_Article_  
Evaluation of the Thermoelectric Energy Harvesting Potential at Different Latitudes Using Solar Flat Panels Systems with Buried Heat Sink  
[PERSON] 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,19,17,18,19,19,17,19,19,17,19,18,19,19,17,19,19,19,17,19,19,19,  
A very common application for energy harvesting systems is in wireless sensor networks (WSN), especially in sensors that stay in hard-to-reach places or when maintenance costs are prohibitive [9]. [PERSON] et al. [10] studied different types of energy harvesting for WSN, ranging from photovoltaic to thermoelectric and RF energy harvesting. Their work concluded that in similar conditions, although solar and vibration energy harvesting seem to provide the most amount of power per cm\({}^{2}\), with an average of 100 \(\upmu\)W per cm\({}^{2}\), the harvesting of ubiquitous thermal energy is still very viable, with an average of 60 \(\upmu\)W per cm\({}^{2}\).  
In particular, TEGs can harvest solar thermal energy using solar-flat panels and different configurations of the heat sink, including the use of a heat sink left at the ambient temperature, panels with dissipation phase changing materials (PCMs) in contact with the heat sink (as seen in the works of [11; 12]), and TEGs with the heat sink buried in the soil [13]. Among the different configurations, the use of the soil as heat sink is particularly interesting as it allows harvesting energy during both day and night time and is significantly more economic than the use of PCMs.  
A simple diagram, showing the heat flow in an energy harvesting flat panel system with buried heat sink is shown in Figure 1.  
A study conducted in [14] has demonstrated that, using a low voltage DC-DC converter such as the LTC3108 from Analog Devices, it is possible to harvest energy when the effective temperature gradient across the junctions of the TEG (difference of temperature between the hot and cold sides of the TEG) is \(\Delta T\geq 1\ ^{\circ}\)C.  
As shown in [15], the electrical power delivered by a TEG, \(P_{elec}\), depends on \(\Delta T^{2}\), and can be written as:  
\[P_{elec}=\alpha_{n}\,\alpha_{p}\frac{R_{L}}{\left(R_{L}+R_{TEG}\right)^{2}}\, \Delta T^{2} \tag{1}\]  
where \(\alpha_{n}\) and \(\alpha_{p}\) are the Seebeck coefficients of the n and p elements of the TEG, \(R_{TEG}\) is the output impedance of the TEG, and \(R_{L}\) is the impedance of the external load.  
Therefore, if we compare the temperature difference between the flat panel and the buried heat sink in different locations (and different depths of the heat sink), we can determine the optimal depth for the heat sink and also provide a good estimation of the energy harvesting capability of a solar flat panel in each location.  
Figure 1: Heat flow in an energy harvesting system flat panel system with buried heat sink: when the temperature of the flat panel is higher than the temperature in the soil, the heat flows from the flat panel to the soil; when the temperature of the soil is higher than the temperature in the flat panel, the heat flows from the soil to the flat panel.  
In this work we present an experimental study of solar TEGs (STEGs) that make use of heat sinks buried in the soil. We study their potential for energy harvesting as a function of the depths of the heat sink, at two latitudes: one at the Northern Hemisphere and one at the Southern Hemisphere. We performed an analysis of the daily temperature difference between the temperature profiles of the panel and the soil (at various depths), determining the optimal depth for the installation of a STEG at different latitudes. A prototype of a solar flat panel with a TEG was fabricated and installed in the Southern Hemisphere in order to evaluate the electrical energy stored in a supercapacitor during 32 days of operation.