**1. Introduction**

The progressive deterioration of the global ecosystem, as well as the decline of fossil fuel reserves, are some of the consequences of the current global energy problem. This is primarily due to the excessive use of conventional energy sources. In recent years, this has led to a great developments in the use of alternative energy sources that are more environmentally friendly and that in turn have renewable characteristics, which would guarantee that are exhausted in the short or medium term. Among these alternative sources of energy, the sources based on direct solar radiation stand out [1], as well as the sources

**Citation:** Castañeda, M.;

Gutiérrez-Velásquez, E.I.; Aguilar, C.E.; Neves Monteiro, S.; Amell, A.A.; Colorado, H.A. Sustainability and Circular Economy Perspectives of Materials for Thermoelectric Modules. *Sustainability* **2022**, *14*, 5987. https://doi.org/10.3390/su14105987

Academic Editors: Luisa F. Cabeza, José Ignacio Alvarez and Asterios Bakolas

Received: 8 February 2022 Accepted: 30 April 2022 Published: 15 May 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of use of indirect solar energy, such as hydraulic [2], wind [3], and tidal sources [4]. Other sources are based on biomass [5] and on the use of energy from waves [6]. Likewise, there is potential for geothermal energy [7] and nuclear energy [8]. However, a significant energy source is based on the use of energy residues from industrial processes, and recently there has been a notable boom in the use of thermoelectric systems, which are based on the use of energy waste in the form of heat that is regularly discarded into the atmosphere. In recent years there have been various advances in technologies that have made it possible to make better use of this energy waste [9], since it is not enough to achieve high efficiency, but it is also important to consider the useful life of thermoelectric systems, especially for systems that operate in low temperature ranges. This enables both long-term reliability and the recyclability of the system's components, in order to certify as sustainable development and conserve natural resources for the benefit of present and future generations. However, based on a recent literature review, it has been found that recent developments in these areas are not paying enough attention to the use of sustainable processes and materials, which are fundamental aspects in the development of current technology. This is why the present study focuses on reviewing the main thermoelectric materials used today, and also analyzes the types of modules used and the main applications of these thermoelectric systems. Therefore, the main objective of this analysis is to establish which of the current technologies are designed in a way that favors recycling processes, and therefore, to determine the degree of sustainability of these technologies. In this sense, the present review seeks to characterize, using recent publications from the last decade, which technologies are being implemented, and to what degree they are contributing to a more sustainable production system.

These developments result from a growing demand from the community to produce cleaner manufacturing processes and materials in sectors such as manufacturing, construction and transport that represent around 75% of the energy consumed throughout the world [10].

Recently, considerable attention has been given to the use of thermoelectric (TE) materials as a complement to enhance the efficiency of renewable energy systems based on solar and biomass energy sources [1]. In addition, other applications in industrial sectors are making progress with these systems to become more efficient and reduce the impact on the environment, which translates to a significant recovery of wasted heat normally disposed into the environment. According to various reports, the energy consumption levels of 2019 will significantly increase by 2040 in sectors such as transport and construction, and the use of renewable energy and natural gas use will grow faster than the use of coal and oil [11].

These increases in the energy consumption of the different sectors and in renewable energies result in a promising outlook for TE materials. Studies have reported that about 60% of the energy consumed worldwide is dissipated as waste heat in the transport sector, with the manufacturing and construction sectors also making large contributions of residual heat [12–14]. The recovery of this residual heat, even partially, would represent a significant saving in world energy consumption, as well as significant reductions in greenhouse gas emissions [15], and would be a major contribution to a better environment and to a better circular economy model [16].

Residual heat can be classified into three categories according to the temperatures of the heat sources [11]. In this classification, low temperature ranges are temperatures below 100 ◦C; average temperatures when the temperatures are between 100 ◦C and 300 ◦C; and high temperatures when they are above 300 ◦C, such as in calcining, forming, heat treating, thermal oxidation and metal reheating processes [11]. Considering these ranges, the operating temperatures of commercial thermoelectric generation modules (TEG) show a great potential for the implementation of heat recovery systems using TEG systems in many different processes [17–20]. It is known that TE materials do not allow 100% heat loss recovery due to their current low efficiency. Furthermore, the high cost of the elements used to construct commercial TEG modules is another factor preventing their widespread implementation in many systems and economies [21–23]. Currently, one of the main goals is to improve the performance of TE materials so that they are competitive and efficient, which is can be obtained with a high Seebeck coefficient and electrical conductivity, while the thermal conductivity has to be low. Factors such as profitability and complexity of processing have limited this efficiency increase [24].

In the recent literature, there is a wide variety of studies that examine the development of sustainable alternatives and promote the development of alternative generation systems to reduce dependence on traditional fossil fuels and to reduce the current rates of emissions. Furthermore, numerous works aim to improve environmental conditions based on the development of clean generation systems. Clearly, the generation alternatives based on thermoelectric systems are viable and reliable; however, the present study goes beyond ensuring the implementation of cleaner generation alternatives to demonstrate that those same proposed solutions are in themselves sustainable. Therefore, in this analysis we intend to present some of the work that is being carried out today to contribute to the construction of a better future for future generations.

This work presents a systematic search on TE materials focused on the sustainability and profitability of TEG modules, and is divided in the following topics: materials, TEG modules, costs, efficiency, comparisons with other types of energy collection or cooling systems, and modeling of properties and construction. Finally, the recycling and end disposal of this type of TE material is reviewed (see Figure 1), with a particular focus on the circular economy for thermoelectric modules.

**Figure 1.** Themes of the article related to the circular economy of thermoelectric modules.
