In recent years, the uncertainty about the prices of marine fuels and the growing attention of the international community to environmental issues (including carbon dioxide emission reduction) have promoted the research of more and more efficient methods of energy conversion on board ships. During research activities carried out also in collaboration with leading industries, the authors have developed simulation models for the recovery of the thermal energy released from diesel engines, proposing different waste heat recovery (WHR) plant configurations.

This previous experience has been applied, in this paper, to an existing cruise ferry, whose propulsion system consists of two controllable pitch propellers moved, each one, by a couple of four stroke diesel engines through a reduction gear. Starting from the available data on the current board systems, a proposal of refitting is presented, based on a comparison of six different WHR plant solutions, able to recover part of the waste heat contained in the exhaust gases coming from both main engines and  diesel generators. The considered WHR plant layouts, optimized in relation to the peculiar requirements of the ship, make use of a heat recovery steam generator (HRSG) with single or dual pressure level. The produced steam can be used, depending on the selected layout, to meet the on board steam needs  or/and to contribute to the production of electric energy, in this last case allowing to reduce the use of the diesel generators.

The comparison of the proposed WHR plant configurations, between them and with the originally adopted very simple one, is carried out taking into account not only the amount of thermal energy recovered and the consequent fuel saving, but also some other important aspects such as the sizes and weights of the WHR plant components and the related problems of installation on board.