Simulations And Experiments Of A Vehicle Waste Heat Recovery System With R123and R245fa As Working Fluids

Energy consumed by vehicles has been increasing with the growth of vehiclepopulation. Much fuel energy has been dissipated into the atmosphere through exhaust,cooling systems and lubricating oils. Waste heat resources of heavy-duty diesel engine canbe recovered by the organic Rankine cycle (ORC) system, which is an effective method toimprove engine fuel economy. In this study, a medium-temperature (M-T) ORC waste heatrecovery experimental system was built. R123and R245fa were selected as ORC workingfluids, and extensive numerical simulations and experiments were conducted. The mainobjects and findings of this study are as follows.The waste heat of a heavy-duty vehicle diesel engine under different workingconditions were determined by energy analysis and exergy analysis according the enginebench test data. The results showed that the engine exhaust gas contained plenty of wasteheat. It had much better performance on recovery compared with that of any other wasteheat such as the engine cooling water.A M-T ORC system model was built, and thermodynamic analysis was carried out onthis system. Extensive numerical simulations were conducted to study thermal efficiency ofthe system under different conditions, especially variations of evaporation pressures, massflow rates of working fluids and temperatures of engine exhaust gases. The results indicatedthat the mass flow rate range varied with the change of evaporation pressure for both R123and R245fa. When it was under low evaporation pressure, the system efficiency increasedinitially and then decreased with the growth of the mass flow rate. The system efficiencydecreased in all cases with the growth of mass flow rate when it was under highevaporation pressure. For both systems, the maximum efficiency points shift towards lowermass flow rate with the growth of evaporation pressure. For Rankine cycle with both R123and R245fa, the system thermal efficiency increased with the increase of evaporationpressure. When the pressure conditions were identical, the R123system has a higherefficiency than that of the R245fa system. The thermal efficiency at different temperaturesof engine exhaust gases was similar when it was under fixed evaporation pressure. The system exergy analysis was carried out according to the test data. The resultsshowed that under different engine speeds, the R123system exergy efficiency increasedwith the growth of both evaporation pressure and engine load, while the R245fa systemexergy efficiency increased firstly and then decreased with the growth of evaporationpressure, and increased with the growth of engine load. The evaporator is the key toimprove the system exergy efficiency. An appropriate evaporator can improve the systemexergy efficiency remarkablly.The M-T ORC waste heat recovery experimental system was built for recovering thewaste heat of the heavy-duty diesel engine. The experiment results showed that both theR123system and the R245fa system could output power. The R123system was moreefficient than R245fa system. The system thermal efficiency increased with the growth ofevaporation pressure for both R123and R245fa in the waste heat recovery experiments.Better thermal efficiency could be obtained at a smaller mass flow rate of working fluids.The system experiment was carried out by taking086and106scroll expanders as theexpanding machine with R123as the working fluid. The experimental results showed thatthe system mass flow rate range of working fluids moved to large flux area and the systempower output increased with the growth of exhaust gases temperature. Under the conditionof constantheat source, the system power output increased initially and then remained stablewith the growth of mass flow rate. Under the condition of constant mass flow rate,increasing system load appropriately could improve the power output.