SUMMARYHydrogen liquefaction systems have been the subject of intense investigations for many years. Some established gas liquefaction systems, such as the precooled Linde}Hampson systems, are not used for hydrogen liquefaction in part because of their relatively low e$ciencies. Recently, more promising systems employing the modi"ed Collins cycle have been introduced. This paper reports on second law analyses of a hydrogen lique"er operating on the modi"ed Collins cycle. Two di!erent modi"cations employing the cycle in question were attempted: (1) a helium-refrigerated hydrogen liquefaction system and (2) a hydrogen-refrigerated hydrogen liquefaction system. Analyses were carried out in order to identify potential areas of development and e$ciency improvement. A computer code capable of computing system and component e$ciencies; exergy losses; and optimum number and operating conditions of compressors, expanders, aftercoolers, intercoolers, and Joule}Thomson valves was developed. Evaluation of the thermodynamic and transport properties of hydrogen at di!erent temperature levels was achieved by employing a hydrogen property code developed by researchers at the National Bureau of Standards (currently NIST). A parametric analysis was carried out and optimal decision rules pertaining to system component selection and design were reached. Economic analyses were also reported for both systems and indicated that the heliumrefrigerated hydrogen lique"er is more economically feasible than the hydrogen-refrigerated hydrogen lique"er.
This study investigated various aspects of thermal storage concept, including material characterization
and analysis of form-stable, unencapsulated phase-change materials (PCM) that underwent solid-solid phase transition and was in direct contact with the working fluid. The study focused on temperature range between 100-140°C (212-284°F). Mathematical heat transfer models were developed to examine the operating characteristics of the thermal energy storage unit, identify key parameters
influencing storage, and conducted parametric studies. Both single-phase and phase-change working fluids were considered in the models. Experiments were conducted using a packed bed of PCM pellets and a single-phase working fluid (tri-ethylene glycol) to evaluate and demonstrate the
heat storage concept during charging and discharging. The experimental results aligned well with the heat transfer models, validating their accuracy. Parametric studies explored a wide range of parameters not feasible in laboratory experiments, shedding light on charging, discharging, and thermal storage characteristics. These models facilitated the development and implementation of optimization algorithms for packed bed latent heat storage units. The findings indicated that form-stable latent heat
units utilizing commercially available polymers undergoing solid-solid phase transition can serve as long-term stable thermal storage candidates for use with several single-phase working fluids as well as two-phase steam.
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