Managing Heat Transfer Issues in Thermoelectric Microgenerators

Abstract

This chapter deals with heat transfer challenges in the microdomain. It focuses on practical issues regarding this matter when attempting the fabrication of small footprint thermoelectric generators (μTEGs). Thermoelectric devices are designed to bridge a heat source (e.g. hot surface) and a heat sink (e.g. ambient) assuring that a significant fraction of the available temperature difference is captured across the active thermoelectric materials. Coexistence of those contrasted temperatures in small devices is challenging. It requires careful decisions about the geometry and the intrinsic thermal properties of the materials involved. The geometrical challenges lead to micromachined architectures, which silicon technologies provide in a controlled way, but leading to fragile structures, too. In addition, extracting heat from small systems is problematic because of the high thermal resistance associated to heat exchanged by natural convection between the surrounding air and small bare surfaces. Forced convection or the application of a cold finger clearly shows the usefulness of assembling a heat exchanger in a way that is effective and compliant with the mechanical constraints of micromachined devices. Simulations and characterization of fabricated structures illustrate the effectiveness of this element integration and its impact on the trade-off between electrical and thermal behavior of the active materials in device performance.

Powering internet-of-things from ambient energy: a review

“…The quality of a TE material is determined by the factor ZT (ZT = σS 2 κ T; where σ, S, κ, and T are electrical conductivity, Seebeck coefficient, thermal conductivity, and the mean temperature between the hot, and the cold end, respectively). Therefore, it suggests that an appropriate design of the device for the propagation of heat flux from the hot to the cold end [41], and the use of high ZT materials are essential to achieve high-energy conversion efficiency. However, the output power is equally important as the efficiency for some miniature devices (such as IoT-based WSNs, micro-temperature sensors, etc.)…”

Roadmap on energy harvesting materials

“…In MEMS, Silicon micromachining can be used to fabricate silicon microplatforms able to convert vertical temperature gradients into internal lateral ones. These devices feature the so-called transversal architecture where low dimensional SiGe instances can be integrated flexibly [347], as shown in figure 59. Alternatively, entirely complementary-metal-oxide-semiconductor-compatible approaches can be used for on-chip thermoelectric generators replicating the π-architecture of standard modules [348].…”

Thermo-electro-optical properties of seamless metallic nanowire networks for transparent conductor applications

“…Transparent conducting electrodes (TCEs) are pivotal components for various thermo-1,2 and optoelectronic devices. 3 TCEs have opened up innovation for an extensive range of unprecedented applications, notably, organic light-emitting diodes (OLEDs), 4,5 solar cells, [6][7][8] touchscreens, 9,10 transparent heaters, [11][12][13][14] neurostimulators, 15 photodetectors, 16 smart watches, 17 thermoelectric generators, [18][19][20][21] to name but a few. TCEs made of metal nanowires, 22 carbon nanotubes, 23,24 conductive polymers, 25 and graphene 26,27 are competing materials against the canonical indium tin oxide (ITO) thin films due to its limited mechanical flexibility and high fabrication cost.…”