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Table of Contents

Cover

Title Page

About the Editors

Series Editors' Preface

List of Contributors

Chapter 1: Utilizing Phase Separation Reactions for Enhancement of the Thermoelectric Efficiency in IV–VI Alloys

1.1 Introduction
1.2 IV–VI Alloys for Waste Heat Thermoelectric Applications
1.3 Thermodynamically Driven Phase Separation Reactions
1.4 Selected IV–VI Systems with Enhanced Thermoelectric Properties Following Phase Separation Reactions
1.5 Concluding Remarks
References

Chapter 2: Nanostructured Materials: Enhancing the Thermoelectric Performance

2.1 Introduction
2.2 Approaches for Improving ZT
2.3 Recent Progress in Developing Bulk Thermoelectric Materials
2.4 Bulk Nanostructured Thermoelectric Materials
2.5 Outlook and Challenges
Acknowledgement
References

Chapter 3: Organic Thermoelectric Materials

3.1 Introduction
3.2 Seebeck Coefficient and Electronic Structure
3.3 Seebeck Coefficient and Charge Carrier Mobility
3.4 Optimization of the Figure of Merit
3.5 N-Doping of Conjugated Polymers
3.6 Elastic Thermoelectric Polymers
3.7 Conclusions
Acknowledgments
References

Chapter 4: Silicon for Thermoelectric Energy Harvesting Applications

4.1 Introduction
4.2 Bulk and Thin-Film Silicon
4.3 Nanostructured Silicon: Physics of Nanowires and Nanolayers
4.4 Bottom-Up Nanowires
4.5 Material Properties and Thermoelectric Efficiency
4.6 Top-Down Nanowires
4.7 Applications of Bulk and Thin-Film Silicon and SiGe Alloys to Energy Harvesting
4.8 Applications of Nanostructured Silicon to Energy Harvesting
4.9 Summary and Outlook
Acknowledgments
References

Chapter 5: Techniques for Characterizing Thermoelectric Materials: Methods and the Challenge of Consistency

5.1 Introduction – Hitting the Target
5.2 Thermal Transport in Gases and Solid-State Materials
5.3 The Combined Parameter ZT-Value
5.4 Summary
Acknowledgments
References

Chapter 6: Preparation and Characterization of TE Interfaces/Junctions

6.1 Introduction
6.2 Effects of Electrical and Thermal Contact Resistances
6.3 Preparation of Thermoelectric Interfaces
6.4 Characterization of Contact Resistance Using Scanning Probe
6.5 Characterization of Thermal Contact Using Infrared Microscope
6.6 Summary
Acknowledgments
References

Chapter 7: Thermoelectric Modules: Power Output, Efficiency, and Characterization

7.1 Introduction
7.2 The Governing Equations
7.3 Power Output and Efficiency
7.4 Characterization of Devices
References

Chapter 8: Integration of Heat Exchangers with Thermoelectric Modules

8.1 Introduction
8.2 Heat Exchanger Design – Consideration in TEG Systems
8.3 Cold Side Heat Exchanger for TEG Maximum Performance
8.4 Cooling Technologies and Design Challenges
8.5 Microchannel Heat Exchanger
8.6 Coupled and Comprehensive Simulation of TEG System
8.7 Power–Efficiency Map
8.8 Section Design Optimization in TEG System
8.9 Conclusion
Acknowledgment
Nomenclature
References

Chapter 9: Power Electronic Converters and Their Control in Thermoelectric Applications

9.1 Introduction
9.2 Building Blocks of Power Electronics
9.3 Power Electronic Topologies
9.4 Electrical Equivalent Circuit Models for Thermoelectric Modules
9.5 Maximum Power Point Operation and Tracking
9.6 Case Study
9.7 Conclusion
References

Chapter 10: Thermoelectric Energy Harvesting for Powering Wearable Electronics

10.1 Introduction
10.2 Human Body as Heat Source for Wearable TEGs
10.3 TEG Design for Wearable Applications: Thermal and Electrical Considerations
10.4 Flexible TEGs: Deposition Methods and Thermal Flow Design Approach
10.5 TEG Integration in Wearable Devices
10.6 Strategies for Performance Enhancements and Organic Materials
References

Chapter 11: Thermoelectric Modules as Efficient Heat Flux Sensors

11.1 Introduction
11.2 Applications of Thermoelectric Modules
11.3 Parameters of Thermoelectric Heat Flux Sensors
11.4 Self-Calibration Method of Thermoelectric Heat Flux Sensors
11.5 Sensor Performance and Thermoelectric Module Design
11.6 Features of Thermoelectric Heat Flux Sensor Design
11.7 Optimization of Sensors Design
11.8 Experimental Family of Heat Flux Sensors
11.9 Investigation of Sensors Performance
11.10 Heat Flux Sensors at the Market
11.11 Examples of Applications
References

Chapter 12: Photovoltaic–Thermoelectric Hybrid Energy Conversion

12.1 Background and Theory
12.2 Different Forms of PVTE Hybrid Systems: The State of the Art
12.3 Optimizations of PVTE Hybrid Systems
12.4 Application of PVTE Hybrid Systems
12.5 Summary
References

Index

End User License Agreement

List of Illustrations

Chapter 1: Utilizing Phase Separation Reactions for Enhancement of the Thermoelectric Efficiency in IV–VI Alloys

Figure 1.1 ZT values of the most efficient IV–VI alloys recently published – the p-type: Pb_0.98Tl_0.02Te [1], Pb_0.99Na_0.01Te [2], Ge_0.87Pb_0.13Te [3], Ge_0.5Pb_0.25Sn_0.25Te [4], Pb_0.96Sr_0.04Te (2%Na) [5], Ag_0.9Pb₅Sn₃Sb_0.7Te₁₀ – LASTT [6], Na_0.95Pb₂₀SbTe₂₂ – SALT [7], Pb_0.97Mg_0.03Te:Na [8], PbTe_0.85Se_0.15:2%Na [9]; and n-type: AgPb₁₈SbTe₂₀ – LAST [10], PbTe (0.1 at%In) [11], PbI₂ doped PbTe with various carrier concentrations of 6.54 × 10¹⁸, 2.19 × 10¹⁹, 4.22 × 10¹⁹ and 6.09 × 10¹⁹/cm³ [11], K_0.95Pb₂₀Sb_1.2Te₂₂ – PLAT [12], and (Pb_0.95Sn_0.05Te)_0.92(PbS)_0.08:0.055 mol% PbI₂ [13].
Figure 1.2 Temperature dependence of the vapor pressures of various IV–VI alloys and the mostly volatile elements in these systems [15–19].
Figure 1.3 Compositional dependencies of the enthalpy, ΔH_m, entropy, −TΔS_m, terms of mixing (a), and Gibbs free energy of mixing, ΔG_m (b) for various temperatures; a phase diagram, built from the thermodynamic terms mentioned above, showing a miscibility gap between two components A and B in a binary mixture (c) and a representative phase separation microstructure showing a continuous variation of the concentration of the components A and B in the A–B binary mixture described above (d).
Figure 1.4 Quasi-binary PbTe−GeTe (a) [3] and PbTe−PbS (black curve [27], red curve [28], and blue curve – calculated for 100% Pb0.95Sn0.05Te instead of PbTe based on the PbTe−SnTe phase diagram [27]) and (b) phase diagrams showing a miscibility gap and a tendency for phase separation. The highly efficient (Pb_0.95Sn_0.05Te)_0.92(PbS)_0.08 and Ge_0.87Pb_0.13Te thermoelectric compositions are clearly indicated. For the PbTe−GeTe phase diagram the transition temperatures from rhombohedral (R) to cubic (C) structures are also indicated.
Figure 1.5 Quasi-ternary GeTe−PbTe−SnTe phase diagram showing a well-defined miscibility gap (a), as well as morphological (b), 3-D ZT (c), and 2-D ZT (d) variations upon thermal treatment at 400 °C for different durations for the p-type Ge0.5Pb0.25Sn0.25Te composition.

Chapter 2: Nanostructured Materials: Enhancing the Thermoelectric Performance

Figure 2.1 Schematic illustration of nanocomposite approach by nano-precipitates within grains (a), nano-lamella within grains (b), and heavy doping with embedded nanoinclusions (c).
Figure 2.2 The materials Figure of merit, ZT as a function of temperature for state-of-the-art bulk thermoelectric materials: (a) for p-type materials and (b) for n-type materials.
Figure 2.3 ZT versus temperature of polycrystalline bulk nanostructured samples of p-type Bi_0.4Sb_1.6Te₃ (a) and n-type Bi₂(Te,Se)₃ (b) prepared by spark plasma sintering (SPS). The properties were characterized perpendicular (in-plane) and parallel (out-plane) to the applied pressure direction. Inset Figure is a typical SEM image observed in-plane.
Figure 2.4 (a) SEM image for a typical nanocomposite of p-type Bi_0.4Sb_1.6Te₃ with 0.05% B-doped single wall carbon nanotube (SWCNTS) and 10% Te. (b) ZT as a function of temperature measured in-plane for nanocomposite as compared with a standard sample.
Figure 2.5 (a) Low-magnification TEM image with mesoscale grains and (b) ZT as a function of temperature for nanocomposite of PbTe−SrTe (4 mol%) doped with 2 mol% Na prepared by SPS.
Figure 2.6 (a) High resolution TEM image showing multi nano-arrays embedded in the matrix of HH. (b) ZT versus temperature of p-type Hf_0.5Zr_0.5CoSn_0.2Sb_0.8 SPS sintered HH alloy as compared with a similar composition of ingot and hot-pressed samples reported in Ref. [33].
Figure 2.7 ZT as a function of temperature for nanostructured p-type (a) and n-type (b) our skutterudite mateials sintering by spark plasma sintering as compared with hot pressed sample reported in Ref. [13]. (c) Photos showing a posible mass production of high performance skuterrudite materials by SPS.
Figure 2.8 (a) Power factor as a function of temperature for Ca_2.95Ag_0.05Co_3.9Ni_0.1O₉₊ nanocomposite. (b) STEM-EDX elements maping of the nanocomposite sample.
Figure 2.9 (a) ZT as a function of temperature and HRTEM image showing a spherical metallic nanoinclusion for heavy doped layered cobaltite nanocomposite. (b) Power factor and ZT at 1100 K under various heat treatment cycles.

Chapter 3: Organic Thermoelectric Materials

Figure 3.1 Chemical structure of few classes of organic (semi)conductors: (a) charge transfer salt TTF–TCNQ, (b) conjugated polymer P3HT, and (c) carbon nanotube, CNT.
Figure 3.2 Historical development of the thermoelectric power factor for various classes of emerging thermoelectric materials. Conducting polymers (blue), carbon nanotubes (red), hybrid inorganic–polymer composites (gray), and metal–organic polymers (yellow). p and n stand for positive and negative conduction. The text in parentheses is the value of ZT at 300 K and the reference from which the data are taken. Data sources: 2007 [13], 2010 [14]; 2011 [15], 2012 [16], 2013 blue point [17], 2013 red point [18], 2014 [19], 2015 red point [20], 2015 grey point [21], 2016 [22]. Figure inspired from [23].
Figure 3.3 (a) Chemical structure of a neutral polythiophene chain, and a chain that carries a bipolaron. At high doping level, the coupling between bipolaron wavefunctions can be either intra-chain or inter-chain, and this is at the origin of the creation a bipolaronic band. (b) Elecctronic structure of a polymer chain with (i) one bipolaron. Sketch of the logarithm of the density of state ln N(E) for an amorphous (ii) bipolaronic polymer solid with localized states around the Fermi level E_F; as well as for (iii) a semi-metallic network of bipolarons with the Fermi level lying in a delocalized band. The slope of ln N(E) at E_F is proportional to the Seebeck coefficient. (c) Seebeck coefficient versus electrical conductivity of various PEDOT derivatives (triangle) compared to Sb-doped Bi₂Te₃ (star) [27]. Further improvement in structural order (higher σ) should in principle result in even larger α and thus a higher thermoelectric power factor PF. For the sake of comparison, good thermoelectric material (nanostructured BiSbTe alloy) [28] display similar conductivity range as that of PEDOT, but a much higher Seebeck coefficient than PEDOT derivatives. Figure adapted from [27].
Figure 3.4 (a) Hopping charge transport between the metallic islands of the chains in an interchain (red) and intrachain (blue) level. (b) The Seebeck coefficient versus the relative mobility for PEDOT:Tos samples with various charge carrier mobilities but constant carrier concentration. The charge carrier mobilities were normalized with respect to the highest value in this study. [40]. (c) The radially averaged intensity of 2D-GIWAXS patterns for PEDOT:Tos samples of varying charge carrier mobility and constant charge carrier concentration. The degree of crystallinity increases with the area under the (1 0 0) peak. (d) The UPS spectra at lower binding energies for the PEDOT:Tos samples presented in Figure 3.4b, where the broadening of the band edge is observed with the increase of μ [41].
Figure 3.5 Seebeck coefficient, electrical conductivity, power factor σS², thermal conductivity, and Figure of merit (ZT) of PEDOT:Tos versus oxidation level at 300 K. (Khan et al. 2016 [27]. Reproduced with permission of Wiley.)
Figure 3.6 (a) Electrical conductivity of BBL and P(NDI2OD-T2) films as a function of doping time. (b) Computed spin density distributions of the BBL and P(NDI2OD-T2) oligomers. (c, d) Electrical conductivity σ and Seebeck coefficient S versus doping time for BBL and P(NDI2OD-T2). Adapted from [48].
Figure 3.7 (a) Photograph by Eliot Gomez of the PEDOT:PSS-silane aerogel. (Reproduced with kind permission of Eliot Gomez.) (b) Press–release for one cycle in several steps while the inset shows cyclic pressing and releasing. (c) I–V curves for the aerogel under various levels of pressures and temperature differences. The curves clearly show a change in slope with pressure and voltage shift with increased temperature difference. (Panels (b) and (c): Khan et al. 2016 [27]. Reproduced with permission of Wiley.)

Chapter 4: Silicon for Thermoelectric Energy Harvesting Applications

Figure 4.1 Seebeck coefficients of (a) p-type and (b) n-type single-crystalline silicon at various doping levels. (Geballe and Hull 1955 [29]. Reproduced with permission of American Physical Society.)
Figure 4.2 Thermal conductivity of single-crystalline silicon. (Glassbrenner and Slack 1964 [33]. Reproduced with permission of American Physical Society.)
Figure 4.3 Anomalous trends of the carrier density in heavily doped hot-pressed ncSi samples. Note the larger hole density at 10% at. nominal concentration. (Data from Vining 1991 [32].)
Figure 4.4 Seebeck coefficient, electrical conductivity, and power factor at 300 K of heavily boron-doped nanocrystalline silicon thin films upon annealing. (Narducci et al. 2015b [40]. Reproduced with permission of Royal Society of Chemistry.)
Figure 4.5 Scheme of the vapor–liquid–solid (VLS) mechanism for growing nanowires. (a) Gas precursor introduction into the chamber, (b) precursor adsorption and dissociation at the catalyst surface, (c) material diffusion across the particle, and (d) precipitation of nanowire material and grow of the nanowire.
Figure 4.6 Schemes of the three kind of silicon-metal phase diagrams.
Figure 4.7 Nanolithography techniques suitable for top-down fabrication of silicon nanowires: electron-beam lithography (a), ion-beam lithography or etching (b), nanoimprint lithography (c).
Figure 4.8 Post-lithography steps in top-down fabrication of silicon nanowires.
Figure 4.9 Top-down fabrication of stacked polycrystalline silicon nanowires.
Figure 4.10 Comparison of the layouts of the on-chip TEG developed by (a) Strasser et al. and (b) Xie et al. [110]. Note in both cases the cavity underneath the legs. In Xie's design additional cavities force the heat to flow out while preventing thermalization with the TEG. Drawing on the left reproduced with permission of Elsevier. ((a) Strasser et al. 2002 [27]. Reproduced with permission of Elsevier.)
Figure 4.11 (a) Sketch showing the design of a thermoelectric micro-generator. (b) Images of the resulting prototypes, showing details of the nanowire growing regions and nanowire structure. (Dávila et al. 2012 [115]. Reproduced with permission of Elsevier.)
Figure 4.12 Lateral TEG based on top-down fabricated stacked silicon nanowires.
Figure 4.13 Thermal flow in lateral TEG during harvesting.
Figure 4.14 Simplified thermal model of the TEG.
Figure 4.15 Vertical TEG based on top-down fabricated silicon nanowires.

Chapter 5: Techniques for Characterizing Thermoelectric Materials: Methods and the Challenge of Consistency

Figure 5.1 (a) The trueness is good but the precision is poor. (b) The precision is good but not the trueness. In both cases the accuracy is low.
Figure 5.2 All parts of the ZT-value depend on the carrier properties, especially the carrier concentration. Therefore, one will find an optimum carrier concentration level for a certain temperature where the ZT-value is at its maximum. Please keep in mind that this point is strongly temperature dependent; thus a material has to be optimized at its desired operation temperature.
Figure 5.3 (a) Four-wire-technique to measure the real resistance of a resistive element avoiding the resistance of the test wires $c05-math-047$ . (b) To measure the resistivity of a thin sheet one can use the four-point method. The sheet has to be thin and compared to the distance $c05-math-048$ between the contact points quasi infinite. Valdes and Smits calculated analytical formulas to measure the sheet resistance in other geometrical situations [10, 11].
Figure 5.4 Arrangement of the contact for the van der Pauw measurement of the specific resistance. For a complete measurement all four contact configurations have to be measured. If a direct current (DC) is used, all configurations have to be measured in both polarities.
Figure 5.5 Schematic contact arrangement to measure the electrical resistivity for standardization.
Figure 5.6 To measure thermoelectric materials beside the van der Pauw method three typical measurement arrangements are used. (a) Sample is lying on metallic block introducing the current into the sample, (b) sample is mechanically clamped between the current input blocks, and (c) the current is introduced through contact tips similar to the four-point method shown previously. All three arrangements are used to measure round, square, and bar-shaped samples.
Figure 5.7 In configuration (a) the sample is clamped between the two current inputs. Within one of the inputs a heater (H) is installed, which will introduce a temperature gradient on the sample for measuring the Seebeck coefficient. Configurations (b) and (c) use the same general arrangement for the thermoelements but different ways to introduce the current into the sample for a parallel measurement of the resistivity. In configuration (b) the current is introduced via the sample supports. Configuration (c) uses needles to introduce the current into the sample.
Figure 5.8 Schematic diagram of the LFA method. A disc sample is placed within a controlled thermal surrounding. An infrared laser pulse is irradiated onto the lower face of the sample. The time the heat pulse needs to propagate to the upper face of the sample is measured with an infrared detector.
Figure 5.9 Schematic view on the TDTR method. A laser pulse heats up the surface and a second probe pulse measures the reflectance of the surface, which is a direct indicator of the temperature of the surface. By measuring at different times after the pump pulse, the thermal conductivity can be derived.
Figure 5.10 Basic 3 Omega setup: An AC current is applied to a simple linear bolometer structure and the voltage drop is measured at two separate terminals. The lengths between the voltage terminal and the width and thickness of the strip heater have to be determined precisely.
Figure 5.11 Signals and their frequency components are important with the 3 Omega experiment. $c05-math-059$ is the thermal transfer function, which includes the thermal conductivity of the sample under the bolometer.
Figure 5.12 Schematic of the Harman method. A block current is applied to a sample, which will introduce a temperature gradient due to the Peltier effect. By measuring the voltage one may obtain ZT directly.

Chapter 6: Preparation and Characterization of TE Interfaces/Junctions

Figure 6.1 Schematic diagram of (a) an n-type and p-type thermoelement; (b) a thermocouple formed by soldering n-type and p-type thermoelements with copper layers; and (c) a thermoelectric module consisting of a number of thermocouples connected electrically in series and thermally in parallel. The contact interfaces formed in fabrication processes have a significant effect on the performances of thermoelectric devices.
Figure 6.2 Schematic diagram of (a) a thermoelement without contact layer; (b) a thermoelement with contact layers at the top and bottom ends.
Figure 6.3 A thermoelectric unijuniction for investigation of electrical and thermal contact resistance, consisting of two thermoelements, a copper contact layer, barrier layer, and brazing materials. (a) A schematic diagram; (b) a photograph of skutterudite/copper junction.
Figure 6.4 Sample holder (a) and furnace (b) for the assembly of thermoelectric unijunctions.
Figure 6.5 Circuit diagram of an apparatus for contact resistance measurement based on scanning probe.
Figure 6.6 Schematic of typical resistance profiles of thermoelectric unijunction: the bottom curve shows a perfect contact at interfaces; the middle curve shows the interfaces with noticeable contact resistance but no interdiffusion; and the top curve shows the interfaces with significant contact resistance and interdiffusion.
Figure 6.7 Photograph of the apparatus for contact resistance measurement based on scanning probe.
Figure 6.8 Skutterudite thermoelectric unijunction and its corresponding resistance profile obtained using the scanning resistance probe, showing a significant interdiffusion region extended into the thermoelectric materials, although the region cannot be seen on the photograph.
Figure 6.9 The resistance profiles of three skutterudite thermoelectric junctions, which were prepared at 654, 663, and 668 °C, respectively. No interdiffusion takes place for the junction prepared at 654 °C, which also exhibits a low contact resistivity. The junction prepared at 663 °C shows a slightly larger contact resistivity but with some degree of interdiffusion. The junction prepared at 668 °C shows significant interdiffusion and larger contact resistivity.
Figure 6.10 Schematic temperature profile across a thermoelectric unijunction with no contact influence (dashed line) and with contact resistance (solid line).
Figure 6.11 Thermal contact resistance measurement system (a) infrared microscope; (b) sample holder.
Figure 6.12 Temperature profiles obtained by infrared microscope of thermoelectric interfaces: (a) interface formed by pressure contact; (b) interface formed by soldering using Sn/Pb solders.

Chapter 7: Thermoelectric Modules: Power Output, Efficiency, and Characterization

Figure 7.1 Scheme of the structure and elements that form a thermoelectric module operating in power generation mode. N and P indicate the n- and p-type semiconductor material legs, respectively. Details of a junction are given in the circle. The plots at the sides represent the resistance and temperature profiles across the ceramic plates, electrodes, and thermoelectric material, whose lengths are L_C, L_TE, and L_M, respectively.
Figure 7.2 Scanning electron microscopy images of the junction between a thermoelectric leg and a copper electrode (a, c) before and (b, d) after thermal cycling. Cracks, voids, pores, and interdiffusion can be identified after device failure.
Figure 7.3 Scheme of steady-state temperature (thick line) and voltage (thin line) profiles in an n-type thermoelectric material of length L_TE under power generation operation contacted by two metallic contacts (dark gray) to an external load resistance R_L. The dashed line represents a chosen position where the drift and thermal-diffusion fluxes are indicated. Open-circuit and short-circuit conditions are presented in (a) and (b), respectively. The thin and thick vertical arrows indicate the potential and temperature difference across the material, respectively.
Figure 7.4 Illustration of the variation of the electrical current density at positions x and x + Δx at two different times (t and t + dt), achieved by the accumulation of one carrier in the AΔx volume of the material. Carriers are represented by circles and fluxes by arrows.
Figure 7.5 Scheme of the different heat fluxes (thin horizontal arrows) and heat absorption/generation (vertical thick arrows) processes occurring because of variations of the Seebeck coefficient S at different positions of an n-type thermoelectric material connected to metallic contacts under the flow of a positive electrical current density J. (a) Only Peltier effect takes place at the junctions (0 and L_TE) when the S is constant in the materials (no temperature difference). (b) Under a T gradient S varies inside the thermoelectric material, which produces heat generation in the bulk (Thomson effect), as occurs between points a and b and also indicated by the extended white thick arrows. Peltier effect also takes place at the junctions.
Figure 7.6 (a) Scheme of an n- and p-type semiconductor couple under power generation mode delivering electric power to a load (R_L). A scheme of the temperature (thick line) and voltage (thin line) profiles at open-circuit condition is given in (b) for positions a, b, and c. S_n and S_p are the average Seebeck coefficient values for the given temperature range for the n- and p-type materials, respectively.
Figure 7.7 Energy balances at the boundaries (positions 0 and L_TE) of a p-type leg under power generation mode without contacts consideration. The arrows pointing in and out of the boundaries indicate heat addition and heat removal, respectively.
Figure 7.8 Microscopic scheme of a thermal contact between two solids. Air gaps are predominant in the contacted area, which limits the heat transfer by conduction between the solids. Only a small fraction of the surfaces are actually contacted.
Figure 7.9 (a) Scheme of I–ΔV curves under constant ΔT (dashed line) and constant heat input (solid line) modes. (b) Steady-state equivalent circuits for both modes. A thermoelectric resistance R_TE in addition to the total ohmic resistance of the device R_Ω is included in the constant heat input mode, which accounts for the variations in ΔT that occur owing to the Peltier effect. A dashed square encloses the elements that define the thermoelectric module.

Chapter 8: Integration of Heat Exchangers with Thermoelectric Modules

Figure 8.1 Typical design of flat plate heat exchanger with parallel fins.
Figure 8.2 Variation of thermoelectrical properties of dissimilar thermoelectric materials [9, 10].
Figure 8.3 Cold junction temperature of the thermoelements and hot junction heat absorbed, $c08-math-039$ , $c08-math-040$ .
Figure 8.4 Maximum power generation and efficiency by a uni-couple with variation of the footprint ratio, $c08-math-044$ .
Figure 8.5 Temperature variation of the cold junction of the TEG thermoelements as a function of the flow direction, $c08-math-050$ . $c08-math-051$ .
Figure 8.6 TEG module with a cold side heat exchanger (cross-sectional view of the device).
Figure 8.7 Temperature difference contour of thermoelements and copper interfaces for two sample pressure drops, $c08-math-108$ and $c08-math-109$ .
Figure 8.8 Effect of the pressure drop in the microchannel heat exchanger on the created temperature difference of the cold and hot junctions of thermoelements.
Figure 8.9 Percentage of mass flow rate in channels and thermal energy removed by each channel for three sample pressure drops.
Figure 8.10 Modified channel configuration based on original heat exchanger design in Figure 8.6.
Figure 8.11 Thermal resistances of the heat exchangers versus cooling energy in the heat exchangers.
Figure 8.12 Configuration of an applied plate-fin heat exchanger (PFHX) and a cross-cut heat exchanger (CCHX) with front view of symmetric calculation domain.
Figure 8.13 Variation of the electric voltage generation and the required cooling energy with flow inlet velocity, $c08-math-126$ .
Figure 8.14 Variation of the TEG net power output with variation of the Reynolds number in the channels.

Chapter 9: Power Electronic Converters and Their Control in Thermoelectric Applications

Figure 9.1 System diagram of thermoelectric generator system.
Figure 9.2 Characteristics of passive electric components: resistor, inductor, and capacitor.
Figure 9.3 Characteristics of the transformer, diode, and switch.
Figure 9.4 (a) Circuit diagram of buck converter (top) during on-state (mid), and off-state (bottom) of the switch. (b) Logic gate signal (top), voltages (mid), and currents (bottom) during on and off states.
Figure 9.5 (a) Circuit diagram of boost converter (top) during on-state (mid), and off-state (bottom) of the switch. (b) Logic gate signal (top), voltages (mid), and currents (bottom) during on and off states.
Figure 9.6 (a) Circuit diagram of non-inverting buck–boost converter (top) during on-state in buck–boost mode (mid), and off-state in buck–boost mode (bottom) of the switch. (b) Logic gate signal (top), voltages (mid), and currents (bottom) during on and off states in buck–boost mode.
Figure 9.7 (a) Circuit diagram of flyback converter (top) during on-state (mid), and off-state (button) of the switch. (b) Logic gate signal (top), voltages (mid), and currents (bottom) during on and off states.
Figure 9.8 Measurements and curve fits of a thermoelectric module. (a) Inner voltage source. (b) Inner resistance.
Figure 9.9 Electrical equivalent circuit diagrams of thermoelectric module. (a) Thévenin equivalent. (b) Norton equivalent.
Figure 9.10 Thermoelectric voltage (a) and power (b) dependency on the current for different temperature gradients.
Figure 9.11 Flowchart of the perturb and observe method.
Figure 9.12 Output conductance (a) and power (b) dependency on the output current for different temperature gradients.
Figure 9.13 Flowchart of the incremental conductance method.
Figure 9.14 System diagram of thermoelectric generator system used as case study.
Figure 9.15 Inner battery voltage dependency on state-of-charge level.
Figure 9.16 Open loop bode plot of current controller and plant. Crosses: poles. Circles: zeros.
Figure 9.17 Current controller performance evaluation. Actual and reference inductor currents.
Figure 9.18 Performance evaluation of the MPPT algorithm. (a) Actual and maximum thermoelectric power. (b) Zoom of power. (c) MPPT efficiency.
Figure 9.19 Open loop bode plot of voltage controller and plant. Cross: pole, Circle: zero.
Figure 9.20 Voltage controller performance evaluation. Actual and reference output voltage.
Figure 9.21 Simulation of thermoelectric power production (a) and battery voltage (b) during charging first in maximum power mode and after approximately 1.2 s in voltage mode. The battery capacity has been reduced by 3600 in order to decrease the simulation time.

Chapter 10: Thermoelectric Energy Harvesting for Powering Wearable Electronics

Figure 10.1 (a) Digital photo and schematic of flexible planar TEM reported in [21]. (b) Schematic, (c) digital photo, and (d) thermal image of 3D-flexible planar TEM with wavy-shaped PDMS/Kapton assembled package, discussed in [22–24].
Figure 10.2 Thermal (a) and electrical equivalent circuits (b) of a thermoelectric module in contact with the human skin.
Figure 10.3 (a) Procedure used in [73] for preparing an air-permeable, fabric-based TEG. (b) Photo of the positive face of the TEG device. (c) Digital photo and (d) SEM image of polyester fabric after PEDOT:PSS coating treatment.
Figure 10.4 TEG developed by [75] and its integration in an electrocardiography shirt. Different TEGs are painted like chameleon for invisibility; only one has a different color to give an idea about its size.
Figure 10.5 (a) Band-type flexible TEG for harvesting thermal energy from human skin. (b) Photo of 196 Bi₂Te₃ and Sb₂Te₃ dots on a glass fabric of 40 mm × 40 mm [83].
Figure 10.6 Pisarenko relation at 300 K for p-type Bi₂Te₃ doped with different acceptor impurities; tin-doped samples showed enhanced Seebeck factor.
Figure 10.7 Thermoelectric power factor of doped and gated [110] NWs with different diameters versus carrier density.
Figure 10.8 Properties and long-term stability of the developed thermoelectric module with a packing density of 0.40.

Chapter 11: Thermoelectric Modules as Efficient Heat Flux Sensors

Figure 11.1 Schematic view of a state-of-the-art heat flux sensor.
Figure 11.2 Advances in miniaturization of thermoelectric modules – example of micromodules technology development.
Figure 11.3 Comparison of the designs of thermoelectric modules: thin-film and bulk.
Figure 11.4 Samples 1 and 2, comparison of calibration using precise external heat source (dashed) and by the method proposed (solid).
Figure 11.5 Samples 3 and 4, comparison of calibration using precise external heat source (dashed) and by the method proposed (solid).
Figure 11.6 Outlines of thermoelectric heat flux sensor.
Figure 11.7 Equivalent diagram of heat flux sensor: R₁, R₂ – thermal resistance of upper and lower substrates; R_c – thermal resistance of upper and lower ceramic sides; R_F – thermal resistance of filling; R_TE – thermal resistance of thermocouples (pellets); T′_c, T′_h – temperature of the hot and cold sides directly on pellets; ΔT′ – temperature difference directly on thermocouples.
Figure 11.8 Dependence of thermoelectric heat flux sensor sensitivity on the pellet form-factor.
Figure 11.9 Estimated dependences of the sensitivity S_a and the time constant τ of the sensor on thermoelements height h for their different widths: 0.2; 0.3; and 0.4 mm.
Figure 11.10 Calculated dependence of the detectivity D* and time constant τ of the height of thermocouples at their different packing density y.
Figure 11.11 Dependence of sensitivity of thermoelectric sensors Se on their size A.
Figure 11.12 The relative sensitivity of thermoelectric sensors depending on the number of thermocouples in the sensor.
Figure 11.13 Outlines of a thermoelectric heat flux sensor.
Figure 11.14 Effect of filler on sensitivity (a) and time constant (b) of sensors of various sections a and height h of thermocouples: with silicone filler and no fill (dotted line).
Figure 11.15 An example of the calculated parameters of the heat flux sensor based on a 40-pair thermoelectric module 1MD02-040-03 of thermocouples 0.2 × 0.2 × 0.3 mm³ depending on the size of the sensor surface A. The dotted line – no filler.
Figure 11.16 Temperature dependences of sensitivity of the sensor series HTX (a) and HFX (b).
Figure 11.17 Average calibration temperature dependence of sensitivity for the sensors of the series HTX (a) and HFX (b).
Figure 11.18 Temperature dependences of thermal resistance of sensors series HTX (a) and HFX (b).
Figure 11.19 Average calibration temperature dependences of thermal resistance of sensors series HTX (a) and HFX (b).
Figure 11.20 Typical temperature dependence of Seebeck coefficient.
Figure 11.21 Calibration temperature dependence of Seebeck coefficient.
Figure 11.22 The sensitivity S_e of heat flux sensors of various manufacturers (1–8) and experimental series of TEC Microsystems (HT, HF, HR) depending on size. The dashed line is the line of the maximum attainable parameters of the technology.
Figure 11.24 Thermal resistance R_T of heat flux sensors of various manufacturers (2, 4, 5, 7) and experimental series (HT, HF) depending on size.
Figure 11.25 Experimental scheme of microcalorimetry of water evaporation: 1 – water drop; 2 – heat flux sensor; 3 – massive base.
Figure 11.26 Experiment of water drop evaporation. The dynamics of heat flux from the base absorbed and the average base temperature for the period considered.
Figure 11.27 Dynamics of water evaporation (mass loss).
Figure 11.28 Model experiment of heat fluxes in the soil.
Figure 11.29 Results of experiment of heat fluxes in the soil.
Figure 11.30 Thermoelectric ice sensor.
Figure 11.31 Ice sensor design: 1 – heat flux sensor; 2 – Peltier module; 3 – thermistor. U_T – signal of thermistor; U_D – signal of heat flux sensor; I – electric current; T – temperature; Q – heat flux.
Figure 11.32 Thermoelectric ice sensor operating modes: (a) cooling; (b) heating. Q – heat flux; 1 – heat flux sensor; 2 – Peltier module; 3 – thermistor. T_a, T₁, T₂, T₃ – temperatures, correspondingly: ambient; surface of water or ice; surface of heat flux sensor; surface of Peltier module.
Figure 11.34 Heating mode: (a) thermistor signal; (b) heat flux sensor signal.
Figure 11.35 Experimental cooling–heating cycle: (a) thermistor signal; (b) heat flux sensor signal; (c) thickness of ice or water layer, correspondingly. T2 – temperature at the surface of ice sensor; T3 – temperature at the surface of Peltier module.
Figure 11.36 Design of thermopile laser power meters: (a) “disk” type; (b) “wafer” type.
Figure 11.37 Thermoelectric laser power meters.
Figure 11.38 Design of matrix thermoelectric laser power meter (16-element matrix example).

Chapter 12: Photovoltaic–Thermoelectric Hybrid Energy Conversion

Figure 12.1 A typical schematic diagram of the PVTE device, in which a TEG device is connected thermally and electrically in series with the PV cell. (Lorenzi et al. 2015 [12]. Reproduced with permission of Springer.)
Figure 12.2 Segmentation of AM1.5G spectrum in three regions for an mc-Si:H solar cell and solar TEG with 4% efficiency. Normalized accumulated radiation power. Pie chart: converted power fraction of each module. (Kraemer et al. 2008 [9]. Reproduced with permission of American Institute of Physics.)
Figure 12.3 Histograms showing the contribution of L₂ for different technologies. The numbers on the bars are L_2a and L_2b + L_2c values. Error bars show the range of minimum and maximum values for each L₂ component. Bars on L_2a result from the variability of the energy gaps of CIGS and a-Si. (Lorenzi et al. 2015 [12]. Reproduced with permission of Springer.)
Figure 12.4 Schematics of the device structures discussed in this work. (a) CASE 1, in which the TEG is just placed underneath the solar cell and is electrically connected to it; (b) CASE 2, where also the low frequency portion of the spectrum is recovered by introducing an additional absorbing layer. (Lorenzi et al. 2015 [15]. Reproduced with permission of Cambridge University Press.)
Figure 12.5 PVTE efficiency (%) versus E_g and T_cell, normalized to the PV efficiency for T_cell = 300 K: (a) Case 1, without absorbing layer (b) Case 2, with absorbing layer. (Lorenzi et al. 2015 [15]. Reproduced with permission of Cambridge University Press.)
Figure 12.6 Generated PV, TE, and total energy for a 10-day period in August for the city of Malaga, Spain. (van Sark 2011 [10]. Reproduced with permission of Elsevier.)
Figure 12.7 (a) Schematic diagram of the PVTE hybrid system; the simulation structure of PV: (b) c-Si PV, (c) p-Si TFPV, (d) polymer PV, (e) CIGS PV; influence of temperature on efficiency of PV: (f) c-Si PV, (g) p-Si TFPV, (h) polymer PV, (i) CIGS PV; efficiency of PVTE as a function of concentrating ratio: (j) c-Si PV, (k) p-Si TFPV, (l) polymer PV, (m) CIGS. (Zhang et al. 2014 [16]. Reproduced with permission of Elsevier.)
Figure 12.8 Schematic illustration and photograph of the novel PVTE hybrid device using DSSC and SSA-pasted TE generator as the top cell and the “bottom cell”: (a) hybrid device; (b) DSSC; (c) SSA; (d) TE; and (e) photograph of the hybrid device. (Wang et al. 2011 [7]. Reproduced with permission of Royal Society of Chemistry.)
Figure 12.9 (a) Transmittance spectra of the FTO and DSSC; (b) reflectance spectrum of the commercial SSA. (Wang et al. 2011 [7]. Reproduced with permission of Royal Society of Chemistry.)
Figure 12.10 Structure and electron generation/transfer of the composite anode-based DSSC. (a) Schematic illustration of the structure and electron generation/transfer process of the DSSC. There are two routes for energy conversion. Route (1) is the photovoltaic effect: from the excited dye to the conduction band (CB) of TiO₂; route (2) is the thermoelectric effect: charge transport from the Fermi level of Bi₂Te₃ nanoplate to TiO₂ upon heating by sunlight irradiation. (b) TEM image of the as-synthesized Bi₂Te₃ nanoplates. (c) SEM image of the composite anode film, in which Bi₂Te₃ nanoplates are embedded in the mesoporous TiO₂ network; Bi₂Te₃ nanoplates are highlighted with dashed circles. (Source: Chen et al. 2012 [17]. Reproduced with permission of Royal Society of Chemistry.)
Figure 12.11 (a) Schematic diagram of the PVTE hybrid system with an optical concentrator; (b) the structure of the PV cell within one period. (Da et al. 2016 [18]. Reproduced with permission of Elsevier.)
Figure 12.12 Schematic description of the PVTE hybrid system. Light from the sun is concentrated and illuminates the MJ PV cell (P_in), generating electrical power (PPV). The cell's input energy that is not converted into electrical power (Q_h) heats a copper block that flattens the temperature and conducts some of the heat, Q_h, TE, toward the TEG's hot side, while some of the heat is lost to the environment, Q_L, Q_h, TE is conducted through the TEG, converting a part of it to an additional electrical power (PTE) while the rest of the heat (Q_c) is removed from the system through a copper heat sink attached to the cold side of the TEG. The hot and cold copper blocks' temperatures, T_h and T_c, respectively, are measured by thermocouples. The materials and energy gap of each layer in the MJ PV cell are also illustrated. (Beeri et al. 2015 [19]. Reproducedw ith permission of American Institute of Physics.)
Figure 12.13 (a) The scanning electron microscopy (SEM) image of 0.16 g/m² CNT films composed of crosswise overlapped multilayers. (b) The SEM image of the CNT films (areal density of 0.16 g/m²) coated with Bi₂Te₃ at a thickness of 5 µm. Inset: photograph of the corresponding sample. (c) Illustration of the series connection of the devices for solar energy conversion. (Xia et al. 2014 [20]. Reproduced with permission of American Chemical Society.)
Figure 12.14 The conversion efficiency of TEG, PV, and PV+TEG type versus thermoelement length in ambient atmosphere. (Hashim et al. 2016 [2]. Reproduced with permission of Elsevier.)
Figure 12.15 (a) Schematic diagram of a solar-thermoelectric module. (b) SEM image of the CuO nanoparticles produced by the proposed process. (Chang et al. 2011 [21]. Reproduced with permission of Elsevier.)
Figure 12.16 (a) Schematic of the thin-film solar cell of the hybrid system with the fishnet embedded in the back passivation layer. (a) 3D schematic of the solar cell; (b) design parameters of the fishnet structure; (c) top view of the schematic. (Oh et al. 2015 [22]. Reproduced with permission of Elsevier.)
Figure 12.17 (a) Structure of the designed c-Si thin-film solar cell. (b) Top view of the structure. (c) Cross- view of bottom patterned ITO–ZrO₂–MgO–SiO₂ antireflection coating. (Zhang et al. 2011 [24]. Reproduced with permission of Elsevier.)
Figure 12.18 Output power and power improvement. (a) With varying TEG output power. (b) With varying PV output power. (Xu 2015 [23]. Reproduced with permission of Elsevier.)
Figure 12.19 (A) Schematic illustration of novel hybrid solar panel; (B) photos of (a) single-crystalline Si solar cell, (b) TE module, (c) HDPE and Al powder, (d) solar panel before attachment of solar cell, and (e) prototype of hybrid solar panel with water tube connections. (Yang and Yin 2011 [8]. Reproduced with permission of IEEE.)
Figure 12.20 (a) Schematic diagram of the experimental system, (b and c) performance test of the combined modules used in model house, with 2000 ml/min cooling flow rate. (Cheng et al. 2011 [25]. Reproduced with permission of Elsevier.)
Figure 12.21 Solar cell cooling system using a TE cell. (Benghanem et al. 2016 [26]. Reproduced with permission of Elsevier.)

List of Tables

Chapter 2: Nanostructured Materials: Enhancing the Thermoelectric Performance

Table 2.1 Calculated efficiency of collected TE materials at the hot side temperatures of 700, 900, and 1100 K, while the cold side temperature is fixed at 300 K

Chapter 6: Preparation and Characterization of TE Interfaces/Junctions

Table 6.1 The required contact resistivities and thermoelement length for ensuring the reduction in ZT being limited to 10% using Bi₂Te₃, which possesses an electrical resistivity of 1 × 10⁻⁵ Ω m and thermal conductivity of 1.5 W/m K
Table 6.2 Systematic error and repeatability of the resistance scanning probe determined using the standard reference sample (SRM 1461) from NIST

Chapter 10: Thermoelectric Energy Harvesting for Powering Wearable Electronics

Table 10.1 Skin temperature (°C) in neutral, warm, and cold sTable conditions

Chapter 11: Thermoelectric Modules as Efficient Heat Flux Sensors

Table 11.1 Seebeck coefficient of different materials
Table 11.2 Three applications of thermoelectric modules
Table 11.3 Thermal resistances of some materials
Table 11.4 Samples of heat flux sensors manufactured to verify the calibration method
Table 11.5 Results of calibration using precision external heat source and by the method proposed for Samples 1 and 2
Table 11.6 Results of calibration using precision external heat source and by the method proposed for samples 3 and 4
Table 11.7 Main parameters of thermoelectric heat flux sensors
Table 11.8 Dependence of sensor properties on design parameters
Table 11.9 Typical parameters of thermoelectric material of p- and n-type at 300 K
Table 11.10 Values of form-factors of the thermoelectric pellets
Table 11.11 Performance parameters of heat flux sensors series HTX
Table 11.12 Performance parameters of heat flux sensors series HFX
Table 11.13 Performance parameters of heat flux sensors series HRX
Table 11.14 Experimental temperature dependences of sensitivity
Table 11.15 Coefficients of the polynomial of the sensitivity temperature dependence of sensors
Table 11.16 Coefficients of the temperature dependence polynomials of the thermal resistance R_T
Table 11.17 Coefficients of the polynomial of Seebeck coefficient
Table 11.18 Manufacturers of state-of-art heat flux sensors
Table 11.19 Types of laser power meters [25]

Chapter 12: Photovoltaic–Thermoelectric Hybrid Energy Conversion

Table 12.1 Energy loss in a single-junction device powered by solar energy

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