I built a box from 5cm thick Styrofoam with outer dimensions 60x55x80cm. I wanted to use it as a fermentation chamber for home brew purposes. The reason why I did not choose regular (some used cheap) refrigerator is I need to have it in a small chamber with floor dimensions of 80x130cm. Also door of that chamber is only 55cm wide. So getting inside some refrigerator would be most likely impossible.
I decided to buy 2 peltier modules - TEC-12715 and attached them to a "hot side heatsink" of dimensions 23x17x4cm and "cold side heatsink" of dimensions 10x17x5cm. I am powering each separately from the dedicated ATX power supply which each can handle 18A max at 12V, so there is a reserve. Hot side is being cooled by 5 fans removed from old cases and atx power supplies. The smaller, cool heatsink is inserted into the top part of the box and sealed. On the cold heatsink there is one 12cm fan.
Unfortunately I can only get max of approx 18 degree Celsius inside (35L barrel with approx 20L of a beer inside + 4x 1.5L bottles with water just as a accumulation of the "cold"). Hot heatsink has most of the time 50 degrees Celsius (I have a temp sensor sticked on it's middle part (near to the peltiers which are also in the middle)). Ambient temperature is now 29 degrees Celsius. So it is a difference of max 11 degrees of Celsius.
Should I consider this to be "normal" when considering inefficiency of peltier modules? Or should I be able to squeeze more cold inside? I read somewhere on the internet the peltier's efficiency is terrible once the hot side is over 40 degrees, but I am not sure if this is true. I planned to do some temp regulation as well, but as I wanted to get closer to 15 degrees Celsius, this does not have the reason now.
Any ideas, suggestions?