[EDIT: Found a couple of 12715 units in the pile that did suck the full amperage, but despite excellent cooling of the hot side, could not deliver much cooling.]
I'm waist deep in project to keep shot glasses full of beverage near or below zero °C.
As I understand it from the tech sheets, a module should can come close theoretical dT as the maximum heat absorption should also be available as long as full voltage and amperage are applied and the hot side is maintained cold enough.
No worries there. A simple cooling loop or even decent air cooling computer heat sink keeps the hot side cold and we get 95% of dT. Achieving cold side temps of -15 °C is no problem. In fact, we have achieved lower temps on the cold side simply by chilling the cooling loop of the hot side.
Clearly temperature is not an issue for us to achieve, but where we run in to issues is current and Wattage. For instance, a TEC1-12715 connected to a 12 VDC PSU capable of supplying 150 Amps is drawing a mere 60 W. Why is this?
In terms of heat dissipation, we have even gone so far as to use Stannol Kristall 511 solder that binds ceramic very well to aluminum and copper for very high W/mk. We have also (for reference) tried high W/mk standard CPU/GPU creams such as Grizzly Kryonaut, Grizzly Hydronaught, and Noctua's best. We also, since this is not a high compression project, tried silicone rubber heat transfer glue of 2.9 W/mk and a special heat transfer epoxy of 8.7 W/mk.
We have tried bonding in every way conceivable - again, there is no problem with temperature, but dissipation of Wattage and module power consumption are low. I think we can all agree that a single TEC1-12715 affixed to cooling source maintaining 30 °C and the cold side being at -15 °C is normal. Literally soldering or using a high W/mk glue or grease to a milled and polished aluminum shot glass should freeze 25 °C liquid in minutes or less. At best, we can remove 1 °C every 3 minutes and at best achieve 8 °C at the coldest.
In tests, the 12715's did cool the intended liquid much faster than the 12706's. Ideally, the TEC units will cool a basin of salt water to sub-zero C and our shot glasses will sit in the water, but we are open to other ideas.
We've seen TEC machines that can cool a bottle of wine or soda to 7 °C in 12 minutes. Clearly we re missing something.
So... what are we not understanding or do simply have a bad batch of TEC units?