## 6 Behavior on a Real Volcano  
On 18 th December 2019, the prototype was installed at Teide volcano (Canary Islands, Spain) with its associated electronics (Figure 9) and on the submission date of this manuscript, the prototype was still in operation. Teide is a stratovolcano located in the Canary Islands (Spain), an archipelago of volcanic origin that emerges in the Atlantic Ocean. With an altitude of 3718 m, Teide is one of the most evident manifestations of the active volcanism of the islands, and therefore, it is widely monitored. Figure 10 details the location of the prototype, in the northern side of the volcano at an approximate altitude of 3500 m, as well as the position of the gateway, 14 km away with direct vision. As shown in Figure 9, in its installation, aluminum deflectors were added so that the volcanic gases are diverted, not coming into contact with the thermoelectric modules nor the cold side heat exchangers. Polyurethane foam was used for sealing the joints. Furthermore, polyethylene was also added in order to insulate the aluminum block of the hot side heat exchanger and the thermoelectric modules from the influence of ambient conditions. Figure 11 depicts the typical operation of the prototype, taking as reference the variables measured between 29 December 2019 and 2 January 2020. More specifically, Figure 11a shows the temperature of the ground \(T_{ground}\), the hot and the cold side of two thermoelectric modules \(M1\_T_{H}\), \(M1\_T_{C}\), \(M2\_T_{H}\) and \(M2\_T_{C}\), and the ambient temperature \(T_{amb}\). Each monitored thermoelectric module belongs to one of the individual generators installed. As can be observed, ground temperature \(T_{ground}\) remains practically constant with an average value of 81.82 \({}^{\circ}\)C. At the opposite side of the prototype, the ambient temperature \(T_{amb}\) presents the typical fluctuations of day and night. Hence, since the considered values correspond to winter, during the night, temperatures close to 0 \({}^{\circ}\)C are found, while during the day, the temperatures rise up to an average of 10 \({}^{\circ}\)C.  
The difference between the ground and the ambient temperatures (\(T_{ground}-T_{amb}\)) represents the maximum temperature difference achievable by the thermoelectric modules, although, in reality,  
Figure 9: Prototype installed at Teide volcano on December 2019, composed of 4 thermoelectric modules. the temperature difference between the hot and the cold side of the modules will be lower. The approximation of the temperature difference of the thermoelectric modules (\(T_{H}-T_{C}\)) to the maximum available one (\(T_{ground}-T_{amb}\)) depends on the heat exchangers installed. The lower the thermal resistances of the heat exchangers, the closer the temperature differences. In the considered hot side heat exchangers, there exists an average temperature difference of \(13.5\,^{\circ}\mathrm{C}\), while this difference is of \(10.5\,^{\circ}\mathrm{C}\) in the case of the cold side ones. Thus, the thermoelectric modules work under a gradient of \(54\,^{\circ}\mathrm{C}\), which is similar for the two thermoelectric modules that are being monitored. This gradient is slightly influenced by the ambient conditions, decreasing during the day and increasing during the night, which will affect generation. Figure 11b-d show the values associated with the measurements performed by means of the INA219s: power, voltage, and intensity respectively. In particular, these sensors, as shown in Figure 6, have been installed monitoring the individual power generated by the thermoelectric modules M1 and M2 of which the temperatures are also being measured (before the MPPT), the generation of all the thermoelectric modules (after the MPPTs and before the battery), and the consumption of the different devices (at the battery output). As explained before, the consumption profile presents a cyclic oscillation, with a fixed power requirement of \(0.3\,\mathrm{W}\) and peaks of \(0.5\,\mathrm{W}\) during \(120\) s every \(12\) min.