**2. Methodology**

The methodology proposed for this study was based on a comprehensive systematic review in which the most relevant publications related to the development of thermoelectric generation systems were taken into consideration. Based on this preliminary investigation, a relationship was established between the most relevant sources in order to establish comparisons between them in order to critically analyze the information collected in relation to the sustainability of these systems and to subsequently classify the documentation collected.

In this review, the most relevant publications were selected by the authors and summarized to identify their most notable aspects, and tables and graphs were developed, seeking to synthesize the collected evidence in the best way.

Based on the proposed methodology, a systematic search was initially made in the Scopus database using the keywords "thermoelectric AND modules" with the AND logical connector. In this initial search, a total of 4676 results were obtained. Then, two filters were used to refine the search. In the first filter, only selected review type documents were selected, and in the second filter, the publication date was restricted to the range from 2015 to 2020, resulting in a final total of 61 reviews, as shown in Table 1. These filters were based on reviews by Alam and Ramakrishna [25] and Zheng et al. [26], who provided an overview of research in the area of thermoelectric materials incorporating semiconductor-doped materials and highlighted efforts to date to increase thermoelectric efficiency from different perspectives and the potential for economic and environmental benefits through the improvement of thermoelectric systems.

**Table 1.** Summary of the initial systematic search for the subject of the review.


Figure 2 presents the classification according to content of the works selected in the initial bibliographic review using four categories: TE materials, TEG modules, applications and sustainability. The classification was applied by grouping the articles into different themes and subtopics after reading the titles, abstract and keywords. From this selection, it was found that 10 articles were not relevant to the topic of TE materials and, out of the 51 relevant articles, some were classified in two of the categories.

**Figure 2.** Topics identified in the articles of the initial systematic search.

In this initial systematic search, a knowledge gap was found in the sustainability category in which themes of profitability, cost/efficiency ratio and recycling, among others, were developed. For this reason, it was decided to focus on this area in the present analysis.

Regarding the development of this review, six basic themes related to the sustainability of these types of materials were considered: efficiency, cost, recycling, sustainability, life cycle and profitability. For this, after the removal of duplication in articles, four filters were applied. The first filter was the year of publication in which only the articles published from 2010 to 2020 were considered. The second filter was the type of document; in this case only the research articles were considered. The third filter was based on reading the title and abstract, and finally, a thorough reading of the selected articles was carried out, which resulted in a total of 36 relevant articles, from which the analysis presented below was developed. Table 2 shows the search strings used and the results of their systematic debugging. Likewise, Figure 3 shows the results of the search carried out, where Figure 3a shows the data obtained by percentage for each search string. Figure 3b shows a classification of the publications analyzed by year, revealing the growing interest in TEG. In Table 3, the publications and topics covered by each of these strings are presented according to the topics to be addressed in this study.


**Table 2.** Search strings and filters used in the systematic search.

Filter 1: year of publication of the article from 2010 to 2020, Filter 2: type of document "article type", Filter 3: reading of title and summary, Filter 4: reading the article.

**Figure 3.** Systematic search results for (**a**) number of articles found per search string, and (**b**) number of articles per year.

**Table 3.** Articles found in the systematic search with their classification by subject matter.



**Table 3.** *Cont.*

MAT: TE materials, MOD: thermoelectric modules, TEG: thermoelectric generators, EFF: efficiency, MDL: modeling, and RE: recycling.
