After the first lesson, I re-designed the experiment as follow.
1. to reduce light level, I close the curtain instead of just stopping down the aperture
2. zero gain, exposure reduced to 111s
3. to make the comparison more realistic, I run the camera for 10 minutes before taking the uncooled shot
4. after the uncooled shot, I switched on the Peltier and fan, wait for 1 minute and begin to capture a series of cooled shots, it will show how noise is gradually reduced by cooling
5. due to sunsetting outside, the light level is dropping (need to re-do at night)
So, after the experiment, I touched the camera body with my hand and it's about 5-10 degrees below ambient, I guess I need a better heat sink and more powerful fan and use the Peltier at 12V rather than 6V now (due to limitation of the heat sink)