Editor's Note: This article originally appeared in the June 2012 issue of SMT Magazine.
Thermal simulation can be used to help create better products--everything from laptops to beer fridges. While refrigerator design is not normally on the top of everyone’s priority list, whether in the office or college dorm, a well-running mini-fridge can be crucial to motivation levels. Because I had access to a beer fridge that wasn’t working properly, I thought it might be a good idea to understand why. Besides, the design lessons learned from it can be applied to many other electronic products.
The mini-fridge works by having a thermoelectric cooler (TEC) pump heat from inside the fridge down to a heatsink, using the Peltier effect. A fan then blows cold air over the heatsink so that the heat is convected away from vents in the lower part of the fridge. The heat flow should ensure the space inside the mini-fridge remains at a low, controlled temperature. This approach is less noisy than the classic evaporator/condenser cycle used in most domestic fridge/freezers; although it’s somewhat less reliable.
A situation like this is a perfect case for computational fluid dynamics (CFD) simulation software. First, I measured the main constituent parts of the fridge and created a 3D representation in FloTHERM CFD software in just over an hour. Then I had to find the characteristic information for the TEC and the fan. Such objects are not modeled explicitly per se; instead, a “compact modeling methodology” is applied in which their key physical behavior is retained but the exact physics of their operations isn’t modeled.
I used parameters such as the current required to pump a certain number of watts against a temperature difference at two (hot side) temperatures as input for the FloTHERM TEC SmartPart object (this particular TEC was not available in the installed libraries). I also used the fan curve that relates to the pressure drop over the fan to the amount of air that it can shift.
As a matter of physics, I expected the air within the fridge to be cold. Here are the surface temperatures on the inside of the fridge, with the side of the fridge hidden for clarity: