[en] Highlights: • A WER system plant scheme for the exploitation of ICE wasted thermal energy is presented. • The study is applied to 2 different marine ICEs: a turbo-charged 300 kW boat engine and a 12.6 MW ship engine. • The study shows that single loop WER power output can be increased up to 8.11% by means of a dual loop WER system. • Dual loop cycle using water and R245fa respectively as the working fluids has been found to be the best solution among the options analyzed. - Abstract: Waste thermal energy recovery systems have assumed an important role in the last decade as an effective way to improve fuel utilization in thermal engines, since they provide an opportunity to produce eco-friendly electrical power from an otherwise wasted energy source, leading to a reduction of the pollution and an increase of the overall system efficiency. In this scenario, the Rankine cycle technology based on simple or Organic Rankine cycle, earned a promising market position, since it allows for the production of additional electric power from relatively low-temperature heat sources (350–650 K); this feature makes these cycles a very suitable solution to recover thermal energy from Internal Combustion Engines, geothermal sources, solar thermal modules and micro-gas turbines. This paper presents the comparison between a single loop and a dual loop waste energy recovery system specifically designed as a bottomer to marine engines of different power range. The particular application considered shows several advantages for the installation of a waste energy recovery system; in particular, the basically infinite availability of the cooling medium represented by the seawater substantially facilitates the condenser design. R 245fa and R600 have been implemented in the second recovery loop and their performance has been addressed. The paper shows how adding a second recovery loop based on organic Rankine cycle to the steam Rankine cycle loop improves the system performance both in terms of recovered electric power (up to 8.11% and 2.67% respectively in small and large application size) and heat source utilization rate, since the heat source temperature could reach values as low as 343.15 K when considering a sulfur free fuel. In addition, R 245fa is to be preferred over the R 600 since it allows for the production of the same power considering lower values for the cycle top pressures.