Design and off-design models of single and two-stage ORC systems on board a LNG carrier for the search of the optimal performance and control strategy

Abstract

A diesel-electric propulsion system is generally used in large scale ships to allow a free placement of Internal Combustion Engines (ICEs), to acoustically decouple engines and hull, and reduce total weight and volume. On long voyages, the speed of a vessel can be more or less constant. Thus, to a first approximation, it can be considered that the engines operate most of the time at steady-state conditions. On this basis, Organic Rankine Cycle (ORC) systems can be conveniently installed aboard to generate additional electric power by recovering ICEs waste heat and increase in turn the overall system efficiency. The ICEs-ORC combined cycle system has to be designed properly to maximize the work production and guarantee at the same time a stable operation during both transient and steady-state working conditions.This paper presents the design and off-design dynamic and steady-state models of three ORC system configurations that exploit the waste heat of four dual fuel diesel ICEs on board a LNG carrier. One single-stage and two (subcritical or supercritical) two-stage ORC system configurations are considered. The available heat is taken from charge air, jacket and oil cooling systems, while the exhaust gases are not considered to be available being already used for other ship internal needs. Dynamic simulations are employed to define the control strategies that lead to stable operation of all the considered ORC system configurations after transients of the ship operation. Steady-state off-design models are then used to identify the best system design point taking into account the real ship operation during a reference year. Results show that the best performing single-stage ORC is designed for a ship speed of 16.5 knots on loaded voyage, and has an annual work production of 1665.8MWh with a peak thermal efficiency of 6.5%. The supercritical configuration reaches the best performance among the two-stage ORC systems, achieving a work production of 2306.6MWh and a thermal efficiency of 12.6% at 15.5 knots.