*3.4. Module Manufacturing*

With the aim of decreasing the production costs without affecting the efficiency of the modules, some authors focus their research on the design of TE modules, the search for new materials in order to decrease costs, or increasing their efficiency. The use of oxides, half-heusler materials, skutterudites, and composite materials (organic/inorganic) can be a solution to overcome the limitations of the TEG modules. It is important, however, that these studies be developed in a holistic context oriented to applicability.

Studies such as those by Salvador et al. [52], on skutterudite encapsulated modules with a ZT of 1, enable this type of module to compete directly with the efficiencies of commercial PbTe modules. However, to date, commercial modules with this type of materials are scarce and costly due to their current manufacturing methods. Fu et al. [44] evaluated the potential of doped half-heusler materials with an efficiency of 6.2% and a power density of 2.2 W/cm<sup>2</sup> . These modules can resist high temperatures (~927 ◦C) and are an economical alternative to commercially used TE materials.

In order to reduce costs in TEG modules and to improve their coupling to any surface, Lee et al. [39] studied the manufacture of TEG devices and basic electronics using titanium oxides (which operate at temperatures ~500 ◦C), and deposited them by plasma sintering, thus obtaining an assembly of thermocouples connected in series and in parallel with an efficiency of 0.85% and an electrical power of 2.43 mW at 450 ◦C. The authors did not conduct an economic analysis on the assembly, only referring to its easy and low manufacturing cost through the elimination of many of the parts that make up the commercial TEG modules. On the other hand, Yazawa et al. [54] proposed the use of flexible modules that incorporate organic/inorganic composite materials and reduce costs but with a reduced performance (ZT between 0.01 and 0.25), a performance that is relatively low compared with commercial TE materials.

Anderson et al. [23] carried out a techno-economic analysis on the total cost of TEG devices, finding that the use of impure TE materials such as oxides or other types of cheaper TE materials are not the most feasible option at a cost level, since TE material only represents 15% of the total value of a TEG module.

Table 6 presents some characteristics of commercial modules such as base material, dimensions, power output, open circuit voltage, operating temperature range and cost. Currently, the most common TEG modules are those manufactured from bismuth telluride. Among these, a great variety can be found in which characteristics such as their configuration, output power and circuit voltage might vary. Most modules can work at maximum temperatures between 320 ◦C and 350 ◦C, but their optimal operating conditions are around 250 ◦C. These types of modules can be found in the market with prices ranging between \$10 and \$28 US.

Likewise, commercially it is possible to find modules that resist higher temperature ranges, such as TEG PbTe-BiTe modules. These TEG modules can work at maximum temperatures of around 360 ◦C, and commercially they can be found with better output powers than the BiTe based ones. Naturally, the improvement of these characteristics is reflected in their cost.

Table 850 ◦C reaching 6% efficiency, being attractive for the recovery of residual heat at high temperatures. However, this type of component is up to seven times the cost of traditional BiTe-based modules. This makes them less attractive due to their cost-efficiency ratio.


**Table 6.** Technical characteristics and cost of commercial modules.
