*3.2.2. Quantitative analysis*

Visual analysis of **Figures 6** and **7** states a pressure distribution which is far from uniformity. In this section, this fact is demonstrated with a statistical analysis based on the median. Since the pressure films used for the experiments are ranged from 0.6 to 2.5 MPa, those parts that work with a pressure out of scale will not be appropriately represented. Thus, the median is the most significant parameter to analyze: half of the pixels work under that pressure while the other half works above it.

distribution in the modules, which is in turn related to the thermal contact resistance. If the module works under an uneven pressure distribution, the generation will be reduced, since

The Importance of the Assembly in Thermoelectric Generators

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The most appropriate configuration seems that in which each module has the biggest amount of screws as close as possible to it. Furthermore, as the torque of these screws increases, the clamping pressure will get bigger as well as better distributed, and therefore, the thermal contact resistance will be reduced, leading to a better generation unless bending is provoked.

Last section has concluded that it is recommended to have an individual tightening of the modules in order to ensure a uniform pressure distribution that leads to a good thermal contact. In order to achieve it, screws are necessary although they represent a type of

By definition, a thermal bridge is an area or component of an object which has higher thermal conductivity than the surrounding materials, creating a path of least resistance for heat transfer [45, 46]. In thermoelectric generators, there are two main sources for thermal bridges. On the one hand, the screws used to ensure a good contact and pressure distribution are normally metallic, and therefore, highly thermal conductive. In order to reduce the amount of heat lost through these screws, it is typical to use nylon washers. Nonetheless, even with this nylon

On the other hand, due to the small thickness of the thermoelectric modules (most commonly 3 mm), it can occur that part of the heat directly flows within the heat exchangers, instead of through the thermoelectric modules. Therefore, insulating materials are usually inserted

In the present section, assuming there is a good thermal contact, three different alternatives of insulating materials that are normally used in order to avoid thermal bridges will be studied:

rings, it is estimated that the thermal resistance of each screw is 52 K/W.

some parts of the module can even be not in contact with the heat exchangers.

**Figure 8.** Median of the pressure against the applied torque for the configuration of two modules.

**4. How to avoid thermal bridges**

thermal bridge.

between the heat exchangers.

On the one hand, for the heat exchanger with two modules, in the case in which there are only four screws, most of the module works under a pressure of 0.6 MPa or less. This value improves if the bending is restricted by means of an additional pair of screws located between the modules. As shown in **Figure 8**, and accordingly to the previous states, the median of the pressure increases with the torque in a proportion that seems linear.

On the other hand, in the heat exchanger with four modules, the lack of macroscopic contact is evident with the median value of 0.6 MPa, i.e. most parts of the modules have a clamping pressure of less than 0.6 MPa. Nonetheless, although the visual analysis evinces a slight improvement when the fifth screw is introduced and a more significant torque exerted, this fact is not appreciable in the median value since the range of pressures of both configurations is much lower than 0.6 MPa, the lower limit of the scale of the film.

Hence, despite not being considered a critical aspect, screw distribution and torque are aspects to definitely take into account. The assembly configuration determines the pressure

**Figure 7.** Pressure films for (a) four screws, 1 Nm (b) five screws, 2 Nm.

**Figure 8.** Median of the pressure against the applied torque for the configuration of two modules.

distribution in the modules, which is in turn related to the thermal contact resistance. If the module works under an uneven pressure distribution, the generation will be reduced, since some parts of the module can even be not in contact with the heat exchangers.

The most appropriate configuration seems that in which each module has the biggest amount of screws as close as possible to it. Furthermore, as the torque of these screws increases, the clamping pressure will get bigger as well as better distributed, and therefore, the thermal contact resistance will be reduced, leading to a better generation unless bending is provoked.
