and 2 resistors
Here it is the electronic design