Harvesting Low-grade Heat Research Articles

Thermo-osmotic ionogel enabled high-efficiency harvesting of low-grade heat

A new thermo-osmotic approach demonstrated a revolutionary peak efficiency of 11.17% in harvesting low-grade heat for power generation.

Thermally regenerative electrochemically cycled flow batteries with pH neutral electrolytes for harvesting low-grade heat.

Harvesting waste heat with temperatures lower than 100 °C can improve the system efficiency and reduce greenhouse gas emissions, yet it has been a longstanding and challenging task. Electrochemical methods for harvesting low-grade heat have aroused research interest in recent years due to the relatively high effective temperature coefficient of the electrolytes (>1 mV K-1) compared with the thermopower of traditional thermoelectric devices. Compared with other electrochemical devices such as the temperature-variation based thermally regenerative electrochemical cycle and temperature-difference based thermogalvanic cells, the thermally regenerative electrochemically cycled flow battery (TREC-FB) has the advantages of providing a continuous power output, decoupling the heat source and heat sink, and recuperating heat, and compatible with stacking for scaling up. However, the TREC-FB suffers from the issue of stable operation due to the challenge of pH matching between catholyte and anolyte solutions with desirable temperature coefficients. In this work, we demonstrate a pH-neutral TREC-FB based on KI/KI3 and K3Fe(CN)6/K4Fe(CN)6 as the catholyte and anolyte, respectively, with a cell temperature coefficient of 1.9 mV K-1 and a power density of 9 μW cm-2. This work also presents a comprehensive model with a coupled analysis of mass transfer and reaction kinetics in a porous electrode that can accurately capture the flow rate dependence of the power density and energy conversion efficiency. We estimate that the efficiency of the pH-neutral TREC-FB can reach nearly 9% of the Carnot efficiency at the maximum power output with a temperature difference of 37 K. Via analysis, we identify that the mass transfer overpotential inside the porous electrode and the resistance of the ion exchange membrane are the two major factors limiting the efficiency and power density, pointing to directions for future improvements.

Ionic thermoelectric materials for waste heat harvesting

Ionic thermoelectric polymers are a new class of materials with great potential for use in low-grade waste heat harvesting and the field has seen much progress during the recent years. In this work, we briefly review the working mechanism of such materials, the main advances in the field and the main criteria for performance comparison. We examine two types of polymer-based ionic thermoelectric materials: ionic conductive polymer and ionogels. Moreover, as a comparison, we also examine the more conventional ionic liquid electrolytes. Their performance, possible directions of improvements and potential applications have been evaluated.

Thermoelectric Converters Based on Ionic Conductors

Thermoelectric materials represent a new paradigm for harvesting low-grade heat, which would otherwise be dissipated to the environment uselessly. Relative to conventional thermoelectric materials generally composed of semiconductors or semi-metals, ionic thermoelectric materials are rising as an alternative choice which exhibit higher Seebeck coefficient and lower thermal conductivity. The ionic thermoelectric materials own a completely different thermoelectric conversion mechanism, in which the ions do not enter the electrode but rearrange on the electrode surface to generate a voltage difference between the hot and cold electrodes. This unique character has inspired worldwide interests on the design of ionic-type thermoelectric converters with attractive advantages of high flexibility, low cost, limited environmental pollution, and self-healing capability. Referring to the categories of ionic thermoelectric conversion, some representative ionic thermoelectric materials with their respective characteristics are summarized in this minireview. In addition, examples of applying ionic thermoelectric materials in supercapacitors, wearable devices, and fire warning system are also discussed. Insight into the challenges for the further development of ionic thermoelectric materials is finally provided.

Liquid Thermocells Enable Low-Grade Heat Harvesting

Recently, two Science papers tackled the challenge of harvesting low-grade heat via liquid thermocells. One combines TGC and TDC to significantly enhance thermopower by an order of magnitude, and the other reveals the selectively thermosensitive crystallization/dissolution at two electrodes. These intriguing and inspiring advances in terms of materials, mechanisms, and performances are briefly introduced, and some remaining challenges and future directions are outlined for further thermopower improvement and practical implementation.

Thermosensitive crystallization-boosted liquid thermocells for low-grade heat harvesting.

Low-grade heat (below 373 kelvin) is abundant and ubiquitous but is mostly wasted because present recovery technologies are not cost-effective. The liquid-state thermocell (LTC), an inexpensive and scalable thermoelectric device, may be commercially viable for harvesting low-grade heat energy if its Carnot-relative efficiency (ηr) reaches ~5%, which is a challenging metric to achieve experimentally. We used a thermosensitive crystallization and dissolution process to induce a persistent concentration gradient of redox ions, a highly enhanced Seebeck coefficient (~3.73 millivolts per kelvin), and suppressed thermal conductivity in LTCs. As a result, we achieved a high ηr of 11.1% for LTCs near room temperature. Our device demonstration offers promise for cost-effective low-grade heat harvesting.

Numerical and experimental study of a two-phase thermofluidic oscillator with regenerator achieving low temperature-differential oscillation

In this work, a two-phase thermofluidic oscillator with regenerator is numerically and experimentally investigated, focusing on the mechanism of regenerator in lowering the onset temperature. A modified acoustic-electric analogy model, considering the thermophysical properties of regenerator, is developed to predict the onset temperature difference and resonant frequency at onset point, which is then verified by experiments. The influences of the material and type of regenerator on the onset temperature difference, resonant frequency and pressure amplitude have been investigated. By inserting the regenerator packed with copper mesh screens, the onset temperature difference is decreased from 35.1 °C to 7.0 °C, which is lowest ever reported in the literatures. Additionally, the pressure amplitude is increased from 4.5 kPa to 10.7 kPa at a hot temperature of 46 °C. The merits of simple construction, free maintenance and low temperature-differential onset enable this updated two-phase thermofluidic oscillator suitable for low-grade heat harvesting.

Dynamic modelling and analysis of an adsorption-based power and cooling cogeneration system

In this study, an electricity and cooling power cogeneration system is investigated to harvest low-grade heat below 80 ℃, which consists of a two-bed adsorption-based desalination (AD) system for thermally separating the salt solution into diluted and concentrated solutions while offering cooling power and a reverse electrodialysis (RED) system for converting the Gibbs free energy of mixing of the generated solutions into electricity. The dynamic response of the cogeneration system is presented first, and the asymmetric operation period is analyzed. Then the effects of adsorption/desorptiontime, switching time, working concentration, working fluid mass, and adsorbents on the electric efficiency, coefficient of performance (COP) and exergy efficiency of the cogeneration system are systematically evaluated. Results reveal that longer adsorption/desorption time leads to degraded electrical efficiency and exergy efficiency, and upgraded COP. Extended switching time contributes to COP and exergy efficiency, however, decreases the electric efficiency. Larger salt concentration improves the electric efficiency, however degrades the exergy efficiency and COP. Increasing working solution mass can augment the electrical efficiency, exergy efficiency and COP. Furthermore, refrigeration performance conflicts with power generation performance for different adsorbents. With CAU-10 as the adsorbent, an exergy efficiency of 30.04% is achieved, meanwhile the electric efficiency and COP is 0.39% and 0.84, respectively at adsorption/desorption time of 900 s, switching time of 10 s and working concentration of 8 mol/kg.

Data on the current-voltage dependents of nickel hollow microspheres based thermo-electrochemical in alkaline electrolyte.

Low-grade waste heat harvesting and conversion into electric energy is an important way of renewable energy development and thermo-electrochemical cells are promising devices to solve this problem. In this paper, we report original data on the current density and maximum output power dependents on voltage of the thermos-cells with nickel hollow microspheres electrodes and different electrolyte concentration (from 0.1 to 3.0 mol/l)which exhibit excellent hypothetical Seebeck coefficient and accordingly high open-circuit voltage values at low source temperature. The composition, microstructure and morphology of the hollow nickel microspheres based electrodes are included here. Because of the low cost of nickel based thermo-cells could be commercially feasible for harvesting low-quality thermal energy, in this connection, the raw data of measurements of their properties are given here.The data is related to “High Seebeck coefficient thermo-electrochemical cell using nickel hollow microspheres electrodes”, Burmistrov et al., Renewable Energy, 2020 [1].

Study on a heat-driven thermoacoustic refrigerator for low-grade heat recovery

Recovering low-grade heat from renewable energy sources and waste heat is crucial for improving energy utilizing efficiency as well as reducing CO2 emissions. Conventional thermoacoustically-driven refrigerators have a high onset temperature and low cooling efficiency, which significant limit their capacity for low-grade heat utilization. This paper investigates a novel thermoacoustically-driven refrigerator with gas-liquid resonators which enable a lower onset temperature and better cooling performance for harvesting low-grade heat. Theoretical analyses were performed on multi-stage systems to explore the onset characteristics and steady performance. Onset characteristics analysis was conducted by using a transfer matrix method. The effects of mean pressure, liquid volume ratio and the expected liquid mechanical damping coefficient on the onset temperature difference and working frequency were studied for systems with different numbers of stages. A comparison of system onset performance was made with conventional systems containing a gas-only resonator. The research illustrated that for a mean pressure of 1 MPa, the proposed system can significantly reduce the onset temperature difference from 144.1 K to below 35.5 K. In addition, an analysis was then conducted to study the parametric sensitivity of the thermodynamic performance. Calculation results show that the proposed system can achieve a baseline cooling power of 2.7 kW and a thermal-to-cooling efficiency of 0.67 at a heating temperature of 420 K and a cooling temperature of 270 K. This represents significant increases by a factor of 5.6 in cooling power and 1.5 in efficiency from a gas-only to a gas-liquid resonator.

Enhanced power factor of n-type Bi2Te2.8Se0.2 alloys through an efficient one-step sintering strategy for low-grade heat harvesting

This work reports the enhanced power factor of n-type Bi2Te2.8Se0.2 alloys through an efficient one-step sintering strategy for thermal energy harvesting.

Pressure-retarded membrane distillation for simultaneous hypersaline brine desalination and low-grade heat harvesting

Membrane distillation (MD), an emerging pre-concentration technology for zero liquid discharge to manage industrial wastewater, has become increasingly attractive when the low-grade waste heat is utilized as its driving force. Pressure-retarded membrane distillation (PRMD) – a derivative of MD by applying a hydraulic pressure lower than the membrane liquid entry pressure at the cold permeate side – has the potential to further recover the low-grade heat in terms of electricity, which is an added benefit for the use of conventional MD. Here, we for the first time experimentally evaluated the feasibility of using PRMD for simultaneous desalination of hypersaline water and harvesting of low-grade heat and explored the mechanisms governing the experimental PRMD performance. Using a commercial membrane, NaCl solution at the concentration as high as 4 M (233.76 g L−1) is desalinated with the salt rejection >99.9% and additional energy is generated in PRMD at the same time. We showed that the membrane operated with its active layer facing a cold solution orientation in PRMD exhibits better mechanical stability at higher applied pressures, which is essential for achieving higher power output. However, in contrast to the other orientation that is used in conventional membrane distillation (MD) processes, the coupled effects of internal temperature polarization (ITP) and internal concentration polarization (ICP) lower effective driving force and membrane permeability, which significantly compromises the water vapor flux and peak power density. These results indicate that unlike MD processes, controlling internal scaling and fouling are critical for PRMD and our findings can serve as useful guides for creating high-performance PRMD membranes in practical environmental applications.

Thermolytic osmotic heat engine for low-grade heat harvesting: Thermodynamic investigation and potential application exploration

The osmotic heat engine is a promising technology for harvesting low-grade heat from different heat sources. However, a better understanding of the system performance, thermodynamic efficiencies, and suitable application circumstances (type of heat sources, system energy generation capacity, etc.) is needed before the transition can be made from conceptual design to practice. Firstly, the energy efficiency (ƞth) and exergy efficiency (ƞX) of a thermolytic osmotic heat engine (NH4HCO3 solution as the working fluid) were investigated in this study. It was found that the osmotic heat engine performs better when the operating temperature (heat source temperature) is lower (323 K). Additionally, a higher draw solution concentration and a lower feed solution concentration can increase both ƞX and ƞth. Subsequently, the energy return on investment with either low-grade industrial waste heat or solar thermal energy acting as the heat source was calculated. It was found that different energy return on investment values can be obtained with different heat sources. The results show that when industrial waste heat is used as the heat source, a much higher energy return on investment value (approximately 55) can be obtained. This finding indicates that it is suitable to generate electricity from industrial waste heat using the osmotic heat engine. When solar thermal energy is used as the heat source the energy return on investment value is 1.3–2.2 because there is a large amount of embodied energy in the flat-plate solar collector. This study represents a step forward towards the practical application of the osmotic heat engine.

A graphic analysis method of electrochemical systems for low-grade heat harvesting from a perspective of thermodynamic cycles

Various electrochemical approaches for low-grade heat harvesting have been investigated in recent years. However, most current studies are based on electrochemistry methods, while thermodynamic methods that focus on the general energy conversion mechanism are relatively rare. In this study, to quantify the chemical energy of electrochemical systems, Gibbs free energy is introduced as the third dimension in the conventional T-S diagram from a perspective of thermodynamic cycles, and a graphic analysis method is proposed consequently. Then the ideal cycle, which could identify the conversion relationship among heat, work and chemical energy, is presented guided by the graphic analysis method. A discussion is presented as well to explain the non-ideality of actual cycles. Finally, as a representative case, the energy conversion mechanism and the efficiency expression of thermally regenerative electrochemical cycle are restated, to valid the graphic analysis method. The graphic analysis method polishes the thermodynamic theoretical system of electrochemical systems for heat harvesting where chemical energy matters, and leads to an in-depth understanding on the intrinsic energy conversion mechanism. Furthermore, the graphic analysis method would provide a theoretical guidance for integration of chemical energy into the construction of thermodynamic cycles, and even enlighten the boundary extension of thermodynamic cycles.

Synthesis and characterization of novel p-type chemically cross-linked ionogels with high ionic seebeck coefficient for low-grade heat harvesting

The low value of the Seebeck coefficient of the order of few μV/K in inorganic conductors, semiconductors and conducting polymers has inspired the researchers to explore alternative thermoelectric materials. In this work, a novel class of ionogels (IGs) has been synthesized with the aim of developing high-performance thermoelectric materials. Chemically cross-linked ionogels were prepared by the immobilization of ionic liquid, 1-butyl-3-methyl imidazolium tetrafluoroborate (BMIMBF4) in polyethylene glycol dimethacrylate (PEGDMA) matrix using azobisisobutyronitrile (AIBN) as the free radical initiator. The concentration of BMIMBF4 in IGs was varied from 60 to 90 wt (wt. %). The impact of variation of ionic liquid content on the thermoelectric properties of ionogels was analyzed by measuring their thermoelectric properties. Thermal stability, glass transition temperature (Tg), Surface morphology, microstructure, the crystallinity of IGs and nature of chemical interaction between the ionic liquid and polymer matrix were observed by performing TGA, DSC, FESEM, FETEM, XRD, and FTIR respectively. The ionic conductivities of neat BMIMBF4 and IGs were determined by electrochemical spectroscopy using SI 1260 Impedance/Gain-Phase Analyzer. It is worth noting that ionic conductivity (49.41 mS/cm) of IG with 90 wt % of BMIMBF4 is 10 times higher than the ionic conductivity of neat BMIMBF4 (4.5 mS/cm). The origin of this promising achievement (very high conductivity) lies in the “breathing polymer chain model”. The breathing in and out of polymer chains dissociates the ion aggregates resulting in a significant increase in ionic conductivity of IGs. A remarkably higher value of the Seebeck coefficient (2.35 mV/K) was achieved for IG with 60 wt % of BMIMBF4 and defined as ionic Seebeck coefficient because of its origin from the diffusion of ions of the ionic liquid. Due to the positive value of ionic Seebeck coefficient (in an analogy to positive Seebeck coefficient of p-type semiconductors) we termed the ionogels as p-type chemically crosslinked ionogels. A decrease in glass transition temperature of IGs with an increase in ionic liquid content was observed from DSC curves corresponding to an increase in mobility of cations and anions. TGA analysis showed that the synthesized IGs were highly thermally stable up to 390 °C. FESEM and FETEM images revealed that ionic liquid is well confined in the PEGDMA scaffold. The results indicate that ionogels may serve as promising candidates for future thermoelectric applications and also opens the perspective of engineering ionogels with high Seebeck coefficients and electrical conductivities.

Novel solar-powered photovoltaic/thermoelectric hybrid power source

A photovoltaic (PV) module converts solar radiation into electric power. Solar collectors convert directly solar heat into hot water (not electric power). A thermoelectric generator (TEG) directly converts solar heat into electric power. Thus, this study combines a PV module with some TEGs to directly convert both solar radiation and solar heat into electric power. The result is a novel solar-powered PV/thermoelectric hybrid power source. The output power of a TEG module is very small when temperature difference between its two surfaces (hot and cold) ranges below about 50C∘. As a result, TEGs cannot be used in low-grade heat harvesting applications such as solar heat harvesting. This study also solves this defect, so that, the mentioned temperature difference is raised by utilizing a two-phase closed thermosiphon (TPCT) heat pipe, a concentrator and a radiator. A prototype of the proposed PV/thermoelectric hybrid power source has been constructed, and experimental results obtained from its operation under realistic conditions are given. The novelties and contributions of this research work can be outlined as below:The PV/thermoelectric hybrid power source directly converts both solar radiation and solar heat into electric power.The technique used to design and construct the PV/thermoelectric hybrid power source enables utilizing TEGs in low-grade heat harvesting, in particular, solar heat harvesting.The technique provides a dc bus with constant voltage, so other power generation devices such as fuel cell and wind power generation system can be easily added to the system by connecting them to the dc bus through a converter.The technique uses a high-efficient maximum power point tracking (MPPT) method which is a modified and high-efficient version of perturb and observe (P&O) method with adaptive and variable step size.

Harvesting Low-Grade Heat via Thermal-Induced Electric Double Layer Redistribution of Nanoporous Graphene Films.

In this work, a closed thermoelectric cell based on a nanoporous graphene electrode is developed to convert low-grade thermal energy to electric energy. The thermoelectric cell consists of two nanoporous graphene electrodes in contact with the hot and cold ends, respectively, encapsulated in a KCl electrolyte, and the energy is harvested from the redistribution of the electric double layer (EDL) of the graphene electrodes under different temperatures. Because of the large specific surface and conductivity of nanoporous graphene electrodes, the electric voltage is 168.91 mV with the hot-end temperature of 61 °C and cold-end temperature of 26 °C, corresponding to the thermoelectric coefficient of 4.54 mV·°C-1, which is much larger than that of the conventional thermoelectric generator. The specific power output achieves 1.38 mW·g-1 and is also significantly larger than the previous EDL-based thermoelectric generator. System performance with the concentration of the KCl electrolyte is examined. The proposed thermoelectric cell can harvest low-grade waste heat from the ambient environment, which may have potential applications in energy supply, wireless powered devices, outdoor survival, and so forth.

Cellulose ionic conductors with high differential thermal voltage for low-grade heat harvesting.

Converting low-grade heat into useful electricity requires a technology that is efficient and cost effective. Here, we demonstrate a cellulosic membrane that relies on sub-nanoscale confinement of ions in oxidized and aligned cellulose molecular chains to enhance selective diffusion under a thermal gradient. After infiltrating electrolyte into the cellulosic membrane and applying an axial temperature gradient, the ionic conductor exhibits a thermal gradient ratio (analogous to the Seebeck coefficient in thermoelectrics) of 24 mV K-1-more than twice the highest value reported until now. We attribute the enhanced thermally generated voltage to effective sodium ion insertion into the charged molecular chains of the cellulosic membrane, which consists of type II cellulose, while this process does not occur in natural wood or type I cellulose. With this material, we demonstrate a flexible and biocompatible heat-to-electricity conversion device via nanoscale engineering based on sustainable materials that can enable large-scale manufacture.

Replacing commercial thermoelectric generators with a novel electrochemical device in low-grade heat applications

A thermoelectric generator (TEG) is a solid-state electronic device. The main defect of a commercial TEG, which makes it useless for most of practical applications such as solar energy harvesting, is its very low power production when the difference of temperature between its two surfaces ranges below 50 °C. The traditional solution is to add some relatively expensive devices such as concentrator, evaporator, condenser and cooling system to TEGs that there is often no technical and economical justification for it. This study provides a new solution by presenting a novel electrochemical device operating based on the thermally regenerative electrochemical cycle (TREC). The proposed device is first analyzed in detail to provide theoretical concepts. A prototype of the device has been constructed, and experimental verifications are given that substantiate the capability of the device in precisely mapping temperature difference between its two cells to its output electric power. A comparison between the proposed electrochemical device and a commercial TEG module TEG1-1263-4.3 is also performed that demonstrates the power production of the electrochemical device is extremely more than that of a commercial TEG module, in particular, in the temperature difference range of 0–50 °C. For instance, 54.5 W of electric power is produced by the device at the temperature difference of 50 °C, while it is only 0.3 W for a TEG module TEG1-1263-4.3. This point explicitly verifies the superiority of the proposed electrochemical device in low-grade heat harvesting. The novelty and originality of this study can be summarized as follows. First, presenting and constructing a novel electrochemical device operating based on the TREC with the size and weight of, respectively, 50×30×32cm3 and 3.1 kg that makes it portable and suitable for industrial applications. Second, the power production of the constructed electrochemical device is extremely greater than that of TEG modules that considering size, weight and portability is the only alternative technology in low-grade heat harvesting applications. Third, this work presents an electrochemical device which is an industrial TREC based system applicable to industrial applications that is clearly different from small-scale TREC cells reported in the literature, which are only applicable to small-scale experimental applications.

Performance evaluation and parametric optimization strategy of a thermocapacitive heat engine to harvest low-grade heat

A novel Stirling-like irreversible electrochemical thermo-capacitive regenerative cycle operating between a heat reservoir and a heat sink for harvesting low-grade heat is proposed, in which the main irreversible losses are considered. Expressions for the efficiency and power output of the cycle are derived. The inherent regenerative heat loss of the cycle is revealed. The influences of several important parameters, such as the capacitance of the supercapacitor, charge ratio, endpoint voltage of the charging process on the power output of the cycle are discussed. The maximum power output and corresponding efficiency are calculated. The results show that when the charging endpoint voltage of the electrochemical thermo-capacitive regenerative cycle is 10 V, the maximum power output is 31.9 W and the efficiency at the maximum power output can reach 12.8%, which is 51.2% of the Carnot efficiency. The optimal values for several parameters at the maximum power output are determined. The parametric optimum selection criteria of the cycle are provided. The results obtained can ensure the cycle to operate in the optimum states.