Thermal Energy Harvesting Research Articles

Topology-optimized freeform thermal metamaterials for omnidirectionally cloaking sensors

The cloaked sensor, which is capable of cloaking itself and still receiving an incoming signal simultaneously, has attracted widespread concern. In the thermal diffusion community, the possibility of designing thermal metamaterials for cloaking sensors has been successfully proved with the help of transformation thermotics and scattering cancelling technology. However, a long-held issue is that designing thermal metamaterials with freeform shape and omnidirectional functionality for cloaking sensors. Here, a synthetic strategy is reported to design freeform thermal metamaterials for omnidirectionally cloaking sensors and making sensors normally receive and measure external heat signals without distortion simultaneously. An irregularly shaped thermal metamaterial is designed by multiscale topology optimization and fabricated by 3D printing, and its thermal functionality is verified numerically and experimentally. This work addresses an intriguingly important challenge in cloaking sensors in terms of freeform thermal metamaterials with omnidirectional functionality that is not possibly achieved via the current design technologies for thermal metamaterials, thus laying a foundation for the application of thermal metamaterials in sensing systems, and it may be extended to thermal energy harvesting, thermoelectric converting, thermal camouflage and illusion metadevices.

Multi-octave metasurface-based refractory superabsorber enhanced by a tapered unit-cell structure

An ultra-broadband metasurface-based perfect absorber is proposed based on a periodic array of truncated cone-shaped text {TiO}_2 surrounded by TiN/text {TiO}_2 conical rings. Due to the refractory materials involved in the metasurface, the given structure can keep its structural stability at high temperatures. The proposed structure can achieve a broadband spectrum of 4.3 µm at normal incidence spanning in the range of 0.2–4.5 µm with the absorption higher than 90% and the average absorption around 94.71%. The absorption can be tuned through the angle of the cone. By optimizing geometrical parameters, a super absorption is triggered in the range of 0.2–3.25 µm with the absorption higher than 97.40% and substantially average absorption over 99%. In this regard, the proposed structure can gather more than 99% of the full spectrum of solar radiation. Furthermore, the absorption of the designed structure is almost insensitive to the launching angle up to 50^circ for TE polarization, while it has a weak dependence on the incident angle for TM polarization. The proposed structure can be a promising candidate for thermal energy harvesting and solar absorption applications.

Flexible Piezoelectric and Pyroelectric Nanogenerators Based on PAN/TMAB Nanocomposite Fiber Mats for Self-Power Multifunctional Sensors.

Self-powered wearable electronics to convert mechanical and thermal energy into electrical energy are important for biomedical monitoring, which highly require good flexibility, comfortability, signal sensitivity, and accuracy. In this work, composite nanofiber mats of polyacrylonitrile (PAN) and trimethylamine borane (TMAB) were prepared by electrospinning, which exhibited excellent piezoelectric and pyroelectric abilities in harvesting mechanical and thermal energy. The PAN/TMAB-4 nanofiber mats not only generated a high voltage of up to 2.56 V and a high power of 0.19 μW upon shape deformation but also exhibited linear voltage response to thermal gradient. The hybrid piezoelectric and pyroelectric output signals were successfully integrated together and have been applied to precisely monitor human vital signs, including elbow bending angles, foot posture, and breathing status, in real time by attaching the flexible sensors to proper human body parts. Overall, good flexibility, bifunctional sensing ability, and self-power make PAN-/TMAB-type sensors very attractive in fabricating high-performance electronics for detecting motion, monitoring health, and making portable microelectronics.

Dual-Level Voltage Bipolar Thermal Energy Harvesting System from Solar Radiation in Malaysia

Harvesting energy from solar radiation in Malaysia attracts the attention of researchers to utilize the potential by ongoing improvement. Roofing material with low albedo absorbs the heat, that can then be harvested using a thermoelectric generator. Previous research only measured the open-circuit voltage with different thermoelectric generator configurations. Low power output limits the potential to be utilized. The low output power can be increased using a DC converter. However, the converter must be tuned concerning low- and high-voltage levels, bipolar, and the maximum power point tracking. Therefore, this paper presents a dual-level voltage bipolar (DLVB) thermal energy harvesting system. The circuit is tested at constant and various time intervals to evaluate the system’s functionality and performance. Experiment results show that the proposed harvesting system can boost from 0.6 and 1.6 V to achieve the optimum level. The mean efficiency of the harvesting circuit obtains 91.92% at various time intervals. Further, the field test result obtains output power from 1.45 to 66.1 mW, with the mean efficiency range of 89.62% to 92.98%. Furthermore, recommendations are listed for future research.

Direct Wet-Spun Single-Walled Carbon Nanotubes-Based p-n Segmented Filaments toward Wearable Thermoelectric Textiles.

Three-dimensional thermoelectric (TE) textiles (TETs) fabricated with TE filaments (TEFs) possess merits over other types such as thickness-direction thermal energy harvesting and excellent conformability with dynamic body curves, revealing the prospect of generating electricity for on-body application. Nonetheless, there is still a lack of a costless but scalable method to automatically and seamlessly produce in-series interconnected p-n segmented TEFs with high TE properties via conventional fiber spinning processes. Here, we developed an alternate wet-spinning strategy to continuously manufacture single-walled carbon nanotube-based p-n segmented TEFs at large scale. The TEF with high electrical conductivity (400-800 S cm-1) displays a low contact resistivity of 189.8 μΩ cm2 between the segments and interelectrode, showing 2 orders of magnitude smaller than that reported in the literature. More importantly, the power factors of p-type and n-type segments are 26.25 and 17.14 μW m-1 K-2, respectively, which are 3 and 4 orders of magnitude higher than those of advanced studies. We finally embroidered it into spacer fabric to fabricate a wearable TET, demonstrating an output power density of 501 nW m-2 at ΔT = 27.7 K. The methodology can inspire the development of fiber-based electronics such as wearable TEs and diodes and so forth.

Large harvested energy with non-linear pyroelectric modules

Coming up with sustainable sources of electricity is one of the grand challenges of this century. The research field of materials for energy harvesting stems from this motivation, including thermoelectrics1, photovoltaics2 and thermophotovoltaics3. Pyroelectric materials, converting temperature periodic variations in electricity, have been considered as sensors4 and energy harvesters5–7, although we lack materials and devices able to harvest in the joule range. Here we develop a macroscopic thermal energy harvester made of 42 g of lead scandium tantalate in the form of multilayer capacitors that produces 11.2 J of electricity per thermodynamic cycle. Each pyroelectric module can generate up to 4.43 J cm−3 of electric energy density per cycle. We also show that two of these modules weighing 0.3 g are sufficient to sustainably supply an autonomous energy harvester embedding microcontrollers and temperature sensors. Finally, we show that for a 10 K temperature span these multilayer capacitors can reach 40% of Carnot efficiency. These performances stem from (1) a ferroelectric phase transition enabling large efficiency, (2) low leakage current preventing losses and (3) high breakdown voltage. These macroscopic, scalable and highly efficient pyroelectric energy harvesters enable the reconsideration of the production of electricity from heat.

High-Power-Density Energy-Harvesting Devices Based on the Anomalous Nernst Effect of Co/Pt Magnetic Multilayers.

The anomalous Nernst effect (ANE) is a thermomagnetic phenomenon with potential applications in thermal energy harvesting. While many recent works studied the approaches to increase the ANE coefficient of materials, relatively little effort was devoted to increasing the power supplied by the effect. Here, we demonstrate a nanofabricated device with record power density generated by the ANE. To accomplish this, we fabricate micrometer-sized devices in which the thermal gradient is 3 orders of magnitude higher than conventional macroscopic devices. In addition, we use Co/Pt multilayers, a system characterized by a high ANE thermopower (∼1 μV/K), low electrical resistivity, and perpendicular magnetic anisotropy. These innovations allow us to obtain power densities of around 13 ± 2 W/cm3. We believe that this design may find uses in harvesting wasted energy, e.g., in electronic devices.

Emerging Solid-to-Solid Phase-Change Materials for Thermal-Energy Harvesting, Storage, and Utilization.

Phase-change materials (PCMs) offer tremendous potential to store thermal energy during reversible phase transitions for state-of-the-art applications. The practicality of these materials is adversely restricted by volume expansion, phase segregation, and leakage problems associated with conventional solid-liquid PCMs. Solid-solid PCMs, as promising alternatives to solid-liquid PCMs, are gaining much attention toward practical thermal-energy storage (TES) owing to their inimitable advantages such as solid-state processing, negligible volume change during phase transition, no contamination, and long cyclic life. Herein, the aim is to provide a holistic analysis of solid-solid PCMs suitable for thermal-energy harvesting, storage, and utilization. The developing strategies of solid-solid PCMs are presented and then the structure-property relationship is discussed, followed by potential applications. Finally, an outlook discussion with momentous challenges and future directions is presented. Hopefully, this review will provide a guideline to the scientific community to develop high-performance solid-solid PCMs for advanced TES applications.

Energy Harvest in Ferromagnet‐Embedded Surface Acoustic Wave Devices

AbstractSurface acoustic wave (SAW) filters are at the center of the radio frequency module in mobile terminals. With the increase of SAW frequency and the miniaturization of devices, the heat generation becomes prominent in SAW devices, providing a potential platform for thermal energy harvesting. Here ferromagnetic materials are embedded into SAW delay lines to demonstrate that thermal energy can be harvested to generate robust transverse electrical signals via anomalous Nernst effect, which are comparable with the anomalous Hall signals when an electric current is intensively applied. The transverse voltage exhibits nonreciprocal features, both amplitude and polarity are dependent on the propagation direction of SAW in contrast to its longitudinal counterpart, which only changes the amplitude but remains polarity with opposite SAW direction. The finding provides a different platform for thermoelectric energy conversion and opens a window for thermal energy harvesting and acoustic or heat flow sensing in SAW devices.

In situ boiling-induced self-assembly of stably self-dispersed crumpled graphene for efficient utilization of thermal energy

Boiling can induce the self-assembly of nanomaterials in nanofluids to generate reliable micro/nanostructures for enhancing boiling heat transfer. The practical applications of most nanofluids in boiling, however, are limited by the poor thermal stability at the boiling point, and improving the thermal stability of nanofluids at such high temperature is challenging. Here, we demonstrate that the crumpled graphene can remain uniformly and stably dispersed within deionized water at the boiling point of 100 °C due to its highly wrinkled structure, while self-assembling to generate a porous structure on the substrate during boiling. Such porous structure can vary the surface micromorphology, wettability and roughness, which increases the nucleation sites and departure frequency of bubbles, thus effectively enhancing the critical heat flux and the maximum heat transfer coefficient by 80.7 and 89.1% compared to the pristine copper substrate, respectively. This in situ boiling-induced self-assembly provides a facile and low-cost strategy to achieve high-performance boiling heat transfer for a wide variety of nanomaterials. The crumpled graphene/water nanofluid used can be directly utilized as a phase change working medium in boiling systems to harvest thermal energy and generate clean steam.

Porous pyroelectric ceramic with carbon nanotubes for high-performance thermal to electrical energy conversion

The recycling of low-grade thermal energy from our surroundings is an environmental-friendly approach to contribute to sustainability, which remains a grand challenge. Herein, a high-performance porous pyroelectric ceramic formed using carbon nanotubes (CNT) is designed and fabricated using a modified solid-state reaction technique. Localized characterization of PMN-PMS-PZT and PMN-PMS-PZT with 0.3 wt% CNT additions by piezoelectric force microscopy suggests that the presence of porosity and defects in grains can restrict the reversal of domains and weaken the local piezoresponse; that is due to the influence of porosity on the electric field, domain morphology, or screening effects induced by defects at the pore surface. More importantly, the porous ceramics showed enhanced figure of merits, including voltage responsibility and energy harvesting figure of merit, compared to the dense ceramic. The harvested energy increased by 208% when the 0.3 wt% of CNT was added to produce porosity, which has a potential application in thermal energy harvesting and sensing system.

Thermally assisted efficient electrochemical lithium extraction from simulated seawater

Extracting lithium electrochemically from seawater has the potential to resolve any future lithium shortage. However, electrochemical extraction only functions efficiently in high lithium concentration solutions. Herein, we discovered that lithium extraction is temperature and concentration dependent. Lithium extraction capacity (i.e., the mass of lithium extracted from the source solutions) and speed (i.e., the lithium extraction rate) in electrochemical extraction can be increased significantly in heated source solutions, especially at low lithium concentrations (e.g., < 3 mM) and high Na+/Li+ molar ratios (e.g., >1000). Comprehensive material characterization and mechanistic analyses revealed that the improved lithium extraction originates from boosted kinetics rather than thermodynamic equilibrium shifts. A higher temperature (i.e., 60 oC) mitigates the activation polarization of lithium intercalation, decreases charge transfer resistances, and improves lithium diffusion. Based on these understandings, we demonstrated that a thermally assisted electrochemical lithium extraction process could achieve rapid (36.8 mg g-1 day-1) and selective (51.79% purity) lithium extraction from simulated seawater with an ultrahigh Na+/Li+ molar ratio of 20,000. The integrated thermally regenerative electrochemical cycle can harvest thermal energy in heated source solutions, enabling a low electrical energy consumption (11.3–16.0 Wh mol-1 lithium). Furthermore, the coupled thermal-driven membrane process in the system can also produce freshwater (13.2 kg m-2 h-1) as a byproduct. Given abundant low-grade thermal energy availability, the thermally assisted electrochemical lithium extraction process has excellent potential to realize mining lithium from seawater.

Harvesting of residuary thermal energy from Mg modified BNT-BKT pyroelectric solid solution using Olsen cycle

The incessant necessity of self-powered applications such as bio-medical devices, a wireless sensor etc has driven the request for high-production lead free pyroelectric materials towards energy harvesting. Herein, magnesium substituted bismuth sodium potassium titanate pyroelectric systems (Bi0.50Na0.41K0.09)(Ti1-xMgx)O3 where x = 0, 0.01 and 0.02 was synthesized. An indispensable role of magnesium on structural, microstructural and dielectric properties has been investigated. The co-existence of rhombohedral and tetragonal phase is confirmed by the X-ray diffraction pattern. The ferroelectric to relaxor/antiferro-electric phase transition was observed to be decrease from 150 °C to 76 °C with increase in doping from 0% to 2% respectively. Thereafter, the Olsen cycle was performed on the ferroelectric loops to demonstrate the energy harvesting properties of fabricated materials. The maximum of 475 mJ cm−1, 479 mJ cm−1 and 588 mJ cm−1 heat energy has been harvested from x = 0, 0.01 and 0.02 samples respectively. The Present work un-plug the various opportunities to develop high-performance pyroelectric energy harvester by utilizing lead free materials.

Effects of internal airflows on the heat exchange potential and mechanics of energy walls

Energy walls are earth retaining structures equipped with pipe heat exchangers that harvest thermal energy sources from underground. Unlike fully embedded energy geostructures such as energy piles, energy walls interact not only with the ground but also with internal airflows under a wide range of conditions. Yet little is known as to how the internal airflows affect the heat exchange potential and mechanics of energy walls. In this paper, we conduct a systematic investigation into the effects of internal airflows on various aspects of energy wall performance, from the harvestable thermal power to the structural behavior. We construct and validate a numerical model of a full-scale energy wall. We then utilize the model to develop a comprehensive understanding of how internal airflows affect the energy, geotechnical, and structural performance of energy walls under a wide range of site and operational conditions. Results show that an internal airflow not only increases the heat exchange potential of energy walls but also leads to marked variations in the thermally induced axial forces, shear forces, and bending moments in the walls. Our findings consistently indicate that even minimal airflows should be considered in the analysis and design of energy walls.

Processing of hybrid shape memory alloy coupled with P(VDF-TrFE) piezoelectric polymer composite for energy harvesting application

Small-scale energy harvesting to power self-powered electronic devices is tremendously increasing. In this context, the ability to combine thermal and mechanical energy harvesting using smart materials deserves more attention for further study. We have presented the feasibility of using P(VDF-TrFE) piezoelectric polymer coupled with NiTi shape memory alloy (SMA) to harvest both mechanical and thermal energy in simple scalable devices. A novel hybrid composite consisting of SMA, and P(VDF-TrFE) has been developed without any complex composites or patterned structures, which couples the piezoelectric effect of the poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) and the shape memory effects of NiTi SMA. We have also fabricated a multi-layer SMA/PEN/P(VDF-TrFE) device using epoxy as an interface and bonding layer. During bending, the P(VDF-TrFE) device with an active area of 3.68 cm2 generated an output voltage of 8 V and an output power of 6.25 µW. When coupled with NiTi SMA to create a hybrid composite thermoelectric material, the device can convert the phase change associated with SMA, and thus thermal energy is converted to mechanical energy and finally into electrical energy. The SMA/P(VDF-TrFE) hybrid composite produced an output voltage of ∼2 V for one cyclic heating and cooling of the device. This is a promising approach for the further development of coupled hybrid energy harvester devices for the powering of small-scale electronic devices such as sensors, MEMS, or biomedical devices.

Energy harvesting from human body heat using highly flexible thermoelectric generator based on Bi2Te3 particles and polymer composite

Energy harvesting technology is in high demand for self-powered devices. Flexible thermoelectric generators (f-TEGs) have attracted considerable attention for application in wearable electronics, which can be applied to healthcare and smart clothing. However, previous f-TEGs still have limitations such lack of novel research focus concerning its wearable applications and complicated and high-cost fabrication procedures. This study reports an f-TEG comprising Bi2Te3-based thermoelectric composite films on a flexible substrate to demonstrate thermoelectric energy conversion from the human body. To verify the thermoelectric energy harvesting efficiency of the energy harvester based on temperature gradients, an f-TEG was developed by placing p-type Bi0.5Sb1.5Te3 (BST) of 133 μWm−1K−2 and n-type Bi2Te2.7Se0.3 (BTS) of 124 μWm−1K−2 thermoelectric films. The fabricated f-TEG harvested an output voltage of 18.13 mV, current signal of 2.74 μA, and output power of ≈ 12.6 nW at ΔT = 25 K. The theoretical thermoelectric potential distribution of the f-TEG was confirmed by finite element analysis using multiphysics simulation. Furthermore, we investigated the biological output performance of the f-TEG on the human body, which showed the output performance of ≈ 0.38 μA and ≈ 2.3 mV. This study presents the possibility of harvesting thermal energy from human body heat, which will be useful for improving wearable device applications.

Giant pyroelectricity in nanomembranes.

Pyroelectricity describes the generation of electricity by temporal temperature change in polar materials1-3. When free-standing pyroelectric materials approach the 2D crystalline limit, how pyroelectricity behaves remained largely unknown. Here, using three model pyroelectric materials whose bonding characters along the out-of-plane direction vary from van der Waals (In2Se3), quasi-van der Waals (CsBiNb2O7) to ionic/covalent (ZnO), we experimentally show the dimensionality effect on pyroelectricity and the relation between lattice dynamics and pyroelectricity. We find that, for all three materials, when the thickness of free-standing sheets becomes small, their pyroelectric coefficients increase rapidly. We show that the material with chemical bonds along the out-of-plane direction exhibits the greatest dimensionality effect. Experimental observations evidence the possible influence of changed phonon dynamics in crystals with reduced thickness on their pyroelectricity. Our findings should stimulate fundamental study on pyroelectricity in ultra-thin materials and inspire technological development for potential pyroelectric applications in thermal imaging and energy harvesting.

A Symmetrical Terahertz Triple-Band Metamaterial Absorber Using a Four-Capacitance Loaded Complementary Circular Split Ring Resonator and an Ultra-Thin ZnSe Substrate

In this research work, a symmetrical four-capacitance loaded complementary circular split ring resonator is proposed, which uses an ultra-thin Zinc Selenide (ZnSe) substrate to realize a low-profile triple-band metamaterial (MTM) perfect absorber for application in the terahertz (THz) frequency range. The electromagnetic properties of the proposed structure were calculated and investigated using the Finite Integration Technique (FIT). The proposed structure exhibited three highly absorptive (nearly perfect) peaks at the resonance frequencies of 15.68 THz, 37.48 THz, and 39.55 THz. Furthermore, the absorber was found to be insensitive to the polarization and incident wave angles, due to its symmetrical design. The effects of the conductor type, substrate thickness, unit cell dimension, resonator gap, and substrate type on the reflection and absorption spectra were investigated. To validate the numerical results, the proposed design was analyzed using High-Frequency Simulation Software (HFSS) and Advanced Design System (ADS). The surface current, electric field, and magnetic field distributions at the three-resonance frequency were analyzed. It was concluded that the overall performance of the proposed MTM structure was superior compared to those reported in the literature. The proposed design could be a good candidate for application in stealth technology, imaging, and thermal energy harvesting.

Energy-Saving Pathways for Thermoelectric Nanomaterial Synthesis: Hydrothermal/Solvothermal, Microwave-Assisted, Solution-Based, and Powder Processing.

The pillars of Green Chemistry necessitate the development of new chemical methodologies and processes that can benefit chemical synthesis in terms of energy efficiency, conservation of resources, product selectivity, operational simplicity and, crucially, health, safety, and environmental impact. Implementation of green principles whenever possible can spur the growth of benign scientific technologies by considering environmental, economical, and societal sustainability in parallel. These principles seem especially important in the context of the manufacture of materials for sustainable energy and environmental applications. In this review, the production of energy conversion materials is taken as an exemplar, by examining the recent growth in the energy‐efficient synthesis of thermoelectric nanomaterials for use in devices for thermal energy harvesting. Specifically, “soft chemistry” techniques such as solution‐based, solvothermal, microwave‐assisted, and mechanochemical (ball‐milling) methods as viable and sustainable alternatives to processes performed at high temperature and/or pressure are focused. How some of these new approaches are also considered to thermoelectric materials fabrication can influence the properties and performance of the nanomaterials so‐produced and the prospects of developing such techniques further.

A cooling design for photovoltaic panels – Water-based PV/T system

This paper proposes an innovative thermal collector for photovoltaic-thermal (PV/T) systems. The thermal behavior of the photovoltaic module and the designed cooling box flow are coupled to achieve the thermal and electrical conversion efficiencies of the water-based PV/T system. Different inlet mass flow rates and temperatures are simulated under normal operating cell temperature conditions (NOCT). The temperature distribution and average temperature of the photovoltaic module layers are investigated. The results show that when the mass flow rate is 0.014 kg/s, and the inlet flow temperature is 15 °C, the PV module reaches an electrical conversion efficiency of 17.79% with 76.13% of thermal efficiency. The designed cooling system exhibits better performance with a significant increase in thermal and electrical conversion efficiency compared to current solutions in the literature. The findings in this paper highlight the utility of PV/T systems and their massive potential to popularize the solar energy field and harvest thermal and electrical energy simultaneously.