Characterization of thermoelectric generators by measuring the load-dependence behavior
Highlights
► This paper is to present a methodology to characterize thermoelectric (TE) generators. ► The measurements were done on a TE module TEC1-12707 and a SPICE model was obtained. ► The SPICE model is Vopen = 53.17 × ΔT [mV] in series with an internal resistance of Rint = 3.88 ± 0.13 Ω. ► The load resistance of 3.92 Ω given by Vout/Iout is also in accordance with the measurements. ► Vout/Iout = 3.92 Ω is within the range [μ − σ, μ + σ] Ω, where μ = R0 = 3.88 Ω and σ = ΔRint = 0.13 Ω.
Introduction
In the present days there is the increased concern to reduce fossil fuel dependence because it is necessary to revert their effects on the environment, e.g., reduction in the ozone depletion or greenhouse effect due to the gas emissions [1], [2]. In this context, the use of renewable sources of power is a concern even more and more high [3], [4], [5], [6]. The development of the materials technology reached a matured phase where the idea of fabricating all solid-state thermoelectric generators was possible to achieve [7], [8], [9], [10]. A solid-state thermoelectric generator is a device that converts temperature gradients into electricity and whose components do not include neither any type of movable parts nor any kind of fluids (for example, Freon gaz) [11], [12] which makes these type of converters very appreciated for stand-alone nodes of wireless sensors networks with harvesting capabilities [10], [13], [14], [15] for running at large periods of time with battery replacement and/or node management [16], [17], [18], [19]. In fact, solid-state thermoelectric converters are generally based on heavily doped semiconductors use the Seebeck effect to produce electrical energy [20], [21]. In a general form, the performance of a TE device is determined by the figure-of-merit of the materials used [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], which is expressed in terms of the dimensionless parameter, ZT, given by:where α [V K−1] is the Seebeck coefficient, ρ [Ωm] is the electrical resistivity, κ [Wm−1 K−1] is the thermal conductivity and T [K] the temperature. The performance parameter ZT is very useful in Seebeck sensing devices, such as infrared thermal detectors. In a thermoelectric converter, another performance factor is more appropriate, which is the power-factor, PF [WK−2 m−1]. The PF is defined as the electric power per unit of area through which the heat flows, per unit of temperature gradient between the hot and the cold sides, and is expressed as:
In this context, this paper presents a measurement methodology for characterizing the behavior and the performance of thermoelectric converters. A set of commercial modules were used to achieve such a purpose. The thermoelectric modules model TEC1-12707 presents a contact area of 40 mm by 40 mm and can work with temperatures up to 450 K without taking the risk to degrade the quality of the measurements and thus, putting in jeopardize the applicability of the proposed method.
Section snippets
Measurement setup
The Fig. 1a shows an artist impression of a typical thermoelectric converter and the respective physical factors that determine its operation. If the junctions at the bottom are heated and those at the top are cooled (producing a temperature differential) then electron/hole pairs will be created at the hot end and absorb heat in the process. The pairs recombine and reject heat at the cold ends. A voltage potential, the Seebeck voltage, which drives the hole/electron flow, is created by the
Experimental results
The first set of tests consisted in the determination of the V/I (output voltage versus the output current) output characteristic of the thermoelectric converter. The Fig. 3 shows the output voltage, Vout [mA], versus the output current, Iout [mA], for several test temperatures, Ttest [°C], and thus, for several temperature gradients, ΔT = TH − TC [°C]. As expected (and the same conclusion applies to the load-less case), the output voltage increases with the test temperature, Ttest, (and thus, with
Conclusions
This paper presented a method for characterizing the behavior of thermoelectric converters. The method is based on measuring the voltages and currents supplied by the thermo-converter as a response to variations of the load resistance. It was used thermoelectric modules model TEC1-12707 for doing the measurements and the characterization of this device. In spite of being used a commercial thermoelectric converter, the method can be applied to other thermoelectric modules that use the Seebeck
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