Thermal Power Conversion Research Articles

Thermodynamic study of semi-closed rankine cycle based on direct combustion of hydrogen fuel

Electrolysis of water for hydrogen-production has enormous potential. This study proposes a semi-closed Rankine cycle for efficiently utilizing direct hydrogen combustion via thermal power conversion. Unlike traditional closed Rankine cycles, combustion occurs inside the combustion chamber with hydrogen and pure oxygen, followed by a mixed heat exchange with the mainstream working fluid, water. The heat absorption process of the semi-closed Rankine cycle occurs within the working fluid and exhibits characteristics of both open and closed cycles. Thus, this cycle can combine the advantages of desirable parameters and isothermal heat rejection of the Rankine cycle. Applying the semi-closed Rankine cycle to hydrogen fuel utilization yields a higher energy efficiency, especially when coupled with a supercritical compression regeneration process. The main steam parameters were 620 °C/30 MPa, the cycle's average heat absorption temperature reached 561 °C, and its energy efficiency was 59.35%. When the main steam parameters were increased to 1200 °C/30 MPa, the average heat absorption temperature rose to 1021 °C and its energy efficiency was 68.27%, demonstrating a significant efficiency advantage. The newly proposed semi-closed Rankine cycle based on direct combustion of hydrogen fuel provides a novel and feasible path for exploring the efficient utilization of hydrogen.

Design optimization and off-design performance analysis of one-dimensional supercritical CO2 Brayton cycle

Supercritical carbon dioxide (SCO2) recompression Brayton cycle (RBC) is an encouraging technology for thermal power conversion and heat harvest for a wide range of heat sources due to its high-efficiency, low auxiliary power consumption, and compact size. However, most studies on its design and off-design performance analysis have been limited to the simple zero-dimensional component models without considering aerothermodynamics and ambient conditions. In this study, we developed a set of fully one-dimensional components models for SCO2 compressors, turbines, and heat exchangers, and, then, applied them to the multi-objective design optimization of a SCO2 cycle targeting a 50 MW net power output. The cycle off-design performance analysis methods and the control strategies were developed to countermeasure the performance degradation against the varying cooling water temperatures and part loads. The multi-objective cycle design optimization demonstrates that the cycle thermal efficiency and total product unit cost can be 0.476 and 11.652$/GJ, respectively, for the 50 MW SCO2 RBC cycle. The choke margin of the main compressor is 0.136 less than that of the re-compressor, and condensation can lead to a sharp decline in efficiency under high-flow conditions, causing the earlier onset of choking. The off-design performance analysis shows that the turbine inlet pressure control improves cycle efficiency below the design point of 317 K but does otherwise above the point. In contrast, inventory control strategies can maintain stable cycle efficiency and achieve adjustable net power output between 10 % and 110 %. The findings have the potential to advance further the commercial application of SCO2 cycles in high-temperature thermal energy systems.

General integration method for system configuration of organic Rankine cycle based on stage-wise concept

Organic Rankine Cycle (ORC) integrated systems with heat sources consist of heat exchange processes and thermodynamic power conversion processes. Fully exploring feasible system configuration while considering fluid selection is crucial for enhancing ORC power generation and balancing economic costs. This study innovatively proposes a theoretical framework that decomposes the Rankine cycle into basic thermodynamic processes. The design of system configuration is regarded as the adjustment of the series-parallel relationships of these basic processes, aiming to achieve simultaneous optimization of the cycle structure and the HEN structure, thereby greatly expanded the search space for feasible ORC system configurations. The optimization problem is solved using a bi-level strategy, The outer layer determines the number of thermal power conversion components, operating parameters, and working fluid selection, then provides the associated stream information to the inner layer for optimizing the heat exchange scheme. Applying this method to two different case studies from the literature validated its effectiveness in optimizing scenarios with single and complex heat sources. The Pareto frontier obtained under the conditions of Case I indicates that the investment cost can be reduced by $33.04 k/yr while maintaining the same net power output. In exchange for an increase of $1.51 k/yr in investment cost, the maximum net power output can be improved by 124.74 kW. Under the conditions of Case II, the maximum net output power can increase by 9.71 %–52.5 %, and the Pareto frontier featuring outputs exceeding 50 kW is provided. By analyzing the relative positions of literature schemes and Pareto front within the solution space, the impact of system configuration improvements on the objective functions is explained. These results confirm the effectiveness of the proposed approach for designing optimal ORC energy recovery systems for heat sources. This contribution advances optimization methods for similar closed cycle configurations, which is significant for advancing the utilization of waste heat and addressing sustainability challenges.

Enhancing thermoelectric effect with BaTiO3-doped ZrO2 tapes and ferromagnetic nanostructures

The Anomalous Nernst Effect (ANE) emerges as a promising candidate for converting heat into electricity through magnetic nanostructures. However, the substrate plays a central role in optimizing this phenomenon for future technological applications. Tapes of ZrO2 and BaTiO3-doped ZrO2, manufactured by tape casting, possess characteristic flexibility in the green state, which makes them suitable for use as coverings on virtually any curved surface. Moreover, BaTiO3 ceramic materials present piezoelectric properties, which, when combined with thermoelectric properties, can generate interesting heterostructures with simultaneous thermal and vibrational power conversion capabilities. This study explores the combination of tape casting and magnetron sputtering to improve the functionalization of ceramic tapes with ferromagnetic nanostructures. Specifically, it investigates the quasi-static and thermoelectric properties of heterostructures created by depositing NiFe thin films onto sintered ZrO2 and BaTiO3-doped ZrO2 tapes using magnetron sputtering. The results indicate a 71 % increase in the effective thermoelectric coefficient as the amount of BaTiO3 in the ceramic substrate increases. These findings suggest a new approach to integrating these techniques for the production of multifunctional materials for thermoelectric energy conversion.

Experimental study and performance enhancement of a novel micro heat pipe photovoltaic/thermal system in a cold region

The photovoltaic/thermal (PV/T) system, which utilizes solar energy to generate electricity and thermal energy, has significant potential in the northwest region of China, an area with abundant solar irradiation. To improve the performance of the micro heat pipe PV/T system, the study analyzed the system's thermal collection efficiency and power conversion efficiency using numerical simulation of the PV/T panel. The model incorporated double-layer glass and replaces water with nanofluid as the working medium inside the airfoil heat exchanger. The average relative error of the model was 0.9 %. In experiments conducted in Lanzhou, located in the northwest region of China, the power conversion efficiency reached its peak of 12.64 % during winter, while the thermal collection efficiency reached its maximum of 39.45 % during summer. Moreover, utilizing R141b as the working fluid in the micro heat pipe resulted in an average increase of 7.14 % in thermal collection efficiency and an average increase of 4.46 % in power conversion efficiency compared to acetone. This indicated that R141b outperforms acetone in terms of system performance. The study observed that the highest overall efficiency is achieved when the air layer thickness of the double-layer glass is 11 mm. The thermal resistance between the inner wall of the airfoil heat exchanger and the water in the PV/T components was the highest, reaching 48.85 %. Therefore, it was necessary to use nanofluids instead of water to reduce the thermal resistance of PV/T modules. The system performance remained relatively stable when the volume fraction reaches 8 %. At this volume fraction, the Al2O3-water nanofluid, Cu-water nanofluid, and Ag-water nanofluid increased the power conversion efficiency by 0.34 %, 0.72 %, and 0.77 % respectively. Moreover, they improved the thermal collection efficiency by 1.16 %, 2.69 %, and 2.78 % respectively. The micro heat pipe PV/T system was utilized for winter heating of farmers in Lanzhou and Jinchang cities. It maintained an indoor temperature of over 16 ℃ throughout the day, thus meeting the indoor heating standards.

Experimental Characterization of Commercial Scroll Expander for Micro-Scale Solar ORC Application: Part 1

In order to reduce greenhouse gas emissions and achieve global decarbonisation, it is essential to find sustainable and renewable alternatives for electricity production. In this context, the development of distributed generation systems, with the use of thermodynamic and photovoltaic solar energy, wind energy and smart grids, is fundamental. ORC power plants are the most appropriate systems for low-grade thermal energy recovery and power conversion, combining solar energy with electricity production. The application of a micro-scale ORC plant, coupled with Parabolic Trough Collectors as a thermal source, can satisfy domestic user demand in terms of electrical and thermal power. In order to develop a micro-scale ORC plant, a commercial hermetic scroll compressor was tested as an expander with HFC-245fa working fluid. The tests required the construction of an experimental bench with monitoring and control sensors. The aim of this study is the description of the scroll performances to evaluate the application and develop optimization strategies. The maximum isentropic effectiveness is reached for an expansion ratio close to the volumetric expansion ratio of the scroll, and machine isentropic effectiveness presents small variations in a wide range of working conditions. The filling factor is always higher than one, due to leakage in the mechanical seals of the scroll or other inefficiencies. This study demonstrates that using a commercial scroll compressor as an expander within an ORC system represents a valid option for such applications, but it is necessary to improve the mechanical seals of the machine and utilize a dedicated control strategy to obtain the maximum isentropic effectiveness.

A global heat flow topology for revealing the synergistic effects of heat transfer and thermal power conversion in large scale systems: Methodology and case study

In the processes of modeling, optimization, heat transfer analysis, and thermal power conversion analysis in large-scale complex systems, challenges are encountered regarding intricate design and optimization. In this paper, a global heat flow topology is proposed that reveals the synergistic effect of heat transfer and thermal power conversion in large-scale systems. The approach involves the construction of a heat flow topology, mathematical model development, analysis of specific parameters, and multi-objective optimization. It integrates various methods and tools, including heat flow topology, numerical simulation tool, entransy transfer analysis, global thermal resistances visualization, genetic algorithm (GA), and technique for order preference by similarity to ideal solution (TOPSIS). By using the LNG cold energy recovery and regasification process in floating storage and regasification units (FSRU) as a case study, the methodology provides a comprehensive understanding of the interactions and conversions between heat transfer and thermal power within the system. The results demonstrate that the optimal configuration achieves a maximum thermal efficiency of 21.73 % and a maximum cold recovery efficiency of 58.48 %. Additionally, the interaction between LNG pressure and seawater temperature exhibits limited synergistic impact on both LNG cold exergy efficiency and entransy transfer efficiency. Similarly, the combined influence of LNG pressure and evaporator temperature shows weak synergistic effects on parameters such as supply/demand ratio, thermal efficiency, and entransy transfer efficiency. The analysis of the global thermal resistance bubble chart reveals the significant impact of evaporator temperature and seawater temperature on the evaporator resistance, as well as the notable impact of LNG pressure on the condenser resistance.

Experimental investigation of droplet propulsion driven by thermal power conversion

Experimental investigation of droplet propulsion driven by thermal power conversion

Numerical analysis of double-diffusive natural convective flow of Ostwald-de Waele fluid in an irregular enclosure with a circular obstacle

PurposeThe current study aims to enhance the double diffusive natural convection in an Ostwald-de Waele fluid filled in a hexagonal enclosures using non-uniform magnetic field embedded with circular obstacle. The upper wall has maximum temperature Th∗ and concentration (Ch∗) whereas the lower wall is kept at minimum temperature Tc∗ and concentration Cc∗. The parameters that play a significant role in the heat and mass transfer are evaluated, including Hartmann number, power law index, Rayleigh number and buoyancy ratio. Design/Methodology/approachIn two-dimensions, the governing formulation is written as a set of balancing equations for velocities, pressures, energies and concentrations. The nonlinear governing PDEs are solved using numerical approach based on Finite Element Method (FEM). The discretization is performed using the stable finite element pair ℘2/℘1. Code validation and results are ensured by conducting a grid convergence and comparison test respectively. FindingsThe influence of flow on velocity, isotherms and iso-concentration field is examined by analyzing a broad variety of variables such as power-law index 0.7≤n≤1.2, Hartmann number 0≤Ha≤50, Rayleigh number 104≤Ra≤106, inclination angle 0°≤γ≤90° and buoyancy ratio -2≤N≤2. The findings are displayed graphically, including the flow, temperature and concentration fields. Heat and mass transfer rates are maximum for shear thinning fluids while for shear thickening fluids it reduces, whereas it is maximum for concentration dominated assistant flow and minimum for concentration dominant counter flow. ApplicationThe novel aspects of the research include its thorough examination of solar thermal power conversion and its inventive energy deficiency devices and a wide range of electrical design applications.

An Efficient Electrothermal Model of a Thermoelectric Converter for a Thermal Energy Harvesting Process Simulation and Electronic Circuits Powering

This paper deals with an electrothermal model of a thermoelectric converter dedicated to performing simulations of coupled thermal and electrical phenomena taking place in harvesting processes. The proposed model is used to estimate the electrical energy gain from waste heat that would be sufficient to supply electronic circuits, in particular autonomous battery-less nodes of wireless sensor networks (WSN) and Internet of Things (IoT) devices. The developed model is not limited to low-power electronic solutions such as WSN or IoT; it can also be scaled up and applied to simulations of considerably higher thermal power conversion. In this paper, a few practical case studies are presented that show the feasibility and suitability of the proposed model for complex simultaneous simulation processes in both the electrical and thermal domains. The first example deals with a combined simulation of the electrothermal model of a thermoelectric generator (TEG) and an electronic harvester circuit based on Analog Devices’ power management integrated circuit LTC3108. The second example relates to the thermalization effect in heat sink-less harvesting applications that could be mitigated by a pulse mode operation. The unique contribution and advancement of the model is the hierarchical structure for scaling up and down, incorporating the complexity of the Seebeck effect, the Joule effect, heat conduction, as well as the temperature dependence of the used materials and the thermoelectric pellet geometries. The simulations can be performed in steady as well as transient states under changing electrical loads and temperatures.

Optimization of a Reverse-Brayton cycle with inter-cooling and regeneration

A Reverse-Brayton cycle represented by the B787 electrical driven environmental control system is used as the analysis object, including the thermodynamic process of two-stage compression, intercooling, and regeneration. An analytical expression for the thermodynamic performance of the cycle is derived based on the endoreversible thermodynamic analysis model (ETM). Combined with a genetic algorithm, an ETM-based optimization approach (ETM-OA) is proposed. Using this method, it is found that the coefficient of performance is increased by 34.1 % and 48.4 % under two typical flight conditions, respectively, and the corresponding system entropy generation number is reduced by 30.2 % and 51.1 %, respectively. Both ETM-OA and entropy generation analysis show that the electric supercharging module, secondary heat exchanger, and air cycle machine are key components of the system. This study provides a convenient method for obtaining the thermal power conversion mechanism of complex thermal cycles and conducting optimization analysis.

Skin-Friction Coefficient Model Verification and Flow Characteristics Analysis in Disk-Type Gap for Radial Turbomachinery

Under the conditions of high speed and density flow, the windage loss in the gap behind an impeller has an important influence on its thermal power conversion efficiency. Daily and Nece took the relative gap and Reynolds number as the characteristic parameters and divided the flow of a rotating disk in a closed chamber into four flow regions through laminar flow, turbulent flow and state of boundary layers. In this paper, the skin-friction coefficient model of Regio III and IV was verified by a numerical method under conditions corresponding to the typical Reynolds number range for a radial impeller. The results show that when the relative gap increases, the relative deviation between the numerical calculation and the model prediction results decreases, and the maximum deviation is −12.4%. With the increase of Reynolds number, the Region IV model is more accurate. Region III has the flow state of boundary layers merging and separation at the same time. Both models have good prediction accuracy with different mediums of air, water-liquid and critical CO2. The deviation in Region III is larger and shows a decreasing trend with respect to Region IV, Based on the two models, a method for predicting the optimal gap is proposed. The method is verified to be reliable and can minimize windage loss.

Experimental and numerical studies of thermal power conversion and energy flow under high-compression ratios of a liquid methane engine (LME)

To improve the performance of a liquid methane engine (LME) under universal characteristics, the in-cylinder combustion process and thermal power conversion under different compression ratios were studied by experiment. The experimental results show that increasing the compression ratio results in a linear increase in EEE (effective expansion efficiency) and EER (effective expansion ratio). Moreover, the thermal efficiency varies significantly with the compression ratio only at low and medium loads. The effective thermal efficiency can reach up to 48.69%. For high-speed conditions, it is recommended to reduce friction losses and pump gas losses. Furthermore, a 1-D simulation model was developed to simulate the in-cylinder energy flow with a compression ratio of 12.6. The simulation results show that exhaust timing has a more pronounced effect on the in-cylinder energy flow (thermal balance) than intake timing. When the engine speed is 800 rpm, advancing the exhaust timing by 20°CA can increase the effective thermal efficiency by about 1%.

An efficient multilayer adaptive self-organizing modeling methodology for improving the generalization ability of organic Rankine cycle (ORC) data-driven model

The efficient and accurate model construction of an organic Rankine cycle (ORC) system is the key to its analysis, prediction, and optimization. As a typical multidisturbance nonlinear dynamic system, the ORC system always operates in a nonstationary state. Under uncertain disturbance, accurately identifying the thermal power conversion process state and quickly realizing the accurate association of various mappings are the key considerations of constructing the data-driven model of the ORC system. From the perspective of data selection, parameter association, and structural design, this study proposes a methodology for the efficient multilayer adaptive self-organizing modeling of ORC systems. This methodology can realize efficient autonomous modeling in the whole design process of the data-driven model of the ORC system. Moreover, the proposed methodology can minimize the structural risk of the model by balancing the empirical risk and structural complexity. By taking real data as the test base, the generalization ability and time cost of the data self-selection layer, information self-correlation layer and adaptive self-organizing part of the structure layer are evaluated. Compared with the direct construction of the ORC system data model, the proposed methodology can reduce the model construction time cost by 75.54% and improve the generalization ability by 61.88%. In addition, maximizing the generalization capability with minimum structural risk is an important part of data-driven model construction of ORC systems. In this study, a data-driven model structural reliability assessment approach for ORC systems is proposed. Then, the proposed adaptive self-organizing optimization methodology is verified on the basis of the structural reliability assessment model. The multilayer adaptive self-organizing modeling methodology proposed in this study can provide new ideas and necessary theoretical guidance.

On the analysis of thermosolutal mixed convection in differentially heated and soluted geometries beyond rectangular

PurposeThis paper aims to study the impact of convexity and concavity of the vertical borders on double-diffusive mixed convection. In addition, the study of entropy generation is performed. This numerical study has been carried out for different patterns of wavy edges to reveal their effects on heat and mass transfer phenomena.Design/methodology/approachFour different flow features are treated by varying the directions of convexity and concavity of the vertical walls. A uniform temperature, as well as concentration distributions, are introduced to the left border while keeping a cold temperature and low concentration for the right border. The horizontal boundaries are in adiabatic condition. The upper border of the chamber is moving in the right direction with an equal speed. The governing Navies–Stokes equations are designed to describe energy and species transport phenomena, and these equations are solved by compact scheme.FindingsThe investigated results are analyzed for various parameters, namely, Prandtl number, Richardson number, thermal Grashof number, Lewis number, Buoyancy ratio and amplitude of the wavy walls. It is observed that the thermal and solutal transfer performance becomes effective with lower Richardson numbers. The results reveal that the concavity and convexity of the side borders of the cabinet can control the thermosolutal performance. It is also observed that among all wavy chambers, Case-4 records maximum thermosolutal transfer rate, while Case-3 attains minimum thermosolutal transfer rate.Originality/valueThis work is an example of solar thermal power conversion, power collection systems, systems of energy deficiency, etc.

Energy, exergy and economic (3E) study on waste heat utilization of gas turbine by improved recompression cycle and partial cooling cycle

ABSTRACT As the bottom cycle of gas turbine waste heat utilization, the S-CO2 cycle can enhance the power output capacity of the system. Based on the energy analysis of the recompression cycle and the partial cooling cycle for gas turbine waste heat utilization, the two cycles are improved from the matching of heat exchange flow of regenerator and heater in this paper. Under the same parameters, compared with the cycle before improvement, increase in heat source utilization rate and net power output of improved cycles is 45.69% and 39.31%, 43.54 MW and 35.43 MW, and the power consumption cost is decreased by 0.6106 $/kWh and 0.6289 $/kWh. However, as part of heat recovery inside the system is replaced by the heat of the heat source, the thermal efficiency is decreased by 10.43% and 8.7%, respectively. Variable parameter study is conducted on the two improved cycles to analyze the cycle characteristics from the perspective of energy, exergy, and economy (3E). The results showed that the improved recompression cycle has a high thermal power conversion capacity, higher system thermal efficiency and exergy efficiency and lower power generation cost, while the improved partial cooling cycle has higher waste heat utilization potential. However, the area of heat exchangers in both cycles tends to increase, which is not conducive to the compactness of the system.

Decomposition analysis of CO2 emissions of electricity and carbon-reduction policy implication: a study of a province in China based on the logarithmic mean Divisia index method

Abstract As the proportion of electricity in final energy consumption gradually increases, CO2 emissions reduction actions in the power system will become the key to achieving China’s carbon-peak and carbon-neutrality goals. It is essential to analyse and quantify the driving forces of CO2 emissions from electricity generation in the fossil-rich area in China. This paper aims to identify the characteristics of CO2 emissions generated by electricity and provide a basis for formulating CO2-reduction policies in power systems. First, we analyse the current state of CO2 emissions from electricity generation in Anhui Province that was dominated by fossil energy during the period 2010–19. Then, we apply the logarithmic mean Divisia index method to find the nature of the factors influencing the changes in CO2 emissions. Finally, we analyse the CO2-reduction measures of each side of the source–network–load–storage of the power system in Anhui through a power-system carbon-reduction path analysis model proposed in this study and provide policy suggestions. The results showed the following. (i) CO2 emissions in Anhui Province continued to increase from 2010 to 2019 and the trend in the growth rate of CO2 emissions presented approximately a u-shaped curve. (ii) Economic activity has always been the dominant factor driving the growth of electricity CO2 emissions. The increase in the proportion of renewable energy in power generation, the improvement in thermal power-conversion efficiency and the decrease in the intensity of power consumption are the three major driving factors for the reduction in CO2 emissions from power generation in Anhui. (iii) The CO2-reduction measures of the power system are provided in each link of the source–network–load–storage, such as developing the photovoltaic industry and building energy storage, upgrading and transforming coal-fired power stations, reducing the loss rate of transmission lines on the grid side and improving the efficiency of the utilization of electricity on the user side.

Thermo-economic analysis and multi-objective optimization of supercritical Brayton cycles with CO2-based mixtures

• A thermo-economic analysis of Brayton cycle with CO 2 -based mixtures is performed. • The Pareto fronts of 3 working fluids are obtained by multi-objective optimization. • SCO 2 -Xe cycle shows the best thermodynamic performance compared to others. • CO 2 -Kr shows the potential for development through thermo-economic analysis. The Supercritical carbon dioxide recompression Brayton cycle (SCO 2 RBC) is regarded as one of the most promising thermal power conversion systems and mixing other gases with CO 2 to change the critical point represents an effective strategy to improve the performance of the Brayton cycle. In this work, the feasibility of using xenon and krypton as additives to the SCO 2 RBC is investigated via thermodynamic analysis. The effects of the key parameters on the thermo-economic performance of recompression Brayton cycles using CO 2 , CO 2 /xenon (mass fraction 0.55/0.45) and CO 2 /krypton (mass fraction 0.755/0.245) as working fluids are discussed by means of parametric analysis. A non-dominated sorting genetic algorithm is employed to simultaneously optimize the cycle exergy efficiency and the levelized cost of energy. The total thermal conductance, the main compressor outlet pressure, pressure ratio and split ratio are selected as the decision variables. The optimum solutions with their corresponding decision variables are selected from the Pareto frontiers. The results show that the supercritical CO 2 /krypton cycle has the huge development potential. Specifically, the optimum exergy efficiencies of Brayton cycles using CO 2 , CO 2 /xenon and CO 2 /krypton as working fluids are 0.585, 0.646 and 0.664. The addition of xenon can improve cycle efficiency up to 12.08 % higher than SCO 2 RBC while the levelized cost of energy also increased by 10.04%. The cycle efficiency of the supercritical CO 2 /krypton cycle is 9.44% higher while the cost is 2.49% higher compared to the SCO 2 RBC. There are suitable decision variables to make the cost of the cycle using CO 2 / krypton lower than that using CO 2 while achieving the same exergy efficiency when the required exergy efficiency is in the range of 0.60-0.62. The exergy destruction distribution of Brayton cycle illustrates that high temperature recuperator is the highest exergy destruction component and using CO 2 -based binary mixtures can reduce the total exergy destruction effectively.

Thermo-hydraulic performance investigation of heat pipe used annular heat exchanger with densely longitudinal fins

To enhance the heat transfer capacity of the condenser end in the heat pipe heat exchanger, heat transfer and flow characteristics investigations of three densely longitudinal fin-equipped annular heat exchangers have been carried out by using a combined method of experimental testing and numerical simulation. One of the heat exchangers as a baseline is featured by the long straight fins. The other featured folded fins and arc-shaped fins, in which 40 rows of short fins were arranged just like the blade-lattice. The attained results showed that the short fold fins and arc-shaped fins had Nusselt numbers that were 27% and 35% higher than the long straight fins, and had friction factors that were 1088% and 2209% higher than the long straight fins. It indicates that the dense fin arrangement has a relatively limited influence on the wall temperature and overall heat transfer capacity. However, the friction loss is significantly sensitive to the fin arrangement because of the severe flow drag and flow separation induced by the blade-lattice like fins. The thermo-hydraulic performance evaluation suggests that the three densely longitudinal fins are suitable for different application scenarios with different heat transfer and pressure loss requirements. This work serves as a reference for the heat exchanger design in the heat pipe-based thermal power conversion system.

Comparative analysis and optimization of pumped thermal energy storage systems based on different power cycles

As the proportion of renewable energy in the world’s energy mix gradually increasing, energy storage technologies are gaining more and more attention. Pumped thermal energy storage (PTES) technology is one of the most promising electrical energy storage technologies. In many power cycles that can be adopted as discharge subsystems of PTES, organic Rankine cycle (ORC) is widely used because of its advantages of efficiently utilizing low-temperature waste heat and converting it into electrical energy. In addition, organic flash cycle (OFC) is considered as a promising candidate for the thermal power conversion subsystem of PTES due to its higher potential performance. In the present work, by taking four different forms of OFC and one basic ORC as power cycles, five PTES systems are established, optimized and analyzed. The results show that the turbine and compressor occupy the largest percentage of the total investment cost, while the throttle valve has the largest exergy destruction. The optimized results illustrate that the PTES system combining basic vapor compression heat pump cycle with basic organic Rankine cycle (BORC-BVCHP) shows the best economic performance with the Levelized cost of storage (LCOS) value as low as 0.4413 $/kWh at the storage temperature of 403.15 K. BORC-BVCHP system also shows the optimal round-trip efficiency (31.15%) and maximum exergy efficiency (23.40%), respectively. This indicates that ORC-based PTES system has obvious advantages over the OFC-based PTES systems in terms of economic performance and thermodynamic performances, and ORC is more suitable to be applied in the study of PTES system.