Analysis and Modelling of Small Scale Organic Rankine Cycle System with Scroll Expander

Abstract

In recent years, the growing concern about pollution from fossil fuels has resulted in an increased effort for utilization of low-grade heat sources such as industrial waste heat, solar energy and other renewable sources. There also exits the problem of supplying electricity to a sizeable proportion of India’s population, which does not have access to the electricity grid. Distributed power generation using solar energy can help solve this energy problem without deteriorating the environment. For this purpose, various thermodynamic cycles such as the organic Rankine cycle (ORC), supercritical steam Rankine cycle, Kalina cycle and organic Brayton cycle have been proposed and studied for conversion of low-grade heat sources into electricity. Among these options, ORCs are reported to be more efficient at a small scale and with medium temperature heat source, and have been commercially implemented in several projects. However, with moderate temperatures, thermal efficiencies are generally low, and hence it becomes critical to choose appropriate process parameters for optimum cycle performance. Also, at the distributed scales (~10 kW), the choice of expander (or turbine) becomes extremely critical, as conventional turbines used in (Steam) Rankine cycle at these scales become incompatible. Positive displacement expanders such as scroll expanders have emerged as attractive options for micro-solar ORC based applications. This PhD work is based on a micro-solar ORC based application, with focus towards designing an optimized scroll expander for such application. Scroll turbomachine, though being used as compressor for quite some time, is not yet available commercially as expander. After a literature review of research work done in the field of distributed solar power generation and scroll turbomachine, a thermodynamic analysis is performed for the ORC to study the effects of various thermodynamic parameters. In the context of optimizing the cycle for a distributed power generation, a basic ORC as well as a regenerative ORC are studied. The thermodynamic analysis is tailored towards selection of appropriate working fluid to further optimize the ORC cycle for such applications, and these candidate fluids also aid in design of an optimized scroll expander. The remaining part of the thesis is focused on analysis and design of scroll expander. Scroll devices have traditionally been used as compressors. In the absence of any commercially available scroll expanders, several researchers have modified scroll compressors to operate in the expander mode to generate electricity. In the present thesis, a numerical model for scroll compressor has been modified and adapted for the study of scroll expander. This scroll expander model is a semi-empirical (or semi-numerical) model formulated using the conservation of mass, first law of thermodynamics and empirical formulation based on experiments conducted on scroll turbomachines. Such semi-empirical methods have been traditionally used and proven to be effective in rapid prototyping of scroll compressors. The presents work has also used the scroll model to analyze the scroll expander with the aid of Ns-Ds diagram, which is a more conventional way of analyzing a turbomachine through non-dimensionalization. It is found that the scroll expander occupies a unique domain in the Ns-Ds space, thus suggesting the scope of novel and unexplored domain of its application. The semi-empirical approach, though fairly robust, is not capable of giving any spatial distribution of thermal-fluid properties inside the scroll expander. Hence, to obtain a better insight into the physics of expansion inside a scroll, the scroll expander is analyzed using computational fluid dynamic (CFD) simulation. The CFD analysis, though requiring much more computational resources compared to the semi-empirical method, is capable of predicting more details such as the distribution of various thermal-fluid property inside the scroll at different locations. The present commercially available CFD software cannot be easily adapted to simulate the thermal-fluid phenomena inside the scroll expander, as the continuously changing geometry of a rotating scroll prohibits the use of standard turbomachinery modelling techniques. A comparative study of semi-empirical method and CFD simulation is performed. It is observed that CFD can very well be used for qualitative studies and complementing the semi-empirical method for study of novel and unconventional scroll expanders. Finally, the thesis is concluded by summarizing the work in the context of designing an optimized scroll expander and micro-solar application and highlighting certain unexplored domain to take this study further.