Energy and exergy analyses of a two-stage organic rankine cycle with low pressure stage regeneration for IC engine waste heat recovery

A two-stage Organic Rankine Cycle (ORC) with Low Pressure Stage Regeneration (LPSR) proposed in this article is intended to utilize the engine coolant energy completely for vaporization of organic fluid in a Low Pressure stage Heat Exchanger (LPHE) and the engine exhaust energy for sensible heating, vaporization and super heating of organic fluid in a High Pressure stage Heat Exchanger (HPHE) besides utilizing the superheated vapor energy of exhaust from Low Pressure stage Turbine (LPT) in a regenerator. Since regeneration is used only at low pressure stage, the energy associated with the engine exhaust gases can be utilized to the maximum extent by lowering its temperature nearer to the temperature of liquid phase working fluid after High Pressure stage Pump (HPP), thereby maximizing the Waste Heat Recovery (WHR) potential of the bottoming two stage ORC. The WHR efficiency of two-stage ORC with and without LPSR is analyzed at a typical operating condition of the engine using a nearly dry fluid R123 and a nearly isentropic fluid R134a as the working substances. It is observed that the thermal efficiency of the two-stage ORC with R123 is higher than that with R134a. The LP stage regeneration has been found to be effective in increasing the thermal efficiency and, in turn, the WHR efficiency of the two-stage ORC with both R123 and R134a. The increase in the fuel efficiency of the IC engine due to the bottoming two-stage ORC is found to be 7.22% with R123 and 6.21% with R134a with LPSR and 6.58% with R123 and 5.51% with R134a without LPSR. The optimum pressure in HPHE is found to be about 2.5 MPa and 3.5 MPa with R123 and R134a respectively.