Elsevier

Energy

Volume 86, 15 June 2015, Pages 300-310
Energy

Experimental assessment of thermoelectric generator package properties: Simulated results validation and real gradient capabilities

https://doi.org/10.1016/j.energy.2015.04.041Get rights and content

Highlights

  • Optimized design of a flexible thermoelectric generator (μTEG) for self-powered wearable devices.

  • Finite element analysis on a 3D generator on wavy-shaped PDMS/Kapton assembly.

  • Experimentally measured thermal gradient ranges between 0.20 K and 0.64 K higher than the simulated value.

  • The warm-up time of 500 s for 4 mm thick package to completely thermalize the generator.

  • Good matching of stabilization times between the experimental and simulated results, error <4.2% for 4 mm package.

Abstract

The optimized design of a flexible micro thermoelectric generator (TEG) suitable for self-powered wearable devices and its real temperature gradient performance is proposed and discussed in this paper. Finite element analysis was performed on a three-dimensional p-n thermocouple on wavy-shaped poly-dimethylsiloxane (PDMS)/Kapton assembly using COMSOL Multiphysics software. Electrical and thermal simulations were carried out to determine the geometric effects of the single thermocouple (length and width of the thermoelements, deposition procedure of junction between p- and n-type legs) on output power and efficiency performance of the TEG. The experimental results confirmed that the experimentally measured thermal gradient ranges between 0.20 and 0.64 K higher than the simulated value and such a result has great importance for correct generator design and determination of effective thermal gradient which can be recovered using the proposed package solution. Heat transfer analysis was performed to optimize the proposed package solution and maximize the thermal gradient that can be recovered between the thermocouples junctions. Experimental results confirmed that the thicker package ensures better insulation, with a real gradient about 0.11 K lower that the simulated one. The warm-up time for 4 mm package to completely thermalize the Kapton upper surface is about 500 s; good matching on thermal response times has been found between the experimental and simulated results for all investigated package thicknesses.

Introduction

The Energy Harvesting technology enables to recover the waste energy dispersed into the environment and convert it into electrical energy useful to supply ultra-low power devices. Vibrations, heat and light are some of the most frequently exploited energy sources, and a comparison of their typical power densities is reported in Table 1 [1]. The thermoelectric generation is a particularly interesting technology for the direct conversion of heat into electrical power by using solid state devices characterized by extended maintenance-free durability and noiseless operation without moving parts. A thermoelectric generator consists of an array of pairs of semiconductor materials connected electrically in series and thermally in parallel. Each pair forms a junction in contact with the heat source, while the other ends of the thermoelements are in thermal contact with the substrate, air or a medium which is at a lower temperature than the heat source (Fig. 1). The performance of a TEG is strongly influenced by temperature distribution inside the materials and thermal gradient which can be recovered between the hot and cold junctions of the thermocouples. The heat transfer from a medium to another one represents an important issue which allows to understand how a physical system containing a heat source and a dissipative medium evolves. The heat can be transferred by three different mechanisms: conduction, convection and radiation. In some cases, depending on the working conditions only one mechanism turns out to be dominating, whereas the others can be neglected.

Recently, the numerical simulation based on Finite Element Method (FEM) has become a very powerful tool to analyze and optimize the performance of thermoelectric devices. In Ref. [2] numerical technique was used to study the effect of geometric design on the performance of a thermoelectric generator (TEG) with constant cross section area, but variable height of the thermoelectric legs. The authors found that the output voltage linearly increases with the legs height until a specific height, after which the output voltage saturates. In Ref. [3] finite element analysis was performed on a 3D model of micro-TEG to investigate the role of the dimensions of the device on the power generation efficiency. To improve the computation efficiency significantly, Chen et al. [4] proposed a 3D compact model of a thermoelectric cooler represented as single “black box” in a computational fluid dynamics simulation environment. In Ref. [5] a 3D TEG model was proposed and implemented to simplify the co-design and co-optimization of the fluid and the thermoelectric device, which are crucial for maximizing the system performance. In Ref. [6] the application of a TEG to harvest energy from the waste heat of a commercial table lamp was investigated experimentally and numerically. Both open-circuit and closed-circuit lamp-TEG system were simulated. A 1D TEG model taking the Peltier and Joule heats into consideration was proposed to predict the power generation rate based on the simulated hot and cold sides thermal conductances of the open-circuit system. In Ref. [7] the optimum efficiency and geometrical dimensions of a segmented TEG module were derived by mathematical methodology and numerical simulations based on FEM calculations were carried out to verify the validity of the optimum segmented TEG model operating in design boundary condition. Wang et al. [8] investigated the performance of a TEG combined with an air-cooling system designed using two-stage optimization: an analytical model was used to model the air-cooling system, a numerical method with a finite element scheme was employed to predict the performance of the TEG. Jang et al. [9] investigated the optimal structure of high-performance micro-TEG using the FEM analysis with 3D models.

In Ref. [10] the authors reported some details about the design and fabrication of a wearable and flexible thermoelectric generator. The proposed TEG was designed to be used as “electronic garment”, in order to recover the heat useful to the thermoelectric generation from the temperature difference existing between the body skin and the environment. Standard UV (Ultraviolet) photolithography and lift-off process were used to deposit 1 μm-thick thermoelectric thin films on Kapton substrate. P-type Sb2Te3 and n-type Bi2Te3 have been chosen as thermoelectric materials, because of their high thermoelectric efficiency at room temperature. The figure-of-merit of Bi/Sb/Te thin films depends on composition and crystalline structure of materials and it also varies with the deposition technique, as exhaustively reported by Goncalves [11]. Kapton is a flexible and low cost polyimide film extensively used in wearable/bending electronics applications, because of its good physical, chemical, and electrical properties over a wide temperature range. The fabrication process required only two photolithographic steps to complete the thermopile, as the single thermocouple consists of a direct p–n junction. Embedded thermometers and metal pads were deposited by e-beam evaporation, in order to monitor the thermocouples junctions temperature and electrically test single partitions of the array. The proposed device integrates 2778 thermocouples of Sb2Te3/Bi2Te3 thin films into an area of 25 cm2 of Kapton substrate. Each thermocouple is 3 mm long, with a width of 50 μm and 145 μm for the p-type and n-type leg, respectively. By a proper package solution (Fig. 2) the device is able to autonomously recover the thermal gradient useful to the thermoelectric energy harvesting, using the temperature difference existing between the body skin and the environment (about 17 K). The realized TEG generates an open-circuit voltage of about 2 V with a thermal gradient of about 5 K between the hot/cold thermocouples junctions, but it exhibits a high internal resistance of about 2.3 MΩ which strongly limits the output current. However, such a drawback can be successfully solved by increasing the thin films thickness and extracting the TEG output voltage from a set of different parallel blocks of thermocouples, in order to divide (multiply) for n the total internal resistance (the total output current). By using 10 μm thick films and configuring the TEG as a parallel set of 8 blocks, an open-circuit voltage of 250 mV and an output current (in matched load condition) of about 2 μA are measured, with an equivalent internal resistance of 29 kΩ.

In this work the authors intend to present the results of numerical FEM simulations performed to aid the design of the thermopile and the packaging of the device. Different design issues were considered (width and length of the thermoelements, type of junction between the p-n pair) and the their effect on the TEG generation performance (harvested power, internal electrical resistance of the array, thermal efficiency) was discussed. Heat transfer simulations were also used to optimize the packaging of the device, in order to maximize the temperature difference between the p-n pairs junctions. A comparison between numerical and experimental results was also done and a good agreement was found between experiments and simulations.

Section snippets

Formulation

The main challenge was optimizing the generation performance of the TEG and, at the same time, ensuring the success of the fabrication and packaging of a low-cost device on flexible Kapton substrate.

The numerical simulation was initially used to design the single thermocouple, defining length and width of the legs and type of junction to be used to electrically connect the thermoelements (p–n junction or p-type material/metal/n-type material). Nolas et al. [12] demonstrated that the conversion

Simulation results

First, the effect of the type of junction on the electrical resistance of the single thermocouple was investigated. The thickness of metal and thermoelectric thin films was set to 10 μm. A normal current density of 0.2 A/mm2 was applied to one of the two terminals of each thermocouple and the ground condition (V = 0) was applied to the other terminal (Fig. 4a). The materials electrical resistivity used in the simulations are reported in Table 2. The simulation results indicated that the

Conclusion

The design and the real properties assessment of a Kapton/PDMS thermoelectric generator package was presented and discussed in this paper. Electrical and thermal FEM simulations were used to investigate the dependence of output power and efficiency performance of the TEG on the length and width of the thermoelements and the type of junction between p- and n-type legs. The temperature difference between body skin and environment is used to harvest energy by Seebeck effect. To create and maximize

Nomenclature

Ap,n
Cross-sectional area of p- or n-type leg, m2
λp,n
Thermal conductivity of p- or n-type leg, Ω/m
ρp,n
Electric resistivity of p- or n-type leg, Ω·m
Pout
Output electric power of the TEG, W
α
Seebeck coefficient, V/K
Τ
Temperature, K
ΔΤ
Thermal gradient, K
Rth
Electrical resistance of the TEG, Ω
Qin
Heat flux through TEG surface, W
η
Thermoelectric conversion efficiency, –
J
Electric current density vector, A/m2
ρq
Charge density, C/m3
σ
Electric conductivity, S/m
Je
External current density vector, A/m2
Qj
External

Cited by (13)

  • Geometrical optimization of a thermoelectric device: Numerical simulations

    2018, Energy Conversion and Management
    Citation Excerpt :

    Studies of the geometry of TE devices using simulations are still ongoing. On Ref. [13], the authors simulate two types of devices with distinct TC’s and study how their efficiency depends on the width of the legs and number of these TC’s. However, their main focus was also the simulation of the heat source and a comparison and validation of the numerical results with the experimental studies.

  • Recovery of thermal energy released in the composting process and their conversion into electricity utilizing thermoelectric generators

    2018, Applied Thermal Engineering
    Citation Excerpt :

    It is the opposite for p-type materials. Thus, a thermoelectric couple is one pair of n- and p-type material, and generally a thermoelectric generator has several couples wired electrically in series and thermally in parallel, as shown in Fig. 1(a) and (b) [22–25]. These couples and their electrical interconnects are enclosed by an electrical insulator, typically a ceramic [26], as shown in Fig. 1(b).

  • Nanogenerators for Human Body Energy Harvesting

    2017, Trends in Biotechnology
    Citation Excerpt :

    In addition, this NG supports over 1000 cycles of bending movements without showing significant degradation in output performance. Another example of a flexible thermo-NG is the wavy-shape module presented by Francioso and colleagues [74,76]. The researchers assembled the p- and n-thermolegs on a wavy substrate made of two PDMS layers covered with a thin layer of Kapton.

  • Modelling, fabrication and experimental testing of an heat sink free wearable thermoelectric generator

    2017, Energy Conversion and Management
    Citation Excerpt :

    The heat flow can be parallel or perpendicular to the substrate surface, in the latter case an optimized package can maximize the thermal gradient between the junctions. A flexible planar TEG formed by sputtered thin films as active p- and n-type thermocouples and structured by a wavy-shaped PDMS/Kapton assembled package was designed, fabricated and tested by authors in [1–5]. An open circuit output voltage of about 2 V at temperature difference of 5 K and internal resistance of about 2.3 MΩ was measured for a thermopile of 2778 thermocouples into an area of 25 cm2.

  • Theoretical modeling of thermoelectric generator with particular emphasis on the effect of side surface heat transfer

    2016, Energy
    Citation Excerpt :

    Thermoelectric power generation technology, on the basis of Seebeck effect, has received much attention in recent several decades due to its various merits, such as light weight, high reliability, environmental friendliness, no moving parts and no real fluids [1–5]. The potential applications of thermoelectricity include utilizing solar energy [6–8] and geothermal energy [9], recovering waste heat in automobile [10,11], iron and steel industry [12], cook stove [13], boiler of power plant [14] as well as body skin [15]. However, there still needs long time to fully commercialize these applications because of the low efficiency of thermoelectric conversion (less than 5%).

View all citing articles on Scopus
View full text