SUMMARYThe hybrid system comprised by a proton exchange membrane (PEM) fuel cell and internal combustion engine shows many advantages for vehicle applications. The hybrid system can recover the un-reacted hydrogen from fuel cell, utilize heat in the combustion product from cylinder, or combine the advantages of both. Based on thermodynamics and electrochemistry, an indirect integration system of the PEM fuel cell and Otto cycle is established for vehicle applications. The irreversibilities such as the entropy production and overpotentials in the fuel cell, the finite-rate heat transfer between the air in the Otto cycle and combustion chamber wall, the irreversible compression, expansion, and regeneration processes in the Otto cycle are considered. The excellence of the PEM fuel cell compared with internal combustion engine is shown in terms of energy conversion efficiency. When the vehicle is speeding or launching suddenly, not only the flow rate of natural gas into the hybrid system should be increased but also a specific coupling mode between two powertrain systems should be found.

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Proton exchange membrane fuel cells (PEMFCs) based on syngas are a promising technology for electric vehicle applications. To increase the fuel conversion efficiency, the low-temperature waste heat from the PEMFC is absorbed by a refrigerator. The absorption refrigerator provides cool air for the interior space of the vehicle. Between finishing the steam reforming reaction and flowing into the fuel cell, the gases release heat continuously. A Brayton engine is introduced to absorb heat and provide a useful power output. A novel thermodynamic model of the integrated system of the PEMFC, refrigerator, and Brayton engine is established. Expressions for the power output and efficiency of the integrated system are derived. The effects of some key parameters are discussed in detail to attain optimum performance of the integrated system. The simulation results show that when the syngas consumption rate is 4.0 × 10−5 mol s−1cm−2, the integrated system operates in an optimum state, and the product of the efficiency and power density reaches a maximum. In this case, the efficiency and power density of the integrated system are 0.28 and 0.96 J s−1 cm−2, respectively, which are 46% higher than those of a PEMFC.