Integrazione di un ciclo a fluido organico in un motore a gas per propulsione navale

The topic of this paper lies on the development of high efficiency and low emissions internal combustion engine (ICE) for electric ship propulsion. The trend in this topic has yielded in recent years to very efficient dual fuel internal combustion engines, using Liquefied Natural Gas (LNG) as fuel, in addition to fuel oil. In the paper the 4 stroke, medium speed, Wärtsilä 6L50DF is regarded as the reference engine. To further improve the electrical efficiency of this kind of engine, heat recovered from exhaust gases and from water cooling circuits can be used as additional energy source. In the study the thermal-to-electrical energy conversion is considered to be realized by means of the ORC (Organic Rankine Cycle) technology, which is generally regarded as a quite effective heat recovery solution, from both thermodynamic and economic points of view. The expectation is that a “combined cycle” (ICE-ORC) of these kind and size could reach a power increase of about 10%, with respect to the simple diesel engine, making available in addition a huge amount of low grade thermal energy, that could be consumed by thermal users in both fixed and on ship board installations. In this study four different possible configurations are considered for the “bottom” ORC group: • simple cycle with heat recovery on engine exhaust gases only; • simple cycle with heat recovery on engine exhaust gases and pre-heating with the high temperature water cooling circuit; • regenerated cycle with heat recovery on engine exhaust gases only; • combined Rankine cycles with two pressure level. Four working fluids (benzene, toluene, cyclohexane and MDM) are first taken into account, focusing, in the following, on toluene Rankine cycles. All simulations are developed by means of modular simulation software (Aspen® has been used in particular), highlighting both energetic (efficiency) and economic (Pay Back) performance of most promising solutions.