Simulation and experimental validation of an ORC system for waste heat recovery of exhaust gas

A couple of characteristics make it favourable to use an Organic Rankine Cycle for heatrecovery, especially for small scale applications. The high vapour density and the smallenthalpy difference in the expansion make it possible to use volumetric expansion machines. Inmost evaluated systems, these units are modified scroll- or screw-type compressors from airconditioningand refrigeration industry. The use of these mass manufactured components,including plate-type evaporators, condensers etc, lead to a cost-effective design of small scaleORC systems. At the Institute for Energy Systems of the Technische Universitat Munchen aprototype system was designed and brought into service. In this article first measurement dataof the system is used to validate a semi-empiric model that was developed with the EngineeringEquation Solver Software (EES). The paper includes a description of the methodology and thecomponents of the model. The heat input composes of exhaust gas heat exchanger, thermaltransfer cycle and evaporator. The working fluid cycle further includes volumetric expander andfeed-pump. In order to enable a realistic behaviour of the model in part-load operation, the heatexchanger areas are fixed and transferred heat is calculated according to the NTU-method withvariable heat transfer coefficients according to changes in flow rates. From first measurementdata, a couple of parameters are fixed in the model and characteristic output parameters arecalculated. In a second step the results of the simulation are compared to the measured outputparameters at several stationary operation points. Results of that comparison show that themodel can achieve a good conformity to reality over the span of the given data. Due tolimitations in terms of heat source mass flow the system could only be operated in low part load.Therefore a validation of the presented model for full load still needs to be done in future.