Figure 6. Experimental results of improved rule-based strategy
Figure 7. Experimental results of proposed ECMS strategy
Table 2. Comparison of fuel consumption over three segments
Dealing with emissions – managing the trade-off between NOx emission and fuel consumption
A minimization of fuel consumption will lead to a reduction in emission only if the emission and fuel consumption are proportional. However, among the major composition of ship emissions (CO2, NOx, SO2, HC, CO, PM), a possible trade-off between NOx emission and fuel consumption has been highlighted. An example is the specific NOx emission (SNOX) and SFOC curve as shown in Figure 8 , where it can be observed that there is an increase in NOx emission when fuel consumption is low in certain operating range. Although there are existing technologies in the market such as exhaust gas recirculation (EGR) and selective catalytic reduction (SCR) to reduce NOx emissions, such devices may have their constraints such as requiring addition operating power or space limitations on smaller vessels. Therefore, the possibility to optimize NOx emission reduction and fuel consumption during engine operation, through the power management control approach, is investigated.
Figure 8: SFOC and SNOX of PERKINS 2506C
With the validated performance of the proposed ECMS, the optimization strategy is extended to further consider the minimization of ship emission. To manage the increase in fuel consumption due to the effects of NOx emissions, a method is proposed to include a control weighting KNOX that defines the amount of priority given to NOx emission reduction. The proposed cost function formulation is expressed as:
where Ctotal,eqv represents the total equivalent fuel consumption defined in Equation (1)-(3), and Etotal,eqv is the total equivalent NOx emission that can be obtained by replacing the SFOC terms in Equation (1)-(3) to the engine’s SNOX values. The value of KNOX is in the range of 0 to 1, where a value of 0 omits the NOx emission and considers only fuel consumption in the optimization problem, while a value of 1 considers only NOx emission. In this way, the ship operators are given the flexibility to prioritize between fuel efficient and emission efficient operation depending on the operation needs. For example, when a vessel is operating near the harbor or in ECAs where the limit for NOx emission is more stringent, a pure NOx emission reduction control can be adopted. In cases when there are lesser requirements on the emission reduction, fuel consumption can be improved by adjusting the KNOX to a lower value. The effectiveness of the proposed method in managing the trade-off between fuel consumption and NOx emission is demonstrated through simulations over the harbor tug operations shown in Figure 5.
Performance of the proposed approach is investigated over the range of KNOX at regular intervals of 0 to 1. The results for KNOX at 0 and 0.6 are presented in Figure 9(a) and (b) respectively to show the effects of KNOX. In Figure 9(a) where NOx emission is not considered (KNOX = 0), it is observed that the engine operation is maximize around 100 percent loading, which is the most fuel-efficient point as observed from the SFOC curve in Figure 8. As weighting factor increases, the emphasis to reduce NOx emission increases. Therefore, it is seen in Figure 9 (b) that there is a shift in the engine operating point to around 40 percent loading when switched on, in attempt to maximize the engine operation around the lowest point of the SNOX curve in Figure 8 to reduce NOx emission.
The average specific fuel consumption and NOx
emission over the range of KNOX
at regular intervals of 0 to 1 for one operation cycle is shown in Figure 10. The increase in KNOX
causes a shift in the engine operating point towards the emission efficient point that brings a reduction in NOx
emission, while compromising on the fuel efficiency. Overall, the proposed approach performs as expected where the increase in the weighting factor KNOX
reflects a reduction in NOx
emission. Using this method, the emphasis on NOx
emission reduction can be controlled by adjusting the weighting factor, hence managing the compromise on the fuel efficiencies
Figure 9. Simulation results of proposed approach at (a) KNOX = 0 and (b) KNOX = 0.6
Figure 10. Effects of KNOX values on NOx emission and fuel consumption
While technologies are rapidly evolving for new system configurations and alternative power sources to achieve more energy efficient operations, advanced control technologies are necessary to achieve the desired benefits of these systems. The proposed strategies in this work have shown the potential of optimization-based power management control strategies for advanced marine power systems to improve fuel efficiency and reduce emission. The proposed strategies are not limited to a hybrid electric vessel and can be easily extended for different combination of power sources, as well as different applications such as offshore rigs. Moving forward, with digitalization bringing greater connectivity and faster communication on ships, there are greater opportunities to be explored in optimization-based methods to improve control strategies, enabling ships to operate more efficiently and environmentally friendly.
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