Waste heat recovery of future marine propulsion systems

Abstract

In response to climate change, the marine sector is increasingly focused on the energy transition. New marine power plants operating more efficiently and on cleaner fuels are receiving significantly more attention. While the transition to different power plants and fuels results in reduced fossil fuel consumption and emissions, waste heat recovery systems can aid in obtaining both as well. Current as well as future marine power plants do not convert all of the energy contained in the fuel into useful power, with significant amounts of energy being wasted as heat. Waste heat recovery technologies can be applied to recover some of this energy and generate additional power, effectively boosting total system efficiency, with the resulting benefits of improved fuel economy and reduced specific emissions.
This study first provides an extensive summary of all the different marine power plants, fuels, and waste heat recovery technologies, of both the present and future. One type of power plant that could be an advantageous alternative for future marine propulsion is the solid oxide fuel cell (SOFC) due to its high efficiency and potential to run on clean fuels. SOFCs generate electricity from chemical energy using a high-temperature electrochemical process, resulting in high-temperature waste heat being expelled. Therefore, the potential for a waste heat recovery technology to boost system efficiency is high, and it is chosen to investigate several power cycles for the waste heat recovery of a case study vessel powered by a 2 MW SOFC system. The aim of this study is to develop and execute an approach to evaluate these waste heat recovery technologies regarding their efficiency, size, and associated cost.
While numerous power cycles for waste heat recovery are in existence, a selection of potentially suitable systems with a number of different setups is made to investigate further. Thermodynamic models are created for the selected power cycles to determine and compare their theoretical efficiencies. Subsequently, the size of the heat exchangers are calculated for the evaluation of system size, considering the size of other components such as turbomachinery as well. Two types of heat exchangers are considered in this study: the compact and innovative printed circuit heat exchanger (PCHE) and the classic but commonly large shell and tube heat exchanger (STHE). Finally, the investigated systems are subjected to an economic analysis based on the cost associated with the main components, with again considerations being made regarding excluded additional components.
The results indicate that various configurations of the (transcritical) Rankine cycle operating on steam and CO2, as well as the (supercritical) Brayton cycle operating on CO2 and air, present with significant theoretical efficiencies ranging from 41 to 52% and electrical power outputs ranging from approximately 530 kWe to over 670 kWe. From the evaluation of the system size it is concluded that the smallest systems are those operated on CO2 equipped with PCHEs, while the largest are the air Brayton cycles equipped with STHEs. The economic analysis revealed that the systems with the lowest costs are the configurations of the (transcritical) Rankine cycle operating on steam, as well as certain air Brayton cycles equipped with PCHEs. The systems with the highest cost are found to be the air Brayton cycles equipped with STHEs, due to the significant sizes of the required heat
exchangers. In general, it is concluded that no system outperforms the others simultaneously across all three investigated aspects of efficiency, size, and cost, and trade-offs will be required when selecting a waste heat recovery technology for the presented case study vessel. Nonetheless, a detailed process-oriented approach has been developed and executed to allow various waste heat recovery solutions to be compared, and it is proven that a significant amount of power can be produced from recovered waste heat. The results from this study can be directly consulted by ship owners and designers considering the application of waste heat recovery to an SOFC powered vessel. Furthermore, the developed approach can also be applied to waste heat recovery in other industries and power generation systems, as it provides a step-by-step guide on relevant calculations and considerations.