The cavity profile of a diaphragm compressor for a hydrogen refueling station

A low flow rate and short diaphragm life are the two disadvantages of diaphragm compressors when applied in hydrogen refueling stations. A new generatrix of the cavity profile of a diaphragm compressor was developed in this study to increase the cavity volume and decrease the diaphragm radial stress. A reduction in the diaphragm radial stress that resulted from the new design was validated by experiment and numerical simulation. The volumes of the cavities with different generatrices and the radial stress distribution of the diaphragm were investigated under various design conditions. The results indicated that the volume of the cavity with the new generatrix was approximately 10% larger than that with a traditional generatrix at the same allowable stress and cavity radius. At a similar cavity volume and radius, the radial stress values of the diaphragm in the cavity with the new generatrix were low. The decrease rate of the maximal radial stress of the diaphragm in the cavity with the new generatrix reached 13.8%. In the diaphragm centric region, where additional stress was induced by discharge holes, the maximal radial stress decrease rate reached 19.6%.     

Design of a hydrogen compressor for hydrogen fueling stations

Hydrogen compressors dominate the hydrogen refueling station costs. Metal hydride based thermally driven hydrogen compressor (MHHC) is a promising technology for the compression of hydrogen. Selection of metal hydride alloys and reactor design have a great impact on the performance of the thermally driven MHHC. A thermal model is developed to study the performance characteristics of the two-stage MHHC at different operating conditions. The effects of heat source temperature and hydrogen supply pressure on the compression ratio and isentropic efficiency are investigated. Finite volume method is used for discretizing the reaction kinetics, continuity, momentum and energy equations. Metal hydrides selected for this analysis are Mm_0.2La_0.6Ca_0.2Ni₅ and Ti_1.1Cr_1.5Mn_0.4V_0.1. The thermal model was validated with the results extracted from an experimental study. Validation results demonstrated that the numerical results are in good agreement with the data reported in literature.     

Effect of hydraulic oil compressibility on the volumetric efficiency of a diaphragm compressor for hydrogen refueling stations

The low volumetric efficiency of the diaphragm compressor under hydrogen refueling process, which hereby results in poor energy efficiency and high cost of hydrogen applications, should be paid attention to. This paper presents theoretical analysis and experimental investigation of the factors affecting the volumetric efficiency of the diaphragm compressor for hydrogen refueling process, focusing on the influence of hydraulic oil compressibility. A mathematical model was established to estimate the volumetric efficiency of diaphragm compressors, in which the effects of clearance volume, superheating of suction gas and pressure loss were taken into account and the emphasis was focused on the compressibility of hydraulic oil. A test rig was built to validate the theoretical model and further experimental investigations were carried out to identify the factors influencing the oil compressibility and hereby the volumetric efficiency. The volumetric efficiency was measured and compared under varied oil compressibility conditions by varying elastic modulus, oil overflow pressure and oil volume. The results indicated that the measured volumetric efficiency agrees well with the calculated value. The compression and expansion of hydraulic oil have a dominant influence on the volumetric efficiency, resulting in a loss of 37% of volumetric efficiency as compared to 2.4%, 18% and 1%, respectively for losses associated with clearance volume, superheating of suction gas and pressure loss, for a diagram compressor under refueling conditions with suction pressure of 30 MPa and discharge pressure of 90 MPa. The volumetric efficiency reduced rapidly with the increased oil overflow pressure, at a rate of 5% decrease with every 10 MPa rise in oil overflow pressure. As the oil volume increased by 100% of the stroke volume, the volumetric efficiency droped by 5.5%.     

A non-destructive fault diagnosis method for a diaphragm compressor in the hydrogen refueling station

Costly and time-consuming maintenance of the hydrogen compressors due to their frequent breakdown severely hinders the deployment and promotion of hydrogen refueling station (HRS), and effective condition monitoring and fault diagnosis is the key to reduce the unscheduled downtime of the compressor. This paper proposes a non-destructive method for fault diagnosis of diaphragm compressors for HRSs based on the acoustic emission (AE) signal. The AE signals in the time domain are segmented into angle-domain signals correspond to a working cycle. The feature events of the moving components are determined through the measured AE signal in both angle-domain and angle-frequency domain based on short-term Fourier transform (STFT). Those feature events signals are innovatively applied to identify the typical abnormal conditions of excessively high oil pressure, slightly inadequate oil pressure and seriously inadequate oil pressure, replacing the traditional and destructive pressure measuring method. The results show that this method can be used to effectively diagnose abnormal working conditions and indicate that this method can be utilized as a powerful tool in the non-destructive condition monitoring and fault diagnosis of the diaphragm compressors.     

Thermal-structural coupled analysis and improvement of the diaphragm compressor cylinder head for a hydrogen refueling station

When applied in the hydrogen refueling station, the diaphragm compressor with super-high pressure ratio will experience a high discharge temperature of over 200 °C, especially for that with large capacity and horsepower. Considering the thermal stress, the structural strength and deformation of the cylinder head are crucial to the reliability and efficiency of the diaphragm compressor. In this paper, a thermal-structural coupled analysis was proposed, based on which the deformation and stress of the diaphragm compressor cylinder head under high-temperature and high-pressure conditions were obtained. And the fatigue life of the studs on the cylinder head was also estimated based on the stress results obtained in the thermal-structural coupled analysis. The experimental method was conducted for verifying the numerical results. The results indicated that during the operation of the compressor, the temperature in the discharge holes was the highest, resulting in not only plastic deformation but also large stress concentration, and the high thermal stress could reach up to the strength limit of the material. A structural improvement was therefore proposed to decrease the stress in the region of the discharge holes by cutting a larger hole and attaching a new separate one for valve installation. Further stress analysis showed that the stress in the same region of the improved structure was significantly reduced, which guaranteed the safety of the compressor.     

Modeling and development of a liquid piston hydrogen compressor with a double buffer structure: A new insight

Liquid piston hydrogen compressors (LPHCs) are promising used in hydrogen refueling stations. Piston braking is crucial to LPHCs since the solid piston works under a high stroke frequency, which may result in piston collision. The on-off valve in the previous studies requires complicated control program to ensure the piston positions at dead centers. In this paper, a novel LPHC with a double buffer structure and an improved hydraulic driving system are proposed. A simulation involving the flow through the buffer aperture confirms the braking effect of the proposed design. Some parameters affecting piston braking are studied. A light piston can produce a lower velocity pulsation and lower peak pressure in the buffer chamber. The minimal piston displacement decreases with an increase in the bottom throttle diameter, while the maximal piston displacement increases with an increase in the top throttle diameter. The results also show that the slight pulsation of the piston velocity is attributed to the spool insertion, the piston reverse motion, and the opening of valves. This study can provide a technical reference for the optimization of LPHCs.     

Numerical investigation on the wave transformation in the ionic liquid compressor for the application in hydrogen refuelling stations

The ionic liquid compressor exhibits excellent advantages in hydrogen refuelling stations due to the specific design based on the hydraulic system and the ionic liquid piston. The application of the ionic liquid column results in a complex two-phase flow issue inside the compression chamber. This two-phase flow behaviour is critical for the compressor design as it influences the wave dynamics during the compression, but it is absent in the open literature. In this paper, transit numerical simulations were carried out to investigate the wave transformation during a compression cycle by the volume of fluid (VOF) method under different heights of the ionic liquid piston. The effect of liquid height on the wave transformation, discharged quantity of ionic liquid and hydrogen gas, and the turbulence kinetic energy was analysed. The minimum crest value of the turbulent kinetic energy was observed as 0.54 kJ in the cases of 30 and 40 mm. The optimal height of the ionic liquid piston was recommended 40 mm under the presented design condition based on the simulation results.     

Study on the dynamic characteristics of the free piston in the ionic liquid compressor for hydrogen refuelling stations by the fluid-structure interaction modelling

The ionic liquid compressor is promising for hydrogen refuelling stations, where the dynamic characteristics of the free piston are crucial for adjusting the compressor performance. This paper presents an investigation of the dynamic characteristics of the free piston in the ionic liquid compressor through a fluid-structure interaction modelling in three typical conditions. The results show that in the typical condition with no impact, phenomenons of buffering, oil charging, and oil overflow are observed in the oil pressure variation. Three features are found in the motion curve: asymmetric motion with a delay of reversal due to the buffering effect, variable location of the dead centre, and fluctuation in the piston velocity. When the impact occurs at the TDC, an opposite variation trend is observed in the gas and oil pressure curve. In the typical condition with impact at the BDC, the oil pressure drops below the atmospheric pressure.     

A new high-pressure clearance seal with flexible laddered piston assembly in oil-free miniature compressor for potential hydrogen applications and investigation on its dynamic sealing efficiency

In this paper, a new flexible laddered piston assembly of clearance seal without any soft seal part was proposed for a long life time run of high-pressure stage in oil-free miniature compressor for potential hydrogen applications. In this assembly, the functions of radial load bearing and gas sealing were undertaken independently by large and small piston. The dynamic sealing performance evaluation was carried out by comprehensively considering the real-time variation of gas properties and piston motion with thermodynamic process in the compression chamber. Simulation study shows that the introduced clearance gap has a great influence on the expansion, compression and discharge process. Leakage through the clearance would lead to the in-cylinder pressure drop during the discharge process and bring about oscillation and earlier closure of the discharge valve. Sealing clearance exert more significant influence on the sealing efficiency in the high-pressure stage compared to sealing length and shaft speed.     

A zero ODP replacement for R12 in a centrifugal compressor: an experimental study using R134a

It is well believed that the hydrofluorocarbons (HFCs) and their mixtures are the most promising candidates to substitute the conventional refrigerants, chlorofluorocarbons (CFCs) and HCFCs which contain chlorine atoms in the molecule. This substitution is necessary for the harmful action of CFCs and of HCFCs toward atmospheric ozone layer damage because the disruption of ozone has been attributed to chlorine. For this reason they must be replaced by more environment‐friendly refrigerants, as the new family, designated as HFCs, that are chlorine free. Centrifugal compressors differ from positive displacement compressors in two major respects: high vapour volume flow for a given physical size and lower pressure ratio. They are particularly suited to applications where differences between evaporator and condenser temperatures are low. The preferred properties for fluids used in centrifugal compressors differ in certain important aspects from those preferred for fluids used in positive displacement units. In particular centrifugal compressors typically utilize fluids such as CFC114, CFC113, CFC12 and CFC11 for which many potential candidate replacements exist; however, for CFC12, HFC134a is the most suitable replacement. A comparison of the refrigerants HFC134a and CFC12 has been carried out and the results from the tests, using data from a refrigerating plant operating with a centrifugal compressor are reported. The chilled water cooling plant, with a refrigerating capacity of 6500 kW is made up of a centrifugal two‐stage compressor, a condenser linked to a cooling tower, an economizer and a flooded evaporator. Experimental results show that a lower coefficient of performance is found when R134a is used as substitute for R12; the difference between the COP values decreases rising the compression ratio. Copyright © 2002 John Wiley & Sons, Ltd.     

A new two-stage design of feedback controllers for a hydrogen gas reformer

In this paper we have first reviewed operations of a hydrogen gas reformer and provided its linearized mathematical model. Then, we have simplified an existing algorithm for a two-stage design of feedback controllers for linear continuous-time time-invariant systems. The proposed design significantly reduces the computational requirements and provides flexibility of designing different type of controllers for different dynamic parts of the system. Since the hydrogen gas reformer (also known as a fuel processing system) possesses slow and fast modes (state variables), the newly proposed design is further simplified and specialized for this class of systems. The obtained algorithm is efficiently applied with very high accuracy to the hydrogen gas reformer. As a matter of fact, the eigenvalue placement problem is solved for the reformer dynamics for both slow and fast modes. The design is so flexible that combined hybrid controllers (optimal, robust, set-point, eigenvalue assignment controllers or any other linear controller) can be designed independently for particular subsystems of the hydrogen gas reformer. The hybrid linear feedback controller design for the hydrogen gas reformer that optimizes its slow subsystem and assigns the desired eigenvalues to its fast subsystem is also presented in the paper.     

A coordinatively saturated cobalt complex as a new kind catalyst for efficient electro- and photo-catalytic hydrogen production in purely aqueous media

It has been shown that coordinatively unsaturated complexes can catalyze hydrogen production via an unstable hydride intermediate. Herein we present a new kind of water soluble catalyst based on a coordinatively saturated cobalt complex, [(phen)₂Co(CN)₂]?ClO₄1 that is formed by the reaction of 1,10-phenanthroline (phen), Co(ClO₄)₂·6H₂O and tetracyanoethylene (TCNE). Under photoirradiation with blue light (λ_max = 469 nm) in air, together with [Ru(bpy)₃]Cl₂ and ascorbic acid in a pH 5.5 aqueous solution, 1 possesses photocatalytic activity for water reduction to hydrogen with an initial turnover number (TON) of 1232H₂ per mol of catalyst at first 10 h, and this activity is sustained for at least 70 h. This can be attributed to that oxidative quenching by 1 (k_q = 1.69 × 10¹⁰ M^?1 s^?1) dominates over reductive quenching to [Ru(bpy)₃]Cl₂ by ascorbic acid (k_q = 1.55 × 10¹⁰ M^?1 s^?1). Additionally, 1 electrocatalyze hydrogen generation from a neutral water with a turnover frequency (TOF) of 1113.1 mol of hydrogen per mole of catalyst per hour (mol H₂/mol catalysts/h) at an overpotential (OP) of 838 mV. We hope this can afford a new method in proton or water reduction catalysis using coordinatively saturated complexes in purely aqueous media.     

A new cathodic electrode deposit with palladium nanoparticles for cost-effective hydrogen production in a microbial electrolysis cell

Microbial electrolysis cell (MEC) provides a sustainable way for hydrogen production from organic matters, but it still suffers from the lack of efficient and cost-effective cathode catalyst. In this work carbon paper coated with Pd nanoparticles was prepared using electrochemical deposition method and used as the cathodic catalyst in an MEC to facilitate hydrogen production. The electrode coated with Pd nanoparticles showed a lower overpotential than the carbon paper cathode coated with Pt black. The coulombic efficiency, cathodic and hydrogen recoveries of the MEC with the Pd nanoparticles as catalyst were slightly higher than those with a Pt cathode, while the Pd loading was one order of magnitude less than Pt. Thus, the catalytic efficiency normalized by mass of the Pd nanoparticles was about fifty times higher than that of the Pt black catalyst. These results demonstrate that utilization of the cathode with Pd nanoparticles could greatly reduce the costs of the cathodic catalysts when maintaining the MEC system performance.     

A new plastic correction for the stress intensity factor of an under-clad defect in a PWR vessel subjected to a pressurised thermal shock

For the assessment of an under-clad defect in a vessel subjected to a cold pressurised thermal shock, plasticity is considered through the amplification β of the elastic stress intensity factor K_I in the ferritic part of the vessel. An important effort has been made recently by CEA to improve the analytical tools in the frame of R&D activities funded by IRSN. The current solution in the French RSE-M code has been developed from fitted F.E. calculation results. A more physical solution is proposed in this paper. This takes into account two phenomena: the amplification of the elastic K_I due to plasticity in the cladding and a plastic zone size correction in the ferritic part.     

A new Fuzzy&Wavelet-based adaptive thresholding method for detecting PQDs in a hydrogen and solar-energy powered EV charging station

This study presents a hybrid fuzzy decision-maker (FDM) and un-decimated wavelet transform (UWT)-based method for detecting power quality disturbances (PQDs) in a developed hydrogen and solar energy-powered electric vehicle (EV) charge station. The proposed adaptive FDM&UWT-based hybrid method eliminated the lack of performance of threshold-based signal analysis methods in noise-containing signals and it is implemented for a reliable PQD detection and integration in a developed microgrid. Also, the proposed method has eliminated the need for a processing-intensive filtering process to reduce noise from the signal. With this adaptive approach, detection errors in boundary conditions in threshold value methods are avoided and at the same time, cost and computational burden are minimized by using only the peak values in the detail coefficients of the voltage signal. The mean test accuracy is 96.13% for the FDM method using pyramidal UWT in noise-free conditions. Besides, the pyramidal UWT-FDM has a mean classification accuracy of 94.96% under 20–40 dB high-level noise conditions. The effectiveness of the UWT-FDM method is also tested using an experimental setup. The mean test accuracy for experimental data is 96.66%.     

A new insight for photocatalytic hydrogen production by a Cu/Ni based cyanide bridged polymer as a co-catalyst on titania support in glycerol water mixture

A two dimensional Cu/Ni based coordination polymer [{Cu^II(4,4ʹ-dipy)₂}{Ni(CN)₄}]_n·0.7(C₂H₆O₂)·1.6(H₂O) (CP-1) (4,4ʹ-dipy = 1,3-di (4-pyridyl)propane) has been demonstrated as a potential co-catalyst on TiO₂ support for hydrogen evolution under UV light. CP-1/TiO₂ composite exhibits considerable hydrogen production in comparison with the pristine CP-1 and TiO₂ (P25), highlighting that the photocatalytic performance is significantly related with the good separation of photo generated e⁻/h⁺ pairs. Different wt. % (2.5, 5 and 7.5%) of CP-1 in CP-1/TiO₂ composites were tested for photocatalytic hydrogen production in 5 vol % glycerol/water mixture. The 5 wt % CP-1/TiO₂ composite displayed the greatest hydrogen production of 9.2 mmolh⁻¹g⁻¹. The concealed mechanism is divulged on the behalf of results obtained by cyclic voltammetry, photoluminescence and diffused reflectance/UV-visible studies which demonstrate that upon irradiation of UV light, electrons transfer from TiO₂ conduction band to CP-1. CP-1 not only grabs the conduction band electrons of titania but also performes as a co-catalyst to reduce the protons into hydrogen. These results are anticipated to direct the forthcoming advancement in creating proficient, cheap semiconductor photocatalysts for solar hydrogen production.