Energy and power density as key metrics for assessing hydrogen carriers as alternative fuels in maritime applications

Abstract

Alternative fuels are essential to decarbonising maritime transport, provided they retain high energy and power density; hydrogen carriers fulfil these criteria and represent a promising fuel option. This research quantifies the power-energy trade-offs of four hydrogen carrier fuels: namely, sodium borohydride, ammonia borane, n-ethylcarbazole, and dibenzyltoluene. Each hydrogen carrier is combined with four energy converters: proton exchange membrane fuel cell (PEMFC), internal combustion engine (ICE), solid oxide fuel cell (SOFC), and gas turbine, respectively, resulting in 16 outcomes, which were assessed using Ragone plots. For each hydrogen carrier, one energy converter is selected as a case study. Four reference cases are also examined: marine diesel oil (MDO) and methanol combined with ICE, ammonia with SOFC, and liquid hydrogen with PEMFC. These 8 case studies (carrier and converter combinations) are evaluated for performance across several shipping applications. The results indicate that ammonia borane combined with ICE and sodium borohydride combined with a PEMFC offer energy and power density profiles suitable for various vessel types, including inland ships and ocean-going vessels. These combinations also show potential for specialised vessels such as fishing boats, dredgers, tugs, and ferries, which operate under unique load profiles and duty cycles. Compared with alternative fuels, such as ammonia, methanol, and liquid hydrogen, hydrogen carriers can demonstrate higher energy and power densities. Under specific circumstances, they may achieve performance comparable to MDO, highlighting their potential for zero-emission maritime propulsion.