Thermally Regenerative Electrochemical Cycle Research Articles

Elevating Low-Grade Heat Harvesting with Daytime Radiative Cooling and Solar Heating in Thermally Regenerative Electrochemical Cycles.

Thermal radiation control has garnered growing interest for its ability to provide localized cooling and heating without energy consumption. However, its direct application for energy harvesting remains largely underexplored. In this work, we demonstrate a novel system that leverages daytime radiative cooling and solar heating technologies to continuously power charging-free thermally regenerative electrochemical cycle (TREC) devices, turning ubiquitous low-grade ambient heat into electricity. Notably, by harnessing a substantial 35 °C temperature differential solely through passive cooling and heating effects, the integrated system exhibits a cell voltage of 50 mV and a specific capacity exceeding 20 mAh g-1 of PB. This work unlocks the potential of readily available low-grade ambient heat for sustainable electricity generation.

Performance Potential of a Concentrated Photovoltaic-Electrochemical Hybrid System

A novel hybrid system model, combining a concentrated photovoltaic cell (CPC) with a thermally regenerative electrochemical cycle (TREC), is proposed. This innovative setup allows the TREC to convert heat from the CPC into electricity. The model incorporates mathematical equations that explicitly define power output, energy efficiency, and exergy efficiency for both the CPC and the TREC individually, as well as for the hybrid system as a whole. The outcomes of the computations reveal that the hybrid system surpasses the performance metrics of the CPC alone. Specifically, the hybrid system achieves a notably higher maximum power density (MPD), maximum energy efficiency (MEE), and maximum exergy efficiency (MMEE) compared to the standalone CPC, with improvements of 392.68 W m−2, 10.33%, and 11.11%, respectively. Through thorough parametric analyses, it was observed that specific factors positively impact the hybrid system’s performance. These factors include higher operating temperatures, increased solar irradiation, specific concentration ratios, and alterations in the internal resistance or temperature coefficient of the TREC. However, it was noted that elevating the operating temperature of the CPC adversely affects the hybrid system’s performance. Furthermore, augmenting solar irradiation and optical concentration ratios amplifies the limiting electric current. Conversely, reducing the internal resistance of the TREC enhances the overall performance of the hybrid system. These discoveries have practical implications for optimizing the design and operation of a functional CPC-TREC hybrid system, providing valuable insights into maximizing its efficiency and effectiveness.

Design and optimization of the novel thermally regenerative electrochemical cycle power device based on machine learning

Utilizing low grade heat and waste heat to generate electricity not only mitigates environmental impacts, meanwhile it enhances energy efficiency and lowers energy expenses. Thermal regenerative electrochemical cycles (TREC) can directly convert low grade heat to electricity, and it has a massive potential for low grade heat utilization. The type and design of a TREC system with practical and continuous power output are of paramount importance, and they can directly influence the performance and efficiency of low-grade heat recuperation systems. In this paper, a novel TREC system of a double layers rotating structure was proposed, and the novel optimal design approach employs CFD and machine learning for this system was presented. The results revealed that the height and number of TREC units had a significant impact on the device's performance. The heat to electricity efficiency of TREC device can be up to 7.34% (equivalent to 48.91% of the Carnot cycle efficiency) when the number of units was 12 and the height of unit was 5 mm. The results indicated that the MLP model was the most suitable, with RMSE values of 0.0077, 0.4415, and 0.5091 for heat to electricity efficiency, heat recuperation efficiency, and heat recuperation, respectively. This work established an effective approach for more rapid and precise estimation of the TREC device's performance metrics. The findings from prediction can serve as a practical guide for the design of TREC devices of various sizes. Furthermore, the method can envision the efficiency before the TREC device is manufactured in case of insufficient numerical calculations, reducing the computational and optimized costs.

A novel high-efficient continuous power generation device employing thermally regenerative electrochemical cycle

The significant attention garnered by thermally regenerative electrochemical cycles (TRECs) is due to their remarkable heat to electricity efficiency in harnessing low-grade heat from energy conversion systems, by addressing the prominent energy challenges associated with waste heat recovery and refrigeration. For a single unit, TREC undergoes four distinct working processes, discharging, heating, charging, and cooling to complete a closed-loop thermodynamic cycle for energy conversion. However, this single unit cannot produce electricity continuously, leading to intermittent power output and significant heat consumption during the heating phase. To address this limitation, current research introduces a novel concept by employing two rotating layers of TREC units, aimed at overcoming the associated problems of discontinuous power output. This system can notably improve energy conversion efficiency by realizing uninterrupted power output and curtails heat absorption. In this work, high-performance TREC device's structure design, working principle, operational processes, and mathematical models are described in detail. Moreover, based on numerical calculations, impact of various key factors including the number, height, and radius of units were thoroughly examined and it was observed that the number and height of units have a substantial impact on the performance of the TREC device. The findings show that, under specific conditions, the device is capable of generating 12.876 mJ of net work at a current density of 124 mA/g. Furthermore, the TREC device achieves an impressive heat-to-electricity efficiency of 10.32 %, equivalent to 68.78 % of the Carnot cycle efficiency. This innovative rotating concept offers a promising solution for achieving continuous and highly efficient energy output from the TREC device, further enhancing its practical application viability.

A comparative study of electrochemical Brayton cycle and thermally regenerative electrochemical cycle

Both the emerging thermally regenerative electrochemical cycle (TREC) and electrochemical Brayton cycle (EBC) harvest low-grade heat by utilizing the temperature effect of electrode potential. Their key difference lies in the cycle configurations, the Stirling cycle and the Brayton cycle, respectively. Determining which cycle configuration performs better could indicate the direction of thermodynamics for further research. However, this remains an open question to be investigated. To address this, a performance comparison of EBC and TREC using KI/KI3 and K3Fe(CN)6/K4Fe(CN)6 solutions as electrolytes is conducted based on an improved system configuration and a two-dimensional model coupling mass transfer and electrode kinetics. Multi-objective optimization results show that there is no significant difference in performance between EBC and TREC. Under a given temperature difference of 50 °C, the power density and exergy efficiency of optimized EBC are respectively 5.28 W m−2 and 16.6%, compared with 5.93 W m−2 and 15.6% of optimized TREC. The corresponding temperature drops of heat sources are 14.49 °C and 12.26 °C, respectively, indicating that EBC is more suitable for heat sources with large temperature variations. The concentration overpotential is found to be the main contributor to entropy generation in both EBC and TREC besides heat transfer. Their power density and exergy efficiency can be further improved by approximately 50% and 40% by eliminating overpotentials, respectively. This study is expected to provide beneficial insights into the performance optimization and electrolyte improvement of EBC and TREC.

Engineering of Solvation Entropy by Poly(4-styrenesulfonic acid) Additive in an Aqueous Electrochemical System for Enhanced Low-Grade Heat Harvesting.

The thermally regenerative electrochemical cycle (TREC) is a reliable and efficient approach to converting low-grade heat into electricity. A high temperature coefficient (α) is the key to maximize the energy conversion efficiency of the TREC system. In this study, we present significant improvement of α of a Prussian blue analogue (PBA)-based electrochemical cell by adding poly(4-styrenesulfonic acid) (PSS) to the electrolyte. Raman spectra showed that water-soluble charged polymers strongly affect the ion hydration structure and increase the entropy change (ΔS) during ion intercalation in PBA. A large α of -2.01 mV K-1 and high absolute heat-to-electricity conversion efficiency up to 1.83% was achieved with a TREC cell in the temperature range 10-40 °C. This study provides a fundamental understanding of the origin of α and a facile method to boosting the temperature coefficient for building a highly efficient low-grade heat harvesting system.

Enhanced efficiency of photovoltaic/thermal module by integrating a charging-free thermally regenerative electrochemical cycle

In order to utilize solar energy and improve overall electrical efficiency, a photovoltaic/thermal (PV/T) module coupled with a continuously operated charging-free thermally regenerative electrochemical cycle (TREC) system is proposed, and thermodynamic models of the continuously operated charging-free TREC system and PV/T module are established. By adjusting the inlet air flow rate of the PV/T module, its output temperature can be kept above 60 °C during the daytime, providing a heat source for the TREC system. The effect of the TREC cell packs arrangement and the cell parameters on the performance of the TREC system is also investigated. Finally, the electrical efficiency of the hybrid system is compared with that of the single PV/T system. When four TREC cells operating under a current density of 5 mA·g−1·PB are used, the total electrical efficiency of the coupled system can be improved by a maximum of 12.58% compared with that of the single PV/T module.

Evaluation of thermally regenerative electrochemical cycle for thermal-to-electrical energy conversion

A significant amount of low-grade heat (< 100 °C) can be found in various sources, such as geothermal/solar sources, industrial plants, vehicles, and biological entities, but it is often wasted due to the absence of cost-effective and efficient recovery technologies. Thermally regenerative electrochemical cycle (TREC) represents a promising solution for effectively harnessing low-grade heat. Rapid advancements in TREC chemistry, materials, and design have established the crucial foundations for high-power, efficient, and long-lasting TREC systems. However, evaluating the potential of reported TREC systems of different types is challenging due to the inconsistency in evaluation metrics and methods. In this Perspective, we examine the working principle of various TREC systems, including the electrically powered TREC systems, charging-free TREC systems that solely convert thermal energy to electrical energy, and TREC systems that simultaneously provide high-power energy storage and thermal energy conversion. The critical performance metrics for each of these three types of TREC systems, such as absolute/apparent thermoelectric efficiency, power density, net electricity generation, various forms of energy loss, and thermal energy input, are presented to compare the thermoelectric performance across different types of TREC systems at various scales. In addition, some practical methods for measuring the critical parameters, current challenges, and future directions for practical applications are also highlighted.

A novel energy harvesting and battery thermal management in hybrid vehicles using a thermally regenerative electrochemical device

The orientation these days is on using high-efficiency devices such as hybrid vehicles. The current study uses a thermally regenerative electrochemical cycle (TREC) for energy harvesting from vehicle exhaust. Also, the TREC is utilized in reverse mode as a thermally regenerative electrochemical refrigerator (TRER) for cooling the battery in hybrid vehicles when the engine is turned off. To study the performance of the proposed system, an analytical model is developed and validated. The effect of different parameters on the TREC's performance, such as fuel mass flow rate, water temperature, mass flow rate, ambient temperature, and the Nusselt number, is investigated. Using the TREC for harvesting the wasted energy from the engine achieved an efficiency of 4.63%. The TRER has a variation in the peak COP from 7.84 to 1.89 based on the cold and hot reservoir temperatures. Under optimum operating conditions, using the TRER can reduce the battery's temperature from 293 K to 287.2 K after 1000 s of operation. Using the TREC and TRER devices provides a valuable solution to improve overall efficiency and protect hybrid vehicles' batteries.

A numerical model for a thermally regenerative electrochemical cycled flow battery for low-temperature thermal energy harvesting

Low-temperature thermal energy (<130 °C) recycling and utilization can significantly increase energy efficiency and reduce CO2 emissions. Among various technologies for heat-to-electricity conversion, thermally regenerative electrochemical cycle (TREC) has garnered significant attention for remarkable efficiency in thermal energy utilization. The thermally regenerative electrochemical cycled flow battery (TREC-FB) in this paper offers several advantages, including continuous power output and operating without an external power supply. The goal of this investigation is to enhance the understanding of how various parameters affect system performance through simulation, thus optimizing cell performance. In this work, based on the conservation equations and electrochemical equations, the two-dimensional steady models coupled with the flow field and electrochemical field of high-temperature cell and low-temperature cell are constructed separately by COMSOL Multiphysics. The diffusion coefficient and kinetic parameters in the model were obtained by cyclic voltammetry (CV), chronoamperometry (CA) and Tafel electrochemical measurements for subsequent application in the models. Experimental results have confirmed the validity of this model. The main focus of this work is to examine how the system performance is impacted by various factors including current density, electrolyte flow rate, temperature coefficient, porous electrode geometry, heat recuperation efficiency, and temperature difference between hot and cold cells. The results indicate that a larger electrolyte flow rate leads to larger power density, but reduces system efficiency. Smaller porous electrode thickness, higher temperature coefficient, higher heat recuperation efficiency and larger temperature difference between the cells can enhance the system performance. This work offers a new guide for further enhancing TREC-FB performance.

Thermal convective effect on the performance of thermally regenerative electrochemical cycle against self-discharge

The effect of convective heat transfer on low-grade heat recovery in thermally regenerative electrochemical cycles (TRECs), which were operated on the basis of electric double-layer capacitors, was investigated at varying flow rates of fluid flowing from thermal reservoirs. Although the TREC performance is primarily determined by the magnitude of an open-circuit voltage rise driven by a temperature difference, it can be significantly restricted by spontaneous charge losses due to self-discharge. To investigate the convective heat transfer effect against self-discharge, cycle performance was evaluated through experiments at flow rates ranging from 56 to 184 mL/min, and further estimated at extended lower flow rates to 14 mL/min through numerical calculations. When TRECs were operated between 20 and 60 °C, the highest net work was obtained to be 81.3 μJ at 184 mL/min, corresponding to 34 % conversion efficiency relative to the Carnot efficiency. Unlike the high flow rate conditions, the TREC performance below 56 mL/min significantly decreased with reduced the flow rate due to the considerable charge loss by self-discharge. This shows the stronger flow dependence of the TREC performance. This study provides a novel approach to estimate energy loss through a time constant analysis, thereby enabling better TREC designs concerning energy efficiency.

Dual power generation modes for thermally regenerative electrochemical cycle integrated with concentrated thermal photovoltaic and phase change material storage

A dual power stroke system that generates electricity during the day and night is proposed. The model consists of concentrated thermal photovoltaic (CPV/T), thermally regenerative electrochemical cycle (TREC), and phase change material (PCM) storage. During the daytime, the concentrated thermal photovoltaic generates electricity directly and supplies hot water to raise the charging temperature of the thermally regenerative electrochemical cycle. To reduce the discharging temperature of the thermally regenerative electrochemical cycle and to avoid losing heat to the environment, a phase change material storage is used to store the heat. During the nighttime, the stored heat in the storage is used back to raise the charging temperature of the regenerative cycle. The discharging temperature is reduced using a finned heat sink. Raising the charging temperature of the regenerative cycle and reducing the discharging temperature will generate electricity during the day and night. An analytical model is developed and validated to investigate the effect of different parameters like solar radiation intensity, water mass flow rate, and temperature in the concentrated thermal photovoltaic and ambient temperature on efficiency and the power produced. The solar radiation, water inlet temperature, and mass flow rate impact the system's performance during the daytime. The ambient temperature is a primary influencer during the nighttime. The efficiency of the regenerative cycle and photovoltaic cells can reach 7.11 % and 15.74 % during the daytime. During the nighttime, the regenerative cycle is the only source of electricity and has an efficiency of up to 6.13 %. Using the proposed model presents a powerful solution for the energy problem in remote areas during the day and night times.

Thermoresponsive ionic liquid for electrochemical low-grade heat harvesting

Thermally regenerative electrochemical cycle (TREC) is a promising technology for low-grade heat harvesting by employing the thermogalvanic effect of the electrodes. Whereas the electrolytes applied in TREC systems have a negligible response to temperature variation. In this study, a thermoresponsive ionic liquid (TRIL) is added to an electrolyte to endow it with temperature-driven phase change behavior, and the electrolyte is then utilized in a copper hexacyanoferrate-based TREC system for ultralow-grade heat harvesting. The TREC system is operated between 10 and 30 °C across the phase change critical point (Tc), so that the solvation states of the ions varied during the charging and discharging process, and a high energy density of 1.30 J g−1 and high energy conversion efficiency of 1.32% (20.0% for the Carnot efficiency) are achieved. The energy efficiency is 10 times that achieved by the conventional non-TRIL system under the same conditions. Moreover, the Tc of the TRIL can be tuned according to the species and concentrations of the electrolyte salt, which enhances the feasibility and resilience of the TRIL-containing TREC system. This study provides a novel perspective for electrolyte design in electrochemical cells, promoting the applicability of electrochemical cells in high-performance ultralow-grade thermal energy harvesting systems.

Towards efficient continuous thermally regenerative electrochemical cycle: Model-based performance map and analysis

Continuous thermally regenerative electrochemical cycle (TREC), which could convert heat to power and realize the continuous power output, is a promising technology for low-grade heat harvesting. However, the mechanism elucidation and performance analysis of continuous TREC are still incomplete, which hinders its performance improvement and practical applications. In this study, a pseudo-two-dimensional flow and electrochemical coupled model for continuous TREC is developed to investigate its performance. Based on the model, the polarization mechanism is revealed, and the effects of operating parameters and physical properties of electrolytes on the cycle performance are analyzed in detail. The results indicate that the concentration polarization is the major contributor to the system overpotential, especially at low electrolyte flow velocities. Regarding operating parameters, although temperatures of heat source and heat sink and electrolyte flow velocity determine the power density, the heat recuperation efficiency shows a decisive effect on the efficiencies. With perfect recuperation, the thermal efficiency and exergy efficiency could reach 8.3 % and 57.5 %, respectively. Regarding physical properties of electrolytes, the electrolyte concentrations affect the power density by changing the limiting current density. In addition, the temperature coefficient significantly affects the power density at any electrolyte flow velocity, while the internal resistance does so only at high flow velocities. With a temperature difference of 50 °C, a high power density of 4.33 W m−2 is expected if the temperature coefficient is increased to 4.0 mV K−1 and the area specific resistance is reduced to 1.0 Ω cm2. Based on the results, beneficial recommendations are put forward for further optimizing the system configuration and screening electrolytes for continuous TREC.

Enhancing heat-to-electricity conversion performance of the thermally regenerative electrochemical cycle using carbon-copper composite electrodes

Low-grade heat is ubiquitous but difficult to be recovered. Thermally regenerative electrochemical cycle (TREC) system is expected to be one of the technologies for efficient recovery of low-grade heat. However, due to the limited temperature coefficient and temperature difference, it is difficult to further improve the heat-to-electricity conversion performance of TREC system. Therefore, a TREC system with carbon-copper composite negative electrode is proposed. By studying various factors (acid treatment, current density, and electrolyte concentration) for the deposition of copper metal on carbon cloth, suitable conditions for the preparation of carbon-copper composite electrodes were determined. The proposed method increased the electrode reaction contact area, reduced the electrical energy charged during the TREC system cycle, and increased the heat-to-electricity conversion efficiency of the system to 1.3 times (from 2.97% to 3.86%) compared with the conventional system with copper sheet negative electrode.

Membrane-free redox flow cell based on thermally regenerative electrochemical cycle for concurrent electricity storage, cooling and waste heat harnessing of perovskite solar cells

Global climate change requires a significant reduction of greenhouse gas emission through developing sustainable energy technologies such as photovoltaic (PV) devices. However, the inherently variable nature of solar irradiance and consequent electricity generation increase challenges for the system stability of electrical grids. Moreover, the PV devices face significant performance loss under full irradiance conditions with 55–65 °C operating temperature. Herein, we demonstrate a novel solar energy conversion and storage (SECS) system by integrating a perovskite PV device with a low-cost membrane-free Zn/Mn-based redox flow battery (RFB) which has a quite negative temperature coefficient. The proof-of-concept SECS system shows great potential for future large-scale deployment of sustainable energy by three concurrent processes: (1) Liquid electrolyte of RFB provides an effective cooling process for the PV device, mitigating its performance loss at high temperature. (2) Thermally regenerative electrochemical cycle (TREC) allows RFB to generate additional electricity from the low-grade heat gathered from the solar cell at a high absolute thermoelectric efficiency, which adds to the overall system efficiency. (3) RFB timely stores the electricity produced by the solar cell, buffering the fluctuation of solar power, stabilizing the electrical grids, and reducing the energy curtailment.

Potential analysis of a system hybridizing dye-sensitized solar cell with thermally regenerative electrochemical devices

To realize broad residual-heat utilization of energy, a novel solar-driven system is proposed by hybridizing a dye-sensitized solar cell (DSSC) with a thermally regenerative electrochemical cycle (TREC) and a thermally regenerative electrochemical refrigerator (TRER). Systems for power and cooling cogeneration consisting of DSSC and two-stage thermally regenerative electrochemical devices are rarely explored. Part of heat dissipated by DSSC is absorbed by TREC to generate power, which drives TRER for refrigeration. Mathematical formulas about the power output and efficiency of the subsystems and coupled system are deduced considering thermodynamic and electrochemical irreversible losses. Moreover, numerical calculations indicate that the maximum power output density (MPOD) and maximum energy efficiency (MEE) of the coupled system are 95.15W m−2 and 24.13%, respectively, which are promoted by 10.68% and 8.89% compared to a single DSSC. Extensive parametric studies are performed for grasping the impacts of several key parameters on system performance, thereby providing data support for system optimization. There are optimal values for performance indicators when TiO2 film thickness is located between 1.0×10-2mm and 4.0×10-2mm, and the performance improvement is extremely inconspicuous after the photoelectron absorption coefficient reaches 5.0 × 105m−1. The conclusions show that this established model is suitable, and it opens up another avenue to design such a functional system.

Efficient Low-Grade Heat Conversion and Storage with an Activity-Regulated Redox Flow Cell via a Thermally Regenerative Electrochemical Cycle.

Efficient and cost-effective technologies are highly desired to convert the tremendous amount of low-grade waste heat to electricity. Although the thermally regenerative electrochemical cycle (TREC) has attracted increasing attention recently, the unsatisfactory thermal-to-electrical conversion efficiency and low power density limit its practical applications. In this work, a thermosensitive Nernstian-potential-driven strategy in the TREC system is demonstrated to boost its temperature coefficient, power density, and thermoelectric conversion efficiency by rationally regulating the activities of redox couples at different temperatures. With a Zn anode and [Fe(CN)6 ]4-/3- -guanidinium as the catholyte, the TREC flow cell presents an unprecedented average temperature coefficient of -3.28mV K-1 , and achieves an absolute thermoelectric efficiency of 25.1% and apparent thermoelectric efficiency of 14.9% relative to the Carnot efficiency in the temperature range of 25-50°C at 1mA cm-2 . In addition, a thermoelectric power density of 1.98mW m-2 K-2 is demonstrated, which is more than 7 times the highest power density of reported TREC systems. This activity regulation strategy can inspire research into high-efficiency and high-power TREC devices for practical low-grade heat harnessing.

Continuous thermally regenerative electrochemical systems for directly converting low-grade heat to electricity

Thermally regenerative electrochemical cycle (TREC) system, which converts heat to electricity by charging at a lower voltage and discharging at a higher voltage, is a promising technology with high energy conversion efficiency for low-grade heat recovery. However, its charging process consumes additional energy and breaks the continuity of power generation. Herein, we present a continuously operated TREC system for direct heat-to-electricity conversion. In this system, two identical electrochemical cells operating at different temperatures are combined in a unit; thus, electricity can be generated continuously by periodically alternating between two temperatures. This concept is mainly demonstrated with a copper hexacyanoferrate cathode and a Cu/Cu2+ anode, with this system achieving an energy conversion efficiency of 1.76% (14.19% of Carnot efficiency) when operated between 10 and 50 °C without heat recuperation effects. Even at an ultralow temperature difference of 10 °C vs room temperature, its efficiency is 0.98%. The proposed system allows great freedom in electrode material selection as proven by another system with nickel hexacyanoferrate cathode and Ag/AgCl anode, thereby improving the flexibility and practicability of TREC systems in low-grade heat harvesting.

Na/K mixed electrolyte for high power density and heat-to-electricity conversion efficiency low-grade heat harvesting system

Low-grade heat widely exists in nature and industry processes. Due to the lack of effective recovery technologies, a large amount of low-grade heat is wasted. Thermally regenerative electrochemical cycle (TREC) system can efficiently convert low-grade heat into electrical energy. However, the low power density limits the practical application of TREC system. To improve the power density and heat-to-electricity conversion efficiency, a TREC system with Na/K mixed electrolytes (4 M sodium nitrate [NaNO3] + 1 M potassium nitrate [KNO3]) is proposed. When the temperature difference of the system is 34°C (between 46 °C and 12 °C), the heat-to-electricity conversion efficiency can be as high as 3.9% (relative Carnot efficiency of 36.4%). At the current density of 40.16 mA/g , the power density can reach 23.29 mW/g, increasing to 6.5 times of conventional system, which is the highest level in TREC system. The comprehensive results of electrochemical test and elemental analysis show that K+ has superior electrochemical reaction characteristics. The proposed sodium/potassium (Na/K) mixed electrolyte system improves the power density and heat-to-electricity conversion efficiency by reducing the voltage hysteresis and increasing the operating current and voltage.