I installed a computer fan above the coils and below the roof cap. My goal was to improve the draft in the stack rather than blow hot air at the coils. The fan is powered by a 1W HF solar panel that is attached to
the top on the roof cap. If there is enough sun to make things warm, there is enough sun to power the fan. No switches or theromstats. It does make a difference. My scientific test was I had ice starting to form in my beer. I decreased the setting by about a half a number. Cost was about 25.00. The only thing I would do different is try and find a
slightly bigger solar panel. The HF unit my say it is 1 watt, but there are some less-than-sunny days that the panel can't start the fan. Oh and clean the panel (all of your panels) once in a while. It helps.