Heat Harvesting Research Articles

Thermodynamic analysis of pavement roads underground heat harvesting through water pipelines for effective management of urban heat island effect integrated to space cooling applications

Urban heat island effect drastically affects the climate by approximately increasing the surroundings temperature by 2°C, thus, rises the energy demand for space cooling. As roads/asphalt pavements makes 20–30% of urban areas that are fully exposed to solar radiations, this unused energy can be utilized by laying down a network of underground cross-linked polyethylene (PEX) pipes full of heat transfer fluids. In this research, the cold-water from a reservoir is pumped to various pipeline configurations to gain effective heat from the pavements/roads. This water circulation absorbs the heat, increasing the temperature (10–25°C) of cold water, also decreasing the pavement temperature by about 15°C. Furthermore, a solar collector is attached to increase temperature of circulated water to be used by low-grade temperature applications. Here, double effect absorption cooling (DEAC) and ejector cooling systems have been analysed thermodynamically for efficient space cooling application. The overall energy and exergy efficiency of DEAC integrated system is calculated to be 40.79% and 5.08%, respectively. The rate of cooling obtained by the evaporator is 6.75 kW, when rate of heat supplied is 4.3 kW. For the ejector cooling integrated system, the overall energy and exergy efficiencies are recorded as 15.27% and 3.38%, respectively. The rate of effective cooling through ejector cooling system is 2.7 kW.

Synergistically Optimized Thermoelectric and Mechanical Properties of Mg3.2Bi1.5Sb0.5-SiC Composites.

Motivated by the surging demand for low-temperature waste heat harvesting, materials with both prominent thermoelectric and good mechanical properties are preferred in practical applications. In this present work, the composite exploration of Te-doped Mg3.2Bi1.5Sb0.5-x vol % nanosized SiC (x = 0, 0.05, 0.1, 0.2, and 0.5) was carried out, where nanosized SiC is physically dispersed in the matrix in the form of a second phase. SiC second phase compositing further optimized the matrix carrier concentration, resulting in a higher power factor in the service temperature range (the highest value from 28.9 to 31.7 μW cm-1 K-2), and the (ZT)ave from 0.91 to 0.96 compared with the matrix sample. In addition, the SiC second phase effectively enhanced the mechanical properties of composite materials, including flexural strength, microhardness, and modulus. Because of the simultaneous optimization of thermoelectric and mechanical properties, the overall performance of Te-doped Mg3.2Bi1.5Sb0.5-0.05 vol % SiC composite is leveraged to meet special requirements of power generation. It is expected that the addition of SiC should be broadly applicable to address the physical performance in other thermoelectric systems.

Synergistic Combination of Sb2Si2Te6 Additives for Enhanced Average ZT and Single-Leg Device Efficiency of Bi0.4Sb1.6Te3-based Composites.

Thermoelectric materials are highly promising for waste heat harvesting. Although thermoelectric materials research has expanded over the years, bismuth telluride-based alloys are still the best for near-room-temperature applications. In this work, a ≈38% enhancement of the average ZT (300-473K) to 1.21 is achieved by mixing Bi0.4Sb1.6Te3 with an emerging thermoelectric material Sb2Si2Te6, which is significantly higher than that of most BiySb2-yTe3-based composites. This enhancement is facilitated by the unique interface region between the Bi0.4Sb1.6Te3 matrix and Sb2Si2Te6-based precipitates with an orderly atomic arrangement, which promotes the transport of charge carriers with minimal scattering, overcoming a common factor that is limiting ZT enhancement in such composites. At the same time, high-density dislocations in the same region can effectively scatter the phonons, decoupling the electron-phonon transport. This results in a ≈56% enhancement of the thermoelectric quality factor at 373 K, from 0.41 for the pristine sample to 0.64 for the composite sample. A single-leg device is fabricated with a high efficiency of 5.4% at ΔT = 164 K further demonstrating the efficacy of the Sb2Si2Te6 compositing strategy and the importance of the precipitate-matrix interface microstructure in improving the performance of materials for relatively low-temperature applications.

Bacterial Cellulose-Based ionogels with synergistically enhanced mechanical and thermoelectric performances for low-grade heat harvesting

Ionogels have a bright prospect for low-grade heat harvesting due to their advantages of non-volatility and high thermal stability. However, it is still challenging to develop ionogels with mechanical robustness and high thermoelectric properties. This work reports a carboxymethylated and Na+ doped bacterial cellulose-based ionogel (CBCIG-Na) with synergistically increased mechanical and thermoelectric performances. The enhancement of thermoelectric properties of ionogels is mainly attributed to the increase of ion transport pathways and the mobility difference between anions and cations in ionic liquid. After the modification, the CBCIG-Na ionogel with 0.5 mol% Na+ (CBCIG-Na0.5%) exhibits high Seebeck coefficient (45.02 mV K−1), ionic conductivity (21.2 mS cm−1) and ionic power factor (4296.82 μW m−1 K−2) when the relative humidity (RH) is 85 %. This is the highest ionic power factor ever reported for bacterial cellulose-based ionogels. Moreover, the CBCIG-Na0.5% shows high tensile strength (7.69 MPa) due to the larger 3D fiber network and enhanced intermolecular force, obvious adhesivity (9.28 kPa) and high transparence (76 %). After that, a flexible i-TE device containing 5 legs and packaged with polydimethylsiloxane (PDMS) was constructed. A high thermovoltage of 264.36 mV is generated when wearing the i-TE device (5 legs) on the forearm at room temperature (ΔT = 1.2 K). The outstanding performances, environmentally friendly and low-cost characters of CBCIG-Na0.5% ionogel indicating its great potential for low-grade heat harvesting.

Coordination enhanced high-seebeck coefficient n-type gel-based thermocells for low-grade energy harvesting and n-p type connected devices

Thermocells (TECs) have shown potential for commercial applications in low-grade heat harvesting, but the development of n-type TECs has always been lagging behind p-type TECs, hindering the practical utilization of integrated devices. Here, n-type and p-type TECs of polyacrylic acid (PAA) and bacterial cellulose (BC) composite hydrogel are developed, respectively. By coordinating Fe3+ with carboxyl groups, the thermal entropy difference between Fe2+ and Fe3+ increases, resulting in a record high Seebeck coefficient (S) of 2.51 mV K−1 of Fe3+/Fe2+ redox couple in n-type TEC. In p-type TEC, a S of −1.29 mV K−1 based on [Fe(CN)6]4‒/[Fe(CN)6]3‒ redox couple is achieved. The 22-pair n-p type connected device generated a voltage of 1.8 V at a temperature difference of 30 K, which is enough to power a calculator and light up an LED without an amplifier. This work offers a fresh perspective for enhancing the performance of n-type TECs and promoting the practical application of n-p type connected devices.

A novel concentrated photovoltaic and ionic thermocells hybrid system for full-spectrum solar cascade utilization

Concentrated photovoltaic-thermoelectric generator (CPV-TEG) hybrid system can harvest the low-grade waste heat derived from photovoltaic cells to generate more electric energy. However, it is economically unfeasible due to the expensive cost of thermoelectric generators and higher photovoltaics temperature. Ionic thermocell based on thermogalvanic effect has a more inexpensive cost and ionic Seebeck coefficient of 1– 2 orders of magnitude higher than thermoelectric generators, which is promising for full-spectrum solar cascade utilization. In present work, a novel concentrated photovoltaic and ionic thermocells hybrid system (CPV-iTEC) is reported, aiming to comprehensively evaluate its feasibility. The ionic thermocell adhered directly to the photovoltaic panel consists the π-type multi-channels, electrodes and p/n-type redox electrolytes flowing in multi-channels, which can simultaneously cool photovoltaic panel and output electric energy for waste heat harvesting. A three-dimensional numerical model is proposed and validated by preliminary experiment to investigate the effects of structural and operating parameters on the performance of CPV-iTEC during energy, exergy and economic analyses. Results show that the optimal structural parameters of ionic thermocell are the 2 mm channel height and 1 mm width, showing a 7.18% higher energy efficiency than that of CPV-TEG at 10 concentration ratios. The presence of ionic thermocell can significantly decrease the photovoltaic panel temperature from 351.30 K to 325.14 K, and thus broaden the available concentration ratio from 1– 21 to 1– 39 compared with CPV-TEG, reaching a higher output power of 16.61 W. During the optimization of advanced redox electrolytes/electrodes, the energy efficiency can reach 49.63% at 21 concentration ratios, which is 5.06% higher than that of CPV-TEG, and show a 22.65% lower overall cost. The application of ionic thermocell in the full-spectrum solar cascade utilization of concentrated photovoltaic system is proved to be feasible with significant advantages of low cost, high performance, and flexible operation.

3D Printed Gelatin Methacrylate Hydrogel‐Based Wearable Thermoelectric Generators

AbstractThe present study focuses on the utilization of a hydrogel consisting of gelatin methacrylate (GelMA) and polyvinyl alcohol (PVA) as a matrix for hosting the redox couple Fe(CN)63−/4−. The hydrogel exhibits a discernable thermopower (Src) of 3 mV K−1. The beneficial effect of the hydrogel microstructure on the mechanical robustness is demonstrated by small‐angle X‐ray scattering (SAXS). Moreover, the hydrogel is used to construct a 3D printed thermoelectric generator (TEG) consisting of eight p‐type thermoelectric legs, which exhibits commendable thermoelectric properties, including an open‐circuit voltage of 64 mV and a power density of 4.0 mW m−2 under a temperature gradient (ΔT) of 2.5 K. These findings demonstrate that 3D printing both enhances the quality of the interface between the hydrogel and electrode and provides a promising method for a more facile TEG fabrication process with the potential for further applications in low‐grade waste heat harvesting.

Zinc-iodine thermal charging cell engineered by cost-efficient electrolytes for low-grade heat harvesting

Emerging ionic thermoelectric devices (i-TEs), with synergistic effect of thermogalvanic effect and thermo-diffusion effect, are promising devices to achieve heat to electricity. We report a zinc-iodine thermal charging cell (ZITCC) enabled by cost-efficient electrolytes for low-grade heat harvesting. Efficient functional electrolytes are obtained through optimization from Zn(NO3)2, Zn(CH3COO)2 and ZnSO4 electrolytes and introducing redox species (I−/I3−). The ZITCCs generate electricity through thermo-diffusion effect of electrolyte ions, as well as the thermogalvanic effect involving I− to I3− conversion and Zn plating, enabling efficient heat-to-electricity conversion. The Seebeck coefficient of 19.2 mV K−1, a high output voltage of ∼1 V and superior normalized maximum power density of 0.44 mW m−2 K−2 is obtained at temperature difference of 15 K. Meanwhile, ZITCCs have good energy storage capacity. In addition, a quasi-solid-state device delivers an 8.5 mW m−2 output power and impressive performance at temperature difference of 8 K can be acquired. This work provides a good strategy for the design of high-Seebeck coefficient, cost-effective i-TEs for low-grade heat harvesting.

Research and development of a new combination of piezo-thermoelectric energy harvester systems from roadways

Due to the problems associated with fossil fuels, scientists and governments are investigating alternative energy sources. In recent decades, there has been an increase in interest in initiatives involving the collection of clean, limitless energy. This paper focuses on two renewable energy harvesting combination technologies: mechanical vibration utilizing piezoelectric technology and thermal sources utilizing thermoelectric technology. Existing scientific literature proposes various techniques for producing and modeling each system individually. This study proposes a novel piezo-thermoelectric pavement model with piezo-thermoelectric coupling. Due to the lack of typical experimentation in the scientific literature, a new laboratory experimental prototype proposes to reproduce artificially and simultaneously heat harvesting on the artificial road surface and mechanical vibration caused by passing vehicles. Testing the laboratory-developed prototype has determined the efficacy of the piezo-thermoelectric coupling electronic model. This study demonstrated that a hybrid piezo-thermoelectric system is more suitable for road pavement applications than a piezo-thermoelectric coupling system. A hybrid combination system can continue to produce energy even if one of the energy sources is unavailable or malfunctioning, whereas a coupling combination system cannot. In laboratory testing, the combined piezo-thermoelectric harvester proposed could generate up to 1.75 μW without optimizing the materials or power generation. This innovative study demonstrates the feasibility and applicability of combining thermoelectric and piezoelectric technology to harvest energy from road surfaces.

Potentials and Limits of PMN-PT and PIN-PMN-PT Single Crystals for Pyroelectric Energy Harvesting

Waste heat is inherent to industrial activities, IT services (e.g., data centers and microprocessors), human mobility, and many other common processes. The power lost each year in this way has been estimated in the 1000 TWh in the EU which, owing to skyrocketing energy prices and not least the urgent need for decarbonizing the economy, has engendered tremendous research efforts among scientists and engineers to recover/recycle this waste energy. Beyond established thermal engineering solutions for waste heat, advances in multifunctional materials open new paradigms for waste heat harvesting. Two smart material types are of particular focus and interest at present; these are thermoelectric and pyroelectric materials, which can both transform heat to electrical power, though via different effects. The present paper summarizes our research work on a new class of pyroelectric materials, namely <111> oriented (1-x)(Pb(Mg1/3Nb2/3)O3–xPbTiO3 (PMN-PT) and x-Pb(In1/2 Nb1/2)O3-y-Pb(Mg1/3 Nb2/3)O3-(1-x-y)-PbTiO3 (PIN-PMN-PT) single crystals that exhibit some of the highest pyroelectric properties ever measured. First, a figure of merit for pyroelectric energy harvesting is derived, followed by a detailed assessment of the properties of the said crystals and how they depend on structure, poling, thickness, and temperature. The properties are further contrasted with those of conventional pyroelectric crystals. It is concluded that the PMN-PT-base single crystals are best suited for harvesting devices with a working temperature range from 40 to 100 °C, which encompasses waste heat generated by data centers and some chemical and industrial processes, affording the highest figure of merit among pyroelectric materials.

Valorization of coconut peat to develop a novel shape-stabilized phase change material for thermal energy storage

Coconut peat (CP) is a product derived from coconut husks and it has been recognized as a valuable resource for the removal of heavy metals, as an oil-sorbent material, and for agricultural purposes. However, large quantities of coconut husks are still discarded yearly, leading to environmental pollution. In this study, a series of CP/polyethylene glycol (CP/PEG) composites were prepared as novel shape-stabilized phase change materials (SSPCMs) by vacuum impregnation to demonstrate a potential new application of CP in the area of thermal energy storage. The composites were investigated by thermogravimetric (TG) analysis, differential scanning calorimeter (DSC), scanning electron microscopy, attenuated total reflection-Fourier transform infrared spectrometer, leakage test, and thermal cycling testing. The results revealed that the CP/PEG composites exhibit minimal leakage of PEG while the composite with a CP/PEG mass ratio of 3:7 has a high latent heat storage of 108.5 J/g and relative enthalpy efficiency of 91.7%. Meanwhile, TG results indicated that it has good thermal stability up to 150 °C and the thermal cycling test showed that it has excellent thermal reliability even after 100 thermal cycles. The heat harvesting and releasing performance of the sample was evaluated as well to understand its thermoregulation capability. Therefore, the obtained CP/PEG composite demonstrates that it can potentially be utilized for thermal storage applications such as in wallboard or building materials for passive cooling, thus fostering a cleaner process by realizing energy savings.

Harvesting Thermal Energy through Pyroelectric and Thermoelectric Nanomaterials for Catalytic Applications

The current scenario sees over 60% of primary energy being dissipated as waste heat directly into the environment, contributing significantly to energy loss and global warming. Therefore, low-grade waste heat harvesting has been long considered a critical issue. Pyroelectric (PE) materials utilize temperature oscillation to generate electricity, while thermoelectric (TE) materials convert temperature differences into electrical energy. Nanostructured PE and TE materials have recently gained prominence as promising catalysts for converting thermal energy directly into chemical energy in a green manner. This short review provides a summary and comparison of catalytic processes initiated by PE and TE effects driven by waste thermal energy. The discussion covers fundamental principles and reaction mechanisms, followed by the introduction of representative examples of PE and TE nanomaterials in various catalytic fields, including water splitting, organic synthesis, air purification, and biomedical applications. Finally, the review addresses challenges and outlines future prospects in this emerging field.

Magneto-Induced Janus Adhesive-Tough Hydrogels for Wearable Human Motion Sensing and Enhanced Low-Grade Heat Harvesting.

Janus hydrogels with different properties on the two surfaces have considerable potential in the field of material engineering applications. Various Janus hydrogels have been developed, but there are still some problems, such as stress mismatch caused by the double-layer structure and Janus failure caused by material diffusion in the gradient structure. Here, we report a Janus adhesive-tough hydrogel with polydopamine-decorated Fe3O4 nanoparticles (Fe3O4@PDA) at one side induced by magnetic field to avoid uncontrollable material diffusion in the cross-linking polymerization of acrylamide with alginate-calcium. The magneto-induced Janus (MIJ) hydrogel has an adhesive surface and a tough bulk without an obvious interface to avoid stress mismatch. Due to the intrinsic dissipative matrix and the abundant catechol groups on the adhesive surface, it shows strong adhesion onto various substrates. The MIJ hydrogel has high sensitivity (GF = 0.842) in detecting tiny human motion. Owing to the synergy of Fe3O4@PDA-enhanced interfacial adhesion and heat transfer, it is possible to quickly generate effective temperature differences when adhering to human skin. The MIJ hydrogel achieves a Seebeck coefficient of 13.01 mV·K-1 and an output power of 462.02 mW·m-2 at a 20 K temperature difference. This work proposes a novel strategy to construct Janus hydrogels for flexible wearable devices in human motion sensing and low-grade heat harvesting.

Inside Back Cover

This review explores the application of thermoelectric hydrogels in self‐powered sensors, integrated modules, and heat harvesting management. More details are discussed in the article by Wang et al. on page 543—556.image

Thermodynamics of Ionic Thermoelectrics for Low-Grade Heat Harvesting

Thermodynamics of Ionic Thermoelectrics for Low-Grade Heat Harvesting

Thermoelectrocatalysis: an emerging strategy for converting waste heat into chemical energy.

This perspective defines and explores an innovative waste heat harvesting strategy, thermoelectrocatalysis (TECatal), emphasizing materials design and potential applications in clean energy, environmental, and biomedical technologies.

Energy Density in Ionic Thermoelectric Generators by Prussian Blue Electrodes

Solid-state ionic thermoelectric generators have emerged as promising solutions for efficient harvesting of low-grade waste heat. However, the main challenge in achieving continuous power supply is the low efficiency of thermoelectric conversion. In this work, substantial achievements have been made in improving the thermoelectric conversion characteristics by introducing redox pairs on the electrode surfaces. This approach takes advantage of the synergistic effect of thermal diffusion and thermoelectric effects to maximize the conversion efficiency. To improve the thermoelectric storage and output power performance, Prussian blue was attached to a carbon woven fabric and used as an electrode. The incorporation of Prussian blue/carbon woven fabric electrodes results in an increase in current density output and an instantaneous power density of 3.7 mW/m 2 ·K 2 . Furthermore, under a temperature gradient of 10 K, the output energy density for 2 h is 194 J/m 2 , and the Carnot relative efficiency is as high as 0.12% at a hot side temperature ( T H ) of 30 °C and a cold side temperature ( T C ) of 20 °C. Our findings validate the efficacy of integrating thermal diffusion and redox reactions in ionic thermoelectric generators, paving the way for the progress of thermocharged devices and their potential commercial applications.

Synthesis and enhanced room-temperature thermoelectric properties of CuO-MWCNT hybrid nanostructured composites.

This work presents the synthesis of novel copper oxide-multiwalled carbon nanotube (CuO-MWCNT) hybrid nanostructured composites and a systematic study of their thermoelectric performance at near-room temperatures as a function of MWCNT wt% in the composite. The CuO-MWCNT hybrid nanostructured composites were synthesized by thermal oxidation of a thin metallic Cu layer pre-deposited on the MWCNT network. This resulted in the complete incorporation of MWCNTs in the nanostructured CuO matrix. The thermoelectric properties of the fabricated CuO-MWCNT composites were compared with the properties of CuO-MWCNT networks prepared by mechanical mixing and with the properties of previously reported thermoelectric [CuO]99.9[SWCNT]0.1 composites. CuO-MWCNT hybrid composites containing MWCNTs below 5 wt% showed an increase in the room-temperature thermoelectric power factor (PF) by ∼2 times compared with a bare CuO nanostructured reference thin film, by 5-50 times compared to mixed CuO-MWCNT networks, and by ∼10 times the PF of [CuO]99.9[SWCNT]0.1. The improvement of the PF was attributed to the changes in charge carrier concentration and mobility due to the processes occurring at the large-area CuO-MWCNT interfaces. The Seebeck coefficient and PF reached by the CuO-MWCNT hybrid nanostructured composites were 688 μV K-1 and ∼4 μW m-1 K-2, which exceeded the recently reported values for similar composites based on MWCNTs and the best near-room temperature inorganic thermoelectric materials such as bismuth and antimony chalcogenides and highlighted the potential of CuO-MWCNT hybrid nanostructured composites for applications related to low-grade waste heat harvesting and conversion to useable electricity.

Wholly degradable quasi-solid-state thermocells for low-grade heat harvesting and precise thermal sensing

Flexible thermocell devices are designed with device-level degradability for low-grade heat harvesting and precise thermal sensing.

Global softening to manipulate sound velocity for reliable high-performance MgAgSb thermoelectrics

A global softening strategy using stearic acid in MgAgSb to reduce sound velocity has been shown to enhance its thermoelectric performance, achieving high zT values and demonstrating strong potential for low-grade heat harvesting.