A Study on the Charge-Discharge Cycle of a Compressed Hydrogen Tank for Automobiles: Fundamentals

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Environmental pollution, global warming, and depletion of fossil fuels compel radical changes in automotive engine technologies in directions that offer both the potential for achieving near-zero emissions of pollutants and greenhouse gases and a diversification of the transport fuel system away from its present exclusive dependence on fossil fuels. Hydrogen-fueled vehicles can be an environment-friendly alternative. Composite high-pressure tanks can be used for storage of hydrogen gas on board road vehicles. Durability and safety of the fuel tanks are the important concerns involved. In this paper a numerical model is developed for the analysis of the cyclic fast charging and discharging process of a high-pressure hydrogen gas tank to determine the effect on tank wall temperature. The flow is considered as compressible, viscous, unsteady and turbulent. Axisymmetric, time-dependent, Navier-Stokes equations were solved with the two-equation realizable k-ε turbulent model for turbulent momentum closure. Redlich-Kwong real gas equation was used for density computations. The numerical model is of relevance to the design of high-pressure tanks for durability and safety, primarily with regard to the wall configuration.

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A major requirement for the filling of hydrogen tanks is the maximum gas temperature within the vessels during the process. Different filling strategies in terms of pressure and temperature of the gas injected into the cylinder and their effects on key parameters like maximum temperature, state of charge, and energy cooling demand are investigated. It is shown that pre-cooling of the gas is required but is not necessary for the whole duration of the filling. Relevant energy savings can be achieved with pre-cooling over a fraction of the time. The most convenient filling strategy from the cooling energy point of view is identified: with an almost linear pressure rise and pre-cooling in the second half of the process, a 60% reduction of the cooling energy demand is achieved compared to the case of pre-cooling for the whole filling
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Experiments have been conducted to measure the rise in temperature of hydrogen and vessel wall during filling of commercially available, practical tanks to 35 and 70 MPa. Three test vessels with volumes 205, 130 and 39 liters are investigated. The filling time ranges from 5 to 20 minutes. The heat transfer process is modeled using a one-dimensional unsteady heat conduction equation for the wall coupled with a flow and heat balance for the compressed gas. The model requires heat transfer coefficients between the hydrogen and the wall and the wall and surrounding air. Values of 500 W/(m2K) during filling, 250 W/(m2K) after filling for the inside wall and 4.5 W/(m2K) for the outside tank wall are tentatively assumed based on results from a previous study on a smaller vessel. The measured temperatures for the hydrogen gas and the wall are in good agreement with the calculations.
Compressed hydrogen gas is a popular mode of fuel storage for hydrogen powered vehicles. When hydrogen gas is filled at high pressure, the gas temperature increases. The maximum gas temperature should be within acceptable safety standards. Numerical studies can help optimize the filling process. There is a high level of turbulence in the flow as the high velocity inlet jet is penetrating the nearly stagnant gas in the tank. Selection of a suitable turbulence model is important for accurate simulation of flow and heat transfer during filling of hydrogen tanks. In the present work, a comparative study is performed to identify suitable turbulence model for compressed hydrogen tank filling problem. Numerical results obtained with different turbulence models are compared with available experimental data. Considering accuracy, convergence and the computational expenses, it is observed that the realizable k-epsilon model is the most suitable turbulence model for hydrogen tank filling problem. Copyright
Conference Paper
Hydrogen holds much promise as a source of energy that is at once abundant, clean, flexible, and secure. It is also environment friendly because it burns without producing carbon dioxide. Physical characteristics of hydrogen make it difficult to store in large quantities without taking up large amount of space. High pressure gas is a widely used storage mode for hydrogen fuel. Filling up process of a high pressure hydrogen tank should be reasonably short. The process should be designed so as to avoid high temperatures in the tank because of safety reasons. Numerical simulation can aid in optimizing the filling up process. The paper reports the numerical simulation of the filling up process of hydrogen tanks using computational fluid dynamics method. Real gas equations are solved to accurately simulate the process at the high temperature and pressure associated with the fast filling. Local temperature distribution in the tank is obtained at different durations of the fill. The numerical results obtained are validated with available experimental data.
High injection pressures are used during the re-fueling process of vehicle tanks with compressed hydrogen, and consequently high temperatures are generated in the tank, potentially jeopardizing the system safety. Computational Fluid Dynamics (CFD) tools can help in predicting the temperature rise within vehicle tanks, providing complete and detailed 3D information on flow features and temperature distribution. In this framework, CFD simulations of hydrogen fast filling at different working conditions are performed and the accuracy of the numerical models is assessed against experimental data for a type 4 tank up to 70 MPa. Sensitivity analyses on the main modeling parameters are carried out in compliance with general CFD Best Practice Guidelines.
The heat transfer process during filling of an evacuated vessel at low Reynolds number was investigated experimentally using air as the flow medium. The data was analysed using a thermodynamic model similar to one currently in use for the design of systems using commercial carbon fibre reinforced plastic vessels for storage of compressed hydrogen gas. Model assumptions included perfectly-stirred conditions within the vessel, one-dimensional unsteady heat conduction through the composite vessel wall, ideal gas and frictional adiabatic flow conditions through the inlet tube. A transition phenomenon from laminar to turbulent flow was observed by decreasing the inlet diameter while maintaining a similar mass flow rate. Based on the measurements, a new empirical correlation for the Nusselt number under low Reynolds number flow conditions is proposed.
Environmental pollution and rapid depletion of fossil fuels had necessitated the search for alternative technologies and energy sources for transportation. Hydrogen fuel can be an environment friendly alternative. High pressure gas is a widely used storage mode for hydrogen fuel. Refueling of a vehicular hydrogen tank should be reasonably short to gain consumer acceptability. However, quick filling at high pressures can result in high temperatures. This should be avoided because of safety reasons. A numerical model can aid in optimizing the filling up process. The paper reports the numerical simulation of the refueling of high pressure hydrogen tanks using computational fluid dynamics method. Real gas equations are included to accurately simulate the process at the high temperature and pressure associated with the fast filling. Local temperature distribution in the tank is obtained at different durations of the fill. The numerical results obtained are validated with available experimental data. The results give an accurate visualization of the thermo fluid dynamic behavior of hydrogen gas during fast filling.
Hydrogen-fueled vehicles offer a clean and efficient alternative for transportation. Compressed gas in high pressure tanks is a popular storage mode for hydrogen fuel. Time required for filling a hydrogen tank for vehicular applications should be short. But quick filling of hydrogen tanks at high pressures can result in high gas temperatures which can damage the tank and lead to its rupture. Hence the real time monitoring of gas temperature is essential during filling. This paper reports the findings of numerical simulation of filling process of hydrogen tanks. Real gas effects are considered. Local temperature distribution in the tank is obtained at different durations of the fill. Effect of changes in ambient temperature and initial and inlet gas temperatures is studied. Results of the study can aid in optimizing the filling time and in identifying the most suitable locations for the feedback devices within on-board hydrogen tanks.
Some complete experimental data sets, not only on the hydrogen temperature within the tank during filling, but also on the supplied temperature and pressure from the station have been opened for analysis of the temperature change with time. The data were independently obtained for 6 different conditions and have been analyzed and checked to validate the Monde et al. model. It is found that the measured temperatures are well predicted using the software based on the model and the heat loss during filling with hydrogen is also well predicted, if a suitable heat transfer coefficient is adopted.
This paper reports a thermodynamic analysis of filling a fuel tank with compressed gaseous hydrogen. The analysis is based on energy and exergy methods. A parametric study is performed to investigate the effect of initial conditions on the exergy destruction and exergy efficiency of filling processes. The transient filling process is studied to determine the temperature and pressure changes inside the storage tank during filling. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Gas with high pressure is widely used at present as fuel storage mode for different hydrogen vehicles. Different types of materials are used for constructing these hydrogen pressure vessels. An aluminum lined vessel and typically carbon fiber reinforced plastic (CFRP) materials are commercially used in hydrogen vessels. An aluminum lined vessel is easy to construct and posses high thermal conductivity compared to other commercially available vessels. However, compared to CFRP lined vessel, it has low strength capacity and safety factors. Therefore, nowadays, CFRP lined vessels are becoming more popular in hydrogen vehicles. Moreover, CFRP lined vessel has an advantage of light weight. CFRP, although, has many desirable properties in reducing the weight and in increasing the strength, it is also necessary to keep the material temperature below 85 °C for maintaining stringent safety requirements. While filling process occurs, the temperature can be exceeded due to the compression works of the gas flow. Therefore, it is very important to optimize the hydrogen filling system to avoid the crossing of the critical limit of the temperature rise. Computer-aided simulation has been conducted to characterize the hydrogen filling to optimize the technique. Three types of hydrogen vessels with different volumes have been analyzed for optimizing the charging characteristics of hydrogen to test vessels. Gas temperatures are measured inside representative vessels in the supply reservoirs (H2 storages) and at the inlet to the test tank during filling.
We have measured the effects of the initial mass and the total fill time on the temperature rise and the temperature distribution within a compressed hydrogen cylinder during refuelling.A type 3, 74L hydrogen cylinder was instrumented internally with 63 thermocouples distributed along the mid vertical plane. The experimental fills were performed from initial pressures of 50, 75, 100, 150, and 200bar at gas delivery rates corresponding to nominal fill times of 1, 3, and 6min. The experimental conditions with larger ratios of final to initial mass produced larger temperature changes. However, the lower ratios generated the largest rates of temperature rise. Longer fill times produced lower final average gas temperatures (compared to shorter fills), and a temperature field with significant vertical stratification due to buoyancy forces at lower gas inlet velocities. A sensor located at the end opposite to the gas inlet could be suitable for fuel metering via temperature and pressure measurements only.
The Generic Research Cryogenic Tank was designed to establish techniques for testing and analyzing the behavior of reusable fuel tank structures subjected to cryogenic fuels and aerodynamic heating. The Generic Research Cryogenic Tank tests will consist of filling a pressure vessel to a prescribed fill level, waiting for steady-state conditions, then draining the liquid while heating the external surface to simulate the thermal environment associated with hypersonic flight. Initial tests of the Generic Research Cryogenic Tank will use liquid nitrogen with future tests requiring liquid hydrogen. Two-dimensional finite-difference thermal-fluid models were developed for analyzing the behavior of the Generic Research Cryogenic Tank during fill and drain operations. The development and results of the two-dimensional fill and drain models, using liquid nitrogen, are provided, along with results and discussion on extrapolating the model results to the operation of the full-size Generic Research Cryogenic Tank. These numerical models provided a means to predict the behavior of the Generic Research Cryogenic Tank during testing and to define the requirements for the Generic Research Cryogenic Tank support systems such as vent, drain, pressurization, and instrumentation systems. In addition, the fill model provided insight into the unexpected role of circumferential conduction in cooling the Generic Research Cryogenic Tank pressure vessel during fill operations.
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