*3.6. Useful Life*

The evolution in the design of TEG modules has also contributed to increasing their useful life. Moreover, the thermal stresses to which the TEG modules are subjected, affect them adversely by the formation of microcracks and the expansion and contraction of TE materials. To address this problem, Skomedal et al. [40] incorporated spring-supported contacts in the legs of TEG modules to dampen thermal expansion and contraction in the modules; these authors concluded that good diffusion barriers and possible coatings can reduce oxidation of the hot side electrodes and interconnections. Likewise, they noted that the TEG module could generate 1 to 3 W/cm<sup>2</sup> . Furthermore, Ming et al. [45] studied, via numerical analysis, how the non-uniform flow causes the junctions of the TEG modules to be damaged and also decreases their output power. In addition, Merienne et al. [32] investigated the effect of thermal cycles on commercial TEG modules, finding that when rapid temperature changes are applied, their output power can decrease by up to 61%, compared with a module in which the heat flow is constant and the temperature changes are minimal. Therefore, it is of utmost importance to analyze in detail the design of TEGs and the operating conditions to which they will be subjected in order to establish applications that allow them to extend their useful life.

The current commercial TEG modules have still not reached the economic feasibility, nor the efficiency required for thermal energy recovery applications. Therefore, as mentioned above, the characterization of commercial TEG modules, in the specific conditions to which they are going to be subjected, requires the use of expensive control equipment, or manual processes, which leads to difficulty in making long-term measurements. Therefore, Elzalik et al. [28] developed a characterization method that allows precise and inexpensive estimation of the maximum power point and the dynamic parameters of the TEG module. The proposed procedure can be used with different sources of residual thermal energy and under different operating conditions.

In relation to the sustainability of TEG modules, authors such as Khanmohammadi et al. [22] and Heber et al. [21] state that the amortization period and the power ratio of commercial TEGs make them not viable to be used exclusively as a generation method. Therefore, they recommend using them as a complement to other methods of recovering residual thermal energy, thus generating an integrated system, or the development of segmented TEG modules that allow greater efficiency and profitability.
