## 6 Conclusions  
In this paper, the waste energy generated by an engine's combustion gases and the waste energy from its jacket cooling water of a prototypical huge two-stroke diesel engine with a low speed (model MAN 8565 ME-C8.6 HL) was recovered by using the basic (BORC) system and preheated (PORC) system. The effects of the two ORCs on the operating performance under different sea temperatures and main engine loads are compared andanalyzed, as well as the annual fuel saving and carbon emission reduction caused by the application of the two ORCs. The main findings are as follows:  
1. The net output power of the PORC system is always higher than that of the BORC system under different outboard sea temperatures. When the seawater temperature is 20 \({}^{\circ}\)C, the BORC and PORC systems can obtain the highest net output power of 445.3 kW and 491.3 kW, respectively. Due to the addition of the preheater, in comparison to the BORC system, the inclusion of a preheater leads to a 10.3% enhancement in the net output power of the PORC system. However, when the seawater increases in temperature from 20 \({}^{\circ}\)C to 30 \({}^{\circ}\)C, the net output power produced by the BORC and PORC systems drops to 405.5 kW and 426 kW, respectively, and the PORC system's net output power ratio falls to 5%. Compared to the BORC system, the net output power of the PORC system experiences a substantial increase when the outboard seawater temperature is lower. Therefore, when the ship mainly operates in the sea area where the sea temperature is low, the PORC system should be preferred to recover unused energy from the main engine.
2. Under various load circumstances encountered in navigation, the PORC system's net output power is always greater than the BORC system. As the main engine load rises from 60% to 100%, the net output power of the BORC system escalates from 237.4 kW to 445.6 kW, while the net output power of the PORC system surges from 270 kW to 491.3 kW. The net output power of the PORC system exhibits a more pronounced increase compared to the BORC system as the main engine load rises. Therefore, when the ship mainly operates under high load conditions (80% SMCR or above), the advantage of using the PORC system to recover unused energy from the main engine is more evident by adding a preheater in the ORC system.
3. If we adopt the CO\({}_{2}\) emission ratio of 3.1t-CO\({}_{2}\)/t-fuel for the heavy oil burned by this main engine, the PORC system studied in this paper can save a large amount of fuel consumption during the ship's voyage. According to the data calculated in this paper, taking MAN 8565 ME-C8.6 HL main engine as an example, it can save 510 tons of fuel oil per year and reduce transportation costs by $230,265 per year, and the payback period for PORC systems is about 4.86 years. PORC system can reduce carbon emission by 1581 tons per year, which is of great significance to realize energy saving and emission reduction plans for ships.  
Conceptualization, methodology, supervision, W.C.; formal analysis, data curation, writing-original draft preparation, B.F. and J.Z.; resources, software, validation, W.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Science and Technology Plan Project of Fujian Province, China (Grant Number.2022J011110).  
Not applicable.  
The authors would like to thank the National Natural Science Foundation of China (grant number 51679107) for the financial support, and [PERSON] for his valuable suggestions and technical support.  
The authors declare no conflict of interest.