Performance analysis of combining solid oxide electrolysis cell hydrogen production and marine hydrogen compressed natural gas engine system

Natural gas holds promise as an alternative energy source for ships, and its combustion blended with hydrogen can achieve better beneficial of energy saving and emission reduction. Nevertheless, the current challenges in hydrogen, characterized by its high cost and difficulties in storage and transportation, hinder its application in maritime technology. To address the issue, this study proposes a system that integrates a marine hydrogen compressed natural gas (HCNG) engine, a supercritical carbon dioxide Brayton cycle (SCBC), and a solid oxide electrolysis cell (SOEC). In this system, SCBC is capable of recovering waste heat for electricity generation, while SOEC serves as an energy storage device to adjust engine load and enhance fuel economy. The generated hydrogen can be utilized as a source of hydrogen in HCNG fuel, thereby improving engine performance and reducing pollutant emissions. Various system numerical simulation schemes with different engine hydrogen volume fractions were studied. The results show that the electrical efficiency of SOEC is improved by 17.86% by heating the feed stream with engine exhaust gas. In addition, the hydrogen fraction in which SOEC can act as a complete hydrogen source ranges from 30% to 40%. The results of the sensitivity analysis show that the cycle efficiency reaches a peak of 31.5% at a compressor outlet pressure of 16 MPa, while the efficiency of the whole system reaches a maximum of 50.8% at 21 MPa. The optimal operating temperature for the SOEC falls within the range of 650-700 degrees C for the 30, 40 HCNG schemes, and between 750 and 800 degrees C for the 50, 60 HCNG schemes.