System sizing

Thermal study Choosing the thermoelectric (Peltier) element System dimensions System design process

Thermal study

The first step is to check the feasibility, and advantage, of having the freezer nested into the cooler. We therefore need to calculate the heat power involved in the process.

There are different sources which give information about peltier devices and support the necessary calculation. I first surfed through a very well made blog on the subject Peltier Device Information Directory.

From there, I found a company which makes available an online tool to get a first idea of the heat energy that must be drawn from cooler and freezer in a stade stade operation:
TE Technology, Inc. - Cooling Assembly and Heat Load Calculator.

This is the data needed:volume of freezer: the freezer size should be sufficient for four tube racks, stacked two by two.

The available volume should be mm 131x210x110 = 3.0261 dm3

The cooler size requirements are mm 291x300x160 = 13.968 dm3

Effective volume of cooler, if the freezer is embedded: 13.968-3.0261 = 10.9419 dm3 which is equivalent, for calculation purposes, to a cube of mm 222x222x222.

When using the online tool it is immediately clear that insulation thickness greatly influences the heat energy that must be pumped out by the cooling system and, in turn, the type of peltier element. It is necessary to run different iterations to set the correct system design.
Let's quickly draw two design scenario:

1. Individual cooler and freezer;
2. Freezer nested into cooler;

WET= warmest external temperature (°C)
HEH= highest external humidity (%)
LDT= lowest desired temperature
IW= internal width (mm)
IH= internal height
ID= internal depth
IT= insulation thickness
ITY= insulation type
HG= heat generated inside the enclosure (W)
IHL= insulation heat load (W)
THL= total heat load
EWT= exterior wall temperature
IWT= interior wall temperature
DPT= dew point temperature
PF= polyurethane foam

Power requirements cooler and freezer: 6.2 + 6.1 = 12.3 W
Power requirements cooler with embedded freezer: 6.2 W.

The second solution halves the power requirements. Moreover, it only needs a single thermoelectric block instead of two.

In this case, to get a correct thermal balance, the heat energy flowing through the cooler must be the same as the thermal energy flowing through the freezer and through the peltier element. The freezer insulation must be adjusted accordingly. However, the final insulation thickness and the corresponding thermal power, will be set with the selection of the peltier element.
The thermal balance scheme is the following:

Choosing the thermoelectric (Peltier) element

The thermoelectric (peltier) element must be selected according to : cold side temperature, hot side temperature, dissipated heat power, operating voltage and current. Key aspects are also local availability and cost.
A brand frequently sold by italian vendors is the american Laird Technology which has a free software tool, AZTECH, to guide the TE selection.

It is important to note that Peltier elements DO NOT pump heat energy but merely, through an applied voltage, create a differential temperature gradient between cold and hot sides. Heat power dissipation via an heat sink on the hot side is fundamental to get the thermal flow going. I played for a day with the tool to understand its rationale and made different iterations to come up with, what I consider the best solution. Here are the steps:

1. Get the smaller power dissipation for the cooler using the highest possible insulation: 3W seems the best compromise. Higher voltages require more than two TE elements and lower ones result in an insulation thickness too cumbersome. I will stick to 3W and 90mm.



2. Check the freezer insulation requirements for the same power dissipation as the cooler. 3W require an insulation thickness of 40 mm.

3. Select the Peltier element for the freezer (cold and hot temperatures + 3W heat power dissipation). What we get are two stages TE elements that can work with cold-hot temperature gradients higher than 80 °C.





4. With the analysis Worksheet section I checked the possibility to reduce the number of the selected peltier elements. With only one peltier element the max power dissipated at cold side reduces to 1.81 W with a total power dissipated at hot side of 29 W and a 3A current.

5. I then checked product market availability and prices:


I selected the Multicomp PK2-15828NC-S which matches the different requirements:
Multicomp TE Module PK2-15828NC

• Cooling power
• Voltage requirements
• ∆T max (105°C at Th=50°C)

System dimensions

Here is the summary of the dimensional constraints:

Freezer

• Required internal volume: mm 131x210x110
• Additional volume for fan (mm 80x80x25) and air circulation
• Insulation thickness mm 50

Cooler

• Required internal volume: volume of insulated freezer + useful volume of mm 291x300x160
• Insulation thickness mm 90

System design process

The design process moved along two parallel routes: the design of the thermal group and of the cooler-freezer bodies. In fact, the dimension of the thermal group, with heatsinks and fans, has an impact on the sizing of the cooling bodies and, vice versa, the need to maintain a required cooling volume and insulation, affects the geometry of the thermal group.
As we will see in the relevant sections, the design evolution started from the initial volume requirements and a preliminary concept of heat sink structure.

Link to Fusion360 full assembly design