With the increasing concerns of the emission of greenhouse gases and other pollutants and the push towards sustainable and greener means of transportation, there is a need for new drive systems running on alternative fuels. One fuel that holds significant potential as a marine fuel for the future is methanol. When produced utilising carbon capture methods and green energy, it has the potential to be a net zero fuel. Among other high-potential future fuels, methanol has the added benefit of being liquid at room temperature. Additionally, methanol has the potential to be utilised in novel drive systems, such as the combination of a solid oxide fuel cell (SOFC) and a reciprocating internal combustion engine (ICE). This concept utilises the high efficiency and negligible NOx formation of the SOFC, while the ICE provides dynamic load capabilities. This study concentrates on the electrical efficiency of this type of plant for maritime applications.
This work presents an in MATLAB & Simulink constructed first principles based model of a methanol fuelled SOFC-ICE combined cycle. The zero-dimensional SOFC model consists of a temperature controlled methanator which maintains an external reforming ratio of 0.5; a cell mass balance; a cell energy balance, and an electrochemical model. The ICE model core consists of a turbocharged five-state Seiliger cycle. It simulates Wärtsilä 12V31DF fuelled with methanol and dehydrated hydrogen-rich anode of gas (AOG). The SOFC efficiency and its separate losses are evaluated for different temperatures, current densities, fuel utilisation factors UF and steam-to-fuel ratio’s in combined cycle operations. Additionally, the standalone SOFC performance, without the use of waste heat from the ICE and disuse of residual fuel in the AOG, is evaluated, but only for the nominal condition. The ICE efficiency and losses are evaluated for a power range between 1% and 100%. By varying the ammount of cells, the following power splits PSOFC/PICE have been evaluated: 0/100 25/75; 50/50; 75/25 and 100/0. The combined cycle performance is evaluated for different temperatures and current densities.
This study found that while varying the steam-to-fuel ratio and fuel utilisation factor (UF) have minimal impact on the electrical efficiency of the SOFC in combined cycle operations, temperature and current density have a significant effect on the efficiency of the SOFC. For a steam-to-fuel ratio of 1:1, a UF of 0.8, a current density of 5000 A · m−2 and a mean cell temperature of 1073K an efficiency of 58.6% was obtained. The standalone SOFC, or 100/0 power split, obtained an efficiency of 48.4%. The stand-alone ICE genset, or 0/100 power split, operates at a nominal efficiency of 42.3%. When the ICE is used in a direct drive configuration, it corresponds to an efficiency of 43.6%. The combined cycle obtained efficiencies of 45.4%, 49.1% and 53.4% for 25/75, 50/50 and 75/25 power splits, respectively. These results are compared to the results of a similar study investigating an ammonia-fuelled SOFCICE combined cycle for maritime applications, which reported efficiencies of 47%, 50% and 52% for 25/75, 50/50 and 75/25 power splits, respectively, under similar operating conditions.
The efficiency gain of the methanol-fuelled 75/25 configuration compared to the direct drive is limited. This raises the questions about whether the added complexity of introducing an SOFC is justified for the limited efficiency gain. The highest efficiency was obtained with the methanol-fuelled 75/25 power split, but due to the large proportion of SOFC power, it is less tolerant to dynamics in the load, making it questionable whether it can fully meet the dynamic power demand of a ship. Therefore, the 50/50 power split configuration is expected to be the most viable option in terms of both technological feasibility and efficiency gain. A change in the power split to 100/0 results in a decrease in system efficiency due to the lack of waste heat from the ICE and the inability to utilise residual fuel in the exhaust. When considering efficiency, the values for the ammonia fuelled and methanol fuelled plant are similar.
The 50/50 powersplit configuration is 5.6 percentpoints more efficient than the methanol fuelled direct drive ICE and 1.2 percentpoints more efficient than the methanol-fuelled standalone SOFC. This clearly shows the synergistic benefits of combining a methanol-fuelled SOFC with an ICE. However, when compared to the ammonia fuelled combined cycle with 50/50 power split, it is 0.9 percentpoints less efficient. Nevertheless, it is important to exercise caution when drawing further conclusions from this last figure as the model has been constructed at a system level and no thorough uncertainty analysis has been conducted. Furthermore, the requirements regarding the power and energy density are strongly dependent on the type of ship and its operational profile. Therefore, future research should include the implementation of dynamic load capabilities in the model to evaluate its technological feasibility and overall net efficiency gain in various operational profiles of ships. Also, further research is required on the methanol-fuelled ICE cylinder process and the heat integration of the combined cycle.