## 4 Discussion  
In Mataro, on a cold but sunny winter day (in January) with an ambient temperature of 10 \({}^{\circ}\)C, we observed that the temperature on the aluminium plate can reach up to 22 \({}^{\circ}\)C and a high \(\Delta T\) can be reached in the TEG. However, if we compare two months in the winter in Mataro and Campinas (respectively January and July), the average daily global solar radiation available in Mataro is \(I_{E}\approx 1.89\,\mathrm{kW/m^{2}/day}\), a much lower value than in Campinas, where the average daily global solar radiation available, also in the winter, is \(I_{E}\approx 4.66\,\mathrm{kW/m^{2}/day}\). Concerning the temperature, in July in Campinas the temperature in the aluminium plate can reach up to 40 \({}^{\circ}\)C with an ambient temperature of 20 \({}^{\circ}\)C. These differences result in significantly distinct amounts of energy that can be harvested in different locations, showing that an open air STEG can be difficult to be used to power an autonomous sensor at the latitude of Mataro, if the sensor is not ultra-low power. To overcome the restrictions of using an open air STEG in countries where, during the winter, the average daily available global solar radiation is low and the low ambient temperature cools the aluminium panel, we implemented a system with a buried TEG. This system can harvest energy when the ambient temperature is in the order of 5 \({}^{\circ}\)C (or below) because a point at 40 cm into the soil is at a much higher temperature, typically 12 \({}^{\circ}\)C. Such systems present a much adequate performance to be used at these latitudes. Figure 14: Values of the stored energy in the supercapacitor, measured during 32 days (July 5 to 5 August 2015), in Campinas. The blue bars are the “forward” energy and the red bars are the “reverse” energy. It was observed that the soil temperature profile in two different latitudes (Campinas, SP--Brazil and Mataro, Catalonia--Spain) are sufficiently different to lead to significant differences in the performance of STEGs with a heat sink buried in the ground. The critical scenario for energy harvesting occurs, as expected, during the winter. Thus, we compared the month of January in Mataro with the month of July in Campinas. In Mataro, due to the low ambient temperature, the maximum temperature measured in the aluminium panel during January, 2015 was 22 \({}^{\circ}\)C. On the other hand, in Campinas, during July 2015, the temperature in the aluminium panel reached more than 38 \({}^{\circ}\)C in 70% of the days. The temperature difference between the aluminium panel and the soil at the depth of 40 cm is very different in Mataro and Campinas, both in the \"forward direction\" (panel temperature higher than the soil temperature) and \"reverse direction\" (panel temperature lower than the soil temperature). On the best days in Campinas, the temperature differences, respectively for the \"forward direction\" and \"reverse direction\", were 22 \({}^{\circ}\)C and 10 \({}^{\circ}\)C, while in Mataro, the differences were only 12 \({}^{\circ}\)C and 13 \({}^{\circ}\)C.  
The STEG prototype was buried into the ground in Campinas, at the depth of 10 cm (depth determined after the study of temperature profile), and the energy harvested on a day reached 34.11 J, which is 50% more than an open air STEG was able to harvest [14]. The average energy harvested during a consecutive period of 32 days was 24.38 J per day. We did not have the opportunity to install a buried TEG energy harvesting system in Mataro. If we assume that the supercapacitor of the energy harvesting system with a buried TEG was at its lowest voltage (1.8 V), since the collected energy during a day reached 34.11 J, this is enough to charge a 3 F supercapacitor up to 5 V (equivalent to 2.66 mAh).