Experimental investigation of a zeotropic working fluid and working fluid filling factor on system performance in a small-scale ORC
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameMaster of Engineering
Organic Rankine Cycle (ORC) systems are capable of utilising low-enthalpy heat sources to generate power. For the performance engineering of ORC systems, it is important to understand process parameters and component behaviour. To maximise performance, modelling of the plant thermodynamics must be coupled with data analysis to develop diagnostic procedures, find optimal operating points, and diagnose problems to schedule the most cost effective maintenance. An existing ORC system at the University of Canterbury has been upgraded from a previous iteration to assist in furthering our knowledge of ORC system design and construction.
This paper presents experimental results from running a 1 kW ORC system using HFC-M1 refrigerant, a zeotropic mixture of R245fa and R365mfc, as the working fluid under a wide range of operating conditions. Hot exhaust combustion products from a 30kW CapstoneTM Gas Turbine are used as the heat source, and heat is transferred via a thermal oil loop to the working fluid through a plate heat exchanger. A scroll expander magnetically coupled to an AC generator is used for work extraction and energy conversion.
Trials focused on testing the full range of performance, and investigated the effect of a zeotropic working fluid, and the influence of varying the working fluid liquid level on system performance.
Trials were prematurely ended by bearing failure in the ORC scroll expander. However, analysis of the results from additional tests suggest the working fluid charge in the system influences operation, corroborating findings in literature. While inconclusive, these initial results support the need for further testing the effect of the DVR in a fully functioning system.
A comparative study was performed between the system actual performance and the theoretical performance to evaluate the degree of impact of the operational issues on the system performance. The unit was disassembled to evaluate the component compatibility and assess functionality over the operation. It transpired the system mass was not conserved during the operation due to leakage, contributing to the overall deterioration in system performance over time. It is concluded the zeotropic mixture was chemically incompatible with system components leading to system failure.
Critiques of components and working fluid, derived from experiences in operating the system, coupled with general trends produced by the results, provide recommendations for the design and testing of future small-scale ORC systems.