Thermal Energy Harvesting Research Articles

Assembling all-wood-derived carbon/carbon dots-assisted phase change materials for high-efficiency thermal-energy harvesters

The collection and storage of renewable, sustainable and clean energy including wind, solar, and tidal energy has attracted considerable attention because of its promising potential to replace fossil energy sources. Advanced energy-storage materials are the core component for energy harvesters, affording the high-efficiency conversion of these new-style energy sources. Herein, originated from nature, a series of all-wood-derived carbon-assisted phase change materials (PCMs) were purposed by incorporating carbon dots-modified polyethylene glycol matrix into carbon skeletons via a vacuum-impregnation strategy. The resultant PCMs possessed desired anti-leakage capability and superior thermophysical behaviors. In particular, the optimum sample posed high latent heat (131.5 J/g) and well thermal stability, where the corresponding enthalpy still reserved 90 % over 100 heating/cooling cycles. More importantly, the as-fabricated thermal-energy harvester presented prominent capability to strorage and release multiple forms of thermal energy, as well as high-efficiency solar-energy utilization, corresponding to a photothermal conversion efficiency of 88 % in simulated sunlight irradiation, far exceeding some reported PCMs. Overall, with the introduction of wood-derived carbon dots and carbon skeletons, the assembled all-wood-derived carbon-assisted PCMs afforded trinity advantages on thermal performance, cycling stability, and energy conversion efficiency, which provide a promising potential for the practical application in thermal-energy harvesters.

Harvesting Thermal Energy and Freshwater from Air through Sorption Thermal Battery Enabled by Polyzwitterionic Gel

Harvesting Thermal Energy and Freshwater from Air through Sorption Thermal Battery Enabled by Polyzwitterionic Gel

Machine learning-based optimization of segmented thermoelectric power generators using temperature-dependent performance properties

Segmented thermoelectric generators (STEGs) provide an excellent platform for thermal energy harvesting devices because they improve power generation performance across a broad range of operating temperatures. Despite the benefit of direct thermal energy-to-electricity conversion, conventional STEG optimization approaches are unable to provide a systematic method for selecting the optimal multiple stacks of p- and-n-type thermoelectric materials (TEs) legs from a set of numerous TE materials. In this study, we propose a systematic optimization method based on machine learning to find the optimal STEG for performance maximization. A deep neural network (DNN) is trained using an initial dataset generated via the Finite Element Method (FEM), with inputs including the temperature-dependent properties of p- and n-type TE materials, the lengths of each segment, and external loads, as well as the corresponding performance outputs. The trained DNN captures the inherent nonlinear relationship between these inputs and outputs. The combination of a trained DNN and a genetic algorithm (GA) efficiently navigates a vast design space using 88 p-type and 70 n-type TE materials along with device factors. It formulates four stacked segment pairs in p- and n-leg TEGs, targeting new superior designs with enhanced power, efficiency, or both. The DNN is iteratively refined via active learning (AL) by incorporating new superior STEG designs to enhance prediction accuracy. The optimized STEGs exhibit power and efficiency that are 1.91 and 1.5 times higher, respectively, than the top designs from the initial training dataset composed of 157,916 STEGs. Furthermore, compared to conventional TEG designs without segmentation, our method discovered numerous high-performing STEG designs.

An integrated thermal and RF energy harvesting system with rectifying combination and storage controller for IoT devices

This work presents an integrated thermal/RF energy harvester. The harvesting system can combine energy from two sources simultaneously (one DC and one AC) and deliver a regulated combined voltage of 1.75 V to a load and a storage controller. The output power of the harvester is in the range of hundreds of μWatts which is suitable for low-power wireless sensor networks. Maximum power point tracking (MPPT) is achieved by reconfiguring the number of stages of the DC path, the frequency driving the charge pump, and the capacitors of the matching network in the RF-path. The storage controller charges a supercapacitor to 3.5 V to power up the system during power shortage periods. The system is implemented in a 180 nm CMOS technology and utilizes a total on-chip capacitance of 2.4 nF. The peak end-to-end efficiency of the system reaches 60.3 % at output load power of 1.275 mW.

Modulation of electronic and ionic conduction in mixed polymer conductors via additive engineering: Towards targeted applications under varying humidity

Polymer solids with mixed ion and electron transport hold great promise for next-generation organic electronics, and rational regulation of ionic/electronic contribution within these materials can enable a broadened spectrum of practical applications. However, a fundamental understanding of the conduction mechanisms and their correlations with morphological characteristics remains limited, especially under varying environmental humidity conditions. In the present work, simple additive engineering enables the effective regulation of electronic and ionic contribution in poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) based conductors, giving rising to ion- and/or electron-dominant conductions. As a demonstration, PEDOT:PSS films with different electrical characteristics are successfully applied for thermal energy harvesting, healthcare monitoring and human motion detection upon humidity exposure. Combining operando alternating current (AC) impedance spectroscopy and grazing incidence small-angle X-ray scattering at low and high humidity levels, additive-dependent charge transport mechanisms are elucidated, and correlations between morphological alterations and conductivity evolutions are revealed. This work achieves highly tailorable PEDOT:PSS conduction utilizing Zonyl, dimethyl sulfoxide (DMSO) and carbon nanotubes (CNTs) as additives with distinct humidity responses and gains an in-depth comprehension of underlying mechanisms, which are expected to pave the way for next-generation organic electronics.

Simplicity is the ultimate sophistication: One-step forming for thermosensitive solid–solid phase change thermal energy harvesting, storage, and utilization

In light of the growing recognition of low-carbon initiatives, renewable energy is gradually assuming a dominant role within the energy framework. Phase change materials can enhance the efficiency of renewable energy utilization by energy peak shaving and time shifting. However, leakage and inherent rigidity signify that a significant amount of energy input is required for scale-up applications of phase change materials to enhance shape stability and processability during industrial installation. In conventional enhancement strategies, high-quality encapsulation means more intricate processes and higher energy consumption. Here we propose a simplified thermally-initiated encapsulation and molding strategy, which involves grafting the phase change materials onto thermosetting polymer backbones by covalently integrated, to construct self-supported phase change materials with complex three-dimensional geometry. The encapsulation process is macroscopically manifested as a transformation from solid-liquid to solid–solid phase change. Therefore, by imposing restrictions on liquid materials, any ultra-high-precision complex structure can be molded, and mechanical properties are imparted through curing to achieve shape stability, ultimately enables the integration of phase change material encapsulation and casting. The complete process requires no curing agents or organic solvents, and no pollution emissions, making it highly maneuverable and environmentally friendly. Furthermore, we demonstrate the universal high value applications of new materials in different fields. In the ancient thermotherapy experiment, the sample exhibited sustained heat release for 8 min. In the advanced chip cooling simulation, the chip exhibits superior heat dissipation performance, as evidenced by its operating temperature being 50 °C lower than that of natural cooling within 20 min. It confirms the remarkable potential of phase change materials constructed through this approach for thermal energy applications.

Flexible Carbon Nanotube-Epitaxially Grown Nanocrystals for Micro-Thermoelectric Modules.

Flexible thermoelectric materials have attracted increasing interest because of their potential use in thermal energy harvesting and high-spatial-resolution thermal management. However, a high-performance flexible micro-thermoelectric device (TED) compatible with the microelectronics fabrication process has not yet been developed. Here a universal epitaxial growth strategy is reported guided by 1D van der Waals-coupling, to fabricate freestanding and flexible hybrids comprised of single-wall carbon nanotubes and ordered (Bi,Sb)2 Te3 nanocrystals. High power factors ranging from ≈1680 to ≈1020 µW m-1 K-2 in the temperature range of 300-480 K, combined with a low thermal conductivity yield a high average figure of merit of ≈0.81. The fabricated flexible micro-TED module consisting of two p-n couples of freestanding thermoelectric hybrids has an unprecedented open circuit voltage of ≈22.7 mV and a power density of ≈0.36 W cm-2 under ≈30 K temperature difference, and a net cooling temperature of ≈22.4 K and a heat absorption density of ≈92.5 W cm-2 .

Multifunctional energy harvesting and storage textile technology based on thermionic effect

The development of effective dual-function energy harvesting/storage technologies is a milestone demand for the emerging generation of low-consumption autonomous electronics. This work reports the development of a multifunctional thermionic power textile device merging thermal energy harvesting and electrochemical energy storage for application in self-powered systems. An everyday cost-effective non-conductive natural fabric was transformed into an all-solid-state textile-based thermally-chargeable supercapacitor (T-TCSC) via dyeing with multiwalled carbon nanotubes and subsequent assembly with H3PO4-doped poly(vinyl alcohol) (PVA) solid-gel polyelectrolyte.When subjected to an out-of-plane temperature gradient, the T-TCSC was able to convert the thermal energy into electrical output and store the harvested energy. A Soret coefficient of 1.85 mV K−1 was obtained, which is the highest value achieved so far for TCSCs developed on non-conductive textile substrates reported in literature and ∼10× higher than those of conventional thermoelectric devices. Additionally, a maximum output potential of ∼30 mV was achieved for ΔT = 24.7 K. Concerning the energy storage performance, an energy density of 9.39 μW h cm−2 was obtained at 1.62 mW cm−2 power density.The smart T-TCSC outputs demonstrated its potentialities as a two-in-one self-chargeable textile device that can harvest the thermal energy generated by a temperature gradient, convert it into electrical energy and simultaneously store it.

The mechanism of the capillary oscillation and its application to fabrics’ sweat permeability

The capillary effect plays an important role in air and moisture permeability, and it can be used for thermal enhancement and energy harvesting. However, the capillary oscillation has not been extremely studied, and its mechanism for fabrics’ sweat permeability was rare and preliminary. This paper studies the frequency property of the capillary oscillation in a zig-zag porosity of a fabric with a multiple layer structure. The theoretical analysis reveals that small porosity and low frequency of the zig-zag porosity are beneficial to the high sweat permeability. The proposed capillary oscillation probably paves a new avenue for designing fabrics with high moisture permeability, particularly in sportswear and military apparel in extreme cold environments.

Enhancing solar thermal energy harvesting efficiency and temperature uniformity of Fe3O4 nanofluid in receiver of direct solar thermal collector using dynamic magnetic field

To improve the solar thermal energy harvesting performance (STEHP) according to the receiver depth of a direct absorption solar collector (DASC), the Fe3O4 nanofluid (NF) was used as a working fluid. Besides, an external dynamic magnetic field (MF) was applied to the working fluid, and the effect of the dynamic MF on the heat transfer performance and temperature uniformity of the working fluid was investigated. As a result, with the increase in receiver depth, the STEHP decreased due to the temperature non-uniformity of working fluid and the increase in heat loss at the top part. When the receiver depth was 6 cm, the highest solar thermal energy harvesting efficiency (STEHE) was observed at the concentration of 0.075 wt%. When an external dynamic MF was applied to the 0.075 wt% Fe3O4 NF, the temperature non-uniformity was reduced; therefore, solar energy absorption and storage performance improved significantly. The STEHE using an external dynamic MF increased by 119.6 % compared to DI-water. It was confirmed that applying a dynamic MF to the Fe3O4 NF could significantly improve the STEHP of DASC by enhancing the temperature uniformity within the working fluid and the solar energy absorption performance.

Thermal Interface Engineering in a 3D-Structured Carbon Framework for a Phase-Change Composite with High Thermal Conductivity.

Phase-change materials (PCMs) are promising thermal storage medium for thermal management due to their efficient thermal energy harvesting capabilities. However, the low thermal conductivity (TC) and poor shape stability of PCMs have hindered their practical applications. Construction of an interconnected three-dimensional (3D) heat-conductive structure is an effective way to build phonon conduits and provide PCM confinement. Phonon scattering at the interface is an unavoidable effect that undermines the TC improvement in the PCM composite and necessitates careful engineering. This study focuses on creating a highly thermally conductive 3D carbon-bonded graphite fiber (CBGF) network to enhance the TC of the PCM, with attention especially on thermal interface engineering considering both filler-matrix (F-M) and filler-filler (F-F) interfaces. The composite with an optimized proportion of F-M and F-F interface area achieves the highest TC of 45.48 W·m-1·K-1, which is 188.5 times higher than that of the pure PCM, and a high TC enhancement per volume fraction of the filler (TCEF) of 831% per 1 vol % loading. This also results in an enhanced spatial construction for PCM confinement during the phase change. The results emphasize the significance of interface engineering in creating high-TC and form-stable phase-change composites, providing insightful guidance for rational structural design.

Designing Poly(3,4-ethylenedioxythiophene) Polystyrene Sulfonate/Graphene Oxide/Graphene Nanosheet/Polyethylene Glycol Phase-Change Composites with Superior Thermal Management for Photo-thermoelectric Generators.

Recently, growing interest in self-powered devices has led to the invention of new energy conversion devices. Photo-thermoelectric generators (PTEGs) have rapidly developed for their ability to harvest both light and thermal energy, but these devices are overly dependent on the continuity of energy input and cannot sustain output in an emergency situation. In the current study, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/graphene oxide (GO)/graphene nanosheets (GNPs)/polyethylene glycol (PEG) phase-change composites (PCCs) were prepared with freeze-drying and vacuum-filling processes to acquire materials suitable for imparting energy storage characteristics to PTEGs. The melting and crystallization enthalpies of the PCCs fabricated based on the PEDOT:PSS/GO/GNP aerogels can reach 211.5 and 207.6 J g-1, respectively, which increase by nearly 5% compared with pure PEG, and the growth rate of thermal conductivity of the composites is as high as 262.7% (1.12 W m-1 K-1). Meanwhile, the excellent photothermal properties and high-temperature shape stability that pure PEG does not possess can also be imparted to PCCs by the aerogels. The PTEG assembled with PCCs and thermoelectric components can achieve a continuous output of over 1500 s after 300 s of light irradiation. After integrating the output of the device during the lamp on/off period, it is found that the total output of the device during the light-off period (8.4 V and 9.6 mW) can far exceed its total output during the light-on period (2.7 V and 4.4 mW). This work provides guidance for modulating the performance of PCCs and giving PTEGs the ability to operate under emergency or extremely harsh conditions and the prepared PTEGs are highly promising for practical use.

Mineral-impregnated carbon-fiber based reinforcing grids as thermal energy harvesters: A proof-of-concept study towards multifunctional building materials

This proof-of-concept study demonstrates for the first time the fabrication of a multifunctional reinforcing grid-building material within a thermoelectric element generator (TEG) configuration. Commercially available carbon fiber yarns, which possess inherent Seebeck coefficient (S) values of −2.5 μV/K (n-type) and +7.4 μV/K (p-type), were thoroughly investigated prior to their impregnation with a geopolymer (GP)-based suspension. The resulting hardened mineral-impregnated carbon-fiber (MCF) reinforcements were subsequently tested regarding their physicochemical and mechanical properties. Afterward, individual MCFs were employed as n-/p-type thermoelements to assemble a grid-like TEG consisting of five serially interconnected junctions. The TEG-enabled reinforcing grid exhibited a voltage output of 1.8 mV, corresponding to a generated power of 22.3 nW upon exposure to an in-plane temperature difference (ΔT) of 50 K. Multifunctional building materials are envisaged to exploit thermal gradients on a large-scale during their service lifetime, contributing towards zero energy consumption constructions.

Acceleration strategy of source iteration method for the stationary phonon Boltzmann transport equation

Mesoscopic numerical simulation has become an important tool in thermal management and energy harvesting at the micro/nano scale, where the Fourier's law failed. However, it is not easy to efficiently solve the phonon Boltzmann transport equation (BTE) from ballistic to diffusive limit. In order to accelerate convergence, an implicit synthetic iterative scheme is developed for the stationary phonon BTE, in which a macroscopic moment equation is invoked and solved iteratively coupled with the typical source iteration of the kinetic equation. Different from previous numerical interpolation, the phonon BTE is solved again at the cell interface along the group velocity direction within a certain length when reconstructing the interfacial phonon distribution function. Fourier stability analysis shows that the present method could converge faster than the source iteration method in the (near) diffusive regime. Numerical results prove that the present scheme can capture the ballistic-diffusive effects correctly and efficiently. The present acceleration framework could be a powerful tool for simulating practical thermal engineering problems in the future.

Molecular Ferroelectric Crystals with Superior Pyroelectricity, Plasticity, and Recyclability.

The pyroelectric effect is used in a wide range of applications such as infrared (IR) detection and thermal energy harvesting, which require the pyroelectric materials to simultaneously have a high pyroelectric coefficient and a low dielectric constant for high figures of merit. However, in conventional proper ferroelectrics, the positive correlation between the pyroelectric coefficient and the dielectric constant imposes an insurmountable challenge in upgrading the figures of merit. Here, we explored superior pyroelectricity in [(CH3)4N][FeCl4] (TMA-FC) and [(CH3)4N][FeCl3Br] (TMA-FCB) molecular ferroelectric plastic crystals, which could decouple this positive correlation due to the nature of improper polarization behavior. Therefore, TMA-FC and TMA-FCB derive a high pyroelectric coefficient and a low dielectric constant simultaneously, yielding record-high figures of merit around room temperature. Furthermore, the favorable plasticity enables ferroelectric crystals to attach surfaces with different shapes for device design and integration. More interestingly, the molecular ferroelectrics could be softened and reshaped at elevated temperatures without decay in pyroelectricity, making them recyclable for cost savings and e-waste reduction. Combined with the facile fabrication process, the findings of this work would open avenues for employing molecular ferroelectric plastic crystals in the manufacture of high-performance pyroelectric devices.

High-performance flexible thermoelectric generator based on silicone rubber and cover with graphite sheet

Harvesting thermal energy through a flexible thermoelectric generator (FTEG) offers an excellent micro-power solution for energizing node sensors in the realm of Internet of Things (IoT) and wearable electronics. Nonetheless, current FTEG suffer from drawbacks including low efficiency, significant thermal resistance, and complex manufacturing procedures. In this study, a high-performance FTEG using silicone rubber was designed and fabricated using a straightforward process. The finite-element method was used to optimize the copper electrode thickness, and the bendable substrate layers with various thermal conductivity were studied for the first time. The copper electrode thickness of 0.1 mm was selected because it offers high flexibility and bendability while still providing a relatively high-power output. The 5 × 5 cm2 FTEG device was fabricated and covered with a bendable substrate. Silicon rubber (0.08 Wm−1K−1), silicon rubber added 5% graphene (0.14 Wm−1K−1), and graphite sheets (15 Wm−1K−1) were used as bendable substrates. The FTEG cover with graphite sheets has a maximum output voltage of 1.1 V under a temperature difference (ΔT) at 65 °C. Its maximum output power is 162.4 mW, corresponding to a power density of 6499.1 µW/cm2 under the same above ΔT. The experimental findings indicated that integrating a bendable substrate with high thermal conductivity and electrical insulation properties enhances the performance of the FTEG.

Toward Self-Powered Sensing and Thermal Energy Harvesting in High-Performance Composites via Self-Folded Carbon Nanotube Honeycomb Structures.

The development of high-performance self-powered sensors in advanced composites addresses the increasing demands of various fields such as aerospace, wearable electronics, healthcare devices, and the Internet-of-Things. Among different energy sources, the thermoelectric (TE) effect which converts ambient temperature gradients to electric energy is of particular interest. However, challenges remain on how to increase the power output as well as how to harvest thermal energy at the out-of-plane direction in high-performance fiber-reinforced composite laminates, greatly limiting the pace of advance in this evolving field. Herein, we utilize a temperature-induced self-folding process together with continuous carbon nanotube veils to overcome these two challenges simultaneously, achieving a high TE output (21 mV and 812 nW at a temperature difference of 17 °C only) in structural composites with the capability to harvest the thermal energy from out-of-plane direction. Real-time self-powered deformation and damage sensing is achieved in fabricated composite laminates based on a thermal gradient of 17 °C only, without the need of any external power supply, opening up new areas of autonomous self-powered sensing in high-performance applications based on TE materials.

A discussion of internal flow characteristics and performance of super long gravity heat pipes for deep geothermal energy extraction

Super long gravity heat pipe (SLGHP) provides a novel way for harvesting thermal energy from a deep geothermal reservoir. Previous research has demonstrated the feasibility of this innovative technique, most studies have focused on the overall performance of the heat extraction system, whereas detailed investigations into the internal flow characteristics remains ambiguous. This hampers the application and further advancement of SLGHPs. In this study, a preliminary analysis of the internal flow resistance characteristics is carried out using of the homogeneous model for two SLGHPs, a regular one and a modified one with an inner coaxial pipe. It indicates that the regular SLGHP has higher flow resistance under the same thermal condition of the reservoir. Specifically, the regular SLGHP experiences approximately 1.75 times larger flow resistance than the modified SLGHP (average temperature of the evaporation section and condenser is 95.6 °C and 55 °C, respectively, with water as the working fluid). While on the other hand, the heat transfer capability of the modified SLGHP was improved 29.73%∼77.77% at various condensation temperature compared with the regular SLGHP. The dominant factor contributing to pressure drop in the regular SLGHP is the frictional loss resulting from the counter-current flow of rising vapor and falling liquid film. Conversely, the SLGHP with a coaxial pipe effectively separates the vapor and condensate flows, reducing frictional resistance and improving heat transfer performance. It is speculated that explosive nucleate pool boiling may occur in the liquid pool at the bottom of the SLGHP, enhancing heat transfer from the liquid bulk to the upward-flowing vapor.

A wide input range, 94% peak efficiency auto‐select multi‐mode boost converter for thermoelectric energy harvesting

SummaryIn this paper, a high‐efficiency DC–DC boost converter is proposed for thermal energy harvesting systems. The converter's efficiency is improved using a low‐power digital operating mode detector circuit. The operating mode detector circuit changes the converter's operating mode between the continuous conduction mode (CCM) and discontinuous conduction mode (DCM) at a specific input voltage (boundary input voltage), which reduces the total losses, thus improves the efficiency and increases the input voltage range. Besides, the proper boundary input voltage is calculated as a function of the boost converter's parameters. Finally, the proposed converter is designed in a standard 0.18 μm CMOS process. Simulation results show that the peak efficiency of the converter can be up to 94%, 60.4%, and 35% at 300, 20, and 15 mV of the open‐circuit voltage, respectively.

Non‐similar heat transfer analysis of magnetized flow of Ag‐Mgo/water hybrid nanofluid flow through darcy porous medium

AbstractThis study aims to examine the magnetized flow of Ag–MgO/water hybrid nanofluid over an extending sheet implanted in Darcy porous medium. Thermal radiations, Joule and viscous dissipations are incorporated into energy equation to account for heat transfer. The convective heat flux boundary condition is imposed at sheet surface. Using non‐similar conversions, governing equations are converted to a system of dimensionless partial differential equations (PDEs). These equations are transformed into ordinary ones by using local non‐similar method. MATLAB's bvp4c function is used to numerically simulate the ordinary differential equations (ODEs). The velocity and thermal profiles for positive variation of essential parameters are illustrated graphically. It was concluded that the velocity profile increases for the rising Darcy number. On the other hand, the temperature profile increased for the positive variation of magnetic number, volume fraction, radiation parameter, Eckert number and Biot number while decreasing for all other parameters. The skin friction coefficient and heat transfer rates are thoroughly investigated and findings are reported through tables. It was found that the magnitude of skin friction coefficient rises with an increase in volume fraction, suction and magnetic parameters while the heat transfer is enhanced by increases in Darcy number, suction parameter, radiation parameter, and Biot number. As per the author's knowledge, no work has previously been published on the current model using the local non‐similarity method. This work may provide insight to researchers interested in thermal systems and solar energy harvesting.