## 1 Introduction
In active volcanoes, the most evident manifestation of geothermal energy coincides with one of the signs of active volcanism: fumaroles, vents in the Earth's surface from which steam and volcanic gases are emitted, normally at temperatures between 70 and 100 \({}^{\circ}\)C [22]. Hence, this solution fits in the proverb _\"if you cannot beat them, join them\"_. Traditional geothermal cycles are not suitable for low-scale power generation from volcanic fumaroles. Nonetheless, thermoelectric generators could become an alternative. Thermoelectric generators are devices based on solid state physics whereby heat (i.e., temperature difference) is directly transformed into electricity by means of the Seebeck effect. Thanks to these devices, the operation of which is more deeply described in Section 2, it is possible to generate electricity continuously from geothermal heat [23; 24], even improving with adverse meteorological conditions, thus permitting a reduction in the capacity of the batteries or the utilization of other storage technologies such as supercapacitors. Furthermore, these devices are modular, compact, and have demonstrated their reliability and durability without maintenance requirements in spatial applications [25]. Considering the former advantages, the use of thermoelectric generators have already been proposed to power ground sensors. Due to the difference in heat capacity and conduction rate between the air and the ground, there normally exists a temperature difference between them [26]. [PERSON] [27] and [PERSON] [28] demonstrated that this temperature difference can be transformed into electricity by means of thermoelectric generators, permitting the power supply of remote sensors. As indicated by [PERSON] et al., this type of self-sufficient power source could serve for unattended ground sensors (UGS) in defense and security [29]. They experimented with this proposal at the laboratory, developing a complete sensor consisting of a thermoelectric generator, a DC-DC boost converter, a wireless sensor transmitter, and supercapacitors, which presented good performance with temperature differences as low as 1 \({}^{\circ}\)C.  
Forest monitoring is another possible application. Similarly to volcano surveillance, forests are also monitored in order to analyze the impact of droughts or fires, to uncover their structure, and to study variations of their biota, for which wireless sensors are used [30; 31; 32]. Nevertheless, these sensors also present energy supply problems due to their remoteness. As a solution, [PERSON] et al. proposed the utilization of thermoelectric generators to provide a stable power supply, taking advantage of the mentioned temperature difference between the ground and the air [33]. [PERSON] et al. improved the previous micro-generator and performed field experiments under natural conditions in two different locations [34]. The results obtained over six months reveal that the location influences the power generation that can be obtained. Hence, in Harbin an average of 0.335 mW were generated, while in Beijing, only 0.076 mW could be produced. In a later article, they concluded that in order to efficiently harvest micro-energy from shallow soil, the thermoelectric generator needs to be placed where the soil moisture is greater than 30% [35]. Roadway and bridge infrastructures also require energy harvesting in order to power a multitude of data collection and communication applications [36]. For this purpose, it is considered that thermoelectric generators are one of the most readily available methods [37], transforming the heat absorbed from the exposure to solar radiation into electricity. For instance, [PERSON] et al. fabricated a system to embed into asphalt pavements, so that with the temperature difference between the pavement surface and the soil below it, electricity could be generated. They optimized and tested a prototype in the field, obtaining 29 mW [38].