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 heat recovery, especially for small scale applications. The high vapour density and the small enthalpy difference in the expansion make it possible to use volumetric expansion machines. In most evaluated systems, these units are modified scroll-or screw-type compressors from airconditioning and refrigeration industry. The use of these mass manufactured components, including plate-type evaporators, condensers etc, lead to a cost-effective design of small scale ORC systems. At the Institute for Energy Systems of the Technische Universität München a prototype system was designed and brought into service. In this article first measurement data of the system is used to validate a semi-empiric model that was developed with the Engineering Equation Solver Software (EES). The paper includes a description of the methodology and the components of the model. The heat input composes of exhaust gas heat exchanger, thermal transfer cycle and evaporator. The working fluid cycle further includes volumetric expander and feed-pump. In order to enable a realistic behaviour of the model in part-load operation, the heat exchanger areas are fixed and transferred heat is calculated according to the NTU-method with variable heat transfer coefficients according to changes in flow rates. From first measurement data, a couple of parameters are fixed in the model and characteristic output parameters are calculated. In a second step the results of the simulation are compared to the measured output parameters at several stationary operation points. Results of that comparison show that the model can achieve a good conformity to reality over the span of the given data. Due to limitations 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.