## 1 Introduction  
The emissions produced by maritime transport, due to the use of fuels with a high sulfur content, highly contribute to air pollution in terms of sulfur dioxide and particulates, which are harmful to the human health and to the environment. In 2008, the International Maritime Organization (IMO) adopted a resolution amending annex VI of the 1997 protocol which updates the 1973 international convention for the prevention of ship pollution, named the International Convention for Prevention of Phips' Pollution (MARPOL convention), which contains regulations for the prevention of air pollution caused by ships [1]. The revised Annex VI of the MARPOL convention came into force on 1 July 2010 and introduced stricter sulfur content limits for marine fuel in the sulfur emission control areas--SECA (1.00% from 1 July 2010 and 0.10% from 1 January 2015) as well as in maritime areas outside the SECA (3.50% from 1 January 2012 and 0.50% from 1 January 2020). The MARPOL convention also addressed NOx polluting emissions, defining three \"tiers\": each level consists of a description of the maximum emission limits in g/kWh imposed on ships in relation to engine rotational speed, with values that are gradually more restrictive over the time. Starting from 1 January 2016, each ship must comply with the tier III emission limits, i.e., from 3.5 to 2.0 g/kWh as the rotational speed of the propulsion engine increases. In recent years, the fleet of large cruise ships is occupying an increasingly significant portion in the maritime transport sector, therefore, ship builders and owners have to face a great effort to develop new design and management methodologies that lead to a reduction in consumption and emissions during the operation of ships [2]. From the point of view of emissions only, it is currently feasible to adapt the traditional propulsion systems, based on internal combustion engines fed by Heavy Fuel Oil (HFO), with the addition of emission treatment devices downstream of the engine, like scrubber and Selective Catalytic Reduction systems (SCR) [3]. However, this solution has the drawback of considerable on board occupied volume, in addition to its cost of installation and management. Furthermore, this solution does not entail any fuel savings and, on the contrary, it implies some additional auxiliary consumption. To achieve both objectives, i.e., reduction of fuel consumption and emissions, three main options can be considered:  
* replacing traditional engines with other systems intrinsically characterized by low emissions and high efficiency (e.g., gas turbine (TG) [4; 5], combined gas-steam cycles [6], hybrid diesel-electric systems [7; 8]);
* replacing current fuels with other \"cleaner\" ones (e.g., Liquefied Natural Gas (LNG) [9; 10; 11]);
* converting current engines into complex Combined Heat and Power (CHP) or Combined Cooling Heat and Power (CCHP) systems, characterized by extensive energy recovery, which can be carried out through various technological solutions, in particular through the integration of the main engines with bottom thermodynamic cycles: Organic Rankine Cycle (ORC) and/or Stirling (as described hereinafter), Rankine [12], Brayton [13], Brayton and Rankine [14], or Rankine and ORC [15]. The most promising alternative consistent with the latter approach and based on sufficiently mature technologies seems to be the installation on board of ORC or Stirling groups, fed by the heat recovered from the main engines. While the formers are recently analyzed in various studies applied to ships [16; 17; 18; 19; 20; 21], for the latter, studies for ground-based applications only can be found in literature (see, for instance [22]). [PERSON] et al. [16] focus on the effect of adding a new heat exchanger for exhaust gas energy recovery on the performance of the main propulsion engine, [PERSON] et al. [17] and [PERSON] et al. [18] consider the performance improvement allowed by bottoming a ship propulsion engine with several ORC of different configurations, including dual loop solutions, [PERSON] et al.