Dynamic and energy analysis of a liquid piston hydrogen compressor

Liquid piston compressor is the most promising compressor to be used for hydrogen-refueling stations. However, their energy transfer and the energy dissipation processes of are poorly studied and not well understood. In this paper, a new energy analysis method for an ionic-liquid type liquid piston compressor is proposed. In the compressor section, porous media is used to promote heat transfer from the hydraulic oil during the compression process. A mathematical model has been formulated considering the heat transfer and damping effects of the porous media on the compressor performance. Moreover, the compressibility of the hydraulic oil and its overflow loss on the compressor performance were also established. In the model, the seven stages of the entire working cycle of the compressor were look into in detail, alongside with its energy efficiency. The results show that the key parameters governing the energy efficiency of the compressor are the heat transfer efficiency of the compressor and the overflow losses of the hydraulic oil.     

Theoretical study of the dynamic characteristics of a self-commutating liquid piston hydrogen compressor

Hydrogen is being more and more widely deployed in various fields for its ‘clean’ character. For applications in automobiles where hydrogen has already been adopted for years, higher pressure means better mileage. To improve the pressure of the hydrogen compressor, a novel self-commutating liquid piston hydrogen compressor is proposed in the present study. A two-stage hydrogen booster is designed on both sides of the hydraulic cylinder piston, which is driven by a spool installed in the cylinder piston. The benefits of the novel hydrogen compressor are reducing the throttling loss and enhancing the response of the piston. Furthermore, the principle of the hydrogen compressor is illustrated, based on which a dynamic model is established while taking oil compressibility, leakage and flow force in the compression process into consideration. Moreover, system simulation model is established by applying the simulation software, verifying the feasibility and validity of the novel structure. Accordingly, the energy efficiency on the mechanical-hydraulic structure is improved.     

Evaluation of ionic liquids as replacements for the solid piston in conventional hydrogen reciprocating compressors: A review

Substituting the solid piston of conventional reciprocating compressors used for the compression of hydrogen with a suitable ionic liquid will solve many practical problems and limitations that conventional reciprocating compressors face. However, because of the large number of cation and anion combinations and many studies on the unique properties of ionic liquids and the role of ionic liquid cations and anions in determining these properties, a systematic review is required to narrow down the choice of ionic liquids. Therefore, in the present review, a comprehensive study to find the most appropriate ionic liquid candidate to replace the solid piston in reciprocating compressors for compressing hydrogen is reported.Specific criteria concerning the applications of ionic liquids are determined and the roles of the cations and anions, as well as the effect of temperature, are extensively reviewed to identify the most suitable ionic liquid that can fulfill the requirements. As a next step, the options are narrowed down to five ionic liquids with the triflate and bis(trifluoromethylsulfonyl)imide as the anion choices and three different cation types, imidazolium-, phosphonium-, and ammonium-based, as the cation choices. Finally, the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide is recommended as the best candidate that can be safely used as a replacement for the solid piston in reciprocating compressors for compressing hydrogen in hydrogen stations.     

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.     

转子式压缩机吸气带液时排气状态的变化

滚动转子式压缩机具有较好的抗湿压缩性能,利用少量吸气带液可有效降低压缩机排气温度,且不造成额外的系统成本.对滚动转子式压缩机少量吸气带液时,排气温度、排气比焓的变化趋势进行了实验研究,并对压缩机功耗、吸气比焓和机壳散热量等三个排气比焓的影响因子进行了分析.结果表明:少量吸气带液能有效降低排气温度,且压缩机运行性能良好;当吸气干度x为0.9x1.0时,三个影响因子均趋向定值,且机壳散热所占比例很小(低于1%).     

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.     

往复式压缩机空冷器加装保温棉提高外输气温度的实践

介绍往复式压缩机的参数和工作概况,针对冬季3台往复式压缩机组频繁停机问题,分析造成压缩机出口排气温度过低的原因;提出通过加装保温棉、控制空冷器的风量来进行改进,以达到提升压缩机的排气温度,保证机组正常运行。     

Electrochemical hydrogen compressor: Recent progress and challenges

Hydrogen has higher specific energy than conventional fuels but compared per unit volume under normal conditions, its energy density is lower. This difference is compensated with compression. Theoretically, compression is possible with a proton exchange membrane electrolyzer (PEME), in the process of hydrogen production, but the hydrogen permeation to the oxygen side forms a potentially explosive mixture. An electrochemical hydrogen compressor (EHC) with an analogous working principle presents the most promising solution due to its noiseless and vibration-free operation, modularity, absence of moving parts, and higher efficiency compared to mechanical compressors. Hydrogen purification and its extraction from gaseous mixtures are additional benefits that give electrochemical compression further advantage. This paper discusses the working principle of electrochemical hydrogen compression technology and its design development. The focus is on research trends, recent advances, and transpired challenges. In addition, reviewed literature aspects not studied sufficiently are highlighted, and future research directions are proposed.     

活塞式压缩机节能增效的研究与实践

本文从改造活塞式压缩机中间冷却器以实现节能增效的理论分析着手 ,探讨了改造活塞式压缩机中间冷却器的节能原理 ,并辅以工程应用实例说明。     

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.     

Evaluation of a thermally-driven metal-hydride-based hydrogen compressor

Currently, the hydrogen storage method used aboard fuel cell electric vehicles utilizes pressures up to 70 MPa. Attaining such high pressures requires mechanical gas compression or hydrogen liquefaction followed by heating to form a high-pressure gas, and these processes add to the cost and reduce the energy efficiency of a hydrogen fueling system. In previous work we have evaluated the use of high-pressure electrolysis, in which hydrogen is generated from water and the electrolyzer boosts the hydrogen pressure to values from 13 to 45 MPa. While electrolytic compression is a novel and energy efficient method to produce high-pressure hydrogen, it has several limitations at present and will require more development work. Another concept is to use hydrogen absorbing alloys that form metal hydrides, in combination with a heat engine (hot and cold reservoirs), to drive a cyclic process in which hydrogen gas is absorbed and desorbed to compress hydrogen. Furthermore, by using a thermally-driven compressor, the hot and cold reservoirs can be obtained using renewable energy such as sunlight for heating together with ambient air or water for cooling. In this work we evaluated the thermodynamics and kinetics of a prototype metal hydride hydrogen compressor (MHHC) built for us by a research group in China. The compressor utilized a hydrogen input pressure of approximately 14 MPa, and, operating between an initial temperature of approximately 300 K and a final temperature of 400 K, a pressure of approximately 41 MPa was attained. In a series of experiments with those conditions the average compression ratio for a single-stage compression was approximately three. In the initial compression cycles, up to 300 g of hydrogen was compressed for each 100 K temperature cycle. The enthalpy of the metallic-alloy-hydriding reaction was found to be approximately 20.5 kJ per mole of H₂, determined by measuring the pressure composition isotherm at three temperatures and using a Van't Hoff plot. The thermodynamic efficiency of the compressor, as measured by the value of the compression work performed divided by the heat energy added and removed in one complete cycle, was determined via first and second law analyses. The Carnot efficiency was approximately 25%, the first law efficiency was approximately 3–5%, and the second law efficiency was approximately 12–20%, depending on the idealized compression cycle used to assign a value to the compression work, as well as other assumptions. These efficiencies compare favorably with values reported for other thermally-driven compressors.     

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%.     

Modelling of a hydrogen thermally driven compressor based on cyclic adsorption-desorption on activated carbon

Thermally driven hydrogen compression by cyclic hydrogen adsorption-desorption on activated carbon is presented therein. Hydrogen compression occurs through heat exchange, which allows physisorbed hydrogen to desorb at higher temperature in a given volume. The physical nature of hydrogen adsorption on porous carbon allows reversible desorption, and a flow of compressed hydrogen is then obtained by running adsorption/desorption cycles repeatedly. We investigated the feasibility of such a system through numerical simulations by taking into account both mass and energy balances, and adsorption thermodynamics. We showed that high-pressure hydrogen, up to 70 MPa, can be obtained by simply lowering and/or increasing the system temperature. Such a system opens new perspectives in the frame of the Hydrogen Supply Chain.     

Performance investigation of a two-stage sorption hydrogen compressor

Metal hydrides (MH) are widely investigated for several thermodynamic applications; sorption hydrogen compressor (SHC) is one among them. In this study, the thermodynamic performance and heat – mass transfer behaviour of a two-stage sorption hydrogen compressor (TSSHC) are investigated with the employment of La_0·9Ce_0·1Ni₅ and MmNi_4.8Al_0.2 alloys in series. The hydrogen supply and the discharge temperatures are chosen as 20 °C and 80 °C, respectively. The thermodynamic performance data, i.e. compressor work and efficiency, are evaluated using the experimentally measured pressure-concentration-isotherm (PCI) and thermodynamic properties. In contrast, the heat and mass transfer behaviour is predicted by solving governing equations through the finite volume method (FVM). The numerical model is validated with experimental PCIs, and the results are in close agreement. The predicted cycle time is 75 min, comprising hydrogen supply, sensible heating and cooling, and hydrogen delivery. The TSSHC possessed a compression ratio of 9.5 and a cycle efficiency of 11.4% in which the hydrogen supply pressure is 9 bar using 0.5 kg of each alloy. Later, the influence of mass transfer on overall compressor work, heat input and efficiency is also presented.     

Effect of hydrogen addition on the operating characteristics of a free piston linear engine

A free piston linear engine (FPLE) with glow plug ignition was designed and manufactured. This paper investigated effects of hydrogen addition on the FPLE performance and emission characteristics. Experimental results showed that the FPLE can be started manually and then steady FPLE operating state is achieved. Compared with the crankshaft model engine, the FPLE has a greater speed around the top dead centre. The peak acceleration of the FPLE (4100 m/s²) is more than three times greater than that of the crankshaft model engine. Results also showed that the FPLE performance including the indicated power, indicated thermal efficiency and indicated mean effective pressure improves remarkably with hydrogen addition. The optimal FPLE performance is achieved when the hydrogen volume fraction is 4%. However, the engine performance cannot be further improved when the hydrogen volume fraction continues to increase due to the occurrence of over-advanced combustion phasing. Moreover, the FPLE operating range is significantly broaden and the operating stability is improved with hydrogen addition since the fast burning speed and wide flammability of hydrogen. Besides, the heat release rate, in-cylinder peak pressure and pressure rise rate are also obviously enhanced. The high-speed flame images showed the flame color becomes lighter and the yellow area is smaller compared with that of pure methane condition, indicating that the hydrogen addition to the fuel contributes to burn completely and reduce the soot formation.     

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.     

A new cavity profile for a diaphragm compressor used in hydrogen fueling stations

The short operating life of metallic diaphragm caused by fracture is one of the main disadvantages for diaphragm compressors used in hydrogen fueling stations. A new generatrix for cavity profile is proposed through optimization using the complex method to decrease the maximal radial stresses on both oil and gas sides of the diaphragm clinging to the cavity surface. In the optimization, the convex part of the cavity generatrix is subjected to a constraint that the generatrix has a lower slope than the deformed diaphragm under a uniform pressure load. This constraint aims to avoid cavity dead volume at the end of the gas discharge process. Thus, an analytical solution for the deflection of an edge-clamped metallic diaphragm under a uniform pressure load is firstly developed. The solution employs the principle of minimum energy and the Rayleigh-Ritz method, which based on the theory of thin plates with large deflections. Experimental measurements, as well as the finite elements method (FEM), are employed to validate the solution. The analytical results are found to be in good agreement with the results of measurements and FEM simulations. Secondly, the stress of the diaphragm with a specific deflection is calculated, and the radial stress concerning both gas side and oil side of the diaphragm is taken as the objective function. Finally, a new generatrix is obtained through the optimization. The radial stress of the diaphragm clinging to the new cavity profile is validated via the FEM simulation, and results match well with each other. It also approves that the cavity dead volume is eliminated by the new generatrix at the end of the gas discharge process. Moreover, the maximal and the centric radial stress of the working diaphragm were compared between the new generatrix and the traditional generatrix under the same design parameters, the maximal and the centric radial stress of the diaphragm decreased by 8.2% and 13.9%, respectively. Based on the proposed design method, effects of the cavity volume, cavity radius, diaphragm thickness and diaphragm material properties on the maximal radial stress of the working diaphragm are further discussed.     

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.     

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%.     

Development of new type Ni-Mg texture-like structure hydrogen absorber

This study demonstrated the feasibility of a novel Mg vapor deposition treatment on Ni foam to synthesize a Ni-Mg texture-like structure as a new type of hydrogen absorber. Energy dispersive spectrometry (EDS) yielded an estimative value of the weight percent ratio of Ni and Mg of 71.8 and 20.5 in as-prepared Ni-Mg texture-like structure. The microstructural changes were also characterized by X-ray diffraction (XRD) and the formed hydride tetragonal-MgH₂ was confirmed. The unique combination of large surface area of catalyst (Ni) and hydrogen acceptor (Mg) reduced the hydrogenation and dehydrogenation temperatures and performed the capability of reversible hydrogen storage capacity up to 0.72 wt.% H₂ at 25 °C. Ni-Mg texture-like structure achieved significant hydriding-dehydriding performances at lower temperature than traditional Mg-based hydrogen storage alloys. A possible hydrogen storage mechanism was also discussed where the catalytic Ni foam with large surface area was shown to be a vital factor in improving hydriding-dehydriding kinetics.