Organic Rankine Cycles (ORCs) have been widely used in low-temperature power production systems in the last decade. The heat sources include a wide range of applications such as geothermal and waste heat recovery systems. Despite the undeniable need for ORC units to increase the efficiency of the power systems, the low efficiency and the relatively high primary cost of these systems are still challenges that researchers are trying to resolve. Indeed, the key component of a small-scale ORC unit in terms of both performance and cost is the volumetric expansion machine. Among different types of volumetric expanders available in the market, scroll expanders have shown the best performance in ORC systems. In this research proposal, an experimental study of a scroll expander in a 2 kW ORC unit is introduced. The objective of the project is to develop a semi-empirical model of the specific scroll expander and then using the available experimental data in the literature, it is aimed to develop a more general model of scroll expanders with acceptable accuracy that can be used in modeling of ORC systems in the specified range of working conditions of the database. In addition, results of the model will be used in off-design and charge-sensitive modeling of the ORC.
In this project, a scroll expander will be tested in an ORC system in order to obtain the semi-empirical model of the machine. The model will be used then in modeling of the ORC system in off-design condition. Furthermore, the model of the scroll expander will be used along with other similar models available in the literature to propose a general model of scroll expanders using their main known characteristics for the first time in the literature. Therefore, a general model of one of the main components of ORC systems will be introduced within a range of validity regarding the working conditions of the experimental database. The final correlation will be compared to the available experimental data of scroll expanders in the literature to determine its range of accuracy and validity.
In addition, the results of the performance of the PHEs will be used to tune the available correlations of OHTC to match the numerical results with experimental ones. The next step is to perform a charge-sensitive modeling of the ORC system and to compare the calculated charge of the organic fluid with the real charge of the system in the experiments. Charge-sensitive modeling of ORC systems are innovative approaches that to the best knowledge of the author, very few research papers have focused on this matter. To have an exact calculation of the charge, correlations of OHTC in both evaporator and condenser are very important since the accuracy of the calculated heat flux in PHEs in known mass flow rate and pressure of the system depends on them. Furthermore, correlations of the two-phase void fraction (the ratio between volume occupied by the gas to the one occupied by the liquid in a channel/tube) are necessary to calculate the density of the flow in the two-phase zone. Using charge-sensitive modeling, the inputs to the model are the same inputs that an operator confronts in reality. Hence, knowing the amount of charge of the organic fluid to the system, the thermodynamic condition of the heat source and cold sink, and mass flow rate provided by the pump, the performance of the system can be determined.