An energy management system for ships with electric propulsion and a modular hybrid power supply

Abstract

To reduce the greenhouse gas emissions of ships, hybrid power systems are becoming more common. Hybrid systems combine power generation with energy storage. The energy storage enables the power generating components to run at more efficient operating points. A well known combination is that of a diesel genset and a battery. The ideal combination of the type, size, and amount of power system components depends highly on the operating profile of the vessel. However, this operating profile can vary greatly between voyages, or during the lifetime of a vessel, thus changing the ‘ideal’ power plant. A modular power system (MPS) can provide the solution: a reconfigurable power plant, where components can be added, removed, or replaced.
To control the power-split between the different components an energy management system (EMS) is required. Most EMSs are designed and optimized for a single power plant configuration. This means they are not capable of dealing with an MPS. For this thesis an EMS was developed which is capable of dealing with an MPS: an MPS EMS. The developed EMS was made for ships with electric propulsion. An additional requirement was for the EMS to be real-time capable, so it can be used outside of a simulated environment, i.e. in a real ship. The objective of the EMS would be to minimize fuel consumption.
It was investigated which EMS control strategy would be suited for an MPS EMS. The equivalent consumption minimization strategy (ECMS) combined with the dual decomposition method was found to be suited for this purpose.
The developed EMS can automatically adapt all parameters responsible for stable control of the system. It does this based on the properties of the installed components. Most important of these properties are: minimum and maximum power output, maximum ramp-rate, and the efficiency curve.
The EMS was tested with four different combinations of installed components, and two different operating profiles. Additionally, the effect of a component failure during a voyage was tested. A rule based (RB) EMS and a mixed integer linear programming (MILP) global optimization were used as benchmarks for the fuel consumption. The results show that the developed EMS is capable of controlling various power plant configurations in different conditions, while keeping all components within their allowed operating ranges. For one of the tested operating profiles the fuel consumption using ECMS was 1.9-4.0% higher than when using the global optimization, which is comparable to the results found in literature. However, for the other tested operating profile the developed EMS was outperformed by even the RB EMS, by 1.1-2.1%. This was caused by inaccuracies of the approximation used for the efficiency curve of the gensets. In the simulations where one of the gensets fails during the voyage the EMS was able to automatically adapt to optimize fuel consumption using only the remaining components. Fuel consumption did increase slightly compared to no failures, as expected.