The growing demand for energy and the environmental problems derived from this problem are arousing interest throughout the world in the development of clean and efficient alternative energy sources, which involve ecological processes and materials. The materials used in the processes associated with thermoelectric generation technology will provide solutions to this situation. Materials related to energy make it possible to generate energy from waste heat residues, which are derived from various industrial processes in which significant fractions of residual energy are deposited into the environment. However, despite the fact that thermoelectric technology represents some relative advantages in relation to other energy generation processes, it in turn faces some technical limitations such as its low efficiency with respect to the high costs that its implementation demands today, and this has been the subject of intense research in recent years. On the other hand, the sustainability of the processes when analyzed from a circular economy perspective must be taken into account for the implementation of this technology, particularly when considering its large-scale implementation. In this article, a systematic search focused on the sustainability of thermoelectric modules is carried out as a step towards a circular economy model. The review aims to examine recent developments and trends in the development of thermoelectric systems in order to promote initiatives in favor of the environment. The aim of this study is to present a current overview, including trends and limitations, in research related to thermoelectric materials. As a result of this analysis, it was found that aspects related to costs and initiatives related to circular economy models have been little explored, which represents not only an opportunity for the development of new approaches in the conception of thermoelectric systems, but also for the conception of optimized designs that address the current limitations of this technology.

This research evaluated the operational conditions for a diesel engine with high compression ratio (CR) converted to spark ignition (SI), under stable combustion conditions close to the knocking threshold. The main fuel used in the engine was biogas, which was blended with natural gas, propane, and hydrogen. The engine limit to test the maximum output power was using the knocking threshold; just below the knocking threshold, the output power and generating efficiency are the highest for each blend. Leaner mixtures increased the engine knocking tendency because the required increase in the % throttle reduced the pressure drop at the inlet stroke and increased the mixture pressure at the end of the compression stroke, which finally reduced the ignition delay time of the end gas and increased the knocking tendency of the engine for all the blends. Therefore, the output power should be decreased to operate the engine below to the knocking threshold. Purified biogas achieved the highest output power and generating efficiency compared with the other blends and the original diesel operation; this blend was operated with five equivalence ratios. Purified biogas exhibits an optimal balance between knocking resistance, low heating value, flame speed, and energy density.