Thermomechanical exergy recovery from liquid hydrogen cryofuel
Date
1996
Authors
Flaherty, Kenneth Michael
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Abstract
Hydrogen (H2) is a potential zero emission fuel for transportation vehicles. A barrier to its implementation is the issue of on-board storage, due to low densities at ambient conditions. Cryogenic liquefaction increases the volumetric energy density of H2 but requires considerable energy input during the liquefaction process. Refrigeration work, required to liquefy the fuel , results in the fuel containing thermomechanical exergy. Vehicle powertrains typically utilize the chemical energy contained in a fuel, but waste the thermomecharucal energy. With liquid hydrogen (LH2), the thermomechanical energy amounts to more than 10% of the chemical energy.
This thesis investigates a hierarchy of options for recovering the thermomecharucal energy on board a LH2 powered vehicle. Operating conditions are determined from the Ballard Phase II fuel cell powered bus. This work explores using the cold ofLH2 as a heat sink for a cryogenic heat engine. Such a heat engine could generate work to augment the main vehicle powerplant. Alternatively, the cold LH2 can reduce the parasitic air compression load of the fuel cell powerplant and improve fuel cell operation. This work provides a thermodynamic comparison of several thermomecharucal exergy recovery options. The systems are modelled and the net performance improvements quantified.