FUNDAMENTAL STUDIES ON ORGANIC RANKINE CYCLE EXPANDERS (NextORC)

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Commercial steam power plants pressurise and heat water to produce steam which is then expanded to produce electricity. However, using an organic fluid permits low temperature heat sources, typically between 80 and 350 degrees Celsius, to be converted into mechanical power more economically than steam. Organic Rankine Cycles (ORC) therefore have a great potential to contribute to the UK's mix of low carbon technologies with promising applications such as combined heat and power, concentrated solar power and waste heat recovery from reciprocating engines and other industrial processes with waste heat streams. However, despite successful commercialisation of ORCs for industrial scale applications, more development is required at the commercial and domestic scales before its potential can be realised. More specifically, at these small-scales, the challenge lies in the design of systems that are efficient but are also low cost. One approach to achieving this is to develop systems that operate efficiently over a range of different conditions. This will enable the high-volume, low-cost production of ORC systems, enabling significant improvements in the economy-of-scale. Furthermore, at this scale, different expander technologies, such as turbo and screw expanders, and system architectures can be considered. However, it is not clear which expander technology or system architecture is the optimal choice to achieve the desired improvements in the economy-of-scale. To answer this question it is important to improve the understanding of how different ORC expanders perform across a wide range of operating conditions, and to investigate how these systems respond to changes in the working fluid.The focus of this proposal is to conduct original research to improve the fundamental understanding on the performance of two different types of ORC expander, namely turbo and screw expanders. Computational and experimental methods will be used to investigate the performance of these expanders across a wide range of operating conditions and with a variety of organic fluids. These studies must account for the complexities of using organic fluids that exhibit complex fluid behaviour not observed in conventional fluids such as air and steam, in addition to considering the high speed flows, and two-phase conditions that are expected in turbo and screw expanders respectively. Ultimately, the results from these studies will improve the existing scientific understanding, and will facilitate the development of new performance prediction methods for these expanders. Understanding these aspects will not only lead to improved performance prediction, but could also lead to improved component design in the future. Within this project the new prediction methods will be used to investigate and compare the performance of different expanders within different ORC system architectures. The results from these comparisons will enable the identification of the optimal systems that can operate across a wide range of operating conditions, and therefore best facilitate improvements in the economy-of-scale of small-scale ORC systems.The primary outcomes of this research will be improved fundamental understanding of the performance of ORC expanders and validated performance models for turbine and screw expanders. Furthermore, recommendations will be made on the most appropriate system configurations that offer improvements in the economy-of-scale, thus enhancing the future commercialisation of small-scale ORC technology. Therefore this project has the potential to stimulate investment and create new jobs within the low carbon energy market, whilst positively contributing to the UK's existing research portfolio in turbomachinery and screw expanders.