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

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Abstract

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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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.
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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.
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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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