## 1 Introduction
For this purpose, more efficient piezoelectric materials, such as lead zirconate titanate (PZT) [26; 27] or relaxor-based ferroelectric single crystals, have been studied [28]. Another important energy recovery technique exploits the known Seebeck effect, for which a thermoelectric material subjected to a thermal gradient generates a potential difference V between the hot and cold ends, proportional to the temperature difference \(\Delta\)T times the Seebeck coefficient S [29; 30]. Therefore, this method allows for recovering electricity from heat. The reverse phenomenon, or the Peltier effect, has been used for many years for the implementation of electronic systems capable of generating heat or cooling, based on the voltage applied to the device, called the Peltier cell. Recently, the Seebeck effect has been analyzed in new generation materials, with the aim of obtaining higher energy conversion efficiencies [31]. However, due to the still high cost and the scarce diffusion of such thermoelectric materials, they do not constitute a suitable solution for low-cost applications. On the contrary, the study of the employment of small low-cost Peltier cells as TEG generators for the recovery of thermal energy and how to maximize the conversion efficiency of such devices can be of great interest, as it constitutes a valid source of alternative energy for portable or wearable devices that require very low power. Among the various applications, the biomedical field is one of the most interesting, since the human body spends energy to maintain a constant temperature level, regardless of the atmospheric conditions of the surrounding environment, within certain limits. It is possible to recover part of this energy dissipated by the human body by using TEG generators, which can supply implantable devices or generic health monitoring sensors [32]. In order to directly feed autonomous, portable devices or to recharge the battery by means of thermoelectric energy harvesting, a constant standard supply voltage should be provided, such as 3.3 V, 1.8 V, etc. However, the output voltage obtained by thermoelectric generators is directly proportional to the temperature gradient and, therefore, it is not fixed to a specific value since it is application dependent. Hence, a DC/DC conversion stage is needed between the source and the equivalent load. This is a key aspect of the thermoelectric energy harvesting system, since the electrical description of the energy source, in terms of SPICE model, is crucial to properly design a DC/DC converter, in order to guarantee the maximum efficiency of operation for the whole system by means of maximum power point (MPPT) algorithms [33; 34]. In addition to the previous work [35] conducted by the authors, where a repeatable characterization method for the Peltier cells has been described, in this paper we aim to implement an equivalent circuital SPICE (Simulation Program with Integrated Circuit Emphasis) model of a \(2\times 2\) cm Peltier module with the goal to easily analyze different multicell configurations in low-temperature difference, as for the human body heat, and provide a circuital design method for DC/DC converters with MPPT, in order to maximize the conversion efficiency by taking into account the real specifications of the thermoelectric generator.