Hybrid Electric Power and Propulsion

Research topic

Throughout the last decade, the focus on sustainability and reducing the environmental footprint has changed- and is still shaping the maritime industry. This transformation is driven by the motivation to combat climate change by reducing harmful greenhouse-gas (GHG) emissions. This has led to the development of hybrid power- and propulsion systems. As a result, modern vessels combine energy storage systems (ESS) with internal combustion engines (ICE) and fuel-cells (FC) as well as wind-assisted propulsion. The mix of energy sources can serve different purposes depending on the operation to minimize fuel-consumption, emissions, and down-time. In addition, all-electric ships (AES) are becoming more common as the quality and lifetime of energy storage systems are being improved. Considering the new era of autonomy and digitalization, all-electric- and hybrid power and propulsion systems will play an integral part to enable reliable, robust, and dependable autonomous vessels. As a result, onboard power systems are becoming an essential part of ships of the future. 
 
The integration of the various energy sources is made possible by power electronic converters (PEC) to interface the onboard distribution and propulsion systems. Converter dominated shipboard power systems are complex systems with a high degree of flexibility. This leads to a situation where the dynamics of the power systems are determined by the structure of the low-level control systems. The low-level control layer is responsible for the behavioral dynamics, droop and current control. To elaborate further, the power converters are often off-the-shelf based, designed to operate in both terrestrial- and marine power systems with a stable predefined low-level behavior. It has been well documented that instabilities and oscillations occur when these power converters are interfaced to a common DC-bus due to their high-bandwidth controllers. This leads to an unintentional phenomenon known as negative incremental impedance, which could have a deteriorating effect on power system stability. Furthermore, the low-level controllers can also cause inter-harmonics when they interact with several power converters- and subjected to mechanical oscillations propagating through the motor drives and into the power system. A solution to this problem could be to preserve stability structurally through the low-level control to enable the flexibility and connectivity of the different power sources in hybrid power and propulsion systems.

Problem formulation

Onboard power systems are becoming increasingly complex. This complexity introduces several new challenges that are not addressed in today’s standards- and practice. Therefore, there is a need to develop more sophisticated methods and control systems to ensure the secure and reliable operation of the different components. This requires an overall system-based approach, with emphasis on the behavioural dynamics of the components and the energy efficient operational philosophy. This means building on the existing knowledge of vessel operation, combined with emerging concepts and a power system perspective. As a result, this will provide the technology and control-methods that enable flexible plug-and-play integration of energy sources in vessels with hybrid power systems. 

Research questions

1. By considering the variety of energy sources and the advanced propulsion units: how can the undesirable interaction between the power converters be mitigated to ensure stable, reliable, and the robust operation of future hybrid power systems?  
2. Based on the lack of standards regarding transient performance and inter-harmonics for hybrid power systems, how should a concrete design framework and guidelines look like? 
3. How should hybrid power systems be designed and controlled to ensure reliable operation for fully autonomous low-emission vessels?

Expected outcomes and industrial goals

1. A mathematical model of the hybrid power system needed for dynamic analysis and control system derivation. 
2. A new low-level control structure that accounts for the complex behaviour of power electronic converters and ensures operational stability over a wide range of operating conditions. 
3. A design framework and suggestive guidelines for the transient behaviour of power electronic dominated hybrid power systems, by mathematical models and advanced stability analysis

Achievements 2021

• A hybrid power system model consisting of a variable speed genset, a permanent magnet propulsion system, and an energy storage system is developed based on the port-Hamiltonian framework. The models have been used to study the dynamic interactions between the different components to identify root causes for instability. 
• Webinar presentation: Zadeh, Mehdi; Yum, Kevin Koosup; Hatlehol, Marius Ulla. Hybrid Power Systems. SFI Smart Maritime Webinar; 2021-03-09
• Webinar presentation: "Regenerative power / potential for heat recovery / hybrid cycle". Zadeh, Mehdi; Hatlehol, Marius Ulla; Gabrielii, Cecilia H. Energy Efficiency Onboard. SFI Smart Maritime Webinar; 2021-06-22
  
• The paper “Super-Twisting Algorithm Second-Order Sliding Mode Control of a Bidirectional DC-to-DC Converter Supplying a Constant Power Load”, was submitted for the IFAC CAMS 2022 Conference.
  

• The paper “Super-Twisting Algorithm Second-Order Sliding Mode Control of a Bidirectional DC-to-DC Converter Supplying a Constant Power Load” was presented at the IFAC CAMS 2022 Conference in Denmark and later published.
• The work concerning the first journal paper started in late 2022 and is still in development. In this work, a bifurcation analysis will be used to identify the unstable dynamics associated with the different components of the power system.