Innovative waste heat valorisation technologies for zero-carbon ships − A review

The growing intensity of international commerce and the high share of total global greenhouse gas emissions by the maritime sector have motivated the implementation of regulations by the International Maritime Organisation to curtail vessel emissions. In this context, waste heat recovery (WHR) is an effective way to improve ship energy efficiency, lower amounts of wasted energy rejection to the environment, and therefore ultimately curb green-house gas emissions. Presently, there exists a heterogeneity within the body of literature concerning WHR technologies for on-board applications, study scope and results, complicating the interpretation and cross comparison of the outcomes. Sporadic attempts have been made to review and systematise this landscape, leaving some key areas uncovered. Therefore, the present article aims at filling these gaps by providing and holistic review of WHR technologies development and on-board applications. Further, the energy systems and available waste heat characteristics in large vessel types are overviewed, before both existing and developmental on-board waste heat recovery technologies for maritime applications are reviewed. Emphasis is placed on the performance of these technologies within the broader on-board energy system. Common key performance indicators are drawn from existing systems, experimental prototypes, and simulations, to quantitatively compare the different technologies. This review indicates that a wide range of technological options for embedding waste heat recovery in on-board energy systems are emerging. In particular, traditional turbocompounding is already fully implemented within the marine waste heat recovery (WHR) context. Conversely, ORC systems and absorption refrigeration systems have not yet been suitably adapted for marine applications due to a lack of research and prototypes, despite their deployment in conventional WHR contexts. Other technologies, such as thermal energy storage devices, hybrid refrigeration systems, isobaric expansion engines, Kalina Cycles, and adsorption desalination and cooling systems, are still at the research and development stage, while thermo-electric generation systems continue to incur high deployment costs. The development of research on these innovative technologies, the reduction of their cost and their synergistic integration could lead to significant improvements inengine fuel efficiency and emissions reduction, especially when coupled with existing waste heat recovery measures.