Thermo-economic analysis of green hydrogen production onboard LNG carriers through solid oxide electrolysis powered by organic Rankine cycles

Abstract

LNG carriers play a crucial role in the shipping industry meeting the global demand for natural gas (NG). However, the energy losses resulting from the propulsion system and the excess boil-off gas (BOG) cannot be overlooked. The present article investigates the H₂ production on board LNG carriers employing both the engine's waste heat (WH) and the excess BOG. Conventional (ORC) and dual-pressure (2P-ORC) organic Rankine cycles coupled separately with a solid oxide electrolysis (SOEC) have been simulated and compared. The hydrogen (H₂) produced is then compressed at 150 bar for subsequent use as required. According to the results, the 2P-ORC generates 14.79 % more power compared to ORC, allowing for an increased energy supply to the SOEC; hence, producing more H₂ (34.47 kg/h compared to 31.14 kg/h). Including the 2P-ORC in the H₂ production plant results in a cheaper H₂ cost by 0.04 $/kg_H2 compared to ORC, a 1.13 %_LHV higher system efficiency when leveraging all the available waste heat. The plant including 2P-ORC exploits more than 86 % of the of the available waste compared to 70 % when using ORC. Excluding the compression system decreases the capital cost by almost the half regardless of the WH recovery system used, yet it plays in favour of the plant with ORC making the cost of H₂ cheaper by 0.29 $/kg_H2 in this case. Onboard H₂ production is a versatile process independent from the propulsion system ensuring the ship's safety and availability throughout a sea journey.