Study On The Performance Of The ORC-HP Combined System And Free Piston Expander For Exhaust Heat Recovery Of Diesel Engine

Using the organic Rankine cycle-heat pump combined system(ORC-HP)can recover the waste heat of the engine effectively to improve the overall efficiency of the internal combustion engine and reduce the fuel consumption,the purpose of adjusting the temperature in the vehicle can be achieved by cooling or heating.Therefore,studying the performance of the combined system is of great value and significance for energy saving and recovery of engine waste heat.In this paper,we have done some related work in ORC-HP combined system and free piston expander,and achieved some results.Based on the law of conservation of energy,the thermodynamic model of the combined system is given.According to the thermodynamic model,the energy change and exergy loss rate of each component in the combined system are calculated.The influences of ambient temperature,condensation temperature and evaporation pressure in the ORC system,and the effects of condensation pressure,condensation temperature and evaporation temperature in the heat pump system on the performance of the combined system are studied respectively.The results show that,based on a specific diesel engine data,the combined system can meet the refrigerating and heat production requirements of the coach at higher exhaust temperature and diesel engine speed.Under the heating mode,the total efficiency of the combined system is 43.14%,and the COP_h is 3.76.Only changing the relevant parameters in the ORC system does not affect the heating power of the heat pump system.When the diesel engine speed and exhaust temperature are 2200 r/min and 818 K,the maximum heating power is108 kW;changing the variables in the heat pump system affects both the heat pump's COP_h and the total efficiency of the combined system,but COP_h and the total efficiency have opposite trends;when the variables of the heat pump system are gradually increased,the trend of the heating power and the total exergy loss rate of the combined system is the same as that of the total efficiency;besides,the trend of the net power output is the same as that of the COP_h;the total efficiency of the combined system varies between 40.0%and 47.06%;the COP_h of the heat pump system reaches the maximum value of 4.78,when the condensation temperature of the heat pump system is 323 K.It can be seen that the effects of condensation pressure and condensing temperature on the net power and total exergy loss rate of the combined system are more obvious;when the condensing temperature is 303 K,the maximum net power of the combined system is 6.58 kW;when the condensation pressure is8MPa,the maximum exergy loss rate of the combined system is 222.84kW.Under the cooling mode,the total efficiency of the combined system decreases with the increase of engine exhaust temperature and diesel speed;the change of condensation temperature and the evaporation pressure in the ORC system have nothing to do with the COP_c,and the COP_c keeps a constant value of 3.34;when the diesel exhaust temperature and diesel engine speed work in a higher zone,the net power output of the combined system can meet the cooling power(15 kW),and increases with the increase of diesel engine speed and exhaust temperature.The changes of condensation temperature,condensing pressure and evaporation temperature in the heat pump system have a great influence on the COP_c of the heat pump system;when the condensation pressure increases,the COP_c increases first and then decreases,the maximum value is 3.89,which appears at the working point that the condensation pressure is 8.5 MPa;the COP_c decreases significantly as the condensation temperature increases;when the condensing temperature is 308K and318K,COPc is 3.34 and 1.89,respectively;the COP_c increases with the increase of evaporation temperature,when the evaporation temperature is 273K,the value of COP_c is 2.18,and then,the COP_c rises to 3.98 when the evaporation temperature is287 K.As one of the key components in the ORC system,the performance of the expander has an important influence on the whole ORC system.This paper presents a preliminary experimental investigation of a free piston expander-linear generator(FPE-LG)prototype using compressed air as working fluid.And the experimental test results show that FPE-LG can run stably under the condition of high inlet pressure.The cycle-to-cycle variations of velocity at the reference position,and the cycle-to-cycle variations of displacement at top dead center and bottom dead center can be ignored.The peak velocity of free-piston is close to 1.2m/s when the intake pressure is 0.2MPa and working frequency is 2.5Hz.The valve timing and intake pressure have an important influence on the motion characteristics of free-piston and power output of linear generator.Decreasing the exhaust duration angle or increasing the intake duration angle can improve the stability,symmetry of free piston,reduce cycle-to-cycle variation of free-piston motion and improve the power output obviously when other parameters are constant.The maximum peak power output is about 19.0W when inlet pressure is 0.19MPa and working frequency is 2.5Hz.According to the FPE-LG prototype,the simulation model of free piston expander is built by GT-Suite software.The relevant results are presented through preliminary study.