UNCERTAINTIES IN RISK ASSESSMENT OF HYDROGEN DISCHARGES FROM PRESSURIZED STORAGE VESSELS AT LOW TEMPERATURES

Evaluations of the uncertainties resulting from risk assessment tools to predict releases from the various hydrogen storage types are important to support risk informed safety management. The tools have to predict releases from a wide range of storage pressures (up to 80 MPa) and temperatures (at 20K) e.g. the cryogenic compressed gas storage covers pressures up to 35 MPa and temperatures between 33K and 338 K. Accurate calculations of high pressure releases require real gas EOS. This paper compares a number of EOS to predict hydrogen properties typical in different storage types. The vessel dynamics are modeled to evaluate the performance of various EOS to predict exit pressures and temperatures. The results are compared to experimental data and results from CFD calculations. 1.0 INTRODUCTION Hydrogen may be thermodynamically considered as an almost ideal gas over a very wide temperature and pressure range. Nevertheless, present technological developments stores hydrogen in the liquid state at about 20K under a low pressure of few bars and in the gaseous state at very high pressures up to 800 – 1000 bars at ambient temperatures. Recently, the operational regime of cryo compressed hydrogen (CcH2) storage was reported to cover pressures of up to 35 MPa and temperatures from +65 °C down to -240 °C [1,2]. Considering these wide ranges for temperature and pressures, the assumption of ideal gas behavior and application of the ideal gas equations of state (EOS) is not adequate for all situations. This has been recognized by the scientific community and different approaches describing high pressure gas releases at ambient conditions from storage tanks [3-7] and within vehicles [8] are described. This discrepancy of behavior between ideal gas and real gas is illustrated in Figure 1, where the state-of-the-art reference data provided by NIST [3] are compared against predictions using the EOS for ideal gas. It is shown that the ideal gas EOS accuracy in predicting the density pressure relationship is limited up to about 35MPa at 500K, and up to about 15MPa at 200K. From the risk assessment point of view a large number of release scenarios have to be analyzed, to give a comprehensive evaluation of the associated risks and to provide useful data for risk management purposes. In many scenarios at ambient conditions and moderate storage pressures the use of engineering equations based on the ideal gas EOS may give sufficiently accurate results to make proper decisions. For scenarios using very high pressure and cryogenic storage real gas behavior needs to be taken into account to reduce the level of uncertainty in the evaluations. For this purpose several EOS are being developed which may be classified into the cubic EOS type (equations by e.g. 1 Accesible from http://webbook.nist.gov/chemistry/fluid “hydrogen” and described in the PhD thesis by Leachman: http://www.boulder.nist.gov/div838/theory/refprop/leachman.pdf 2 http://en.wikipedia.org/wiki/Equation_of_state#Cubic_equations_of_state