Waste Heat Recovery systems from marine diesel engines: Comparison between new design and retrofitting solutions

Abstract

Waste Heat Recovery (WHR) systems are more and more common on board new commercial ships, and their good efficiency now is to be considered also for retrofitting solutions. The paper describes the performance and cost analysis of a WHR system, based on an auxiliary steam turbine for electric power generation, supplied by the steam plant adopted for the waste heat recovery from the exhaust gas of the main diesel engines. The application concerns a passenger ship, powered by six diesel generators to drive the electric propulsion motors, to feed the several auxiliary systems and to fulfill the whole hotel service demand. The analysis is carried out on the basis of a comparison between new design and retrofitting solutions for the same application, considering that a complete optimization design process is really possible only for an “ex novo” ship project. In fact, the present paper considers the minimum retrofitting intervention (mere addition of turbo-generator and vacuum condenser) where the optimization process can be reasonably restricted to the steam turbine sizing, while for a new ship it can be extended to the selection of the whole machinery. The different results are based on an optimization numerical code, developed by the authors in Matlab software environment, and they are analyzed in terms of performance and cost, in order to provide helpful guidelines to assist ship-owners and shipbuilders in making their decisions. With regard to these particular systems, for instance MAN suggests the following options, depending on ship power: – under 15 MW, it is proper to introduce only a single gas power turbine to avoid the expensive and heavy installation of the “steam-cycle”; – from 15 MW to 25 MW, the system may be characterized by a single gas turbine or a single steam turbine; – for engines power superior to 25 MW, it is worthy adopting new generation WHR systems equipped with gas and steam turbines. As for the last option, the use of both gas and steam turbines is suggested mainly for the ships powered by a single powerful two strokes diesel engine and it becomes profitable when dealing with ship electric power that is 10% superior to the propulsion power. On the contrary, a gas turbine installation for electric generation could be less favorable in that kind of ships that uses four strokes diesel engines, essentially due to the high number of these engines usually adopted onboard. In fact, it is certainly more complex to bypass exhaust gas of many diesel generators (as in passengers ships)