Thermoelectric Energy Harvesting Research Articles

Optimizing thermoelectric performance of MoS2 films by spontaneous noble metal nanoparticles decoration

Two-dimensional (2D) layered transitional metal dichalcogenides (TMDCs) materials have been theoretically and experimentally proven to possess a good potential in thermoelectric (TE) energy harvesting. As a member of TMDCs, molybdenum disulfide (MoS2) was severely hindered by poor electrical conductivity for better TE performance. Herein, noble metal nanoparticles (NPs) was employed to optimize the electrical conductivity of MoS2 films by spontaneous decoration. As a result, the electrical conductivity and Seebeck coefficient of MoS2 films with a little Ag NPs load were simultaneously enhanced, which may result from the formation of more electrically conductive path and the stronger energy filtering effect generated by Ag NPs decoration. These results indicate that the conflict between thermoelectric factors can effectively resolve by spontaneous Ag NPs decoration. This technique may shed light on a new way for optimizing the TE energy harvesting efficiency for 2D TMDCs materials.

A Colpitts Oscillator-Based Self-Starting Boost Converter for Thermoelectric Energy Harvesting With 40-mV Startup Voltage and 75% Maximum Efficiency

This paper presents a Colpitts oscillator-based self-starting boost converter for thermoelectric energy harvesting. An enhanced swing Colpitts oscillator with a 10-stage voltage multiplier is adopted as a startup circuit so that the minimum startup voltage of 40 mV is achieved. As a main converter, a zero-current-switching boost converter with a dead time controller achieves the maximum end-to-end efficiency of 75%. The inductor used in the Colpitts oscillator is reused in the main boost converter to minimize the use of the inductor. Implemented in a 65-nm CMOS, the proposed converter that requires only two external inductors provides a regulated output voltage of 1.1 V from a thermoelectric generator with an internal resistance of 2.2 $\Omega$ over the input range of 40–200 mV.

An On-Chip Transformer-Based Self-Startup Hybrid SIDITO Converter for Thermoelectric Energy Harvesting

A hybrid single-inductor dual-input-triple-output (SIDITO) converter with an on-chip transformer-based starter for thermoelectric energy harvesting is presented. The on-chip transformer-based LC starter makes the system capable of self-startup at an open-circuit-voltage of 160 mV. With the proposed hybrid SIDITO architecture, additional inductor-sharing power switches are eliminated. An adaptive-on-time control scheme is also proposed to improve the efficiency at low input voltage levels. The chip is fabricated in a 0.18- ${\mu }\text{m}$ CMOS process. The measurement results show that the converter can provide an output current range from $1~{\mu }\text{A}$ to 10 mA with a peak efficiency of 83.3%.

An experimental study of a-Si/ZnO-stacked hetero-structures for potential thermoelectric energy harvesting applications

Here, we present a study of thermoelectric devices with amorphous silicon/zinc oxide (a-Si/ZnO)-stacked hetero-structures fabricated using both radio frequency magnetron sputtering and rapid thermal annealing techniques. Overall, the Seebeck coefficient (S) and power factor (S2σ, where σ is the electrical conductivity) of the a-Si/ZnO-stacked hetero-structures were found to be superior to those of pure a-Si structures. In particular, the Seebeck coefficient and power factor of the a-Si/ZnO (9/8 layers)-stacked hetero-structures were about 1.6 and 23.8 times those of the pure a-Si structures, respectively. These improvements can be attributed to hole blocking by the a-Si/ZnO potential barriers formed at the interface between the ZnO layer with a wide energy bandgap and the a-Si layer with a relatively narrow bandgap. In addition, the a-Si and ZnO materials used in this work are non-toxic, earth-abundant, and cheap, and the fabrication processes were simple and cost-effective, making the hetero-structures suitable for use in non-toxic and biocompatible thermoelectric devices and also for micro-energy harvesting applications.

A 60 mV Input Voltage, Process Tolerant Start-Up System for Thermoelectric Energy Harvesting

This paper presents a 60 mV input voltage start-up system for thermoelectric energy harvesting. A new process tolerant inverter cell is proposed, which is functional at supply voltages as low as 60 mV. Using the proposed unit cell, a ring oscillator has been implemented. The ring oscillator is followed by 40 charge-pump stages, an ultra-low-power level detector, and a boost converter. The energy harvesting system can generate an output voltage of 1 V and delivers a maximum power of $4.5~\mu \text{W}$ from a 60 mV supply. This system has been implemented in a standard $0.18~\mu \text{m}$ CMOS technology, uses neither zero-threshold voltage (normally-on) negative-channel metal-oxide semiconductor nor microelectromechanical systems switches and occupies 3.3 mm2.

Thermoelectric Energy Harvesting Interface Circuit With Capacitive Bootstrapping Technique for Energy-Efficient IoT Devices

This paper presents a low-input-voltage (100 mV), low-output-voltage (500–600 mV) thermoelectric energy harvesting interface circuit for near-threshold energy-efficient Internet-of-Things (IoT) devices. The capacitive bootstrapping technique is used to generate a positive and negative bias pair for alleviating the significant conduction losses of power MOSFETs in a near-threshold operation. Internal bias voltages are automatically boosted to different levels as per the loading conditions to extend the output power range. The deployment of constant on-time digital pulse skip modulation with digital zero current detection (ZCD) achieves an ultralight load operation, and precluding a reverse current. The digital ZCD is capable of dynamically adjusting the off-time ( ${T}_{\mathrm {OFF}}$ ) of the power transistors, which vary according to the input and output voltage levels. The proposed step-up dc-dc power converter implemented using a 180 nm CMOS technology demonstrates a maximum conversion efficiency of 76.4% over a $1~{\mu }\text{W}$ – $500~{\mu }\text{W}$ load range, significantly evaluating the feasibility of the near-threshold interface circuit architecture for energy-efficient IoT devices.

An Efficient and Fast Li-Ion Battery Charging System Using Energy Harvesting or Conventional Sources

This paper presents a multi-input battery charging system that is capable of increasing the charging efficiency of lithium-ion (Li-ion) batteries. The proposed battery charging system consists of three main building blocks: a pulse charger, a step-down dc–dc converter, and a power path controller. The pulse charger allows charging via a wall outlet or an energy harvesting system. It implements charge techniques that increase the battery charge efficiency of a Li-ion battery. The power path controller (PPC) functions as a power monitor and selects the optimal path for charging either via an energy harvesting system or an ac adapter. The step-down dc–dc converter provides an initial supply voltage to start up the energy harvesting system. The integrated circuit design is implemented on a CMOS 0.18 μm technology process. Experimental results verify that the proposed pulse charger reduces the charging time of 100 mAh and 45 mAh Li-ion batteries respectively by 37.35% and 15.56% and improves the charge efficiency by 3.15% and 3.27% when compared to the benchmark constant current-constant voltage charging technique. The step-down dc–dc converter has a maximum efficiency of 90% and the operation of the PPC is also verified by charging the battery via a thermoelectric energy harvesting system.

Development of a Thermoelectric and Electromagnetic Hybrid Energy Harvester from Water Flow in an Irrigation System.

A hybrid energy harvester is presented in this paper to harvest energy from water flow motion and temperature difference in an irrigating pipe at the same time. The harvester is based on the integration of thermoelectric and electromagnetic mechanisms. To harvest the water flow motion, a turbine fan with magnets that are attached on the blades is placed inside of the water pipe. Multiple coils turn the water flow energy into electricity with the rotation of the turbine. The thermoelectric generators (TEGs) are attached around the pipe, so as to harvest energy due to temperature difference. For a maximum temperature difference of 55 °C (hot side 80 °C and room temperature 25 °C), twelve serial-connected TEGs can generate voltage up to 0.346 V. Under a load resistance of 20 Ώ, the power output of 1.264 mW can be achieved. For a maximum water flow rate of 49.9 L/min, the electromagnetic generator (EMG) can produce an open circuit voltage of 0.911 V. The EMG can be potentially used as a water flow meter due to the linear relationship between water flow rate and output voltage. Under the joint action of TEG and EMG, the maximum terminal voltage for TEG is 66 mV and for EMG is 241 mV at load resistances of 10 and 100 Ώ, respectively, resulting in a corresponding power output of 0.435 and 0.584 mW.

A 12 mV Input, 90.8% Peak Efficiency CRM Boost Converter With a Sub-Threshold Startup Voltage for TEG Energy Harvesting

This paper proposed a high efficiency boost converter targeting thermoelectric generator energy harvesting. The proposed converter adopts the critical conduction mode rather than the discontinuous mode to reduce the conduction loss, which can improve the peak efficiency at high input power. To reduce the minimum input voltage, an adaptive on-resistance switch, which can automatically change the high side and low side on-resistance according to the input voltage, is used. To maximize the output power, a maximum power point tracking circuit is designed to track the internal source resistance. Radio frequency (RF) kick-up and low voltage startup are used to improve the cold start voltage. The proposed converter is implemented in a 0.18- $\mu \text{m}$ standard CMOS process, which occupies a chip area of about 0.48 mm2. Measurement results show that the peak efficiency of the converter can be up to 90.8% and 60% at 180 and 20 mV of the open-circuit voltage, respectively. Owing to the cold startup circuit, the proposed converter can start successfully with 260-mV open-circuit voltage or −16-dBm RF input. Moreover, the open-circuit voltage can be further lowered to 12 mV with 1.5- $\mu \text{W}$ output power after startup.

Evaluating the Relationship between Phonon and Thermal Properties of Group IV Alloys Using Molecular Dynamics Simulation

Introduction Group IV alloy semiconductor material is anticipated to be used for the miniaturized thermoelectric (TE) energy harvester, because it has high TE properties, which is called ZT (= S 2 σTκ -1), is related to the Seebeck coefficient S, the electrical conductivity σ, the thermal conductivity κ, and the temperature T. In the previous study, it was indicated the drastically low thermal conductivity in group IV alloy is mainly caused by the mass disorder scattering of phonon [1]. The low thermal conductivity of group IV alloy was reproduced by our molecular dynamics (MD) simulations [2]. In classical MD simulation, it was considered the difference of the atomic mass and the interatomic potentials. Both mass and potential disorder scattering of phonon is deeply linked with the decreasing thermal conductivity. In this study, we quantitatively investigated which disorder is more affected to thermal conductivity in the group IV alloys whose atomic mass and interatomic potentials were varied independently by MD simulations to clarify. Simulation Procedure The models in MD simulation were 28Si x 73Si(1−x), 28Si x 119Si(1−x), 28Si x 28Ge(1−x), 28Si x 28Sn(1−x), 28Si x 73Ge(1−x), and 28Si x 119Sn(1−x) wire with periodic boundary condition. The initial model length and the one side width of square were 30ai and 4ai Å, respectively. The employed interatomic potential is the Stillinger-Weber (SW) potential, which is defined by the sum of two- and three-body potential energy terms that depend on local geometric positions. This potential was optimized to reproduce the lattice constant and the phonon frequencies in the group IV alloy [2]. The thermal conductivity was calculated by equilibrium MD method. The phonon related properties were conducted from the Fourier analysis. Results and Discussion Fig. 1 shows 28Si concentration dependences of thermal conductivities. Red lines represent results of 28Si x 73Si(1−x) and 28Si x 119Si(1−x) alloys, purple lines represent results of 28Si x 28Ge(1−x) and 28Si x 28Sn(1−x) alloys, blue lines represent results of 28Si x 73Ge(1−x), and orange lines represent calculation results from the research of Khatami et al. [1]. Fig. 2 shows the phonon dispersion of 28Si0.5 73Si0.5, 28Si0.5 28Ge0.5, and 28Si0.5 73Ge0.5 alloys. The changing of the thermal conductivities and the phonon dispersion is caused by mass and potential disorders. The amount of the changing by potential disorder is smaller than that of mass disorder. Fig. 3 shows frequency spectra of vibration density of states (VDOS) and phonon lifetimes in 28Si0.5 73Si0.5, 28Si0.5 28Ge0.5, and 28Si0.5 73Ge0.5 alloys. The VDOS D(ω) is closely related to the phonon lifetime due to alloying τalloy, which is defined as τalloy -1(ω) = (π/6)V 0Γalloy D(ω)ω 2, where V 0 is volume per atom, Γalloy is scattering strength due to the mass and potential disorders, and ω is phonon frequencies. The phonon lifetime in 28Si0.5 28Ge0.5 is almost ten times than that of 28Si0.5 73Si0.5 and 28Si0.5 73Ge0.5. The thermal conductivity is defined as κ(ω) = (1/3)C(ω)v 2(ω)τall(ω), and the phonon lifetime is defined as τall -1(ω) = τpure -1(ω) + τalloy -1(ω) = τpure -1(ω) + τmass -1(ω) + τforce -1(ω), where C is heat capacity, v is phonon velocity, τpure is phonon lifetime in pure crystal, τmass is mass disorder phonon lifetime, and τforce is potential disorder phonon lifetime [1]. When we assumed Cv 2 = 100, τpure = 1, τmass = 1/100, and τforce = 1/10, the thermal conductivity scattered by only potential disorder is 9.09, that scattered by only mass disorder is 0.99, and that scattered by both mass and potential disorder is 0.90. This tendency is agree to the results of MD calculation. The mass disorder is dominant factor for the changing of the thermal and phonon properties. Moreover, the potential disorder is ignorable in phonon and thermal calculation by MD method. Acknowledgements This study was partially supported by the Japan Science and Technology Agency’s (JST) CREST program (JPMJCR15Q7) and the Japan Society for the Promotion of Science (JSAP) through a Grant-in-Aid for JSPS Fellows (15J07583).

Performance Evaluation of Thermoelectric Energy Harvesting System on Operating Rolling Stock.

During rolling stock operation, various kinds of energy such as vibration, heat, and train-induced wind are dissipated. The amount of energy dissipation cannot be overlooked when a heavy railroad vehicle operates at high speed. Therefore, if the wasted energy is effectively harvested, it can be used to power components like low power sensor nodes. This study aims to review a method of collecting waste heat, caused by the axle bearing of bogie in a rolling stock. A thermoelectric module (TEM) was used to convert the temperature gradient between the surface of the axle bearing housing and the outdoor air into electric energy. In this study, the output performance by temperature difference in the TEM was lab-tested and maximized by computational fluid analysis of the cooling fins. The optimized thermoelectric energy harvesting system (TEHS) was designed and applied on a rolling stock to analyze the power-generating performance under operation. When the rolling stock was operated for approximately 57 min including an interval of maximum speed of 300 km/h, the maximum open circuit voltage was measured at approximately 0.4 V. Based on this study, the system is expected to be utilized as a self-powered independent monitoring system if applied to a low-power sensor node in the future.

An Integrated Approach to Thermoelectrics: Combining Phonon Dynamics, Nanoengineering, Novel Materials Development, Module Fabrication, and Metrology

AbstractThis review discusses the longstanding efforts to develop advanced thermoelectrics through a multidisciplinary approach by combining condensed matter physics, nanotechnology, solid‐state chemistry, electrical engineering, mechanical engineering, and metrology. The phonon dynamics of skutterudites, clathrates, tetrahedrites, and layered LaOBiSSe are investigated through inelastic neutron scattering, allowing insights into their low lattice thermal conductivity due to rattling in a cage as well as under planar coordination. The electrical resistivity, Seebeck coefficient, and Hall coefficient of Bi‐nanowires are successfully measured with a home‐made system, demonstrating a size effect in thermoelectric and galvanomagnetic phenomena. For PbTe‐based bulk thermoelectrics, an exceptionally high figure of merit ZT (≈1.8 at 800 K) is achieved through nanostructuring. Moreover, correspondingly high conversion efficiency (≈11% for a temperature difference of 590 K) is demonstrated in nanostructured PbTe‐based modules. Sulfides (tetrahedrite, colusite, and CdI2‐type layered systems) and arsenides (LnFeAsO and BaZn2As2) are developed as environmentally friendly and emerging thermoelectric materials, respectively. The output power and efficiency of modules with novel materials, including nanostructured PbTe, Zn4Sb3, and clathrates, are measured with the highly accurate self‐made system. Future opportunities and challenges for the widespread use of thermoelectric waste heat recovery and energy harvesting are also discussed.

Thin Film Thermoelectric Materials: Classification, Characterization, and Potential for Wearable Applications

Thermoelectric technology has the ability to convert heat directly into electricity and vice versa. With the rapid growth of portable and wearable electronics and miniature devices, the self-powered and maintenance of free thermoelectric energy harvester is highly desired as a potential power supply. Thin film thermoelectric materials are lightweight, mechanically flexible, and they can be synthesized from abundant resources and processed with a low-cost procedure, which offers the potential to develop the novel thermoelectric devices and hold unique promise for future electronics and miniature accessories. Here, a general classification for thin film thermoelectric materials varied by material compositions, and thermoelectric properties depended on different measurement technique. Several new flexible thermoelectric strategies are summarized with the hope that they can inspire further development of novel thermoelectric applications.

Multi-parameter optimization design of thermoelectric harvester based on phase change material for space generation

In this study, a thermoelectric energy harvesting device based on phase change material is presented which can be applied with large temperature variation in space for power supply. Aiming at multi-parameter optimization of thermoelectric harvester, an assessment of the generalized design rules for the proposed harvester has been implemented. The effect of thermal conductivity, melting temperature and mass of phase change material on the thermodynamic process were investigated to obtain the design criterion for thermoelectric harvester. Besides, both simulation and experiment validated that choosing PCM with a suitable melting temperature is quite essential to the temperature control by balancing the heat storage and release process, consequently enhancing the power output. This work offers a unique powering solution for wireless sensor involving location with temperature variation in space application.

Energy Neutral Sensor System With Micro-scale Photovoltaic and Thermoelectric Energy Harvesting

Minimizing power conversion losses is critical for energy neutral operation of micro-scale energy harvested sensor nodes. These small form-factor sensor nodes rely on miniature harvesters with low output voltages that must be boosted with large conversion ratios to recharge batteries or super-capacitors. Selective Direct Operation (SDO), a technique to selectively avoid power conversion and thereby eliminate conversion loss in energy harvested systems has been demonstrated as an effective technique for light harvesters. This paper extends SDO to thermoelectric generators (TEGs). SDO exploits the ultra-low circuit functional voltages, enabling sensor systems to effectively harvest energy from cm-scale TEGs which output few 10’s of mW but at low output voltages (100’s of mV). PV cell construction from prior-work, TEG characterization and field measurements are presented in this paper to demonstrate the effectiveness of SDO and co-designing energy harvesters, power conversion circuits and digital sub-systems.

Development of a batteryless VHF-Beacon and tracker for mammals

In this paper, we present the design, set-up and testing of a fully energy-autonomous wildlife tracking collar that is supplied by thermoelectric energy harvesting from the animal’s body heat. Attached to the neck of a mammal, the device determines the animal’s activity, body-temperature and its own energy state. The data is then transmitted in regular short beacons which are also used for triangulation of the animal position. The characterization in laboratory and a field test show that the system design with an ultra-low power switch and a low-voltage step-up-converter with over 42 % efficiency is capable to measure and transmit continuously in intervals of 25 s.

A fully integrated autonomous power management system with high power capacity and novel MPPT for thermoelectric energy harvesters in IoT/wearable applications

This paper reports a fully integrated autonomous power management system for thermoelectric energy harvesting with application in batteryless IoT/Wearable devices. The novel maximum power point tracking (MPPT) algorithm does not require open circuit voltage measurement. The proposed system delivers 0.5 mA current with 1 V regulated output based on simulations, which is the highest output current for a fully integrated converter reported in the literature for ultra-low voltage applications, to the best knowledge of the authors. Regulated 1 V output can be achieved for load range >2 kΩ, and input voltage range >140 mV. The circuit has been implemented in UMC-180nm standard CMOS technology and simulated.

Field-effect and chemical charge-type modulations of carbon nanotubes using functional polymers for thermoelectric energy harvesters

As devices continue to shrink in size and grow in abundance, power supply methods using clean and abundant materials become increasingly important. Single-walled carbon nanotubes (SWCNTs) are promising materials as the thermoelectric (TE) energy-harvesting candidate due to their flexibility, nontoxicity, and relatively large TE power factors. This paper reviews our current studies on the charge-type conversion of SWCNTs for readily pairing several p- and n-type SWCNTs to improve the TE voltage output. First, we describe the polarity tuning of SWCNTs by electric dipoles of ferroelectric polymer. The sign of Seebeck coefficient of SWCNTs could be tuned by the direction of the electric dipoles adjacent to nanotubes in the ferroelectric-gate field-effect transistor-like device configuration. Second, we demonstrate the electron doping into SWCNTs by several versatile polymer-dopant charge transfer. The obtained negative Seebeck coefficients of SWCNTs were correlated to the highest occupied molecular orbital energy levels of the dopant polymers. Our findings enables the direct use of low-temperature waste heats around dwelling environments using eco-friendly materials and techniques.

Graphene embedded form stable phase change materials for drawing the thermo-electric energy harvesting

We introduce a reusable energy harvesting system that can recover discarded thermal energy by utilizing temperature variation of the environment. The system consists of two different phase change materials (PCMs); poly ethylene glycol (PEG) and 1-tetradecanol (1-TD), and a cell of N and P type semiconductor. Since a large amount of heat energy is absorbed during phase transition, PCMS are generally utilized to maintain isothermal temperature on the opposite sides of the N and P type semiconductor as long as possible. The resulting temperature difference between two sides of semiconductor induces an electric current during the heating and cooling process. It is important for the phase change material (PCM) to possess a good shape stability without significant loss of latent heat. A 3D porous graphene aerogel is selected for the pure phase change material to infiltrate into the aerogel and to increase the shape stability and thermal conductivity of the PCM composite. The thermal conductivity of PEG and 1-TD composites is significantly increased to 0.4268 W/m∙K, and 0.3408 W/m∙K, respectively. Thermal and electrical analyses are performed to predict the energy harvested by the device. The electrical energy is generated in the harvesting system due to the Seebeck effect. The maximum value of the electric current reaches 10 mA in both heating and cooling processes and an LED lamp was turned on successfully. Additionally, the energy harvesting system is modelled by using the finite element method (FEM) and the numerical prediction is in good agreement with the experimental results.

A Sub-10 mV Power Converter With Fully Integrated Self-Start, MPPT, and ZCS Control for Thermoelectric Energy Harvesting

An inductive power converter for thermoelectric generator (TEG) is presented in this paper. A novel redundant inverter ring oscillator is proposed to self-oscillate in a very low supply voltage down to 45 mV. An additional self-start assist circuit with a differential clock booster and an exponential charge pump is implemented in the power converter to achieve lower self-start voltage. Boundary continuous-conduction mode, zero current switch (ZCS), and maximum power point tracking (MPPT) features have been incorporated into the power converter. Selective turn-ON dynamical-comparators have been used in ZCS and MPPT circuits to save power and achieve minimum operating supply voltage. Fabricated in 65-nm CMOS process, the chip can self-start at 210 mV. After self-start, only 7-mV input voltage is needed to sustain the power converter operation. With a TEG open circuit voltage of 80 mV and internal resistance of $5~\Omega $ , the chip can provide a maximum output power of $229~\mu \text{W}$ with end-to-end efficiency of 71.5%.