Gradient Utilisation of Engine Waste Heat and Lng Cold Energies in a Large Dual-Fuel Power Ship Using Group Sequential Optimisation Strategy

Abstract

Owing to its abundant reserves, low carbon and its characteristics of high efficiency and clean combustion, liquefied natural gas (LNG) has been regarded as a more feasible fuel of ship engines to meet the increasingly stringent toxic and CO2 emissions regulations proposed by international maritime organization (IMO). Although the thermal efficiency of dual-fuel ship engines fueled with diesel and LNG can be as high as 50% (it is highest in all thermal power plants), nearly half of the fuel energy is lost through exhaust, liner heat transfer or thermal radiation of the engines if these energy is not recycled and reutilized, meanwhile before LNG enters the engine cylinder to do work, it has to be heated to the ambient temperature for avoiding its low-temperature injury to engines, this means that using the thermal energy from engines to heat LNG will leads to the double loss of engine thermal energy and LNG cold energy. To achieve the recycle and highly efficient utilisation of engine waste heat and LNG cold energies of large dual-fuel power ships, this paper proposes a state-of-the-art topology architecture of integrated energy recovery system consisting of a high temperature organic Rankine cycle (HT-ORC) with a 2-stage heated and 2-stage heat release system coupling with a low temperature organic Rankine cycle (LT-ORC), low temperature storage (LTS), freeze desalination (FD) and air conditioning (AC). A thermo-economic analysis model of the system is established, the design parameters of the system are matched and optimised using a dimensional reduction method based on group sequential optimisation, and a multi-objective optimisation strategy is used to improve the integrated energy efficiency and cost. The gradient utilisation of engine waste heat and LNG cold energy of a large dual-fuel power ship was investigated under wide engine load conditions. The results show that the topology architecture of the 2-stage heated and 2-stage heat release ORC systems is beneficial to reduce the average heat transfer temperature difference and the exergy loss during the heat transfer process, and therefore to improve the integrated energy recycle efficiency. For the large ship with a dual fuel engine of 25800 kW, our integrated energy recovery system can achieve a power recycle of 1397.31 kW at the rated power point, and the maximum relative efficiency improvement of 11.73% occurs at a lower ship load of 35%, and a payback period (PBP) of this system is about 0.55 years. The research not only provides a topology architecture of ship cold and heat energy recovery system, but also offers new strategies or paths for optimising the integrated energy efficiency of the system.

Keywords: LNG cold energy, Waste heat recovery, Marine dual-fuel engine, Multi-objective optimisation