Research On The Characteristics Of The ORC System For Marine Diesel Engine Exhaust Energy Recovery With Oil-free Screw Expander

For the marine diesel engine,about one third of fuel combustion energy was taken away by the exhaust.The Organic Rankine Cycle(ORC)system was used for waste heat recovery specially for the exhaust with the temperature under 200 ?.In this paper,the oil-free screw expander was first used as the power machine in the ORC system designed for a marine diesel engine exhaust heat recoevery.The thermal and dynamic characteristics of the oil-free screw expander were analyzed.The reliability about the seal and bearing were studied for the engineering practice.First of all,the exergy was analyzed for the ORC system with different heat exchangers.It found that the biggest exergy loss was in the evaporator.Though the thermal efficiency can be improved by increasing the preheater or regenerator,but the cost will increased at the same time.Through the theoretical analysis and experimental research on this ORC system,a design method approprit for engineering was proposed,and the results showed as follow: superheat degree for the evaporator and supercooling degree can set up 1~2 ?,and the increase of the expansion ratio and efficiency of the expander can help improving the efficiency of the ORC system.Second,the thermal performance of the oil-free screw expander in the ORC system was analyzed and tested.The results showed that: in the working process of the oil-free screw expander,isentropic efficiency is impacted by leakage and flow loss,and the leakage loss is the main factor.The calculation model of each leakage channel is detailed.The variation trends of volumetric efficiency with the meshing clearance between rotors,the tip clearance between rotors and shell,and the inlet face gap were compared respectively,among which the impact of tip clearance on volume efficiency was the largest.The design method of screw expander clearance was summed through the analysis thermal deformation of the rotors and shell.The thermal deformation of the rotor was more than the force.The force deformation of the rotor would increase the tip clearance.The thermal deformation of the shell would increase the meshing clearance.The deformation of the shell is irregular,and remading the internal cylinder in the shell is needed to eliminate the adverse effects.Meanwhile,dynamic performance of the oil-free expander was studied,the shaft calculation model and the calculation method of rotor critical speed was established.The high vibration value of the torsion frequency was captured by the variable speed vibration test,which was basically consistent with the theoretical calculation.Finally,the ORC system test platform was built.The new-type evaporative condenser was used.The efficiency of ORC system was increased by loading the cold source,and was affected by the relative humidity and temperature in the environment.The experimental data has shown that the actual thermal efficiency was only 5.3%,which was lower than the theoretical design efficiency of 5.7%.The reasons for the deviation were that: the actual clearances of the expander were bigger than the theoretical design,and the non-condensing inlet gas inlet can also decrease the system efficiency.In addition,the self-consumption of the ORC system was higher without frequency conversion.By comparing the sealing leakage with different materials in the testing,the medium R245 fa was incompatible with the material NBR,but instead of the polyurethane.Meanwhile,it also verified that common metals such as carbon steel and stainless steel were not corrosion.Through the above theoretical analysis and experimental comparison,it was found that the ORC system was the optimum way to recovery the energy of the exhaust gas with the temperature under 200 ? from the marine diesel engines.The optimized design method of the ORC system for the exhaust gas was realized,which has proved that the design method for the system and expander was feasible in practice.