Maritime application of sodium borohydride as an energy carrier

Abstract

The interest in alternative fuels in the maritime sector is growing due to increasing concerns of the effect of harmful emissions. Hydrogen in combination with a fuel cell has a high potential to be a more environmental friendly solution than the commonly used fossil fuel systems. However, the use of hydrogen encounters problems looking at the fuel storage density, safety and the operating profile of a vessel. Hydrogen storage in a material, such as sodium borohydride, has the potential to overcome or mitigate these problems.
The goal of this research is to provide an insight in the technical feasibility of this technology for the maritime industry. Therefore, the following research question will be answered: How does a technical design of a power train using sodium borohydride as hydrogen storage material on-board of vessels perform in terms of power and energy density and in a transient load? There are three important elements in this system: the sodium borohydride, water and the spent-fuel. The hydrogen in sodium borohydride can be released in a reaction with water, resulting in a hydrogen gas and liquid spent-fuel. The energy and power density is determined using the weights and volumes of these separate elements. Both the least, and most compact configurations are considered to determine the operating range of this technology. A model is made and measurements are done on the power demand of the `Stad Amsterdam' to investigate the performance of the system on dynamic behaviour. The most slow components, such as the mix-chamber, fuel cell and the battery system, are dynamically modelled to identify the critical elements in the design.
In the first configuration, all fuels are stored in separate tanks and all the water needed for the process is stored on-board. This results in a low volumetric energy and power density. To improve the density of the system multiple concepts are considered: a concentrated or dry fuel, a volume-exchange tank, on-board water generation and a filtered spent-fuel. Implementing these concepts results in a compact configuration which is more competitive with diesel systems and other alternative fuels. The hydrogen buffer in the mix chamber, the fuel cell and the battery systems are the most important elements influencing the performance of the system in a transient load. A more constant behaviour can be expected by making use of multiple mix chambers. The safety on-board of the vessel still needs attention, especially around the places containing hydrogen gas and where hydrochloric acid is present. It can be concluded that this technology has a high potential for many types of applications in the maritime industry. It will be technically feasible to implement such a system on-board of a vessel. The harmful emissions in this industry can be decreased significantly using this system. Cost and safety of the system are a challenge and, therefore, further research has to be done to realise the implementation of these systems on-board of vessels.