Thermally Regenerative Electrochemical Cycle Research Articles

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.

Power generation by integrating a thermally regenerative electrochemical cycle (TREC) with a biofuel stove

This paper proposes a system that aims to extract the wasted energy from biofuel stoves using a thermally regenerative electrochemical cycle (TREC). While the biofuel stove heats the space, the wasted heat will be extracted to heat the TREC device. A metal heat sink is used to cool down the TREC device. A theoretical model was developed and validated for the first time for the integrated system to test three types of fuel (Wood, peat, and manure) and three different cooling types (natural convection, forced convection, and evaporative cooling). Based on the parametric study, the net electrical power produced and the efficiency of the system depend on fuel type and mass flow rate, water inlet temperature and mass flow rate, the cooling type used in the heat sink, and the ambient temperature. The efficiency of the TREC varies 0.13%–7.48% based on the operating conditions. A comparison between the new TREC device and the conventional thermoelectric generator (TEG) to extract the wasted heat from the stove is carried out under the same operating conditions. Using the TREC is three times more efficient than TEG. The system represents a simple and low-cost suitable solution for the energy problem in remote areas.

Power generation by integrating a thermally regenerative electrochemical cycle (TREC) with a solar pond and underground heat exchanger

A new system to generate electric power by integrating a thermal regenerative electrochemical cycle (TREC) with a solar pond and underground heat exchanger is proposed. Thus far, there are relatively limited available studies in the literature about integrating TREC with renewable thermal systems. To the best of the authors’ knowledge, this work is a first attempt to investigate the integration of TREC with solar ponds. The solar pond serves two purposes: It is the hot driving source for operating the TREC power system and an energy storage medium to maintain, sustain, and stabilize its performance. The underground heat exchanger serves similar purposes as well but as the cold thermal sink. The novelty of the proposed system is in being a stable and sustainable electric power generator because of having low-cost hot and cold reservoirs that serve as the source and sink of thermal energy and storage domains. A theoretical model has been initially proposed and validated by verifying its predictions with published experimental results. The model is utilized to describe, investigate, and understand the performance of the integrated system. The most critical parameters that affect the performance of the integrated system have been detected and tracked.

Effect of electrolytes on performance of CuHCF electrode of thermally regenerative electrochemical cycle system for harvesting low-grade heat

A huge amount of low-grade heat exists in nature and industrial sectors, but is difficult to harvest. Recycling low-grade heat is of great significance for saving energy and reducing pollutant emissions. The thermally regenerative electrochemical cycle (TREC) system has attracted much attention because of its high conversion efficiency for low-grade thermal energy. The electrolyte has an important influence on the performance of the electrochemical system, but few researches have been done in the field of TREC. Here, the influences of the electrolyte, that is, the cation, electrolyte concentration and anion, on the performance of the CuHCF electrode are studied. Experiments have proved that the K+ and NH4+ cation systems have superior heat-to-electricity conversion performance. In addition, appropriately reducing the electrolyte concentration is beneficial to increase the net work output. As the electrolyte concentration is reduced from 3 M to 1 M, the CuHCF electrode obtains a larger temperature coefficient (−0.629 mV K−1) and net output power (0.96 mWh g−1). Among the different anion systems, the NO3– system has more superior heat-to-electricity conversion performance, with a net output work of 1.06 mWh g−1. This research is instructive for the performance improvement of TREC system through electrolyte optimization.

Low-hysteresis manganese hexacyanoferrate (MnHCF) aqueous battery for low-grade thermal energy harvesting

It is desirable to gather enormous low-grade waste heat energy using effective thermal energy-harvesting systems since most of the waste heats are at the temperature below 170 °C. A thermally regenerative electrochemical cycle (TREC) is a reliable approach for converting heat into electricity with a thermodynamic cycle. Most up-to-date approaches to increase the heat-to-electricity conversion efficiency have focused on improving a temperature coefficient of a full cell. Herein, we report a manganese hexacyanoferrate (MnHCF) as a TREC cathode material, which has the temperature coefficient of −0.72 mV K−1. Also, the heat-to-electricity conversion efficiency of MnHCF is 5.17% (at ηHX = 0%), greater than that reported for a TREC. The low-hysteresis TREC cathode material imparted by the largest interplanar distance guides the practical direction for studying the thermally regenerative electrochemical cycle.

Simulation analysis and experimental validation of enhanced photovoltaic thermal module by harnessing heat

A novel thermally regenerative electrochemical cycle (TREC) has been developed recently for converting low-grade heat into electricity, which is suitable but rarely researched for harnessing the thermal energy generated by the photovoltaic/thermal (PV/T) module to enhance the electrical performance. This study conducts experimental works on the heat-to-electricity performance of the TREC battery to demonstrate the feasibility and superiority of the PV/T-TREC hybrid system. Based on the experimental results, a detailed transient three-dimensional MATLAB simulation model of the PV/T-TREC system is developed to predict the overall electrical performance. For practical applications, the annual behaviors of the PV/T-TREC system employed in three typical climate areas of Hefei, Beijing, Harbin are simulated and compared. The results show that the electrical performance of the hybrid system is better than the single PV and PV/T module during different areas of various weather conditions except for cold season of February and November in Harbin. In comparison to the PV/T, the hybrid system can generate additional electricity of 45.95 kWh/m2, 50.20 kWh/m2, 29.99 kWh/m2 in the locations of Hefei, Beijing, Harbin annually. To reach the maximum electrical efficiency at different amounts of solar radiation, optimal tank volumes are analyzed and suggested in different months. The optimal working temperatures for the maximum electricity output on ideal theoretical analyses are 72 ℃, 83 ℃, 91 ℃, 100 ℃ at the ambient temperature of 5 ℃, 15 ℃, 25 ℃, 35 ℃ in constant heat cases. This study may provide a feasible plan for designing the water tank and water temperature of the PV/T-TREC for achieving high-efficient annual electricity performance of solar power in different application locations.

Performance analysis of a novel hybrid electrical generation system using photovoltaic/thermal and thermally regenerative electrochemical cycle

A combination of photovoltaic/thermal (PV/T) and thermally regenerative electrochemical cycle (TREC) is an effective and promising method to improve the electricity generation efficiency of solar energy by converting the low-grade heat from the hot water of PV/T into electricity. The concept and mathematic model of the hybrid PV/T-TREC system are constructed based on the working principle and characteristics of PV/T and TREC. A parametric analysis (ambient temperature, solar radiation, water mass flow rate, water inlet temperature) and a comparative study of PV, PV/T, PV/T-TREC systems are conducted to demonstrate the performance of the novel system. The results show that the performance of the hybrid PV/T-TREC system is better than PV and PV/T modules in the majority of working conditions. The electrical efficiency of the PV/T-TREC system is 0.92% and 0.35% higher than the single PV/T and PV modules on average as ambient temperature changes. The heat-to-electricity conversion efficiency of the PV/T-TREC system tends to decrease, increase, decrease, increase first and then decrease as factors of ambient temperature, solar radiation, water mass flow rate, water inlet temperature increase respectively.

Effect of air gap on a novel hybrid photovoltaic/thermal and thermally regenerative electrochemical cycle system

Electricity efficiency improvement of photovoltaic/thermal (PV/T) collector is still received much attention recently because solar energy is regarded as an effective and promising alternative to fossil fuels. A novel PV/T combined with a thermally regenerative electrochemical cycle (TREC) hybrid system is proposed in this paper to improve the overall electricity efficiency by means of converting the thermal energy from the hot water of the PV/T into electricity. As the heat gathering in the PV/T reduces the electrical efficiency of PV but improves both the thermal efficiency of the PV/T and electrical efficiency of the TREC, mathematic models are developed to assess and compare the performance of PV/T-TREC systems with and without air gap. The effects of heat recuperation as well as various working conditions on the overall electricity performance of hybrid systems are studied and analyzed. The results show that the PV/T-TREC hybrid system with air gap is superior to the one without air gap. Hybrid systems are better than PV/T and PV collector in all scenarios except for large wind velocity scenarios. The maximum improvement of the overall electrical efficiency is up to 1.79% at the solar radiation of 1000 W/m2. Additionally, at the ambient temperature of 5 °C, most cases at water inlet temperature below 15 °C can achieve a peak power generation. This paper may guide the design, optimization and application of the novel PV/T-TREC hybrid system to improve the overall electricity efficiency of solar energy.

Thermal energy recovery of molten carbonate fuel cells by thermally regenerative electrochemical cycles

Molten carbonate fuel cell (MCFC) is combined with the thermally regenerative electrochemical cycle (TREC) to form a novel hybrid system. TREC is considered for harvesting the waste heat of the MCFC and converting it into electricity. The hybrid system performance analyzed in terms of energetic and exergetic, and optimum criteria for critical parameters, like the current density, power output, and efficiency, are specified. Moreover, the effects of the MCFC and TREC critical design parameters on the system performance are discussed. It was obtained that a higher operating temperature of the MCFC increases the power output and efficiency of the hybrid system while decreases exergy destruction. Higher temperature coefficient and regeneration efficiency and lower internal resistance for the TREC lead to higher power output. Multi-objective optimization was applied, and for choosing the final optimal solution, Fuzzy, TOPSIS, and LINMAP decision-making techniques are employed. It was revealed that the maximum power output density and exergy efficiency for the hybrid system was about 6.4%–26.5% and 7.5%–26.4% higher than a standalone MCFC, respectively. Furthermore, the MCFC-TRECs system is evaluated against other MCFC-based hybrid systems, and it was found that TREC could be considered a favorable option for waste heat recovery of the MCFC. The outcomes of the present paper are useful for the design and optimization of the MCFC hybrid system and improving the thermal performance.

Efficient Low-Grade Heat Harvesting Enabled by Tuning the Hydration Entropy in an Electrochemical System.

Harvesting of low-grade heat (<100°C) is promising, but its application is hampered by a lack of efficient and low-cost systems. The thermally regenerative electrochemical cycle (TREC) is a potential alternative system with high energy-conversion efficiency. Here, the temperature coefficient (α), which is a key factor in a TREC, is studied by tuning the hydration entropy of the electrochemical reaction. The change of α in copper hexacyanoferrate (CuHCFe) with intercalation of different monovalent cations (Na+ , K+ , Rb+ , and Cs+ ) and a larger α value of -1.004mV K-1 being found in the Rb+ system are observed. With a view to practical application, a full cell is constructed for low-grade heat harvesting. The resultant ηe is 4.34% when TREC operates between 10 and 50°C, which further reaches 6.21% when 50% heat recuperation is considered. This efficiency equals to 50% of the Carnot efficiency, which is thought to be the highest ηe reported for low-grade heat harvesting systems. This study provides a fundamental understanding of the mechanisms governing the TREC, and the demonstrated efficient system paves the way for low-grade heat harvesting.

Thermo-osmosis-Coupled Thermally Regenerative Electrochemical Cycle for Efficient Lithium Extraction.

Lithium (Li) production based on the soda evaporation process is time-consuming and unsustainable. The emerging electrochemical Li extraction is time-efficient but requires high-concentration Li sources and significant electrical energy input. Here, we demonstrate a fast, energy-saving, and environment-friendly Li production process by coupling a thermally regenerative electrochemical cycle (TREC) using lithium manganese oxide (LMO) and nickel hexacyanoferrate (NiHCF) electrodes with poly(vinylidene fluoride) membrane-based thermo-osmosis (denoted as TO-TREC). The characterization of LMO and NiHCF electrodes confirmed that the relatively high temperature of TO-TREC has negligible adverse effects on the ion intercalation in LMO and NiHCF electrodes. The LMO/NiHCF pair has a positive temperature coefficient of 0.843 mV K-1. In the TO-TREC process, Li ions are selectively extracted from a Li-containing brine warmed by low-grade heat and then released into a room-temperature recovery solution such as LiCl with a production rate of 50-60 mmol Li+ m-2 h-1. Li source solutions are concentrated by thermo-osmosis simultaneously, making it possible to utilize previously unusable Li-containing sources, such as concentrated brines from desalination plants and industrial effluents. Besides, the TREC harvests thermal energy from the heated brine, saving >20% of electrical energy compared to conventional electrochemical methods. The new process shows the potential to meet the growing global Li demands for many applications.

A review of thermally regenerative electrochemical systems for power generation and refrigeration applications

Nowadays, waste heat recovery and refrigeration are leading energy challenges. For the generation of additional power and refrigeration purposes, many systems were used. Among these systems, a modern electrochemical cycle based on the temperature dependence of the electrodes called thermally regenerative electrochemical cycle (TREC) was considered widely for converting heat to electricity and refrigeration. These systems have a high-temperature coefficient, which is an advantage over thermoelectric systems, high efficiency, low weight and volume, silent operation, sustainably generating power, no moving parts such as compressors, non-use of vulnerable refrigerants for environmental and clean technology, and ability to couple to various energy technologies to increase the energy efficiency of these systems. Also, an enormous amount of low-grade heat exists around us in energy technologies, such as photovoltaic technology, solar/geothermal heat, the waste heat of fuel cells, industry section and conversion and recovery of this low-grade heat is crucial. Therefore, reviewing the current state-of-the-art research and the progress on the thermally regenerative electrochemical systems for power generation and refrigeration focusing on the new material and design mechanism is essential. First, the principle of power generation cycle, TREC, and the refrigeration cycle, thermally regenerative electrochemical refrigeration (TRER), were utterly discussed for a better understanding of the cycles. Furthermore, the continuous system for these cycles was introduced for continuous power generation and refrigeration. Forasmuch as theoretical studies form the basis of experimental works, the power generation and refrigeration applications of these cycles were divided into theoretical and experimental studies. A literature review revealed that theoretical studies on both the TREC and TRER cycles are comprehensive, paving the way for experimental studies. In experimental studies, useful studies have been performed on power generation by this cycle and its continuous form system. However, despite the high potential of refrigeration cycle, very few studies have been done in this regard. Finally, some recommendations for future studies were made.

Insights into the Thermopower of Thermally Regenerative Electrochemical Cycle for Low Grade Heat Harvesting

The thermally regenerative electrochemical cycle (TREC) is a promising technology for converting low-grade heat (<100 °C) to electrical power. In this work, the TREC with the NiHCF cathode and Zn anode achieves a markedly high thermopower (α) of −1.575 mV K–1 and a heat-to-electricity efficiency of 2.41% at the temperature difference of 30 °C (equivalent to 25.15% of Carnot efficiency), surpassing all the existing TREC systems. For the first time, the mixed membranes with mixed pH electrolytes are introduced in the TREC systems to boost α to a record-high value of −2.270 mV K–1. The proposed thermodynamic framework advances the understanding on the origin of α and electrochemical potential, which will guide people to engineer TRECs.

Redox Targeting-Based Thermally Regenerative Electrochemical Cycle Flow Cell for Enhanced Low-Grade Heat Harnessing.

A large amount of low-grade heat (<100°C) is produced in electrical devices and mostly wasted. This type of heat without effective dissipation also causes compromised device performance, reliability, and lifespan. To tackle these issues, a redox targeting (RT)-based flow cell with judiciously designed thermoelectrically active redox materials is demonstrated for the first time for efficient heat-to-electricity conversion through a thermally regenerative electrochemical cycle (TREC). Compared with the conventional TREC systems, the RT-based flow cell not only reveals considerably enhanced thermoelectric efficiency, but the flow of redox fluids also provides a cooling function to the system. In this work, solid material Ni0.2 Co0.8 (OH)2 and redox mediator [Fe(CN)6 ]4-/3- , both of which have negative temperature coefficient and share identical redox potential, are paired via RT-reactions to boost the capacity and meanwhile thermoelectric efficiency of a [Fe(CN)6 ]4-/3- /Zn0/2+ -based flow cell. Upon operating over the TREC cycle, the RT-based flow cell converts heat to electricity at an unprecedented absolute thermoelectric efficiency of 3.61% in the temperature range of 25-55°C.

Improving the Power Efficiency of a PV Power Generation System Using a Proposed Electrochemical Heat Engine Embedded in the System

An important issue in the photovoltaic (PV) power generation systems is the massive amount of heat wasted by PV modules/panels. For the first time, this paper provides a new solution to recover a portion of the mentioned waste heat by utilizing a proposed electrochemical heat engine. A thermally regenerative electrochemical cycle (TREC) system has been designed, constructed, and embedded in a PV power generation system including a 200-W PV module to harvest the waste heat of its PV module, so a PV-TREC hybrid system has been organized and built. The hot cell of the TREC system has been positioned in direct contact with the backside of the PV module, and the cold cell is in direct contact with a heat sink located in the shade of the PV module. Experimental verifications demonstrate that the proposed TREC system efficiently converts the waste heat into electric power ranging from 0.1 to 54.5 W. In this manner, up to 6% of the waste heat of the PV module is recovered. It also substantiated that the daily energy efficiency of the PV-TREC hybrid system is improved by 27.5% compared to a usual PV system, and this is the main contribution and novelty of this paper.

Performance assessment of a biogas fuelled molten carbonate fuel cell-thermophotovoltaic cell-thermally regenerative electrochemical cycle-absorption refrigerator-alkaline electrolyzer for multigenerational applications

In recent years, there has been increasing interest in fuel cell hybrid systems. In this paper, a novel multi-generation combined energy system is proposed. The system consists of a molten carbonate fuel cell (MCFC), a thermally regenerative electro-chemical cycle (TREC), a thermo photovoltaic cell (TPV), an alkaline electrolyzer (AE) and an absorption refrigerator (AR). It has four useful outputs, namely electricity, hydrogen, cooling and heating. The overall system is thermodynamically modeled in a detailed manner while its simulation and modeling are done through the TRNSYS software tool. Power output, cooling-heating and produced hydrogen rates are determined using energetic and exergetic analysis methods. Results are obtained numerically and plotted. The maximum power output from the system is 16.14 kW while maximum energy efficiency and exergy efficiency are 86.8% and 80.4%,. The largest exergy destruction is due to the MCFC.