Analysis of Thermoelectric Coolers as Energy Harvesters for Low Power Embedded Applications Yannick Verbelen, Sam De Winne, Niek Blondeel, Ann Peeters, An Braeken, Abdellah Touhafi Abstract—The growing popularity of solid state thermoelectric devices in cooling applications has sparked an increasing diversity of thermoelectric coolers (TECs) on the market, commonly known as “Peltier modules”. They can also be used as generators, converting a temperature difference into electric power, and opportunities are plentiful to make use of these devices as thermoelectric generators (TEGs) to supply energy to low power, autonomous embedded electronic applications. Their adoption as energy harvesters in this new domain of usage is obstructed by the complex thermoelectric models commonly associated with TEGs. Low cost TECs for the consumer market lack the required parameters to use the models because they are not intended for this mode of operation, thereby urging an alternative method to obtain electric power estimations in specific operating conditions. The design of the test setup implemented in this paper is specifically targeted at benchmarking commercial, off-the-shelf TECs for use as energy harvesters in domestic environments: applications with limited temperature differences and space available. The usefulness is demonstrated by testing and comparing single and multi stage TECs with different sizes. The effect of a boost converter stage on the thermoelectric end-to-end efficiency is also discussed. Keywords—Thermoelectric cooler, TEC, complementary balanced energy harvesting, step-up converter, DC/DC converter, embedded systems, energy harvesting, thermal harvesting. I. I NTRODUCTION S OLID state thermoelectric devices have a long history of being used for cooling purposes [12] in applications where size is a more important factor than thermoelectric efficiency [48], [17]. These thermoelectric coolers (TECs) are also appreciated for their silent cooling capabilities in stealthy applications, as opposed to noisy pumps and fans. Integration in consumer products has increased demand and fuelled their large scale production in recent years, in turn decreasing their price and allowing them to spread outside the specialized market segments they were originally used in [24], [34]. The combination of decreasing prices and better availability have opened opportunities in new domains, including low power electronics as energy harvesters [33]. Traditional solid state thermoelectric coolers are entirely bidirectional from a thermoelectric perspective because of the Seebeck effect on which they rely, and can be used both as cooling devices or as thermoelectric generators (TEGs). When a temperature difference is applied to a junction of two materials with a different thermoelectric constant, a small potential difference Yannick Verbelen is with the Vrije Universiteit Brussel, Pleinlaan 2, 1050 Etterbeek, Brussels, Belgium (corresponding author, e-mail: yannick.verbelen@vub.ac.be). Sam De Winne, Niek Blondeel, Ann Peeters, An Braeken and Abdellah Touhafi are with the Vrije Universiteit Brussel, Pleinlaan 2, 1050 Etterbeek, Brussels, Belgium. will be created over this junction [9]. The voltage is a function of the thermoelectric materials being used, and the temperature difference that is applied. As heat flows through the device from the ‘hot’ side of the junction to the ‘cool’ side, a heat flux is effectively converted into electric power [4]. The maximum thermodynamic power available [14] is given by P max = ΔT 2 4T 0 (K c + K h ) (1) For thermoelectric generators, the heat flux to electric power conversion depends on the Seebeck coefficient S, the thermal conductivity κ, the electrical conductivity σ and the temperature difference ΔT (1) [12], and can be approximated as the dimensionless figure of merit zT [13], [17, p. 190]: zT = S 2 ΔT σκ (2) Higher Seebeck coefficient and temperature difference, and better thermal and electrical conductivity of the material, will result in better heat flux to power conversion. The figure of merit zT as shown in (2) is a parameter of thermoelectric efficiency. Although zT is straight forward to model, the thermal (κ) and electrical (σ) conductivity of thermoelectric materials is difficult to measure, especially at elevated temperature differences [13], because of the Thomson effect [22], [7]. The design and simulation of accurate models for thermoelectric generators [26] is subject of ongoing research efforts [14], [17], and advanced modeling efforts have resulted in sophisticated models [37], [27]. These models unfortunately rely on an extensive set of parameters [27, p. 188, Table 1] which are not easily obtained for the commercial thermoelectric generators that are typically integrated in electronic devices for energy harvesting purposes. A second uncertainty is the physical construction of the TEG, including but not limited to the ceramics used as contact surface, insulating materials, and of course the thermoelectric junctions themselves. For bismuth telluride (Bi 2 Te 3 ), the thermal conductivity κ is 1.6 W/mK for example [7], but values strongly differ between materials. Many thermoelectric materials have been used to construct TEGs, of which PbTe [8], [19] and Bi 2 Te 3 [5], [16] are the most common. Other materials with high zT are Mg 2 Si [49], PbSnTe [2], AgPbSbTe 2 [21], Yb 14 MnSb 11 [3], CeFe 4 Sb 12 [38], FeSi 2 [47], SiGe [39], InGaAs [2], and recently also perovskite and graphene [46]. Current research focuses on making better use of existing materials by means of performance optimization through nanotechnology [15] and MEMS structures [20], rather than discovering new materials with an even higher figure of merit. World Academy of Science, Engineering and Technology International Journal of Energy and Power Engineering Vol:11, No:3, 2017 295 International Scholarly and Scientific Research & Innovation 11(3) 2017 scholar.waset.org/1307-6892/10006681 International Science Index, Energy and Power Engineering Vol:11, No:3, 2017 waset.org/Publication/10006681