Regenerative Cycle Research Articles

Triple-Objective Optimization of SCO2 Brayton Cycles for Next-Generation Solar Power Tower

In this paper, the SCO2 Brayton regenerative and recompression cycles are studied and optimized for a next-generation solar power tower under a maximum cycle temperature of over 700 °C. First, a steady-state thermodynamic model is developed and validated, and the impacts of different operating parameters on three critical performance indexes, including the cycle thermal efficiency, specific work, and heat storage temperature difference, are analyzed. The results reveal that these performance indexes are influenced by the operating pressures, the SCO2 split ratio, and the effectiveness of the regenerators in complex ways. Subsequently, considering the three performance indexes as the optimization objectives, a triple-objective optimization is carried out to determine the optimal operating variables with the aim of obtaining Pareto solutions for both cycles. The optimization indicates that the regenerative cycle can achieve the maximum heat storage temperature difference and the maximum specific work of 396.4 °C and 180.6 kW·kg−1, respectively, while the recompression cycle can reach the maximum thermal efficiency of 55.95%. Moreover, the optimized maximum and minimum pressure values of both cycles are found to be around 30 MPa and 8.2 MPa, respectively. Additionally, the distributions of the optimized values of the regenerator effectiveness and the SCO2 split ratio show different influences on the performance of the cycles. Therefore, different cycles with different optimized variables should be considered to achieve specific cycle performance. When considering thermal efficiency as the most important performance index, the recompression cycle should be adopted. Meanwhile, its SCO2 split ratio and the regenerator effectiveness should be close to 0.7 and 0.95, respectively. When considering heat storage temperature difference or specific work as the most important performance index, the regenerative cycle should be adopted. Meanwhile, its regenerator effectiveness should be close to 0.75. The results from this study will be helpful for the optimization of superior SCO2 cycles for next-generation solar tower plants.

A Comprehensive Review of Organic Rankine Cycles

It has been demonstrated that energy systems driven by conventional energy sources like fossil fuels are one of the main causes of climate change. Organic Rankine cycles can help to reduce that impact, as they can be operated by using the industrial waste heat of renewable energies. The present study presents a comprehensive bibliographic review of organic Rankine cycles. The study not only actualizes previous reviews that mainly focused on basic cycles operating on subcritical or supercritical conditions, but also includes the analysis of novel cycles such as two-stage and hybrid cycles and the used fluids. Recuperative and regenerative cycles are more efficient than reheated and basic single-stage cycles. The use of two-stage cycles makes it possible to achieve higher thermal efficiencies and net power outputs of up to 20% and 44%, respectively, compared with those obtained with single-stage cycles. Theoretical studies show that hybrid systems, including Brayton and organic Rankine cycles, are the most efficient; however, they require very high temperatures to operate. Most organic Rankine cycle plants produce net power outputs from 1 kW up to several tens of kW, mainly using microturbines and plate heat exchangers.

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.

Uterocutaneous Fistula Complicating Caesarean Section: A Case Report

A uterocutaneous fistula (UCF) is a rare clinical presentation following Caesarean section (CS) and other pelvic surgeries. Presently, there are limited reports on the clinical management of UCF. This report describes a 35-year-old woman who presented with a four-year history of abdominal pain and cyclical bleeding from the skin after CS. Fistulography was carried out, and a diagnosis of UFC was made. She had a fistulectomy with the fistula tract excised, followed by debridement of the necrotic tissue and repair of the uterus. At the third-month postoperative follow-up, there was no recurrence of symptoms. A UCF is a rare condition that needs detailed investigation and timely medical and surgical management.

A contemporary description of the Carnot cycle featured by chemical work from equilibrium: The electrochemical Carnot cycle

The emerging electrochemical heat engines featured by chemical work output necessitate a contemporary description of the Carnot cycle, as it was previously described in terms of volume work. Starting from the quasi-static process and relaxation time, this study recognizes the risk to the consistency of thermodynamic logic caused by multiple relaxation times in the same process. To address this issue, a rigorous description of the Carnot cycle featured by chemical work, namely, the electrochemical Carnot cycle, is presented. Using an ideal mixture capable of phase transitions and chemical reactions as the working fluid, a one-to-one correspondence between the destruction and restoration of the equilibrium in each process is achieved in the electrochemical Carnot cycle. This description effectively avoids interference from other relaxation times when determining equilibrium. In practical terms, the electrochemical Carnot cycle could be implemented by available heat exchangers and electrochemical cells with flow battery structures. Furthermore, an investigation of existing electrochemical heat engines reveals that the cycle configuration of electrochemical Brayton cycle is more thermodynamically rational than that of thermally regenerative electrochemical cycle. This study is expected to provide guidance for the cycle construction of electrochemical heat engines and inspire further thinking in this field.

Heat Transfer and Flow Structure Characteristics of Regenerative Cooling in a Rectangular Channel Using Supercritical CO2

At an extremely high Mach number, the regenerative cooling of traditional kerosene cannot meet the requirement of the heat sink caused by aerodynamic heating and internal combustion in a scramjet propulsion system. As a supplement of traditional regenerative cooling, supercritical CO2 is regarded as an effective coolant in severe heating environments due to its excellent properties of heat and mass transportation. In this paper, the heat transfer and flow structure characteristics of regenerative cooling in a rectangular channel using supercritical CO2 are analyzed numerically using a validated model. The effect of heat flux magnitude, nonuniform heat flux, acceleration and buoyancy and flow pattern are considered to reveal the regenerative cooling mechanism of supercritical CO2 in the engine condition of a scramjet. The results indicate that the heat transfer deterioration phenomenon becomes obvious in the cooling channel loaded with relatively high heat flux. Compared with the cooling channels loaded with increased heat flux distribution, the maximum temperature increased for the channel loaded with decreased heat flux distributions. When larger acceleration is applied, a relatively lower wall temperature distribution and higher heat transfer coefficients are obtained. The wall temperature distribution becomes more uniform and the high-temperature region is weakened when the coolants in adjacent channels are arranged as a reversed flow pattern. Overall, the paper provides some references for the utilization of supercritical CO2 in regenerative cooling at an extremely high Mach number in a scramjet.

Experimental investigation of a regenerative organic Rankine cycle (ORC) under different cryogenic cooling conditions

ABSTRACT This study experimentally investigates a Regenerative Organic Rankine Cycle (RORC) system that operated under varied cryogenic conditions using liquid nitrogen (LN2) as the heat sink. A three-fluid heat exchanger (TFHE) is applied to this system instead of a condenser and regenerator to realize the RORC system. The condensing pressure changes between 0.08 MPa and 0.6 MPa, and applying the R290 as the working fluid. The condensing pressure effect on system performances, including electrical power output, cold thermal efficiency, and exergetic efficiency, are investigated under different LN2 flow rates. The comparisons between basic ORC (BORC) and RORC under the same working conditions are conducted. The results demonstrate that as the condensing pressure remains constant, the higher LN2 flow rate is beneficial to the overall performance of the RORC system. The maximum cold thermal efficiency and power generation obtained in the RORC system are 13.35% and 738.81 W, which are obtained at the LN2 flow rate of 120 kg/h and the condensing pressure of 0.25 MPa. Additionally, the RORC shows improved performance with an exergetic efficiency of 10.68%. The results would be a technological guide for the construction and improvement of the system with cold energy recovery.

Heat Transfer Analysis and Structural Optimization for Spiral Channel Regenerative Cooling Thrust Chamber

There is currently a lack of efficient heat transfer analysis methodologies for spiral channel regenerative cooling that has been increasingly applied in liquid rocket engines. To figure out the heat transfer characteristics of the spiral channel regenerative cooling thrust chamber, a simple 1D method based on the traditional semi-empirical formula after correcting the flow velocity is proposed. The accuracy of this approach is verified by the 3D numerical simulation. The verified method is further used to analyze the distribution of inner wall temperature in the test case and optimize the channel’s parameters. The research shows that the maximum inner wall temperature cooled by the spiral channel is 8.5% lower than that of the straight channel under the same channel size and boundary condition, indicating that the application of the spiral channel significantly improves the cooling effect. In addition, the 1D model combined with the second-order response surface model is employed to optimize the channel width, channel height, pitch, and inner wall thickness aiming for the best cooling effect. The calculated maximum temperature of the inner wall after the structure optimization is 586.6 K, which is 29.8% lower than the initial structure before optimization.

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.

Thermo-economic optimization and comparative analysis of different organic flash cycles for the supercritical CO2 recompression Brayton cycle waste heat recovery

It is well known that the energy efficiency of the supercritical carbon dioxide recompression Brayton cycle (sCO2RBC) can be improved by employing the bottoming cycles to recover its large waste heat. To achieve better performance, three bottoming cycles with a good temperature matching with the sCO2 stream, namely the basic organic flash cycle (BOFC), the regenerative organic flash cycle (ROFC), and the organic flash Rankine cycle (OFRC), are employed to combine with the sCO2RBC in this work. The parametric analysis is performed to determine the variation trends of systems’ thermodynamic and exergoeconomic performance with five important parameters. The single-objective optimization is conducted for the sCO2RBC/BOFC, sCO2RBC/ROFC and sCO2RBC/OFRC systems with six organic fluids. The single-objective optimization results show that the highest exergy efficiency of the sCO2RBC/OFRC is higher than that of the rest combined sCO2RBC/OFCs by up to 1.95%. And the lowest total product unit cost for the sCO2RBC/OFRC is up to 0.93% lower than that of the rest combined sCO2RBC/OFCs. Finally, a comparative analysis is performed for the three combined sCO2RBC/OFCs, sCO2RBC/ORC and stand-alone sCO2RBC systems under the multi-objective optimization conditions. Results show that the integrated systems can improve the ηex by 4.62–6.06% and reduce the cp,tot by 2.68–3.14% when taking the sCO2RBC system as the baseline system. The sCO2RBC/OFCs and sCO2RBC/ORC are recommended to use R1336mzz(Z) and R245fa as working fluids, respectively. Moreover, the sCO2RBC/OFRC system achieves an improvement of 0.41%–1.37% in exergy efficiency with a comparable total product unit cost, compared to other aforementioned integrated systems.

Eyes, menstruation and endometriosis.

Vicarious menstruation refers to cyclical bleeding outside the uterine cavity during the menstrual cycle. Haemolacria, or blood in tears, is a rare medical phenomenon that can occur with menstruation or in association with endometriosis. Endometriosis, defined by the presence of endometrial-like tissue in extra-uterine sites, affects around 10% of fertile women; the ocular system is one of the rarest sites it can be present in. Endometriosis usually requires a biopsy for diagnosis, and the anatomic difficulty of obtaining a biopsy of the ocular system makes ocular endometriosis diagnosis more obscure. However, few cases have been described in the literature and due to the psychological, physical, and social implications of haemolacria on the patient, treatment is of utmost importance. We reviewed the literature on ocular endometriosis and ocular vicarious menstruation with the aim to discuss the clinical presentation, necessary workup and various treatment modalities, while also shedding light on the connection between the eyes and endometriosis in general. It is hypothesised that uterine endometrial cells can travel lymphatically or haematogenously and deposit extra-uterine endometriotic lesions that bleed in response to hormonal changes in the menstrual cycle. Additionally, the conjunctival vasculature has been found to be responsive to hormonal changes due to the presence of oestrogen and progesterone receptors, causing bleeding at the corresponding sites, even without endometriotic lesions. Clinical correlation of the haemolacria with the menstrual cycle can suffice for a diagnosis of vicarious menstruation, and thus opens the possibility of treatment to provide symptomatic treatment for the patient.

Thermodynamic design and power prediction of a solar power tower integrated system using neural networks

In spite of the fact that the solar power tower system is considered as one of the most valuable power generation facilities, it still faces challenges such as insufficient utilization of the solar salt temperature range and the dependence on high solar radiation intensity. To address this issue, an integrated recompression Brayton cycle and trans-critical regenerative organic Rankine cycle in parallel layout is proposed. The comparison with other literature shows that the specific work of the integrated system (147.8 kW/kg) outperforms the partial cooling Brayton cycle (130.1 kW/kg) under the same solar salt temperature range. In order to forecast future electricity generation and serve as a reference for plant operation, a power prediction strategy using neural networks is developed. Findings indicate that the integrated system can increase power production and maximize solar salt temperature utilization by adjusting the physical constraint strategy based on the changing temperature interval. Its equivalent work and thermal efficiency reach 12.7 MW and 38.36%, respectively, and it can recover the cost in 8 years. These results suggest that the proposed integrated system is a promising solution to enhance the performance of solar power tower systems with significant economic benefits.

Continuous decentralized hydrogen production through alkaline water electrolysis powered by an oxygen-enriched air integrated biomass gasification combined cycle

This research work presents an innovative approach for continuous decentralized production of renewable hydrogen from woody biomass. Alkaline water electrolysis (AWE) is used to produce high-purity hydrogen, while the oxygen by-product is mixed with ambient air and used to fire a biomass-fueled downdraft gasifier in order to produce an upgraded producer gas with a lower heating value (LHV) between 7–8 MJ/Nm3. This fuel gas is then subjected to a conditioning stage and eventually fed to a combined cycle consisting of a recuperative gas turbine as topping unit and a regenerative subcritical organic Rankine cycle as bottoming unit, which together allow for a combined electric power generation efficiency close to 40%. Most of the net AC power from the integrated gasification combined cycle (IGCC) is rectified to DC power and ultimately used to power an alkaline electrolyzer, with a minor share allocated to all the required utilities and ancillary equipment, including hydrogen compression to 200 bar. The results from simulation of the hybrid IGCC-AWE plant under steady-state operating conditions in Aspen Plus V.11 indicate an optimal efficiency of 17.6% based on the LHV of hydrogen. Thus, if sized for a biomass consumption of 1 t/h, the proposed plant is capable of providing around 26 kg/h of compressed hydrogen at 200 bar.

Energy and exergy analyses of a regenerative Brayton cycle utilizing monochlorobiphenyl wastes as an alternative fuel

This paper develops a thermodynamic model for the regenerative Brayton cycle that uses monochlorobiphenyl waste as fuel in a combustion chamber to power a gas turbine. The regenerator increases the air temperature entering the combustion chamber to react with monochlorobiphenyl wastes to produce a higher turbine inlet temperature. The pressure ratio and turbine inlet temperature can negatively or positively affect power plants' thermal efficiency. Thus, different pressure ratios and turbine inlet temperatures are used to calculate the overall thermal efficiency. A stoichiometric reaction-based gas composition model is used to determine the combustion gas composition at the combustion chamber exit and to calculate the heat capacity of air and combustion gases as a function of temperature. The exergy analysis of the proposed power plant is presented to determine the effect of the pressure ratio on the exergy efficiency of the turbine, compressor, and regenerator. According to the results, an increase in pressure ratio increases cycle thermal efficiency and turbine inlet temperature when using a regenerator, whereas the temperatures decrease when not using a regenerator. The Brayton cycle with a regenerator increases thermal efficiency from 30% to 100% for a pressure ratio of 6–30. This contrasts with the Brayton cycle without a regenerator. In addition, for the regenerative Brayton cycle, the turbine inlet temperature rises from 1050 K to 1200 K as the pressure ratio rises from 1 to 30. In the absence of the regenerator, however, this variation's trend is in the opposite direction. Furthermore, the results show that an increase in pressure ratio will increase the exergy efficiency of the compressor and regenerator, while a decrease will occur in the exergy efficiency of the turbine. Findings indicate that the regenerator has the highest exergy efficiency at a pressure ratio higher than five, followed by turbine and compressor exergy efficiency.

Evaluating electricity generation from low-grade heat sources

Thermally regenerative electrochemical cycle offers a way to extract usable energy from waste heat.

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.

Applying position and time dependent nusselt number and friction factor into 1D model to improve regenerator design for solid-state caloric cooling cycles

Active caloric regenerative cycles have recently attracted much attention as a promising cooling technology that can potentially replace the vapor compression cycle based on its high efficiency and eco-friendly characteristics. The use of modeling is necessary in optimizing the active caloric regenerative cycle due to the system’s complex structure and numerous parameters that need to be taken into account. The cycle uses oscillatory flow to efficiently exchange heat between the caloric material and the heat transfer fluid and to generate a desired temperature gradient in the regenerator. When the oscillatory flow is used, hydrodynamically and thermally developing regions occur all over the regenerator when the direction of the fluid flow changes.To the best of the authors’ knowledge, no previous research has considered the oscillatory developing regions in a 1D model. Therefore, this study incorporated the developing regions generated by the oscillatory flow as well as the entrance developing regions into a regenerator model for the first time. Based on the modeling results, the influence of the oscillatory developing regions and the average Nusselt number increase with higher cycle frequency. Additionally, the new 1D model produces average Nusselt number similar to the 2D model with a maximum error of 4 %. Considering that the 2D model requires very high computational cost to solve the transient condition of the caloric cycle, the advanced 1D model can accurately assess the performance of the caloric cycle and provide a detailed analysis of heat transfer phenomena and fluid dynamics in the regenerator at modest computational cost.The parallel plate type, optimized using the new model, has a hydraulic diameter of 0.1 mm. However, maintaining such a small space between the plates can be challenging from an application standpoint. Therefore, this study proposes the use of a wide rectangular microchannel matrix as the regenerator. The rectangular microchannel matrix, with an aspect ratio of 30, produces a coefficient of performance of 5.65, which is comparable to the parallel plate matrix. Consequently, the wide rectangular microchannel matrix can achieve both high Nusselt number and structural stability simultaneously.

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.