Charge Pressure Research Articles

Preliminary thermodynamic analysis of a carbon dioxide binary mixture cycled energy storage system with low pressure stores

The liquid CO2 energy storage has considerable potential for power supply-demand management, but its low energy density, harsh condensation condition and high operation pressure are substantial obstacles. It is the first time to design energy storage system with high energy density and low-pressure stores by cycling the CO2 binary mixtures. By using CO2 mixtures, the pressure in storage tanks can be as low as ambient pressure (0.1 MPa) and two-tank cold energy storage with liquid storage materials can be used to complete the evaporation and condensation processes of working fluid in subcritical pressure. A large number of refrigerants are discussed and nine of them are selected as preliminary candidate additives into CO2. The performance of newly proposed system cycled with these additives is further analyzed by utilizing an in-house code developed with established thermodynamic models. The numerical results demonstrate that the mass fraction of additives is suggested to be higher than 0.12, otherwise there is high required capacity of cold storage material although a larger energy density. By and large, the mixtures CO2/R161 (0.7/0.3) and CO2/R1270 (0.82/0.18) are the first two most suitable selections among the CO2 mixtures, the round trip efficiency and energy density of which at basic design condition are 52.95% and 29.74 kWh/m3 and 52.12% and 29.83 kWh/m3, respectively. Both the optimal charge and discharge pressures can be identified for round trip efficiency while the energy density increases monotonically with charge pressure and the increasing trend is first steep and gradually becomes slow.

Effect of silicone oil additive on swelling stress alleviation in the metal hydride reactor

Considerable swelling stress associated with hydrogen absorption process caused by metal hydride expansion is extensively observed in a metal hydride tank which is usually designed for hydrogen storage application. Such swelling stress being applied to tank wall may cause potential safety issue such as tank failure. In the present investigation, silicone oil is selected as an additive incorporating into MlNi 4.5 Cr 0.45 Mn 0.05 alloy in an attempt to alleviate the swelling stress. The results obtained by a self-built direct swelling stress testing apparatus show that the addition of silicone oil can significantly reduce alloy particle swelling stress. The addition of 3 wt% silicone oil is appropriate to acquire efficient swelling stress alleviation. During cycling the maximum swelling stress increases with charging pressure. The formation of silicone oil thin film on the surface of alloy particles, acting as a “cushion” among alloy particles, would reduce particle agglomeration and enhance particle movement during hydrogen absorption and desorption cycling. This is the reason for the observed swelling stress alleviation by silicone oil. • Swelling stress was reduced by addition of silicone oil. • The maximum swelling stress increased with charging pressure. • The swelling stress alleviation was ascribed to oil thin film on particle surface.

Design, Manufacturing and Testing of a Stirling Engine with Slider-Crank Mechanism

In this study, a beta type Stirling engine with slider-crank mechanism having swept volume of 365 cm3 was designed, manufactured and performances tested. The design phase was first started by determining the operating parameters of the engine. The necessary mathematical calculations were performed by considering the operating conditions of the Stirling engine with a slider-crank mechanism to be manufactured. After determining the engine parameters, the dimensional design phase was started within the tolerance limits of the engine parts. The parts were designed by the computer-aided SolidWorks program in solid modeling and by the AutoCAD program in two-dimensional design and projecting. Each part used in the manufacturing of the Stirling engine was assembled delicately in the assembly process. A prony-type dynamometer, liquefied petroleum gas (LPG) fuel, and electronically controlled electric heater systems were developed to perform the performance tests and analyses of the manufactured engine. Experimental studies were conducted at hot end temperatures of 627 °C, 727 °C, and 827 °C and at a cold end temperature of 27 °C by utilizing an electrical heater as a heat source and air as a working fluid. According to the results obtained in experimental studies for different heater temperatures and different charge pressures, it was revealed that engine power values increased depending on the heater temperature and charge pressure increase. The maximum power values at all heater temperatures were acquired at a charge pressure of 4 bar. In this study, the maximum engine power was obtained as 69.5 W at a hot end temperature of 827 °C, at a charge pressure of 4 bar, and at an engine speed of 200 rpm when a stainless-steel displacement piston and air as a working fluid were utilized, and the maximum engine torque value was obtained as 4.21 Nm at a charge pressure of 4 bar and an engine speed of 135 rpm. The lowest engine power among the maximum engine power values obtained in all experimental studies was found as 17.09 W at a hot end temperature of 627 °C, at a pressure of 1 bar, and at an engine speed of 185 rpm. The maximum power values of the engine developed within the scope of this study at hot end temperatures of 627 °C, 727 °C, and 827 °C were determined to be 31.2 W, 48.3 W, and 69.5 W, respectively. Upon examining the results obtained from experimental studies, it is observed that the heater temperature and charge pressure have significant impacts on the performance values of Stirling engines. Within the scope of this study, a new power generation system that could use renewable energy sources was put into operation.

Reduction on the inefficiency of heat recovery storage in a compressed carbon dioxide energy storage system

A growing boom can be noticed at the area of energy storage in recent years as this technology can address the supply-demand problem of power generation. As one of the most cutting edge energy storage technologies, the compressed carbon dioxide energy storage captures an increasing number of eyes all over the world. However, large inefficiencies occur within heat exchangers in supercritical pressure with resulting in low system efficiency. Therefore, a new thermal energy storage configuration is designed by separating the supercritical heat exchangers into two parts in which different mass of thermal storage medium is provided to adapt to the specific heat of supercritical CO2, such as 0.45 kg/s for the high-temperature part and 0.76 kg/s for the low-temperature part. Detailed thermodynamic analysis is conducted to identify the superiority of the new configuration compared to the traditional one. The analysis results indicate that the heat transfer between supercritical CO2 and water enjoys much better temperature match in the improved system. The round trip efficiency of improved system can arrive at 57.85% with the optimized final cooler temperature at 32 °C, 5.26% higher than the baseline system with optimized final cooled temperature located at 48 °C. In the improved system, the optimized operation ranges of charge pressure and discharge pressure are at 15–17 MPa and 14–16 MPa, respectively. Equal split ratio should be suggested to the charge and discharge CO2 to properly utilize the self-cycling cold energy.

Performance of a multi-stage thermoacoustically-driven pulse tube cryocooler: Uncertainty quantification and sensitivity analysis

• An octa-engine four-stage cryocooler utilizing low-grade heat source is proposed. • A temperature difference of 132 K at a no-load cold temperature of 77 K is achieved. • Increasing stages reduces hot temperature, but with a lower exergetic efficiency. • Thermal conductivity and engine’s mesh number strongly influence cooling features. • The onset hot temperature has a linear relation with the connecting tube’s length. The current paper presents the broad-range sensitivity analysis and uncertainty quantification for the performance of a thermoacoustically-driven pulse tube cryocooler to make it robustly perform using a low-grade heat source. Accordingly, the onset and steady-periodic operations of the cryocooler are numerically simulated using Rott’s one-dimensional equations. The effects of increasing hot/cold core’s number, engine’s phase-shifter length, and mean pressure on the cooling features and hot-source temperature are investigated. In this regard, a looped-branched, octa-engine, four-stage cryocooler is proposed that operates with a steady temperature difference of 132 K at a no-load cold temperature of 77 K. In contrast to previous findings, the results show that increasing the number of stages or the charge pressure does not necessarily improve thermodynamic performance. Moreover, the uncertainty in the operation of a quad and octa-engine, four-stage cryocooler caused by the geometric and material characteristics is estimated using the Morris method. It is demonstrated that the overall operation is more sensitive to the hot-core dimensions than to the cold-core dimensions. Besides the hot-core dimensions, gas properties and solid thermal conductivity strongly influence the transient and steady-periodic features of the cryocooler, respectively. Furthermore, doubling the hot cores in the four-stage system increases the uncertainty of the acoustic power up to five times, leading to more uncertainty in the cooling features. Drawing on the results of this paper, a multi-stage cryocooler can be designed to optimize the overall performance and reliability.

Recent advances in two-dimensional van der Waals magnets

Two-dimensional (2D) magnets have evoked tremendous interest within the research community due to their fascinating features and novel mechanisms, as well as their potential applications in magnetic nanodevices. In this review, state-of-the-art research into the exploration of 2D magnets from the perspective of their magnetic interaction and order mechanisms is discussed. The properties of these magnets can be effectively modulated by varying the external parameters, such as the charge carrier doping, thickness effect, pressure and strain. The potential applications of heterostructures of these 2D magnets in terms of the interlayer coupling strength are reviewed, and the challenges and outlook for this field are proposed.

Localized ignition delay prediction from temperature measurements of n-heptane in the combustion chamber of the ignition quality tester

The constant-volume combustion chamber apparatus, known as the Ignition Quality Tester (IQTTM), is used in some refineries to measure the ignition delay (ID), the time between fuel injection and fuel auto-ignition, of compression ignited fuels using pressure measurements, and to calculate the derived cetane number (DCN) from correlations developed with the cetane number (CN). The IQTTM is also used in research due to its capability to manipulate combustion controller parameters, including pressure, temperature, oxygen content and mass of fuel injection. In this study, in addition to the traditional global pressure measurements, the temperature was recorded at 46 locations inside the IQT chamber during fuel (n-heptane) injection and combustion. Two sets of experiments were conducted: n-heptane injection into nitrogen (no chemical reaction or combustion), and n-heptane injection into air (chemical reaction or combustion) and the ranges of ambient air temperatures and pressures were 530–590 °C and 10 – 21.4 bar, respectively. A method developed to estimate the physical and total ignition delay based on the temperature measurements is introduced in this study. The combustion initiation location was predicted from the results of the ignition delay, and it was determined that during long periods of ID, the fuel–air mixture becomes pseudo-homogeneous and the combustion initiates everywhere in the main part of the IQT chamber. At higher pressure and temperature conditions, the combustion was initiated at the end of the main part of the chamber. The validation of the ID by temperature method showed good agreement with the predicted ID from pressure measurements. The charge air temperature did not show a significant effect on the average percentage of the total physical ignition delay, while the charge pressure has a significant effect on the average percentage of the total physical ID, which increases with the charge pressure.

Numerically investigated on the transient performance of fast cool-down J-T cryocoolers with non-isometric structure

To achieve fast cool-down is important for the miniature Joule-Thomson (J-T) cryocooler in infrared detectors and other applications. A novel non-isometric helically coiled heat exchanger, with a first-sparse-then-dense type structure, was adopted in this study to improve the cool-down performance of miniature J-T cryocoolers. A transient model that takes real gas characteristics, heat leakage and compressible flow into account, was established and verified. The transient thermodynamic behaviors (temperature, internal heat load, mass flow rate and cooling capacity) of J-T cryocoolers with the typical, first-dense-then-sparse and first-sparse-then-dense type structure were analyzed and compared to obtain the better performance. It can be concluded that the first-sparse-then-dense structure can increase the cool-down rate by 13.4% with a smaller structural mass. Besides, different operating conditions with charging pressure and cylinder capacity were also simulated and analyzed. The results show that the first-sparse-then-dense structure always has the fastest cool-down rate in any operating condition. The simulation results can be used for the accurate prediction of system performance and structural design.

Understanding doctors’ emergency department antibiotic prescribing decisions in children with respiratory symptoms in the UK: a qualitative study

ObjectiveExploration of the factors that influence hospital doctors’ antibiotic prescribing decisions when treating children with respiratory symptoms in UK emergency departments.MethodsA qualitative study using semistructured interviews based on a critical...

Numerical Optimization of the β-Type Stirling Engine Performance Using the Variable-Step Simplified Conjugate Gradient Method

This study focuses on optimizing a 100-W-class β-Type Stirling engine by combining the modified thermodynamic model and the variable-step simplified conjugate gradient (VSCGM) method. For the modified thermodynamic model, non-uniform pressure is directly introduced into the energy equation, so the indicated power and heat transfer rates can reach energy balance while the VSCGM is an updated version of the simplified conjugate gradient method (SCGM) with adaptive increments and step lengths to the optimization process; thus, it requires fewer iterations to reach the optimal solution than the SCGM. For the baseline case, the indicated power progressively raises from 88.2 to 210.2 W and the thermal efficiency increases from 34.8 to 46.4% before and after optimization, respectively. The study shows the VSCGM possesses robust property. All optimal results from the VSCGM are well-matched with those of the computational fluid dynamics (CFD) model. Heating temperature and rotation speed have positive effects on optimal engine performance. The optimal indicated power rises linearly with the charged pressure, whereas the optimal thermal efficiency tends to decrease. The study also points out that results of the modified thermodynamic model with fixed values of unknowns agree well with the CFD results at points far from the baseline case.

Constructal design of a circular micro-channel Stirling regenerator based on exergy destruction minimization

Regenerator is a key component in Stirling engines, and its characteristics have a significant influence on engine performance. Herein, subjected to the global constraints of the fixed total volume and porosity, constructal design of a circular micro-channel regenerator was conducted to achieve its optimal structures. The total exergy destruction rate in the irreversible regenerative process, which includes three types of individual exergy destruction contributions caused by the heat transfer with a finite temperature difference, flow resistance, and axial heat conduction of the solid matrix, was adopted as the optimization objective. The optimal channel diameter was obtained, and the effects of the regenerator inner radius, total volume of regenerators, regenerator porosity, engine speed, piston phase angle, hot part temperature, charge pressure, and thermal conductivity ratio due to the segmentation of regenerators on the optimization results were analyzed. The results show that the total exergy destruction rate offers a minimum at a relatively low channel diameter of around 0.5 mm under the given parameter values. All the parameters studied have an obvious effect on the minimum total exergy destruction rate, and the regenerator inner radius, total volume and porosity of regenerators, and engine speed also have an evident influence on the optimal structure of the regenerator. These indicate that the regenerator performance can be further improved by selecting appropriate global geometric parameter values of regenerators and operating parameter values of engines. The findings of this study can provide some guidelines for the optimal design of micro-channel regenerators and the performance improvement of Stirling engines.

Rapid Depressurization Based Controlled Ice Nucleation in Pharmaceutical Freeze-drying: The Roles of the Ballast Gas and the Vial

Controlled ice nucleation offers several key benefits to the pharmaceutical lyophilization process, including reducing lyophilization cycle time, control of ice crystal morphology, and increased consistency of lyophilized product quality attributes. The rapid depressurization based controlled ice nucleation technique is one of the several demonstrated controlled ice nucleation technologies and relies on the rapid discharge of an inert pressurized gas to induce ice nucleation. In this work, a series of custom wireless gas pressure and temperature sensors were developed and applied to this process to better understand the mechanism of controlled ice nucleation by depressurization. The devices capture highly transient conditions both in the chamber near the vial and within the vial headspace throughout the entire process. The effects of ballast gas composition, initial charge pressure, and vial size on gas pressure and headspace/chamber temperature are explored individually. We model the depressurization as an isentropic process, allowing the influence of these parameters to be evaluated quantitatively. It is demonstrated that monatomic gases (e.g. argon) with low thermal conductivity produce the most favorable conditions for ice nucleation at the end of depressurization, based on temperature drop in the vial headspace. Experimental data also reveal a correlation between initial charge pressure and vial size with the temperature drop within the vial headspace, during depressurization. These findings ultimately provide deeper insight into the rapid depressurization based controlled ice nucleation process and help lay the foundation for a more robust process development and control.

Optimal Design of Accumulator Parameters for an Electro-Hydrostatic Actuator at Low Speed

The electro-hydrostatic actuator (EHA) is a type of highly integrated, compact, closed pump control drive system composed of a servo motor, a metering pump, a hydraulic cylinder and other components. Compared with the traditional valve control system, the electro-hydrostatic actuator has the advantages of a high power-to-weight ratio, high integration, environmental friendliness, and superior efficiency and energy saving. However, due to the complex mechanical–hydraulic coupling mechanism of the system and the existence of non-linear multi-source disturbances, the dynamic and static performance of the system is limited, particularly the pressure pulsation phenomenon under low-speed conditions, which seriously affects the high precision control requirements of the system. In order to address the low-speed pressure pulsation problem of the electro-hydrostatic actuator, first, the mathematical models of the servo motor, metering pump and hydraulic cylinder are established, and the simulation model of the EHA system is created based on MATLAB/Simulink. Second, from aspects of the servo motor and the quantitative piston pump, the causes of the pressure pulsation under low-speed working conditions are analyzed, and the parameter selection method of the accumulator is proposed to eliminate the pressure pulsation based on ωn and ζ of the EHA system. Finally, the optimal charging pressure of the accumulator is simulated and experimentally analyzed. The simulation and experimental results show that the charging pressure range of the accumulator calculated with this method can effectively improve the pressure pulsation phenomenon of the EHA system under low-speed working conditions, and it plays a positive role in the engineering popularization and application of the EHA system.

Study on the establishing-process of piston offset in the helium valved linear compressor under different operating parameters

The piston offset in the helium valved linear compressor (VLC) deteriorates the cooling capacity and operation life of the Joule-Thomson (JT) refrigerator, especially in aerospace applications. To clarify the influence mechanism of operating parameters on the offset characteristic of the VLC, a relative quantitative investigation is theoretically and experimentally proposed. Based on the reliable VLC developed in our lab in the hope of using for the space infrared detector, the main parameters involving the charging pressure, operating frequency, piston displacement and valve opening are systematically studied. The offset amplitude, establishment time and start-up characteristic of the offset under different conditions are presented. The equation that contains all the operating parameters capable of describing the offset values is expressed. The polynomial fitting from the experimental data verifies the deduced equation, and further explains the interaction between the offset and the operating parameters quantitatively. This study can be an effective guidance in reducing or even eliminating the offsets to improve the performance of the VLC.

Parametric assessment, multi-objective optimization and advanced exergy analysis of a combined thermal-compressed air energy storage with an ejector-assisted Kalina cycle

Compressed air energy storage has attracted worldwide attention owning to its low capital investment, scalability, eco-friendliness and long life. In this paper, a new combined thermal-compressed air energy storage with ejector-assisted superheated Kalina cycle is comprehensively investigated. Parametric assessment at the aspect of thermodynamic and economic performances is first conducted to investigate the influence of several crucial design parameters. Multi-objective optimization is then carried out based on genetic algorithm to maximize the round-trip efficiency and economic profits. Finally, the advanced exergy analysis is performed to achieve more valuable information by taking the component interconnections and technological limitations. Results demonstrate that increasing charging pressure, separation temperature, and ammonia mass fraction of basic solution and decreasing compressor efficiency are beneficial to reduce the investment cost per product. Meanwhile, the investment cost per product exists an optimum value along with the pinch temperatures of cooler and heater, discharging pressure and turbine efficiency. The system round-trip efficiency and investment cost per product are respectively 52.92% and 0.087 $/kWh based on multi-objective optimization. The advanced exergy analysis results indicate that interactions among components are weak and the system has a large potential for improvement due to higher avoidable exergy destruction than unavoidable part.

Development of a vuilleumier refrigerator with crank drive mechanism based on experimental and numerical study

• Development of Vuilleumier refrigerator by theoretical and experimental methods. • A real Vuilleumier refrigerator is built and tested. • At 22 bar, 600 rpm, operating temperatures 900 K and 276 K, 221 K is reached. • A novel thermodynamic model is developed and validated. The present paper is focused on development of the thermodynamic model and a heat-driven Vuilleumier refrigerator. Thermodynamic properties and performance are predicted by thermodynamic model. On the other hand, a prototype Vuilleumier refrigerator with crank drive mechanism is built and the performance is measured. In the thermodynamic model, the refrigerator is divided into five working spaces for analysis, including hot chamber, hot regeneration chamber, middle chamber, cold regeneration chamber and cold chamber. Also, the influence of clearance leakage between middle chamber and buffer chamber is investigated. The effects of geometrical parameters, charged pressure, rotating speed, heating temperature, cooling temperature and heat loading on the cold head temperature are evaluated. In addition, the numerical results and the experimental data under different operating parameters are compared for model validation. As the Vuilleumier refrigerator operated at 22 bar pressure, 600 rpm rotating speed, 900 K heating temperature and 276 K cooling temperature, the cold head can reach a zero-load temperature of 221 K. When a 0.7 W heating load is applied on the cold head, cold head temperature of 252.6 K and coefficient of performance of 0.0068 can be obtained.

Performance of cryogenic demountable indium seal at high pressures.

An essential challenge in seal designis to provide an ultra-low leak rate at cryogenic temperatures and high pressures. In this paper, the performance of demountable indium seals under a charging pressure of 8.5 MPa A and at cryogenic temperatures down to -190°C was investigated. Three indium seal structures with a diameter of 30mm were specifically designed and tested. All three structures went through both room temperature and cryogenic temperature tests in cycles with a pressure of up to 8.5 MPa A. In addition, leak rate experiments regarding the creep relaxation effect of the indium ring were conducted. The results showed that the leak rates of all three structures were lower than 1 × 10-10 Pa m3s-1 at both room temperature and cryogenic temperature with the pressure up to 8.5 MPa A when the torque was 8 or 12 N m. It was concluded that the linear loads for achieving a successful indium seal were 163, 171, and 220 N mm-1 alongside its circumference for the 2mm indium M-T structure, the 3mm indium M-T structure, and the Z-shaped seal structure, respectively. Furthermore, although the torque slightly dropped after the assembly due to the creep relaxation effect, the leak rates of the structure were still lower than 1 × 10-10 Pa m3s-1 three days after the assembly. The present work is helpful for designing ultra-low leak rate demountable indium seals at cryogenic temperatures and high pressures.

Thermodynamic analysis on the feasibility of a liquid energy storage system using CO2-based mixture as the working fluid

Pioneering investigation is conducted on the feasibility of designing novel liquid energy storage system by using working fluid blending CO2 with organic fluids to address the condensation problem of subcritical CO2. Organic substances are cautiously screened according to the criteria of environment effect, temperature glide, critical temperature and flammability of working fluid as well as the system performance. Mathematical model of the system is built for thermodynamic examination. An in-house code is developed to complete the system simulations combing with REFPROP subroutine. Results demonstrate that compared to the system with pure CO2, the system with mixture produces an improvement of net power output and energy density and a reduction of charge pressure at an expense of slightly decreasing round trip efficiency. The payment of 6.45 % for round trip efficiency can reduce 55.59 % of charge pressure by taking CO2/R32 as an instance. The system round trip efficiency, energy density and charge pressure decrease with the increase in organic fluid composition. An optimal compression ratio can be identified to reach a maximal round trip efficiency for all mixtures. The cooler outlet temperature is suggested being at the critical temperature of working fluid to reach better system performance.

Carbon natural gas storage performance as predicted by multiscale modeling

Adsorbed natural gas (ANG) technology could help to facilitate the transition to environmentally friendly automobiles. Limits on total energy storage and the accumulation of higher hydrocarbons (C3+) present in natural gas are factors that restrict ANG broad application. Because experimental bed deactivation studies are extremely time-consuming, we propose a multiscale technique to study tank deactivation involving a mathematical model capable of predicting compositions of ANG tank cycles. The model is based on multicomponent ideal adsorption solution theory calculation and molecular simulation of the monocomponent adsorption equilibrium data using the representative pore method. We expect that this approach will save experimental effort and bring new insight to understanding the property-structure relation in activated carbons. The technique is validated using literature experimental data from tanks filled with activated carbons. Two target limits for the charge and discharge pressures (35–1 bar and 65–5.8 bar) are tested. The pressure range of 65–5.8 bar shows the best energy delivery for the three commercial carbons examined. The simulation shows that the amount of energy delivered is maximized for a pore size of 18.5 Å, larger than the 11.5 Å pore previously suggested for storage of pure methane. Maxsorb activated carbon showed similar performance to the metal-organic framework (MOF) NU-125 after approximately 100 charge and discharge cycles. In general, activated carbons are less expensive and more stable than MOFs. By accelerating bed deactivation assessment, the technique will allow the synthesis community to produce more efficient carbonaceous materials and facilitate ANG technology feasibility.

Risk Analysis of Voltage Violation With PHEV Inter-Area Mobile Charging Strategy Under Gas Station Networks Attacked

To reduce the risk of voltage violation after gas station networks (GSNs) are attacked, this study investigates an inter-area mobile charging strategy of plug-in hybrid electric vehicles (PHEVs) to decrease the charging load by taking full advantage of charging resources. First, considering the location of the charging station, the waiting time, and the charging fee, an inter-area mobile charging strategy of PHEVs is proposed, and a mobile charging model of PHEVs among regions is established to relieve the charging pressure. Second, the risk index is developed to analyze the risk of voltage violation in terms of the results of probabilistic load flow (PLF). Finally, the proposed strategy is tested on a modified coastal active power distribution network, and simulation results show that the charging load of PHEVs is dispersed among regions and the risk of voltage over-limit can be reduced.