Selection of Working Fluids for Organic Rankine Cycle Utilizing Waste Heat from Vessels

As a part of endeavor for Net Zero 2050, the ORC (Organic Rankine Cycle) utilizing the waste heat is attracting interest in recent decades. Its characteristics is that the organic substance is used as working fluid instead of water of RC (Rankine Cycle). Many kinds of waste heat sources can be found in various industries, and this research project targets utilization of waste heat from exhaust gas from engine of vessels. As the initial step of the research, this paper analyzes ORC efficiency according to various working fluid. When selecting working fluid, following things should be considered. First of all, the working fluid of nowadays should be environmental-friendly substance. GWP (Global Warming Potential) and ODP (Ocean Deplete Potential) of the selected working fluids should satisfy the current environmental standard. Secondly, the safety grade of the working fluid should be guaranteed to avoid explosion, especially being operated near the engine of the vessel. Lastly, the operating temperature should be lower than the critical point of the selected working fluids. If it is operated at the transcritical points, the operation optimization requires far more complicated consideration, and designing turbine may quite difficult. This paper sets the heat source temperature of two different levels: direct exhaust gas around 200°C from the engine, and indirect heat source around 90°C from the water jacket of the engine. To analyze the variation of system efficiency according to the heat source temperature, the value at several steps near the heat source temperature are calculated: 150~200°C for the exhaust gas, 60~90°C for the water jacket coolant. Based on several standards above, R1233zd(E), R245fa, R600a, R236ea and R1234ze(E) are selected as the working fluids. The basic cycle and the same categories of working fluids can be applied for both case of heat source temperature level. Simulation analysis using ASPEN HYSYS presented that the system efficiency of case applying R1233zd(E) is the highest when the heat source is exhaust gas. On the other hand, R245fa is more effective when the heat source is water jacket of the engine. System efficiency is decided by the enthalpy difference between inlet and outlet of the turbine and the evaporator. This enthalpy difference is dependent on the shape of saturation curve, isotherm curve, and isoentropy curve etc, meaning characteristics of properties of working fluid. This research was supported by Korea Institute of Marine Science&Technology Promotion(KIMST) funded by the Ministry of Oceans and Fisheries. (20220634)