Current progress and future challenges in thermoelectric power generation: From materials to devices

In recent decades, by continuously enhancing the figure of merit ZT of various thermoelectric (TE) materials, solid state TE technology has matured and is on the verge of making an impact in real industrial settings as a promising approach to harvest waste industrial heat and convert it to useful electricity. Nevertheless, actual TE module development has remained stagnant with rather poor efficiencies. This has raised an urgent need to design rational module structures that rely on complex parameter optimization and utilization of efficient integration technologies that minimize energy losses during bonding of various interfaces. Here, we demonstrate a three-dimensional numerical analysis model of a segmented TE power-generating device, which takes into account the temperature-dependent materials' properties and various parasitic losses. The model generates an optimized design with predictive performance to realize maximum conversion efficiency. Combined with the developed bonding schemes and assembly techniques, the segmented modules consisting of Bi2Te3-based alloys and CoSb3-based filled skutterudites were successfully fabricated with a record-high efficiency of up to 12% when operating under a temperature difference of 541 °C. The rational structure design based on the numerical analysis model and the extremely low thermal and electrical losses enable the heat-to-electricity conversion efficiency to reach up to 96.9% of the theoretical efficiency based on the TE materials themselves. These findings highlight the importance of the optimization strategy for TE power generation devices based on the TE materials' intrinsic properties and demonstrate that realistic high temperature TE modules with predictive high efficiency and high power density can be fabricated, which provides a useful guide to achieve a high conversion efficiency in large-scale TE applications.

Realizing a thermoelectric conversion efficiency of 12% in bismuth telluride/skutterudite segmented modules through full-parameter optimization and energy-loss minimized integration

Key issues in development of thermoelectric power generators: High figure-of-merit materials and their highly conducting interfaces with metallic interconnects

Power harvesting devices which harness ambient surrounding energies to produce electricity could be a good solution for charging or powering electronic devices. The main advantages of such devices are that they are ecologically safe, portable, wireless, and cost effective and have smaller dimensions. Most of these power harvesting devices are realized by utilizing the microelectromechanical systems (MEMS) fabrication techniques. In this paper, the capabilities and efficiencies of four micro-power harvesting methods including thermoelectric, thermo-photovoltaic, piezoelectric, and microbial fuel cell renewable power generators are thoroughly reviewed and reported. These methods are discussed in terms of their benefits and applications as well as their challenges and constraints. In addition, a methodological performance analysis for the decade from 2005 to 2014 are surveyed in order to discover the methods that delivered high output power for each device. Moreover, the outstanding breakthrough performances of each of the aforementioned micro-power generators within this period are highlighted. From the studies conducted, a maximum energy conversion of 2500 mW cm −2 is reached by thermoelectric modules. Meanwhile, thermo-photovoltaic devices achieved a rise in system efficiency of up to 10.9%. Piezoelectricity is potentially able to reach a volumetric power density of up to 10,000 mW cm −3 . Significantly in microbial fuel cell systems, the highest power density obtained reached up to 6.86 W m −2 . Consequently, the miniaturized energy harvesters are proven to have credibility for the performance of autonomous power generation.

Micro-scale energy harvesting devices: Review of methodological performances in the last decade

We report promising thermoelectric properties of the rock salt PbSe–PbS system which consists of chemical elements with high natural abundance. Doping with PbCl2, excess Pb, and Bi gives n-type behavior without significantly perturbing the cation sublattice. Thus, despite the great extent of dissolution of PbS in PbSe, the transport properties in this system, such as carrier mobilities and power factors, are remarkably similar to those of pristine n-type PbSe in fractions as high as 16%. The unexpected finding is the presence of precipitates ∼2–5 nm in size, revealed by transmission electron microscopy, that increase in density with increasing PbS concentration, in contrast to previous reports of the occurrence of a complete solid solution in this system. We report a marked impact of the observed nanostructuring on the lattice thermal conductivity, as highlighted by contrasting the experimental values (∼1.3 W/mK) to those predicted by Klemens–Drabble theory at room temperature (∼1.6 W/mK). Our thermal con...

Thermoelectrics from Abundant Chemical Elements: High-Performance Nanostructured PbSe–PbS

The thermoelectric characteristics of commercial polycrystalline Mg2Si doped with Bi, Al + Bi, Ag, and Cu were examined. The samples for the thermoelectric measurements were prepared using the plasma-activated sintering (PAS) technique. The measured values of the Seebeck coefficient were compared with values calculated using the all-electron band-structure calculation package (ABCAP) based on a full-potential augmented-plane-wave (FLAPW) band-structure calculation in a local density approximation (LDA). For the Bi + Al-co-doped samples, the observed values of the dimensionless figure of merit, ZT, were higher than those of solely Bi-doped samples. The maximum value obtained for Bi + Al-doped Mg2Si was 0.77 at 862 K. For the Ag-doped samples, ZT was significantly lower than that of the Bi + Al-doped samples, with the maximum value being about 0.11 at 873 K.

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Thermoelectric Characteristics of a Commercialized Mg2Si Source Doped with Al, Bi, Ag, and Cu

The thermoelectric (TE) characteristics of Sb- and Al-doped n-type Mg2Si elemental devices fabricated using material produced from molten commercial doped polycrystalline Mg2Si were examined. The TE devices were prepared using a plasma-activated sintering (PAS) technique. To complete the devices, Ni electrodes were fabricated on each end of them during the sintering process. To realize durable devices for large temperature differences, thermodynamically stable Sb-doped Mg2Si (Sb-Mg2Si) was exposed to the higher temperature and Al-doped Mg2Si (Al-Mg2Si) was exposed to the cooler temperature. The devices consisted of segments of Sb-Mg2Si and Al-Mg2Si with sizes in the following ratios: Sb-Mg2Si:Al-Mg2Si = 4:1, 1:1, and 1:4. A device specimen composed solely of Sb-Mg2Si showed no notable deterioration even after aging for 1000 h, while some segmented specimens, such as those with Sb-Mg2Si:Al-Mg2Si = 1:1 and 1:4, suffered from a considerable drop in output current over the large ΔT range. The observed power generated by specimens with Sb-Mg2Si:Al-Mg2Si = 1:1 and 1:4 and sizes of 2 mm × 2 mm × 10 mm were 50.7 mW and 49.5 mW, respectively, with higher and lower temperatures of 873 K and 373 K, respectively. For the sample composed solely of Sb-Mg2Si, a power of 55 mW was demonstrated. An aging test for up to 1000 h for the same ΔT range indicated drops in output power of between ∼3% and 20%.

Thermoelectric Behavior of Sb- and Al-Doped n-Type Mg2Si Device Under Large Temperature Differences

Mg2Si unileg structure thermoelectric (TE) modules, which are composed only of n-type Mg2Si legs, were fabricated using Sb-doped Mg2Si. The Mg2Si TE legs used in our module were fabricated by a plasma-activated sintering method using material produced from molten commercial doped polycrystalline Mg2Si, and, at the same time, nickel electrodes were formed on the Mg2Si using a monobloc plasma-activated sintering technique. The source material used for our legs has a ZT value of 0.77 at 862 K. The TE modules, which have dimensions of 21 mm × 30 mm × 16 mm, were composed of ten legs that were connected in series electrically using nickel terminals, and the dimensions of a single leg were 4.0 mm  × 4.0 mm × 10 mm. From evaluations of the measured output characteristics of the modules, it appeared that the electrical resistance of the wiring that is used to connect each leg considerably affects the power output of the unileg module. Thus, we attempted to reduce the wiring resistance of the module and fabricated a module using copper terminals. The observed values of the open-circuit voltage and output power of the Sb-doped Mg2Si unileg module were 496 mV and 1211 mW at ΔT = 531 K (hot side: 873 K; cool side: 342 K).

Development of an Mg2Si Unileg Thermoelectric Module Using Durable Sb-Doped Mg2Si Legs

The electrical and thermoelectric characteristics of n-type Mg2Si equipped with electrodes of Ni and the transition-metal silicides CoSi2, CrSi2, TiSi2, and NiSi were examined. To form the electrodes on the Mg2Si matrix, a monobloc sintering method, i.e., simultaneous sintering of the electrode material during Mg2Si sintering, was used. To obtain dense electrodes and to keep an appropriately low sintering temperature for the Mg2Si matrix, a Ni binder was used for the CoSi2, CrSi2, and TiSi2 monobloc sintering. The mixture ratio between the transition-metal silicide and the Ni was 50:50 in wt.%. The room-temperature I–V characteristics of the fabricated CoSi2, CrSi2, and TiSi2 electrodes with the Ni binder and NiSi electrodes were considered to be adequate for practical applications in as much as ohmic contacts were obtained. The contact resistance at the Mg2Si/electrode interface decreased by 35% and 28%, respectively, for the CoSi2 and CrSi2 electrodes compared with our standard Ni electrode. The thermoelectric power output was measured at the practical operating temperature of 600 K, with ΔT = 500 K. The observed output powers for 3.0 mm × 3.0 mm × 7.5 mm samples equipped with CoSi2, CrSi2, and NiSi electrodes were 153 mW, 149 mW, and 125 mW, respectively, representing increases of 27%, 24%, and 4%, respectively, compared with the 120 mW measured for the sample with Ni electrodes.

The Use of Transition-Metal Silicides to Reduce the Contact Resistance Between the Electrode and Sintered n-Type Mg 2 Si

Mg2Si thermoelectric (TE) elements were fabricated by a plasma-activated sintering method using a commercial polycrystalline n-type Mg2Si source produced by the Union Material Co., Ltd. This material typically has a ZT value of ∼0.6. A monobloc plasma-activated sintering technique was used to form Ni electrodes on the TE elements. The dimensions of a single element were 4.0 mm × 4.0 mm × 10 mm, and these were used to construct a TE module comprising nine elements connected in series. To reduce the electrical and thermal contact resistance of the module, each part of the module, i.e., the elements, terminals, and insulating plates, was joined using a Ag-based brazing alloy. In addition, to maintain the temperature difference between the top and bottom of the module, a thermal insulation board was installed in it. The observed values of open-circuit voltage (VOC) and output power (P) of a uni-leg structure module were 594 mV and 543 mW, respectively, at a maximum ΔT = 500 K.

Tatsuya Sakamoto

Papers

Thermoelectric Characteristics of a Commercialized Mg2Si Source Doped with Al, Bi, Ag, and Cu

Thermoelectric Behavior of Sb- and Al-Doped n-Type Mg2Si Device Under Large Temperature Differences

Development of an Mg2Si Unileg Thermoelectric Module Using Durable Sb-Doped Mg2Si Legs

The Use of Transition-Metal Silicides to Reduce the Contact Resistance Between the Electrode and Sintered n-Type Mg 2 Si

Power Generation Characteristics of Mg2Si Uni-Leg Thermoelectric Generator