Energy modelling and optimization of PEM fuel cells power plants in view of shipping decarbonization

The need to reduce pollutant and greenhouse gases emissions in the shipping sector is leading to a growing interest in fuel cells propulsion. In particular, hydrogen (H2) fuelled Polymer Electrolyte Membrane Fuel Cells (PEMFC) power plants could be a promising solution for zero-local-emission navigation. However, the use of PEMFC in the maritime sector is still in its early stages, as several issues such as the high costs and the incomplete regulatory framework related to PEMFC and H2 use on board still need to be overcome. A further aspect to consider is related to the dynamic performance of PEMFC. As widely demonstrated in the literature, the hybridization of PEMFC with Electric Energy Storage Systems (EESS) could improve the system dynamics and efficiency. However, the power allocation between PEMFC and EESS is a complex issue, which involves the performance degradation of both the energy units. Considering the whole degradation effects in the energy management of hybrid PEMFC systems to determine the best tradeoff between costs and plant lifetime is a challenging yet crucial aspect, rarely addressed in the literature. In addition, the low temperature of PEMFC waste heat makes Waste Heat Recovery (WHR) on board challenging. While this aspect may not influence the overall operation of PEMFC systems for small vessels, it could hamper future installation of PEMFC power systems for larger vessels, where WHR is essential for efficiently covering thermal demands onboard. Recent studies address PEMFC WHR for stationary systems, but it appears to be a lack of studies on PEMFC WHR for ship applications. To fill these gaps, the thesis aims to develop a general methodology that allows to optimize the Synthesis, Design, and Operation (SDO) of PEMFC ship power plants while considering the power units degradation and PEMFC WHR. The proposed optimization models have been developed following a Mixed-Integer Linear Programming approach and have been applied to different case studies. A small size RoRo passenger ferry was considered as case study for the application of a health-conscious energy management strategy for a hybrid PEMFC/Lithium Ion Batteries (LIB) propulsion system, where a multi-objective optimization was set to concurrently minimize the operation cost and the PEMFC/LIB degradation over the entire lifetime of the plant. Subsequently, an uncertainty analysis was carried out to analyze the impact of uncertainties in the input parameters on the optimization results. As for the analysis of WHR integration for PEMFC installed on board, a cruise ship was considered as case study, and a multi-objective optimization was set to concurrently minimize the fuel oil consumption of the ship and the total operating and investment costs. The proposed cruise ship energy system includes PEMFC to supply auxiliary electrical power, WHR solutions as high temperature heat pumps and absorption chillers to recover PEMFC heat, and internal combustion engines for the mechanical power. The SDO of the cruise ship’s energy system is optimized to match the mechanical, electrical, heating, and cooling power demand while minimizing the ship’s fuel oil consumption and the investment and operation costs. The results show that the multi-objective optimization of the hybrid PEMFC/LIB power plant of the ferry can effectively improve the performance of the energy system over time, ensuring an effective operation in terms of costs and efficiency, avoiding stressful events that would decrease the overall plant lifetime. The daily operation cost and H2 consumption increase over time, affecting the overall volume and weight of the plant. As for the cruise ship, the results show that the optimal plant configuration allows to cut the vessel’s marine diesel oil consumption by about 53% with respect to the current ship energy system, highlighting the need to substitute also the main propulsion engines with PEMFC for achieving higher decarbonization rates.