Transient flow performance and heat transfer characteristic in the cylinder of hydraulic driving piston hydrogen compressor during compression stroke

With the advantages of large flow capacity and high pressure, the use of hydraulic driving piston compressors in hydrogen refueling stations is becoming the development trend. Understanding transient flow and heat transfer characteristic is the key issue for the design and application of hydrogen compressors. The transient model of the hydraulic driving piston compressor is constructed by dynamic mesh and the National Institute of Standards and Technology (NIST) real hydrogen model, which accurately predicts flow field and heat transfer. Moreover, the effect of piston reciprocating cycle frequency on hydrogen parameters variation and heat transfer characteristic is investigated. Adiabatic compression theory is commonly applied in the design of reciprocating compressors. The results show that due to the heat transfer, the exhaust temperature predicted by the adiabatic compression theory is 6.29 K higher than the actual value. This study provides beneficial references for the design optimization and reliable operation of hydraulic driving piston hydrogen compressors.     

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.     

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.     

Liquid piston gas compression

A liquid piston concept is proposed to improve the efficiency of gas compression and expansion. Because a liquid can conform to an irregular chamber volume, the surface area to volume ratio in the gas chamber can be maximized using a liquid piston. This creates near-isothermal operation, which minimizes energy lost to heat generation. A liquid piston eliminates gas leakage and replaces sliding seal friction with viscous friction. The liquid can also be used as a medium to carry heat into and out of the compression chamber. A simulation is presented of the heat transfer and frictional forces for a reciprocating piston and a liquid piston. In the application of an air compressor, with a pressure ratio of 9.5:1 and a cycle frequency of 20 Hz, the liquid piston decreased the energy consumption by 19% over the reciprocating piston. The liquid piston and the reciprocating piston exhibited a total efficiency of 83% and 70% respectively. The liquid piston demonstrated significant improvements in the total compression efficiency in comparison to a conventional reciprocating piston. This gain in efficiency was accomplished through increasing the heat transfer during the gas compression by increasing the surface area to volume ratio in the compression chamber.     

Influence of the surface microstructure of the fuel cell gas diffusion layer on the removal of liquid water

The effective removal of water on the gas diffusion layer (GDL) surface and low flow channel resistance are essential for the water management of proton exchange membrane fuel cells (PEMFCs). In this paper, a 3D two-phase volume of fluid (VOF) model is used to compare and analyze the influence of different GDL surface microstructures on the liquid hydrodynamic behavior and optimize the design of the sine wave microstructure. The results show that the surface microstructure of the GDL has a more significant impact on the water removal and flow resistance coefficient in the flow channel, and the sine wave microstructure has substantial advantages. The sine wave peak and period significantly influence the water removal and flow resistance coefficient. As the peak increases, the average relative change rate and the flow resistance coefficient also increase; the influence of the period is opposite to the peak, and the continuous decrease of the period will accelerate the water removal in the flow channel. The sine wave's height and width have little impact on water removal and the flow resistance coefficient. When the sine wave A = 75 μm, T = 1.5, H = 15 μm, and L = 25 μm, good flow channel water removal and low resistance are achieved. This work has particular guiding significance for removing liquid water on the GDL surface and obtaining low flow channel resistance.     

Investigation of boiling hydrogen flow characteristics under low-pressure conditions - Flow regime transition characteristics

Understanding the thermal-fluid characteristics of boiling hydrogen is of great significance for applications of liquid hydrogen, such as alternative clean energy and space vehicles. The boiling temperature of liquid hydrogen under atmospheric pressure is 20.3 K; thus, it is easy to boil to form a gas–liquid two-phase flow. Fuel transfer under the boiling state has been avoided in the space industry because of its unstable flow characteristics; precise control of the fuel, including the boiling flow, is necessary to improve the space-vehicle performance. This study aims to understand the flow-regime transition characteristics of boiling hydrogen through experimental investigation. The experimental conditions were as follows: the flow direction was horizontal, the inner diameter of the heating pipe was 15 mm, the mass flux ranged from 50 to 110 kg/m²s, and the pressure ranged from 250 to 300 kPa A. The flow-regime transition characteristics were obtained by a high-speed camera. Fully liquid phase (LP), dispersed bubbly flow (DB), intermittent flow (IN), and annular flow (AN) were observed during the experiment. Each flow-regime boundary model is constructed using two dominant forces from the experimental result based on a Taitel–Dukler model. For the DB/IN boundary, a large-bubble sustainable condition is derived by the balance between the shear and buoyancy forces acting upon the bubble; for the IN/AN boundary, a droplet-sustainable condition is derived in terms of the force balance between the drag and gravity acting on the droplet. The semi-theoretical model predicts the experimental data with 96.7% accuracy.     

滚动转子式压缩制冷系统润滑油的流动

润滑油的流动对制冷系统的性能和可靠性有重要影响.建立滚动转子式压缩系统实验台,观测和研究膨胀阀出口和蒸发器出口制冷剂/油混合物的两相流流型.结果发现:在蒸发器出口处混合物的流动表现为"油渍"蠕动、"油膜"线状流、"油膜"环状流和雾状湿蒸汽流等流型;在膨胀阀出口有液气分相流和泡气分相流等流型.在一定的运行工况下,压缩机正过热度越小,"油膜"流动速度越快,越利于压缩机回油;当压缩机排气温度等于冷凝温度时,高含油量的液体节流后形成泡状流,使得系统性能恶化甚至造成压缩机损坏.     

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.     

Two-phase wavy-annular flow in small tubes

Base liquid film thickness distribution, wave behavior, and pressure loss measurements have been obtained for 237 horizontal two-phase (air–water) flow conditions in 8.8 and 15.1 mm I.D. tubes in the wavy and annular regimes. The behaviors measured indicate the presence of a transitional wavy-annular regime at flow rates traditionally labeled as annular in these small diameter tubes. Data for a 26.3 mm I.D. tube do not show these same trends.     

The hydrogen flow characteristics of the multistage hydrogen Knudsen compressor based on the thermal transpiration effect

Multistage hydrogen Knudsen compressor based on the thermal transpiration effect has very exciting prospect for the hydrogen transmission in the micro devices. Understanding of the hydrogen flow characteristic is the key issue for the designs and applications of the hydrogen energy systems. Firstly, the numerical models of the multistage hydrogen Knudsen compressor are established. The distributions of the rarefaction, velocity and temperature at different stages of the hydrogen flow are calculated and presented. Moreover, the dimensional pressure increases of the hydrogen gas flow are analyzed, and the flow behaviors in the microchannel and the connection channel are discussed. Secondly, the numerical simulation at different connection channel height is implemented, and the hydrogen gas flow characteristics in the connection are analyzed. Especially, the performances of the pressure drop in the connection channel under different channel heights are studied, and the hydrogen gas compression characteristics of different cases are compared and discussed. Also, the effect of the connection channel height on the hydrogen gas pressure increase in the microchannel is investigated. The studies presented in this paper could be greatly beneficial for the hydrogen detection and transmission.     

Demonstration of a single-stage metal hydride hydrogen compressor composed of BCC V40TiCr alloy

In this study, demonstration of a one-stage metal hydride hydrogen compressor (MH compressor) by using a BCC alloy was performed. It was estimated that V₄₀Ti₂₂Cr₃₈ could compress approximately 1.6 wt% of hydrogen from 1.0 to 10 MPa in 20–140 °C temperature range from equilibrium theory via pressure-composition-isotherm measurements. For demonstration of an actual MH compressor, a kg-scale experimental system was set up; V₄₀Ti₂₂Cr₃₈ (1.4 kg) was introduced into a 1-inch cylindrical vessel with a heat-medium flow tube outside. As a result, 1.0 MPa of hydrogen can be compressed into the hydrogen cylinder at >10 MPa by hydrogen absorption at 10 °C and desorption at 160 °C for 30 min each (1 cycle/h) to achieve a compression rate of 0.23 Nm³/h and indicate the potential of the practical MH compressors by using BCC alloy.     

Dehydrogenation of perhydro-dibenzyltoluene for hydrogen production in a microchannel reactor

A preliminary study regarding the dehydrogenation of perhydro-dibenzyltoluene as a liquid organic hydrogen carrier with switching from a stirred tank reactor to a continuous flow microchannel reactor is presented. The hydrogen production percentage in the case of a continuous flow microchannel reactor was found greater when compared to that of a stirred tank reactor. The hydrogen production was increased from 64.1% to 82.2% with the increase in bottom plate temperature from 260 to 320 °C for 0.01 mL/min flow rate. A maximum of 88% of hydrogen was generated for a 40 hours of operation, at a bottom wall temperature of 290 °C. The kinetic model for the microchannel reactor dehydrogenation was presented with a pre-exponential factor of 3.272 s^?1 and activation energy of 13.79 kJ/mol. The results revealed that a continuous microchannel reactor can be an appropriate technology for the dehydrogenation of perhydro-dibenzyltoluene.     

Compressor speed control design using PID controller in hydrogen compression and transfer system

Hydrogen is an energy carrier which can be processed by high pressure compressor and they can be transported, stored and converted to electricity for later use. This paper proposes a hydrogen compression model development and modeling of hydrogen transportation between two tanks using MATLAB software version 22. The proposed model provides amount of hydrogen required in volumes (m³) and compressor power required in (KW) for compressor speed of 500 rad/s, 1000 rad/s and 1500 rad/s. This model provides hydrogen volume of 1 m³ and 10 KW compressor power requirement at 500 rad/s compressor speed. For compressor speed of 1000 rad/s, the proposed model provides hydrogen volume of 10 m³and 20 KW compressor power requirements and for 1500 rad/s this model provides volume of 30 m³and 30 KW compressor power requirements which indicates that the increase in compressor speed increases hydrogen volume generated and increase the power requirement also. For maintaining compressor speed at desired value, a PID (Proportional + Integral + Derivative) controller has been designed and the results were compared with Proportional (P), PI (Proportional + Integral), and PD (Proportional + Derivative) controllers. PID controller performance can be measured using the parameters delay time and settling time. Low values of settling time and delay time indicate the better performance of PID controller. P controller achieves the desired compressor speed with delay time of 228 ms; settling time of 1250 s. PI controller achieves the desired compressor speed with delay time of 210 ms, settling time of 1210 s. PD controller achieves the desired compressor speed with delay time of 185 ms, settling time of 1280 s. PID controller provides better speed regulation with low delay time of 110 ms and settling time of 1120 s when compared with P, PI, PD controllers. From the simulation results it is observed that PID controller can be a good option for slow process like hydrogen gas flow through pipeline for effective speed regulation.     

The effect of absorber packing height on the performance of a hybrid liquid desiccant system

A hybrid system consisting of vapour compression unit, a liquid desiccant system, and a flat solar hot water collector were designed, fabricated and tested. This combination allowed for a separate control of humidity and temperature without energy penalty. Various packing heights of the absorber component were tested to determine the optimal performance of the combined unit. A 1000 mm packing height with cross-sectional area of 600×600 mm, proved to be the best height that gives promising improvements in the coefficient of performance of the vapour compression unit.     

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.     

Improving hydrogen refueling stations to achieve minimum refueling costs for small bus fleets

Fuel cell vehicles using green hydrogen as fuel can contribute to the mitigation of climate change. The increasing utilization of those vehicles creates the need for cost efficient hydrogen refueling stations. This study investigates how to build the most cost efficient refueling stations to fuel small fleet sizes of 2, 4, 8, 16 and 32 fuel cell busses. A detailed physical model of a hydrogen refueling station was built to determine the necessary hydrogen storage size as well as energy demand for compression and precooling of hydrogen. These results are used to determine the refueling costs for different station configurations that vary the number of storage banks, their volume and compressor capacity.It was found that increasing the number of storage banks will decrease the necessary total station storage volume as well as energy demand for compression and precooling. However, the benefit of adding storage banks decreases with each additional bank. Hence the cost for piping and instrumentation to add banks starts to outweigh the benefits when too many banks are used. Investigating the influence of the compressor mass flow found that when fueling fleets of 2 or 4 busses the lowest cost can be reached by using a compressor with the minimal mass flow necessary to refill all storage banks within 24 h. For fleets of 8, 16 and 32 busses, using the compressor with the maximum investigated mass flow of 54 kg/h leads to the lowest costs.     

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%).     

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.     

Two-phase flow patterns of nitrogen and nanofluids in a vertically capillary tube

Two-phase flow patterns of nitrogen gas and aqueous CuO nanofluids in a vertically capillary tube were investigated experimentally. The capillary tube had an inner diameter of 1.6 mm and a length of 500 mm. Water-based CuO nanofluid was a suspension consisted of water, CuO nanoparticles and sodium dodecyl benzol sulphate solution (SDBS). The mass concentration of CuO nanoparticles varied from 0.2 wt% to 2 wt%, while the volume concentration of SDBS varied from 0.5% to 2%. The gas superficial velocity varied from 0.1 m/s to 40 m/s, while the liquid superficial velocity varied from 0.04 m/s to 4 m/s. Experiments were carried out under atmospheric pressure and at a set temperature of 30 °C. Compared with conventional tubes, flow pattern transition lines occur at relatively lower water and gas flow velocities for gas–water flow in the capillary tube. While, flow pattern transition lines for gas–nanofluid flow occur at lower liquid and gas flow velocities than those for gas–water in the capillary tube. The effect of nanofluids on the two-phase flow patterns results mainly from the change of the gas–liquid surface tension. Concentrations of nanoparticles and SDBS have no effects on the flow patterns in the present concentration ranges.     

Improving chiller performance and energy efficiency in hydrogen station operation by tuning the auxiliary cooling

In a hydrogen station that operates with direct fueling through the use of a 700 bar boost compressor, the outlet hydrogen temperature can significantly increase, stressing the chiller system. This paper evaluates improvements that can be made to the auxiliary cooling system integrated with the compressor. Both theoretical modeling and experiments were performed at Cal State LA Hydrogen Research and Fueling Facility. The findings suggest that adjusting the auxiliary closed-loop cooling system from 15 °C to 10 °C reduced the station energy consumption and decreased the demand on the station chiller that needed to provide ?20 °C hydrogen at the hose. The overall energy consumption for a single fueling reduced by between 2.86 and 9.43% for the set of experiments conducted. After the temperature of the closed-loop cooling system was reduced by 5 °C, the boost compressor outlet temperature dropped from 46-50 °C–40 °C and consequently at the hose the hydrogen temperature declined by 3 °C. Results were scaled up with a forecast on the number of daily refueling events. With a low number of daily fuelings, the proposed set-up showed a minor influence on the overall station energy consumption. However, the benefits were more pronounced for a connector station with sales at 180 kg/day, where the energy efficiency improved by between 1.4 and 5.5%, and even more so for a higher capacity station at 360 kg/day, where the improvement was between 2.9 and 8%.