Anecdotally, I never had any success with this. I bought several cheap but powerful (100W-300W) 12VDC Peltier elements off Amazon. With the hot side bonded onto either a massive chunk of aluminium or a CPU cooler, the cold side never reached less than minus a few degrees C. Open air mounted or with some cling foil insulating it from air (ambient was probably around 25-30C). I also tried stacking a second (up to 50W) Peltier on top of the first, but that was even less successful. Just to say that it's not quite that trivial to get to -20C using Peltiers. If anyone likes to share their more successful experience, I'm all ears!