Evaluation of the prospects for waste heat recovery on board liquefied natural gas-fuelled ships

Abstract

Several solutions can lead to a reduction of the emissions from shipping, one of them being the use of cleaner fuels for propulsion. In this context, liquefied natural gas is an attractive candidate, because it could represent a bridge between heavy fuel oils and future biomass-based fuels. On the other hand, the carbon dioxide emissions of a ships can be reduced by up to 20 % by implementing waste heat recovery units harvesting the heat released by the ship’s engine system and converting it into heating, cooling or electricity. The objective of this thesis is to analyze the synergies emerging when combining these two solutions on a single vessel, namely the use of liquefied natural gas as a fuel, and the implementation of waste heat recovery units. The focus lies on the investigation of the prospects for recovering the low temperature heat released by the liquefied natural gas fuel during its preheating phase, before injection to the engine; and on the development of methods to assess the potential for the onboard installation of waste heat recovery units based on the organic Rankine cycle technology. Lastly, this thesis describes a novel concept that is suitable to provide zero-emission power on board cruise ships during harbor stays. The findings of this thesis indicate that fuel savings in the range from 0.3 % to 2.4 % the main engine consumption can be attained by recovering the liquefied natural gas cold energy, and that the highest potential is obtained when installing an organic Rankine cycle. Considering the optimal design of organic Rankine cycle units for marine applications, the results indicate that the introduction of the Tier III  legislation to constraint the nitrogen oxides emissions leads to increased prospects for waste heat recovery. In particular, waste heat recovery units installed on board ships featuring an exhaust recirculation system are estimated to result in energy productions up to 7.4 % the main engine energy production. A novel method to design organic Rankine cycles was developed. The use of the proposed method in a case study indicated that designing the organic Rankine cycle unit by accounting also for the impact of the additional backpressure supplied to the engine can result in an increase of the attainable fuel savings in the range from 0.52 g/kWh to 1.45 g/kWh compared to the traditional design approaches. A simplified approach based on the use of regression models was also developed. The use of the proposed simplified approach is shown to have an average deviation of 4.5 % and 2.5 % in comparison with the thermodynamic models for the estimation of the unit annual energy production and levelized cost of electricity, respectively. With respect to the possibility to produce emission-free power during harbor stays, the proposed concept is proven to be technically feasible and to be more cost-effective than the installation of lithium batteries when considering life-times over 15 years. This work lays the foundation for future research works in the area, and supports industry by providing a method to optimally design of organic Rankine cycle units tailored for maritime applications.