I'd estimate that a black painted roof will reach temperatures of about 50°C / 122F ?
it'd be best to take some measurement though to confirm this
and the thermo generators I've seen work best when the temperature difference ( hot side to cold side ) is at least 40°C so if you were to rig up some kind of cooling circuit on 1 side then this might be enough but then you have losses with running a pump to circulate the water etc
or use this firms product which incorporate a fan on the cold side
However to be honest I'd say you're better off with a water heater like this, as I don't think your thermal difference is going to be hot enough for it to pay for itself
rather than trying to make electricity from a thermo generator
do you still need help?
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