Elsevier

Energy

Volume 238, Part B, 1 January 2022, 121771
Energy

U-type unileg thermoelectric module: A novel structure for high-temperature application with long lifespan

https://doi.org/10.1016/j.energy.2021.121771Get rights and content

Highlights

  • U-type unileg TEG combined unileg structure with pn-junction is proposed firstly.

  • The σmax and N for U-type module are 46 % and 132 % of those of traditional module.

  • The optimal σmax and Pmax are obtained at (ru,rl) of (0.1,0) and (0,0.5).

  • Optimal LR (1.5–2 mm) and HAg (35 μm) lead to high Pmax and low σmax simultaneously.

Abstract

Strong thermal stress caused by high temperature and difference of thermal expansion coefficient (CTE) will negatively influence the lifespan of the thermoelectric module. In this work, a new high-temperature CaMnO3-based U-type unileg thermoelectric module, combining a unileg structure with pn-junction, is proposed and investigated. The novel design avoids the device failure due to different CTEs and high temperature gradients. As a result, the maximal thermal stress (σmax,TEM) of 3.31 GPa and fatigue life of 41686 cycles are 46 % and 132 % of those of traditional modules at 6 W and 300 K, respectively. To further relieve stress concentration, the effect of rounded corners (ru, rl), Ag layer thickness (HAg) and length of right legs (LR), have been studied. It has been found that larger ru, and rl are suitable to relieve the local stress concentration, and the lowest σmax,TEM and highest power (Pmax) are achieved at (ru,rl)=(0.1,0) and (0,0.5). Moreover, larger LR and HA are beneficial for mechanical properties by decreasing the peak stress and dispersing the high thermal stress regions, while module performance is improved at lower LR and HAg. Results obtained from this U-type unileg thermoelectric module should influence and guide the design and optimization of high-temperature thermoelectric generators.

Introduction

Renewable energy sources, and technology for their exploitation, are urgently needed due to the resources shortage, environmental deterioration, and ecological damage [[1], [2], [3]]. Thermoelectric power generator (TEG), as a novel energy conversion technology, can realize the direct conversion between heat and electricity [4]. In contrast to traditional waste heat recovery technologies, the TEG is advantageous because of its compactness, noiselessness, vibration-less operation, no moving mechanical components, and low ecological hazards. Thus, TEGs have been applied in micropower generators, the internet of things, automobiles, spacecrafts, etc., in last decades [[5], [6], [7]].

The thermoelectric modules, as the main components, of TEGs are typically composed of thermoelectric legs, electrodes and ceramic substrates. Based on the Seebeck effect, thermoelectric modules can directly convert waste heat into useful electricity when a temperature difference (ΔT) is stablished across the module [8]. The theoretical conversion efficiency (η) mainly depends on ΔT and figure of merit of thermoelectric materials (ZT); it is enhanced and approaches the Carnot efficiency as ΔT increases; consequently, larger ΔT benefits to obtain higher power output. However, higher temperatures will induce larger thermal stresses within the material and among the different materials in the thermoelectric module, resulting in serious deformations, fractures and even device failure. Thus, much new designs and strategies have been proposed to solve the issue of these high stresses.

Poor contact between different materials is a crucial reason for high stress and low stability. Clin et al. [9] proved that the stress distribution in the thermoelectric legs was greatly affected by thermal expansion coefficient (CTE) mismatch between electrodes and thermoelectric materials. In this case, Chavez et al. [10] proposed a new pn-junction TEG concept. In a pn-junction TEG, as shown in Fig. 1(a), the metal contact and substrate on the hot side are diminished, and the electrical connection is made by a direct junction of the p- and n-type thermoelectric materials. This design completely gets rid of contact issues between metal and semiconductor on the hot side and, therefore, shows great potential for constructing reliable and long-lifetime thermoelectric devices.

Besides, the contact issues of thermoelectric material/electrode interface, and the CTE difference between n- and p-type thermoelectric materials, are also a big challenge for low-stress design. To solve this problem, Marquesn et al. proposed a unileg structured module for the first time in 2007. In 2008, Lemonnier et al. [11] fabricated and tested a prototype of oxide thermoelectric module only composed of Ca0.95Sm0.05MnO n-legs. The unileg structure reduces the problems associated with CTE difference between p- and n-type thermoelectric legs. As a result, this architecture contributed to good mechanical strength and increased lifespan during thermal cycling. Afterwards, an improved unileg-TEG design was proposed by Wijesekara et al. [12], as shown in Fig. 1(b). Here, the integration of electrode and conductor reduced the total number of contacts in the thermoelectric device and further decreased the thermal stress. Therefore, the unileg module with pn-junction can be considered as a promising structure for low-stress design [13].

As mentioned above, the high stress intensity can be effectively reduced through novel structural design. However, for any structure, the stress at the local structure such as the edge of a hole, or at a right-angle beam, has a higher value than the remote stress [14,15]. The phenomenon of stress concentration will result in fatigue cracks and the failure of an object or component. In this case, Al-Merbati et al. [16] proposed that thermal efficiency is improved for certain geometric configuration of the device. Yilbas et al. [17] investigated the impact of tapered and rectangular pin configurations on thermal stress in thermoelectric generators, and found that thermal stress developed in tapered configuration attains lower values than that of rectangular cross-section. Wang et al. [18] focused on studying the feasibility of an X-type thermoelectric module with different draft angles. They found that the X-type structure can enhance the performance of the thermoelectric module with regard to both electrical power and mechanical reliability. The effect of soldering thickness has been researched by Wu et al. [19], and they proposed choosing a suitable tin soldering thickness will not only alleviate thermal stress intensity in the module, but also increase thermal efficiency. All in all, unreasonable geometric structure is a main reason for high local stress, which will degrade the module performance and lifespan [[20], [21], [22]]. Geometry optimization, such as rounded or chamfered corners and conductor with optimal thickness, is a good method to solve problem of stress concentration.

In this work, an U-type unileg thermoelectric generator, combining a unileg structure with pn-junction, is proposed and investigated to relieve the crucial problem of high thermal stress and short lifespan caused by CTE difference or high heat flux. Moreover, the geometric parameters, such as rounded corners (ru, rl), thickness of Ag layer (HAg), and length of right legs (LR), are optimized to further relieve stress concentration. The results showed that the U-type unileg thermoelectric module has excellent mechanical strength and long lifespan, which would be highly beneficial for commercializing high-temperature thermoelectric devices.

Section snippets

Numerical model

The schematic diagram of the U-type unileg thermoelectric module with ru = rl = 0 mm, as an example, is constructed and drawn by COMSOL and SOLIDWORKS in Fig. 1(c). The designed thermoelectric module is assembled with Al2O3 plates, Ag electrodes, and Dy-doped CaMnO3 unileg with an Ag-coated side. The dimensions (lx × lx × lz) of upper and lower Al2O3 plates are 17 × 2 × 0.1 mm3 and 19 × 2 × 0.1 mm3, respectively. The electrodes, with 5 × 2 × 0.027 mm3 size are used to connect the unilegs and Al2

The performance of the U-type unileg module

Fig. 2 shows the temperature and voltage distributions of the U-type unileg module without rounded corners at Qh = 6 W and Tc = 300 K under open-circuit conditions. As presented in Fig. 2(a), the temperature fluctuates up and down along x-axis, while maintained unchanged along y-axis, and monotonously increased along z-axis. It means that the heat flux flow from the hot-side substrate to the left legs and Ag-coated right legs, and then to the lower electrodes and cold-side substrate. To gather

The effect of the radius of round corners

As observed in the previous results, the novel U-type unileg structure is beneficial for relieving stress intensity and enhancing mechanical properties; however, it is found that sharp change in local structural configuration also results in heavy stress concentration. Thus, in order to minimize stress and enlarge lifespan, round chamfering has been considered, and presented in Fig. 6(a). In this figure, ru, and rl are the fillet radius of upper and lower corners, respectively. Fig. 6(b) and (c)

The influence of geometric size

As above mentioned, geometric size is an essential factor for the location of the maximum stress and the level of thermal stress intensity. For the U-type unileg structured module, the Ag-coating right leg as conductive wire has more complex structure, which has great influence on module output and mechanical performance. Hence, in this section, the influence of right leg length (LR), and Ag-coating layer thickness (HAg), as the two most important geometric factors, on the performance and

Conclusion

A CaMnO3-based U-type thermoelectric module combining unileg structure and pn-junction has been for the first time, in the best of our knowledge, proposed and investigated in this paper. In this structure, right leg is practically short-circuited through coating it with conductive material, acting as an electrical conductor for the unileg structure. The proposed structure avoids the module failure due to high thermal stress, produced at its hot side, between the thermoelectric materials and the

Credit author statement

Xue Wang: Conceptualization, Data curation, Investigation, Formal analysis, Visualization, Writing – original draft preparation. Hongchao Wang: Conceptualization, Supervision, Project administration, Writing-Reviewing and Editing. Wenbin Su: Resources. Tingting Chen: Software. Chang Tan: Validation. María A. Madre: Validation. Andres Sotelo: Conceptualization, Writing-Reviewing and Editing. Chunlei Wang: Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The work is financially supported by National Key R&D Program of China of 2017YFE0195200, the Natural Science Fund of China under grant Nos. 51871134 and 52111530034, the Science Fund of Shandong Province under grant No. ZR2019MEM007, and Qilu Young Scholar Program of Shandong University. M. A. Madre, and A. Sotelo also acknowledge the MINECO-FEDER (MAT2017-82183-C3-1-R) and Gobierno de Aragón-FEDER (Research Group T54-17R) for funding.

References (34)

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