Electrical – Adding Peltier module to an evaporative cooler

It will be difficult to build something which is intrinsically limited by its heat output to a low temperature but still brings the contents up to temperature quickly.

Making some assumptions about the box:

• Wall thickness: 3 inches
• Surface area: 4460 square inches (random 7 cu ft freezer online, inset dimensions by half a wall thickness on each side)
• Insulation R-value: 1 (m^2 * K) / (W * in) [estimate based on Wikipedia numbers]
• Temperature difference: (100 - 72) = 28 deg F = 15.6 K
• Power to maintain temperature difference = \$ \frac{\text{surface area } * \text{ temperature difference}}{\text{wall thickness }*\text{ R-value}} = \frac{\left(4460 \text{ in}^2 \left(\frac{25.4 \text{ m}}{1000 \text{ in}}\right)^2\right) (15.6 \text{ K})}{ \left(3 \text{ in}\right) \left(1 \frac{\text{ m}^2\text{ K}}{\text{W in}}\right) } = 15 \text{ W} \$

To guarantee that the temperature inside the box passively stays around 100 F, we would have to limit the heater's power to 15 W, which isn't much compared to the thermal mass of big hunks of wax. If the box is better insulated, then the power must be even lower. This also assumes that the temperature inside is completely uniform. If not, the power must be lower still to guarantee no hot spots.

I like the aquarium heater idea. It limits the possibility for overheating the wax both by the safety features of the commercial heater, and by the boiling point of water.

tl;dr: you don't want to run at 15VDC. Really. 12V is plenty.

There are a number of factors that come into play. To get the first one out of the way - as has already been said, you can't get 15VDC from a computer supply by using the -12V rail, and while it would seem to be simple to just use the +12VDC and the 3.3VDC rails in the same fashion, the fact that they have a common ground makes that not work. [One of the supplies will be a dead short. Not pretty.] The other thing to consider is what the efficiency curves say. Most, if not all, TEC will cool better at a lower voltage than maximum. Since you didn't give us the part number I can't check it, but using the 12708 (which looks pretty close to your stated specs) - running at 12V/6A is almost as effective for a lot less power (72W vs 112W). Remember, your heat sink has to reject all of the power you put into the TEC, plus the heat transferred from the cold side. You may find that the 40W difference matters. A lot. More is not always better. Use google to find the data sheet for your exact TEC (the number is on the "hot side") - you'll find you can run a bit lower on the voltage, and still get almost the same cooling.