Thermally Regenerative Electrochemical Cycle Research Articles

Simulation of continuous thermally regenerative electrochemical flow cycle and coupled with photovoltaic/thermal system

Thermally regenerative electrochemical cycle (TREC) is an efficient, clean and feasible low-temperature thermoelectric conversion technology. Coupling it with flow battery (FB) can not only achieve continuous operation, but also can be used to realize the heat energy recovery of the photovoltaic/thermal (PV/T) system. In this paper, a continuous TREC-FB system including two charging-free cells is proposed for outputting power at both high and low temperatures. An electrochemical-thermodynamic coupling algebraic model is established to explore the effects of electrolyte flow rate, temperature difference and design parameters of heat exchanger on the power density (PTREC) and thermoelectric efficiency (ηele,TREC) of the TREC-FB. Experimental study on TREC-FB is carried out for model validation. The results show that PTREC is determined by the temperature difference and the electrolyte flow rate, which contribute to enhanced voltage and mass transfer, respectively. For specific heat source scenarios, the ηele,TREC is mainly affected by the flow rate, followed by the heat transfer area AH, and finally is the flow channel height and the thermal conductivity of the heat exchanger. Moreover, for the continuous TREC-FB-PV/T coupled system, the energy conversion efficiency reaches an average of 11 % within one day by optimizing the designed temperature difference.

High power density charging-free thermally regenerative electrochemical flow cycle for low-temperature thermoelectric conversion

Low-temperature thermal energy widely exists in nature and is sustainable. The thermally regenerative electrochemical cycle (TREC) based on the Seebeck effect has attracted widespread attention due to its high thermoelectric efficiency and good cycle stability. However, it has a low power density and cannot sustain continuous operation for extended periods. This work proposes a continuous charging-free thermally regenerative electrochemically cycled flow battery (TREC-FB) system, which can discharge continuously at both high and low temperatures without the assistance of external power source. By adding guanidine hydrochloride (GdmCl) and inorganic salt ions (Cs+, Ca2+, Mg2+, etc.) to the K3Fe(CN)6/K4Fe(CN)6 and I2/KI electrolytes respectively, precipitates are formed in the electrolytes at low temperature and dissolve at high temperature, which creates the concentration gradient of ions. As a result, the temperature coefficient of the full cell increases from −2.24 mV/K to −3.6 mV/K, and both voltage and power density are doubled. Theoretical analysis of the temperature coefficient enhancement is conducted based on the Nernst equation, and the mechanism of the temperature coefficient enhancement is verified using UV–vis spectra. The discharging capacity and energy of the charging-free TREC-FB with high temperature coefficient reach 5 Ah/L and 1.9 kJ/L, respectively, and the thermoelectric efficiency reaches 8.9 % under sufficient heat recuperation. Two TREC-FBs at high and low temperatures are connected to carry out continuous operation experiments of the charging-free TREC-FB system. The use of charge-reinforced ion-selective membrane can effectively inhibit the migration of I− and I3− ions, achieving about 36 h of continuous operation, and the average power density achieves 0.6 W/m2.

Parametric optimization for enhancing the electrical performance of hybrid photovoltaic/thermal and thermally regenerative electrochemical cycle system

Globally, the efficient utilization of solar energy has garnered significant attention. A hybrid system of photovoltaic/thermal (PV/T) modules integrated with thermally regenerative electrochemical cycle (TREC), i.e., PV/T-TREC has emerged recently and shows higher efficiency for solar-to-electrical conversion than a standalone PV system. However, there is a trade-off between the electricity generation from PV/T and TREC because the thermal energy from PV/T degrades the efficiency of the PV/T but improves that of TREC. Previous studies have failed to investigate this problem. This study therefore focuses on the key parameters that significantly affect the thermal output of PV/T, i.e., different PV materials, air gaps, heat storage tank volumes, and working fluids to investigate the maximum electrical efficiency of the hybrid system. The mathematical and transient-state numerical models are developed with the validation/refinement from the experiments. The results show that the hybrid system with PV material of cadmium telluride presents the best overall electrical efficiency of 25.37 %. The case of the air gap fosters the solar-to-electrical conversion. The electrical efficiency versus heat storage tank volume shows a convex curve, peaking at 200 L. The nanofluid performs the best. This study may help guide the practical application of such a high-efficiency electricity generation system.

Power-to-X system utilizing an advanced solar system integrated with a thermally regenerative electrochemical cycle

This article discusses power-to-X technology by proposing an advanced system to generate green electricity and hydrogen. The proposed system is powered by an advanced concentrated parabolic photovoltaic thermal (CPV/T) hybrid system. A proton exchange membrane (PEM) water electrolyzer system is used for the hydrogen production process, with a multistage compression system to increase hydrogen pressure and regulate hydrogen temperature. Additionally, a heat recovery system is employed through the thermally regenerative electrochemical cycle (TREC). A parametric analysis was performed based on thermal resistance to investigate the effect of solar radiation, Reynolds number, lengths of both the receiver side and tube and ambient temperature. A mathematical model was developed and validated to investigate the integrated system performance. CPV/T achieved a maximum system efficiency of 85.4% at 1000 W/m2 and a side length of 0.2 m, with electrical and thermal contributions of 21.1% and 64.3%, respectively. With a tube length of 20 m, the maximum hydrogen flow rate reached 280 kg/h, requiring 2.5 TJ/h for production under a pressure of 750 bar. The TREC system showed a maximum electrical production contribution of about 25.4% at a Reynolds number equal to 3000 compared to the contribution of CPV/T cells.

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.

A novel high-efficiency integrated system combining a thermally regenerative electrochemical cycle and a flow battery

A thermally regenerative electrochemical cycle (TREC) harnesses the temperature effect of electrode potential to achieve efficient heat to electricity conversion but suffers from low power density.

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.