An investigation into the possibilities concerning Solid Oxide Fuel Cells in naval vessels

Abstract

The Royal Netherlands Navy (RNLN) seeks to eliminate net green house gasses and other harmful- emissions. In this search, the operability of her vessels need to be secured. One of the options for eliminating net carbon emissions is Power to Liquid (PtL) conversion to Methanol. Fuel cells, and in particular Solid Oxide Fuel Cells (SOFCs) may be useful tools to power naval vessels efficiently and without the production of pollutants. The dynamic behaviour of these systems however is where a challenge is found. Contrary to methane fuelled SOFCs, little research has been done on the behaviour of methanol fuelled SOFCs in the higher temperature range. It was decided that for this thesis the SOFC system would be pressurised to 20 bar to increase power density, a pre-reformer would be incorporated to improve dynamic behaviour, a Plate Heat Exchanger (PHE) would be used to re-integrate heat from the cathodic outflow to the cathodic inflow, and Anode Off Gas Re-Cycling (AOGRC) would be used to provide steam and heat to the pre-reformer. A model was built that incorporates the following sub models: a pre-reformer that includes reaction kinetics, a dynamic SOFC stack model which incorporates heat losses normal to the cell orientation, and a \textcolor{cyan}{PHE} which includes its dynamic behaviours. These models are interconnected and controlled such, AOGRC is mimicked. These models were verified by checking energy- and mass-balances. The eventual chemical equilibrium of the (kinetics based) model of the pre-reformer was used to validate the pre-reformer model. The leading research by (Aguiar et al.) was used to validate the SOFC model. Typical thermal behaviours of PHEs were used to validate the PHE model. Simulation results show that the pre-reformer could improve the system dynamics by providing a pre heated, hydrogen rich gas mixture to the SOFC. Running it at a temperature of higher than 530 K can pose danger due to more prominent exothermic, methane producing, reactions in the reactor vessel. Results from testing the SOFC show that the SOFC is best operated at an inflow temperature of around 800K for maximum efficiency (including outflow enthalpy). It shows that the SOFC is barely influenced by higher partial pressures of the reactants, and has slow load following characteristic, especially for low terminal current densities. The testing results for the PHE show that the this subsystem achieves higher heater effectiveness when more conductor plate area is used, but at the cost of dynamic response. It also shows that the PHE is the most important factor in the poor load following capabilities of the entire system. This is largly due to the limited convective heat coefficient. AOGRC has been shown to be an effective method to control the temperature of the pre-reformer, low AOGRC ratios make for large preventable energy losses. The interconnected methanol fuelled SOFC system simulation shows that external heating of the ingoing anode flow is detrimental to the efficiency of the system, but it could be instrumental in preventing thermal stresses on the Positive-Electrolyte-Negative (PEN) structure of the cell. Increasing the fuel and airflow excess is also detrimental to the system efficiency. These values are best kept at 1.1 due to uncertainty around for the models` accuracy for very low fuel- and air- flow excess. The settling time for the power output is in the order of several hundreds of seconds, and is largest for large sudden changes in power demands, and low terminal power demands. It was concluded from these results that the here proposed system is not suitable to power a naval vessel of the Snellius class in a satisfying way. The investigation however, has produced valuable tools for future work. Recommendations for future work are to implement condenser/heat exchanger for the anode anode off gas, implement a burner to make use of the remaining chemical energy in the anode off gas and install a turbine to make use of potential energy in the outflowing gasses.