# Application of Thermoelectric Generators for Low-Temperature-Gradient Energy Harvesting
## 3 Selection of the Best TEG + DC/DC Converter Combination
### Experimental Setup  
After a careful literature review, two commercial low-temperature TEGs and two commercial DC/DC converters were selected, which are compared in this paper. As the output voltage generated by the TEGs is in the range of some mV, two DC/DC converters with extreme low startup voltage were selected to amplify the voltage from the millivolt range to the volt range to supply WSs.  
Tables 2 and 3 present the main characteristics of the TEGs and DC/DC converters.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Combination** & **Devices Involved** \\ \hline
1 & TEG \#1 + DC/DC converter \#1 \\
2 & TEG \#1 + DC/DC converter \#2 \\
3 & TEG \#2 + DC/DC converter \#1 \\
4 & TEG \#2 + DC/DC converter \#2 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Combinations of TEGs and DC/DC converters.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**TEG Modules** & **TEG \#1** & **TEG \#2** \\ \hline Model & GM250-157-14-16 & TGL2-8 \\ Manufacturer & European Thermodynamics & Marlow Industries \\ Dimensions (mm) & 40 \(\times\) 40 & 40 \(\times\) 40 \\ Thickness (mm) & 4.1 & 3.6 \\ Matched load resistance (f) & 3.65 & 3.46 \\ Hot side temperature (\({}^{\circ}\)C) & 250 & 230 \\ Cold side temperature (\({}^{\circ}\)C) & 30 & 50 \\ Optimum output voltage (\({}^{\circ}\)V) & 5.05 & 5.25 \\ Optimum output power (W) & 6.99 & 7.95 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Analyzed TEG modules.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**DC/DC Converters** & **DC/DC \#1** & **DC/DC \#2** \\ \hline Model & LTC3108 & LTC3109 \\ Manufacturer & Analog devices & Analog devices \\ Harvesting board & Adaptive ADEH & Demo circuit 1664 A \\ Input voltage range & 50\(-\)400 mV & 30\(-\)500 mV \\ Voltage regulation & 2.35\(-\)5.00 V & 2.30\(-\)5.10 V \\ MPPT technology \({}^{\star}\) & Yes & No \\ \hline \hline \end{tabular} \({}^{\star}\)MPPT stands for maximum power point tracking.  
\end{table}
Table 3: Analyzed DC/DC converters.  
To compare the behavior of the different TEGs, they were placed on the top of a flat aluminum busbar, which was exposed to heat cycle tests. To this end, six resistances (HS50 1R F from ARCOL,1 \(\Omega\), 50 W) connected in series to a BK9205 power supply (BK Electronics, Southend On Sea, England) were used, jointly with two TEGs connected in series, as shown in Figure 3. A rechargeable BM2000C1450 AA251 PATP Nickel battery from GlobTek (GlobalTek, Miami, FL, USA) was also used to store the energy generated by the TEGs and converted by the DC/DC converter.  
To study the performance and efficiency of thermoelectric energy harvesting systems, it is necessary to determine the input/output power of TEG modules and DC/DC converters. Two Fluke 289 data-logger multimeters (Fluke, Everett, WA, USA) were used in ammeter mode to measure the value of the small currents generated by the TEGs (microamp to milliamp range) due to the low temperature gradients analyzed. An NI USB-6210 data acquisition system (National Instruments, Austin, TX, USA) was used to acquire the voltages from the output terminals of the TEG module and the DC/DC converter. Three T-type thermocouples together with an NI-9211 thermocouple acquisition module (National Instruments, Austin, TX, USA) were used to measure the ambient temperature and the temperatures of the hot and cold sides of the TEGs. To ensure a simultaneous acquisition, the Fluke multimeters, the NI USB-6210 and NI-9211 modules were synchronized using a Python code programmed by the authors of this work.  
A thermal joint compound (120-SA, Wakefield-Vette, NH, USA) was used to ensure good thermal contact between the heat source, TEG and heat sink. This compound fills the tiny air gap between mating surfaces with a grease-like paste containing zinc oxide in a silicone oil carrier. It is used for several reasons such as its ability to fill the air gap between different surfaces, its high thermal conductivity and its adhesive ability.