Liquid Ammonia Fuel Storage Alternatives and Minimizing Integration Impact on Ship Design

Abstract

Modern developments in both ammonia internal combustion engines and fuel cells have created the possibility of using ammonia as a carbon-free fuel for propulsion onboard ships. Coupled with these developments has been the recent funding for projects to develop ammonia crackers which offer on-site hydrogen production, enabling the use of a fuel with a high mass energy density without sacrificing large volumes onboard. In this study the requirements for storing ammonia as a fuel are investigated with application to a small passenger ferry. Ammonia, known for its toxicity, needs special containment arrangements and ventilation systems to safely store the substance on manned platforms. These systems affect the space available for ammonia storage in a fixed boundary system and as a consequence may require major changes on a ship to maintain operation specifications. Most scientific research to date has focused on feasibility cases of using ammonia fuel on large ships primarily where ammonia is assumed available from cargo tanks. Storage arrangements, tank types and methods of storage on board have yet to be investigated in detail. In this work, a case study focused on retrofitting a fully electric city ferry to use ammonia as the energy vector instead of lithium-ion batteries. The work offers insight into safety requirements that should be considered, and a detailed comparison of key performance indicators (KPIs) based on tank type and corresponding storage method. KPIs of greatest importance were the mass of fuel that could be stored, the cost of implementing the storage method and the additional space available for extra cargo. Using a multi-criteria analysis seventeen alternatives were analyzed to find that for the case under study an arrangement of three horizontal cylinders would provide the most well-suited alternative primarily due to lower cost. For larger ships the use of lattice pressure vessels is highly recommended as an ammonia fuel tank as these tanks reduce sloshing, allow pressurized storage and can be built using relatively cost-effective methods.