Methodological reviews and analyses on the emerging research trends and progresses of thermoelectric generators

Thermoelectric generator is among the earliest initiated electricity‐harvesting methods. It is a very potential power harvester that can convert wasteful thermal energy into electricity. However, it often suffers from low energy conversion rate due to its inconsistent heat source, inefficient thermoelectric material (or thermoelement) performance, and incompetent structural issues. Progressively for the first time, detailed methodological surveys and analyses are made for bulk, thick, and thin films in this review. This is in order to accommodate better insights and comprehensions on the emerging trends and progresses of thermoelectric generators from 1989 to 2017. The research interests in thermoelectric generators have started back in 1989, and have continuously experienced emerging progresses in the number of studies over the last years. The methodological reviews and analyses of thermoelectric generator showed that almost 46.6% of bulk and 46.1% of thick and thin film research works, respectively, are actively progressed in 2014 to 2017. Nearly 86.2% of bulk and 44.1% of thick and thin film thermoelectric generators are realizing in between 0.001 and 4 μW cm^?2 K^?2, while 43.1% of thick and thin films are earning among 10^?6 to 0.001 μW cm^?2 K^?2. The highest achievement made until now is 2.5 W cm^?2 at a temperature difference of 140 K and thermoelectric efficiency factor of 127.55 μW cm^?2 K^?2. This achievement remarked positive elevation for the field and interest in thermoelectric power generation. Consecutively, the research trends of fundamental devices' structure, thermoelement, fabrication, substrate, and heat source characteristics are analyzed too, along with the desired improvement highlights for the applications of thermoelectric generators.     

Enhanced thermoelectric properties of PEDOT:PSS composites by functionalized single wall carbon nanotubes

This contribution investigates the utilization of carboxylic acid and hydroxyl functionalized single wall carbon nanotubes (SWNTs) for enhancing thermoelectric (TE) performances of the composites prepared with an inherently conductive polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Our results indicated an atypical thermoelectric property which is a simultaneous increase in electrical conductivity and the Seebeck coefficient which leads a greater power factor (PF) up to 22 μW m⁻¹ K⁻² while it is only 0.7 μW m⁻¹ K⁻² for the sample prepared with pristine SWNT. The functionalities on the carbon nanotube walls facilitate blend dispersion in aqueous media without requiring any surfactants and also enhance PFs as a result of improved charge transport routes. The improvement in the thermoelectric performance can be ascribed to hydrogen bonds with the -COOH or -OH groups on the nanotube walls which help induce the polymer chains to adopt an extended conformation.     

Phonon phase stability,structural, mechanical,electronic, and thermoelectric properties of two new semiconducting quaternary Heusler alloys CoCuZrZ (Z = Ge and Sn)

For meeting the energy demand, the development of new and novel thermoelectric (TE) materials for power generation is very vital. In this draft, we have theoretically investigated two new quaternary CoCuZrZ (Z = Ge and Sn) Heusler alloys for their structural, mechanical, electronic, and TE properties. In the energy minimization process, the alloys are found to be non-magnetic in the ground state. Based on calculated phonon dispersion curves, formation energy, and elastic constants, we propose that both CoCuZrGe and CoCuZrSn are stable. Furthermore, the mechanical properties indicate that CoCuZrGe (CoCuZrSn) has a brittle (ductile) nature. The electronic properties examined in Perdew-Burke-Ernzerhof (PBE), PBEsol, and modified Becke-Johnson (mBJ) potential, all predict that reported systems are narrow-gap semiconductors (SCs). In addition, the temperature dependent TE properties have been studied by calculating the electronic thermal conductivity (κ), Seebeck coefficient (S), power factor (PF) and electrical conductivity (σ/τ). The obtained positive value of S conveys the materials as p-type SCs, with a maximum value of 26.2 μV/K for CoCuZrGe and 28 μV/K for CoCuZrSn. The σ/τ, κ, and PF show increasing trends with rising temperature. The PF is found to be 1.55 × 10¹² WK⁻²m⁻¹s⁻¹ for CoCuZrGe and 1.38 × 10¹² WK⁻²m⁻¹s⁻¹ for CoCuZrSn. The proposed semiconducting Heusler alloys may receive attention for a range of TE and spintronic applications.     

Optimized thermal coupling of micro thermoelectric generators for improved output performance

There is a significant push to increase the output power of thermoelectric generators (TEGs) in order to make them more competitive energy harvesters. The thermal coupling of TEGs has a major impact on the effective temperature gradient across the generator and therefore the power output achieved. The application of micro fluidic heat transfer systems (μHTS) can significantly reduce the thermal contact resistance and thus enhance the TEG's performance. This paper reports on the characterization and optimization of a μTEG integrated with a two layer μHTS. The main advantage of the presented system is the combination of very low heat transfer resistances with small pumping powers in a compact volume. The influence of the most relevant system parameters, i.e. microchannel width, applied flow rate and the μTEG thickness on the system's net output performance are investigated. The dimensions of the μHTS/μTEG system can be optimized for specific temperature application ranges, and the maximum net power can be tracked by adjusting the heat transfer resistance during operation. A system net output power of 126 mW/cm² was achieved with a module ZT of 0.1 at a fluid flow rate of 0.07 l/min and an applied temperature difference of 95K.It was concluded that for systems with good thermal coupling, the thermoelectric material optimization should focus more on the power factor than on the figure of merit ZT itself, since the influence of the thermal resistance of the TE material is negligible.     

Investigation of solar hybrid system with concentrating Fresnel lens,photovoltaic and thermoelectric generators

An experimental model of a solar hybrid system including photovoltaic (PV) module, concentrating Fresnel lens, thermoelectric generator (TEG), and running water heat extracting unit was created and studied. The PV module used was of c‐Si and TEG of Bi₂Te₃; the Fresnel lens (solar concentrator) and TEG share an optical train, whereas PV module was illuminated separately with non‐concentrated light. Heat extracting unit operated in thermo siphon mode. In climatic conditions of Mexico (Queretaro, 20^o of North latitude, summer time), the Fresnel lens accepted 120 W of solar radiation power, and the system generated 7.0 W of electric power and 30 W of thermal one. The discussion is made of the possible characteristics of a hypothetical hybrid system where all its elements share the same optical train. Copyright © 2016 John Wiley & Sons, Ltd.     

Design and characterization of a cylindrical,water‐cooled heat sink for thermoelectric air‐conditioners

Thermoelectric air‐conditioners (TEACs) are becoming much concerned due to their many advantages, but the low COPs limit their broad applications. The two key factors to raise the COPs of TEACs are both the improvement of thermoelectric materials and the optimum design of hot side heat sinks. This paper provides a thermoelectric air‐conditioning system with a water‐cooled sleeve heat sink in the hot side of the thermoelectric pellets, and compares the overall heat transfer rates q_t, the total heat resistances R_t between the water‐cooled and air‐cooled heat sinks as well as the optimum fin length, the optimum fluid flow velocity and the optimum fin gap distance. The simulation results show that the overall heat transfer rate of water‐cooled heat sink is more than 20 times that of air‐cooled heat sink under the other same circumstances, as a result of the improvement of heat sink, the optimum COP of the thermoelectric air‐conditioning system with the water‐cooled heat sink proximately doubles that with the air‐cooled heat sink. This novel system could be simply installed and applied all the year round for cooling in summer and heating in winter. Copyright © 2005 John Wiley & Sons, Ltd.     

Experimental evaluation of electrode conversion efficiency of alkali metal thermal to electric converter

The alkali metal thermal to electric converter (AMTEC) system which utilizes the sodium ion conductivity of a beta″‐alumina solid electrolyte (BASE) is expected to have high conversion efficiency above 30% including practical heat losses. However, the achieved experimental efficiencies have been around 15%. In this paper, current–voltage characteristics and heat and mass transfer processes on a single cell have been examined experimentally and thermal electrode conversion efficiency has been discussed. Measured electrode conversion efficiency without thermal losses showed that it was about 40% at a power density of 0.3 W/cm². A theoretical analysis on the thermal losses has also been conducted and these losses are estimated to be 0.3 W/cm² in a practical tube type cell, so that an actual cell system efficiency of 30% is expected. © 2001 Scripta Technica, Heat Trans Asian Res, 30(3): 234–244, 2001     

Properties of Ag-doped Bi-Sb alloys as thermoelectric conversion materials for solid state refrigeration

The energy conversion properties of Bi-Sb system thermoelectric materials doped by Ag was investigated. Bi₈₅Sb_{15 − x} Ag _x (x = 0, 1, 2, 3, 4) alloys with Ag substitution for Sb were synthesized by mechanical alloying and then pressed under 5 GPa at 523 K for 30 min. The phase structure of the alloys was characterized by Xray diffraction. The electric conductivities and the Seebeck coefficients were measured at the temperature range of 80–300 K. The results reveal that the electric conductivities of the Ag-doped Bi-Sb alloys are highly improved. The power factor of Bi₈₅Sb₁₄Ag₁ reaches a maximum value of 2.98×10⁻³ W/(K²·m) at 255 K, which is about three times that of the un-doped sample Bi₈₅Sb₁₅ at the same temperature.     

MODELING AND OPTIMIZATION OF SINGLE-ELEMENT BULK SiGe THIN-FILM COOLERS

Abstract

Modeling and optimization of bulk SiGe thin-film coolers are described. Thin-film coolers can provide large cooling power densities compared to commercial thermoelectrics. Thin-film SiGe coolers have been demonstrated with maximum cooling of 4°C at room temperature and with cooling power density exceeding 500 W/cm². Important parameters in the design of such coolers are investigated theoretically and are compared with experimental data. Thermoelectric cooling, joule heating, and heat conduction are included in the model as well as non-ideal effects such as contact resistance, geometrical effects, and three-dimensional thermal and electrical spreading resistance of the substrate. Simulations exhibit good agreement with experimental results for bulk Si and SiGe thin-film coolers. It turned out that in many spot cooling applications using two n- and p-elements electrically in series and thermally in parallel does not give significant improvement over single leg elements. This is in contrast to conventional thermoelectric modules and is due to the aspect ratio and special geometry of thin film coolers. With optimization of SiGe thin-film cooler, simulations predict it can provide over 16°C with cooling power density of over 2000 W/cm².     

Efficient photocatalytic H2 production using visible‐light irradiation and (CuAg)xIn2xZn2(1 − 2x)S2 photocatalysts with tunable band gaps

The band structures of semiconductor photocatalysts fundamentally determine the photocatalytic activity and the H₂ production from the visible‐light‐driven water‐splitting reaction. We synthesize a suite of multicomponent sulfide photocatalysts, (CuAg)_xIn_2xZn_{2(1 ? 2x)}S₂ (0 ≤ x ≤ 0.5), with tunable band gaps and small crystallite sizes to produce H₂ using visible‐light irradiation. The band gap of the photocatalysts decreases from 3.47 eV to 1.51 eV with the increasing x value. The (CuAg)_0.15In_0.3Zn_1.4S₂ (x = 0.15) photocatalyst yielded the highest photocatalytic activity for H₂ production owing to the broad visible‐light absorption range and suitable conduction band potential. Under the optimized reaction conditions, the highest H₂ production rate is 230 µmol m^?2 h^?1 with a visible‐light irradiation of 2.7 × 10^?5 einstein cm^?2 s^?1, and the quantum yield reaches 12.8% at 420 ± 5 nm within 24 h. Furthermore, the photocatalytic H₂ production is shown to strongly depend on their band structures, which vary with the elemental ratios and could be analyzed by the Nernst relation. Copyright © 2014 John Wiley & Sons, Ltd.     

A study on Sn ion implantation into lead telluride thermoelectric material

Polycrystalline PbTe samples have been implanted by Sn⁺ ion at an energy of 200 KeV with doses of 6×10¹⁶ and 1×10¹⁷ ions/cm² in order to create a Pb_1−xSn_xTe layer with higher carrier concentration. The thermoelectric properties of the implanted and unimplanted samples have been measured at room temperature. The effects of Sn⁺ ion implantation on the structure of PbTe were investigated using XRD and XPS. The results show that Pb_1−xSn_xTe phase has been created in the surface layer after Sn⁺ ion implantation, which can modify the thermoelectric properties of PbTe.     

Parametric study on the thermoelectric conversion performance of a concentrated solar‐driven thermionic‐thermoelectric hybrid generator

A concentrated solar‐driven thermionic‐thermoelectric hybrid generator composed of solar heat collector, thermionic generator (TIG), thermoelectric generator (TEG), and radiator is introduced in this paper. A theoretical model of thermoelectric conversion performance for the hybrid generator is built up based on the heat source of the concentrated solar radiation rather than isothermal heat source. Based on the model, the impacts of related parameters on the internal temperature distributions, output power, and efficiency have been discussed. Moreover, the optimal operating conditions of the TIG‐TEG hybrid device at its maximum output power and efficiency have been determined. Results show that when cascading the TEG with the TIG, there is very little change of the TIG cathode temperature in most conditions, namely, T_C ≈ T_C′. Meanwhile, the anode temperature becomes higher, and the TEG cold end temperature T₂ is close to the anode temperature T_A′ for the single TIG system, ie, T_A > T_A′ ≈ T₂. In theory, the optimal concentrated solar radiation I₀ for the maximum output power P_max and the maximum efficiency η_max differs, which are I_0,P = 2.5 × 10⁶ W/m² and I_0,η = 2 × 10⁶ W/m², respectively, whereas the output power and efficiency of the TIG‐TEG hybrid system simultaneously reach their maximum values when the optimal TIG anode temperature T_A,opt = 1025 K, the optimal TIG output voltage V_opt = 2 V, and the optimal ratio of load resistance to internal resistance (R₂/R)_opt = 2. However, in practice, the parameter values of I₀, Φ_A, and T_A should be strictly controlled under 1.8 × 10⁶ W/m², 1.4 eV, and 660 K, respectively. Generally, the maximum output power and efficiency of the hybrid TIG‐TEG system are, respectively, 35% and 4% higher than that of the single TIG.     

Subamorphous Thermal Conductivity of Crystalline Half-Heusler Superlattices

The quest to improve the thermoelectric figure of merit has mainly followed the roadmap of lowering the thermal conductivity while keeping unaltered the power factor of the material. Ideally an electron-crystal phonon-glass system is desired. In this work, we report an extraordinary reduction of the cross-plane thermal conductivity in crystalline (TiNiSn):(HfNiSn) half-Heusler superlattices (SLs). We create SLs with thermal conductivities below the effective amorphous limit, which is kept in a large temperature range (120–300 K). We measured thermal conductivity at room temperature values as low as 0.75 W m^?1 K^?1, the lowest thermal conductivity value reported so far for half-Heusler compounds. By changing the deposition conditions, we also demonstrate that the thermal conductivity is highly impacted by the way the single segments of the SL grow. These findings show a huge potential for thermoelectric generators where an extraordinary reduction of the thermal conductivity is required but without losing the crystal quality of the system     

Optimization of multiple‐module thermoelectric coolers using artificial‐intelligence techniques

Genetic algorithm (GA) and simulated annealing (SA) methods were employed to optimize the current distribution of a cooler made up of a large number of thermoelectric (TE) modules. The TE modules were grouped into several clusters in the flow direction, and the electric currents supplied to different clusters were adjusted separately to achieve maximum energy efficiency or minimum refrigeration temperature for different operating conditions and cooling requirements. Optimization results based on the design parameters of a large TE cooler showed considerable improvements in energy efficiency and refrigeration temperature when compared to the results of uniform current for the parallel‐flow arrangement. On the other hand, results of the counter‐flow arrangement showed only slight differences between uniform‐ and non‐uniform‐current optimizations. The optimization results of GA and SA were very close to each other. SA converged faster and was more computationally economical than GA for TE system optimization. Copyright © 2002 John Wiley & Sons, Ltd.     

Investigation of thermoelectric generators connected in different configurations for micro‐grid applications

This research work investigates the power‐current (P‐I) and voltage‐current (V‐I) characteristics of the thermoelectric modules (TEMs) in series‐parallel configurations under homogeneous and heterogeneous temperature difference (ΔT) condition. To study its performance, 5 different series‐parallel combinations were formed using 16 TEMs. The comparisons among the different configurations have been done to determine the optimal series‐parallel configuration. The total load power extracted from 16 individually connected TEMs was 18.2 W, which was placed as a reference load power. The optimal series‐parallel combination for maximizing the load power is square series‐parallel configuration, whose maximum load power is 95.5%, compared to the reference load power. Moreover, in square series‐parallel configuration, the total internal resistance value that remains constant is equal to the internal resistance of a single TEM, and the total open‐circuit voltage increases gradually on adding any number of TEMs. Thus, it produces higher load voltage and higher load current simultaneously, which is recommended to power DC micro‐grid applications. Furthermore, the series, parallel, and square series‐parallel configurations are connected as star to obtain 3 separate DC output to power the same application. The performance of TEMs under various configurations is analyzed, and the obtain results are verified experimentally.     

Effects of the cross‐sectional area ratios and contact resistance on the performance of a cascaded thermoelectric generator

Thermoelectric generator offers many advantages such as high durability, environmental protection, and high reliability. Since the geometric optimization of a thermoelectric generator is a proper way to improve the performance, this study considers the multiparameter optimization of thermoelectric generators to investigate the effect of cross‐sectional area ratios and contact resistance on the performance of thermoelectric generator. Here, a ∏ ‐type cascaded thermoelectric system with two stages including two p‐legs and two n‐legs is examined numerically by ANSYS Workbench. The effectiveness and efficiency are obtained for different electric contact resistances. The results show that with a decreasing of the electric contact resistance, the efficiency and effectiveness are increased. Also, higher output power and efficiency of the TEG device are observed with a suitable ratio of cross‐sectional area.     

Performance effect on the TEG system for waste heat recovery in automobiles using ZnO and SiO2 nanofluid coolants

Enhancement in heat transfer of the cold side is vital to amplify the performance of a thermoelectric generator (TEG). With enriched thermophysical properties of nanofluids, significant improvement in heat transfer process can be obtained. The current study concerns the performance comparison of an automobile waste heat recovery system with EG‐water (EG‐W) mixture, ZnO, and SiO₂ nanofluid as coolants for the TEG system. The effects on performance parameters, that is, circuit voltage, conversion efficiency, and output power with exhaust inlet temperature, the total area of TEG, Reynolds number, and particle concentration of nanofluids for the TEG system have been investigated. A detailed performance analysis revealed an increase in voltage, power output, and conversion efficiency of the TEG system with SiO ₂ nanofluid, followed by ZnO and EG‐W coolants. The electric power and conversion efficiency for SiO ₂ nanofluid at an exhaust inlet temperature of 500K were enhanced by 11.80% and 11.39% respectively, in comparison with EG‐W coolants. Moreover, the model speculates that an optimal total area of TEGs exists for the maximum power output of the system. With SiO ₂ nanofluid as a coolant, the total area of TEGs can be diminished by up to 34% as compared with EG‐W, which brings significant convenience for the placement of TEGs and reduces the cost of the TEG system.     

Feasibility of palm‐biodiesel fuel for a direct internal reforming solid oxide fuel cell

The feasibility of a direct internal reforming (DIR) solid oxide fuel cell (SOFC) running on wet palm‐biodiesel fuel (BDF) was demonstrated. Simultaneous production of H₂‐rich syngas and electricity from BDF could be achieved. A power density of 0.32 W cm^?2 was obtained at 0.4 A cm^?2 and 800 °C under steam to carbon ratio of 3.5. Subsequent durability testing revealed that a DIR‐SOFC running on wet palm‐BDF exhibited a stable voltage of around 0.8 V at 0.2 A cm^?2 for more than 1 month with a degradation rate of approximately 15 % / 1000 h. The main cause of the degradation was an increase in the ohmic resistance. Copyright © 2012 John Wiley & Sons, Ltd.     

Parametric study of asymmetric thermoelectric devices for power generation

Thermoelectric devices are considered a promising technique for recycling waste heat. In the present work, a three-dimensional numerical model is developed to study the output performance of thermoelectric devices. A comprehensive analysis is performed based on a conventional π-type thermoelectric couple. The results indicate that the maximum power of thermoelectric devices generally increases with a decrease in height and an increase in cross-sectional area; the maximum efficiency exhibits the opposite trends. The best way to reduce heat losses is by using ceramic plates with higher thermal conductivity. Moreover, the parasitic internal resistance exists in the thermoelements, and its influencing factors are studied. To minimize electric losses, an asymmetric structure is proposed for thermoelectric devices. The results exhibit that the optimal cross-sectional area ratio of the p-type and n-type legs (S_p/S_n) is mainly contingent upon the thermoelectric material parameters; the greater the differences in the parameters of p-type and n-type thermoelectric materials, the greater the gains provided by the asymmetric structure. Furthermore, the experimental data present great consistency with the numerical results. The research results may help guide the design of thermoelectric devices with relatively lower power losses.     

Experimental performance of borehole heat exchangers and grouting materials for ground source heat pumps

The system performance of a ground source heat pump (HP) system is determined by the HP characteristics itself and by the thermal interaction between the ground and its borehole heat exchanger (BHE). BHE performance is strongly influenced by the thermal properties of the ground formation, grouting material, and BHE type. Experimental investigations on different BHE types and grouting materials were carried out in Belgium. Its performances were investigated with in situ thermal response tests to determine the thermal conductivity (λ) and borehole resistance (R_b). The line‐source method was used to analyze the results, and the tests showed the viability of the method. The main goal was to determine the thermal borehole resistance of BHEs, including the effect of the grouting material. The ground thermal conductivity was measured as 2.21 W m^?1 K^?1, a high value for the low fraction of water‐saturated sand and the high clay content at the test field. The borehole resistance for a standard coaxial tube with cement–bentonite grouting varied from 0.344 to 0.162 K W^?1 m for the double U‐tube with cement–bentonite mixture (52% reduction). Grouting material based on purely a cement–bentonite mixture results in a high thermal borehole resistance. Addition of sand to the mixture leads to a better performance. The use of thermally enhanced grouts did not improve the performance significantly in comparison with only a low‐cost grouting material as sand. Potential future applications are possible in our country using a mobile testing device, such as characteristics, standardization, quality control, and certification for drilling companies and ground source HP applications, and in situ research for larger systems. Copyright © 2011 John Wiley & Sons, Ltd.