In this study, a main marine engine with a rating power of 21,840 kW for a ship sailing in an actual voyage was obtained as the research object. The engine’s exhaust gas and jacket cooling water were adopted as the heat source of the organic Rankine cycle (ORC) system developed for the main marine engine. The engine can consume high-sulfur or low-sulfur fuel oil, respectively, according to the different emission control requirements. The impact of the use of high-sulfur or low-sulfur fuel oil, and variations in engine load, amount of recoverable waste heat, outboard seawater temperature, and the ship’s steam demand were comprehensively considered, and the validated ORC system model was used for the analysis of the system’s performance and the ship’s energy saving for the whole voyage. The results demonstrated that when the ship adopted high-sulfur or low-sulfur fuel oil, the maximum total net power output of the ORC system was 449.3 kW and 753.1 kW, respectively. During the whole voyage of 1610.7 nautical miles, when high-sulfur fuel oil was used, the ORC system reduced carbon emission by 40.3 tons and 33.8 tons, respectively, in summer and in winter, and the fuel saving rates were 2.53% and 2.12%; when low-sulfur fuel oil was used, the ship’s carbon emissions were reduced by 62.1 tons and 61.8 tons, respectively, in summer and in winter, and the fuel saving rates were 3.91% and 3.89%.

Marine air-conditioning systems face high energy consumption, particularly in humid marine environments. This study is an experimental investigation of the effects of rotating wheel speed and regeneration temperature on the performance of the system, which is a dual-stage desiccant dehumidification fresh-air pre-treatment system using ship waste heat as the regeneration heat source and seawater-assisted cooling to improve the efficiency of energy use. The results showed that the dehumidification capacity and efficiency of the system improved with an increase in the rotating wheel speed from 6 to 10 r/h and in the regeneration temperature from 80 °C to 110 °C. Optimal performance was achieved with a rotating wheel speed of 10 r/h and a regeneration temperature of 110 °C, balancing the maximum dehumidification capacity, energy efficiency, and waste heat utilization.