Dynamic Modelling of an Organic Rankine Cycle and Applications In
Total Page：16
File Type：pdf, Size：1020Kb
Dynamic Simulation and Experimental Validation of an Organic Rankine Cycle Model BRADEN LEE TWOMEY BACHELOR OF ENGINEERING,MECHANICAL A thesis submitted for the degree of Doctor of Philosophy at The University of Queensland in 2015 School of Mechanical and Mining Engineering Abstract The aim of this thesis is to develop a methodology to model and simulate the dynam- ics of Organic Rankine Cycle (ORC) power plants, and to demonstrate and validate this methodology by performing experiments on a laboratory-scale ORC plant. Typi- cal plant models for ORCs provide steady state analysis of thermodynamic cycles and losses, but for a system that has complex starting mechanisms and undergoes fluctua- tions in operating conditions due to environmental effects, dynamic models are needed to predict how the system will behave. Two main questions are interesting to an ORC designer. What is the start-up and shut-down time of the plant? And: What effect does an unprecedented slow or sharp transient in one or multiple physical variables have on the system? There are a number of modelling libraries available for the simulation of Rankine cycles, however there is no complete package that covers all potential dynamic sce- narios and is fully documented with guidance on how to develop a stable dynamic model. Issues such as component selection and parameterisation criteria, compilation of stable models in large systems, simulation initialisation strategies, and heat transfer model selection present many challenges to the inexperienced modeller. This thesis aims to address these issues and document strategies to overcome them. Existing modelling libraries do not include extensive heat transfer models that can accurately simulate the heat exchange that occurs during dynamic transients in ORC heat exchangers. Void fraction is a critical variable for the dynamics of a closed thermo- dynamic cycle that depends on accurate heat transfer models, and models that switch between single-phase and two-phase heat transfer correlation are beneficial here. De- velopment of an extension of the existing heat transfer models to better model these effects is another aim of this thesis. A smaller ORC laboratory that is separate to the main facility used in this project was available early in the thesis to test initial heat exchanger and cycle modelling re- sults. An intermediate project goal was to use this laboratory to model and analyse a novel small-scale solar cogeneration unit that uses cheap and available components to heat water and produce power using a scroll expander. The completion of this goal was seen as a fundamental step toward understanding the physical characteristics of an ORC that is producing power, and to observe the system dynamics. The results of i the study include a typical day’s power output for various times of the year, and show the competitiveness of this type of system. The major contributions that originate from the main body of work on the larger ORC facility are: • Development of extended pipe models that include an extra wall for heat transfer to a shell or the environment; • Development of detailed, deterministic plate heat exchanger models with de- scriptive parameters that can be quickly configured by an inexperienced mod- eller. These include extensions to heat transfer models and a new phase switch- ing method that is more accurate and robust than those currently available; • Implementations of a number of heat transfer correlations into the existing li- braries, and discussion on when and how the best place to use them is; • Development of an extended expansion valve model and discussion on how best to characterise, parameterise, and initialise it; • Tutorial on how to choose the correct models and initialisation parameters for a compiled organic Rankine cycle model; and • Discussion on initialisation strategies for the compiled ORC model, and presen- tation of a preferred initialisation method. The developed models were configured for simulation of the target experimental ORC. Experimentally stable operating state points representative of the entire operat- ing range of the ORC were chosen as target conditions to compare steady state and dynamic simulation results against. The steady state points at pressure and temper- ature sensors between each component in the ORC were used to record data for six steady state cases. Two pressure sensor locations were chosen as representative of the ORC low-side and high-side pressures, based on their proximity to the primary pres- sure drop driver, the expansion valve. Twelve dynamic test cases using variations in expansion valve closure and pump speed were used to record dynamic test data for comparison against simulation data. A pressure deviation of less than 6 % was observed in all steady state test cases except one. The outlying highest deviation case showed 16 % deviation in the low- side pressure variable. The simulation results for the dynamic test cases matched up closely to the experimental data. The observed deviation between simulation and experiment results for settling time was less than 20 seconds for the worst matching cases. In five ii out of twelve cases, the dynamic pressure transient matched the experimental data with almost zero deviation. The approach taken in this project can be used as a guide for future researchers and model developers to overcome the aforementioned issues in order to further advance research in the area of dynamic modelling of Organic Rankine Cycles. iii Declaration by Author This thesis is composed of my original work, and contains no material previously published or written by another person except where due reference has been made in the text. I have clearly stated the contribution by others to jointly-authored works that I have included in my thesis. I have clearly stated the contribution of others to my thesis as a whole, includ- ing statistical assistance, survey design, data analysis, significant technical procedures, professional editorial advice, and any other original research work used or reported in my thesis. The content of my thesis is the result of work I have carried out since the commencement of my research higher degree candidature and does not include a substantial part of work that has been submitted to qualify for the award of any other degree or diploma in any university or other tertiary institution. I have clearly stated which parts of my thesis, if any, have been submitted to qualify for another award. I acknowledge that an electronic copy of my thesis must be lodged with the Univer- sity Library and, subject to the policy and procedures of The University of Queensland, the thesis be made available for research and study in accordance with the Copyright Act 1968 unless a period of embargo has been approved by the Dean of the Graduate School. I acknowledge that copyright of all material contained in my thesis resides with the copyright holder(s) of that material. Where appropriate I have obtained copyright permission from the copyright holder to reproduce material in this thesis. iv Publications during candidature Peer-reviewed journal articles Twomey, B.; Jacobs, P. and Gurgenci, H. (2013). “Dynamic performance estimation of small-scale solar cogeneration with an organic Rankine cycle using a scroll expander”. Applied Thermal Engineering, vol. 51, pp. 1307 – 1316. Internal reports Twomey, B. (2013). “PSHE and STHE models in a small ORC plant”. Technical report 2013/11, School of Mechanical Engineering, University of Queensland. URL http://espace.library.uq.edu.au. Twomey, B. (2012). “Experimental test results from QGECE laboratory small-scale or- ganic Rankine cycle using a scroll expander”. Technical report 2012/03, School of Me- chanical Engineering, University of Queensland. URL http://espace.library.uq.edu.au. Publications included in this thesis Twomey et al.(2013) – incorporated as Chapter4. Contributor Statement of contribution Twomey, B. Designed experiments (90%) Wrote the paper (100%) Jacobs, P. Critically examined paper (40%) Proofread paper (70%) Gurgenci, H Designed experiments (10%) Critically examined the paper (60%) Proofread paper (30%) v Contributions by Others to the Thesis This thesis includes experimental work on a laboratory located at the University of Queensland, St. Lucia. Hugh Russell and Jason Czapla, past students and staff of the Queensland Geothermal Energy Centre of Excellence, were responsible for setting up the laboratory to its original working configuration. Andrew Rowlands was respon- sible for writing the data aquisition software for the facility. This thesis also includes experimental work on a laboratory located at the University of Queensland, Pinjarra Hills. Andrew Rowlands, Jason Czapla, Carlos De Miranda Ventura, and Rajinesh Singh were primarily responsible for the design and engineering of the facility. Statement of parts of the thesis submitted to qualify for the award of another degree None. vi Acknowledgements I would like to acknowledge the many different people who not only contributed to the work put into making this thesis materialise, but who also contributed to the up- keep of the author’s sanity. I would like to thank Professor Hal Gurgenci and Associate Professor Peter Jacobs for endorsing my candidature and providing valuable supervi- sion over the course of the thesis program. Special thanks, also, to the anonymous reviewers of journal papers other academic works submitted for peer review. I would also like to thank the QGECE staff and students, with special mentions to Andy Rowlands, Hugh Russell, Jason Czapla, Carlos De Miranda Ventura, and Raji- nesh Singh for their efforts in getting the Pinjarra Hills Renewable Power Generation Laboratory off the ground. Further shoutouts to Jessey Zhang, Berto Di Pasquale, and Glenda Heyde, for their assistance in the St.Lucia laboratory and administration office. Thanks is also due to the University of Queensland and the QGECE for their scholar- ship assistance. Finally, I would like to sincerely thank my beloved family and friends for encour- aging and inspiring me to do my best throughout the thesis, especially in the hardest times, with special thanks reserved for Jack, Jim, Cooper and Boagsy.