Electricity and hydrogen co-production via scramjet multi-expansion open cooling cycle coupled with a PEM electrolyzer

Abstract

A scramjet multi-stage open cooling cycle for co-production of electricity and hydrogen is proposed here in which the fuel of the scramjet is used as coolant of the cooling cycle. Energy and exergy analysis of the devised system are conducted to evaluate the performance of the system and the effects of multi-expansion deliberation. In this integrated system, the waste heat of the scramjet cooling cycle drives the power sub-cycle in which a portion of the overall produced power supplies the required electricity of the proton exchange membrane (PEM) electrolyzer for hydrogen production. The results indicated that a four-stage open cooling cycle can be the best scenario in terms of providing more cooling capacity, electricity, and H2 production as well as a reasonable investment cost of its turbines. However, in the case when weight and size of the proposed cogeneration system are very important, a two-stage open cooling cycle can be appropriate. For the fuel mass flow rate of 0.4 kg/s and freestream condition of MaA=6, TA = 223 K and PA = 2.5 kPa, the cooling capacity, net electricity, and hydrogen production of the proposed system with four stages are computed 11.167 MW, 4.48 MW and 55.23 kg/h, respectively. On the other hand, the exergy results showed that PEM electrolyzer has the highest exergy destruction ratio by 70.66% among different components of the set-up. Moreover, the results of exergy analysis exhibited that employing the concept of multi-expansion outstandingly reduces the overall exergy destruction of the system. In this case, the energy and exergy efficiencies of the overall set-up (four stages) are computed 14.07% and 17.44%, respectively. The results of parametric evaluation demonstrated that increasing the back pressure of pump leads to more electricity and hydrogen production. But, increasing the mass flow rate of fuel hasn’t any tangible impact on energy and exergy efficiency.