## 6 Behavior on a Real Volcano

Table 3: Average temperatures, generation and thermal resistances of M2 during 19 May 2020 between 11:00 and 20:00 h.  
increases, also raising the generation, which reaches values of up to 0.66 W in the case of M2. Despite its operation with temperatures below 0 \({}^{\circ}\)C, the cold side heat exchanger does not present signs of freezing, except a very punctual moment. The right between 22 and 23 January, an uncoupling between the ambient and the modules' cold side temperature can be observed, which could be due to the freezing of the cold side heat exchangers. Nevertheless, this fact only causes a slight reduction in the power generated, which is not even enough to reduce the total generation below the demand. On the other hand, the prototype has also dealt with other extreme meteorological conditions such as the typical Saharan air layer. This phenomenon, also known as _Calima_ in the Canary Islands, is characterized by a hot, dry, and dust-laden atmosphere. As shown before, high temperatures are not a problem for the operation of the prototype. The most critical aspect of this phenomenon is the reduced visibility caused by dust plumes, which could affect the communication with the gateway, located 14 km away. Nevertheless, since its installation, there have not been data losses. Not even in February, when one of the worst _Calima_ episodes in the last 30 years affected the Canary Islands [68]. The visibility loss can also occur due to the formation of clouds between the emitter and the gateway. At Teide, it is common to observe a temperature inversion that leads to the so-called _sea of clouds_ due to its appearance from above. Nevertheless, despite occurring quite often, this phenomenon neither affects the communication system. Thus, the prototype, and its associated electronics, have demonstrated to withstand several types of meteorological conditions, being able to supply the required energy mostly with only one thermoelectric module and ensuring communication at every moment. Table 4 summarizes the number of hours that each thermoelectric module has been in operation between 19 December 2019 and 20 August 2020. As can be observed, M2 is the thermoelectric module that leads generation, with more than 4400 h of operation. M3 is the other module that supplies power occasionally, with less than 270 h being active, while M1 and M4 are never necessary. On average, M2 is in operation 86.2% of the time, being in open-circuit the rest of the time. Nonetheless, depending on the ambient conditions, this percentage of time in operation can vary. In the table, apart from  
Figure 14: (**a**) Temperature, (**b**) power, (**c**) voltage and (**d**) intensity measurements during several cold days. the average values, it is also represented by the percentage under the typical conditions shown in Figure 11, under the hot days depicted in Figure 12, and under the cold days considered in Figure 14. Hence, when the ambient temperature decreases, due to the higher generation of the thermoelectric modules, M2 needs to be active a shorter period of time, while in hot days, the operation time increases. It is in these hot days when it is sometimes required that another thermoelectric module, M3, starts its operation, when the generation of M2 is lower than the consumption and the battery is being discharged. Nevertheless, the rest of the time is in open circuit, leading to an average of 0.9% of the time in operation. Thus, it can be seen that during 93.95% of the time that the prototype has been installed at Teide volcano, the generation of M2 has been enough to supply the required power. Finally, Figure 15 shows the power generated by M2 with respect to the temperature difference between its sides \(\Delta T=T_{H}-T_{C}\) during the previous period. As can be observed, there exists a linear correlation between the generated power and the temperature difference between its sides.