Investigation of supersonic impulse turbines for application to geothermal binary power stations

Abstract

This thesis was conducted within the University of Queensland’s Geothermal Energy Centre of Excellence (QGECE). The centre had four major challenges: 1. Optimum energy extraction and sustainable resource management. 2. Efficient power conversion. 3. A cooling system for a desert zone in the world’s driest inhabited continent. 4. To resolve transmission issues inherent to a power plant which is located more than 500km from major load centres and the national grid. This thesis is related to the second major challenge of improving power conversion efficiency. That challenge had an associated milestone to develop working laboratories for testing power conversion systems. This thesis aligns itself with the goals of the centre to develop working facilities and to investigate opportunities to improve power conversion efficiency. The synergies of the centres goals and this thesis’s objectives are based on binary power plant technology for use with geothermal applications. This thesis postulates the question; what is the impact of incorporating a real turbine loss model into a binary cycle analysis? A binary power plant test facility was designed and built to test turbines operating in organic Rankine cycles. For the operating conditions of the power plant test facility, a single-stage, supersonic, axial impulse turbine was designed, built and tested across a range of conditions and experimental performance data was collected for analysis. The gathered data was used to calibrate a computer program written to calculate losses in the stator and rotor passages of a single stage axial impulse turbine. The calibrated loss model was then incorporated into another program written to calculate the performance of organic Rankine cycles. The incorporated loss model into the cycle analysis program allowed for calculating organic Rankine cycle performance based on calculated turbine efficiency. Cycle analyses conducted over a range of pressures, working fluids, and temperatures showed a clear trend that each working fluid had a unique optimum evaporator pressure for each geothermal source temperature. High pressure supercritical cycles were shown to have good cycle performance as they tend to have a good thermal profile match between the thermal fluid and the working fluid in the evaporator, thus maximising the utilisation of the energy in the thermal fluid. However, in certain conditions, the implementation of a recuperator may achieve similar if not better performance than supercritical cycles but at much lower pressures. This is achieved by initiating evaporation of the working fluid before the recuperator outlet. The calibrated loss model showed that the losses in the single stage impulse turbine were dominated by the windage and the partial admission sector losses. At low admission rates the partial admission pumping losses became a dominant source of losses. Also at low admission rates the other losses (clearance, incidence, trailing edge and passage) all become a larger part of the overall losses on a percentage basis. This leads to low admission turbines having relatively low efficiencies and being more sensitive to operating conditions. Smaller power systems will generally have lower mass flow rates and will lead to lower admission turbines. The influence of an incorporated turbine loss model was more pronounced for lower power systems. For a wide range of conditions, performance maps of optimum operating conditions were generated along with preliminary designs of single-stage, axial, impulse turbines. The incorporated loss model provided insight into a holistic design approach that optimises cycle and turbine performance concurrently. The major achievements of this thesis are the successful design and build of an ORC test facility and a single stage axial impulse turbine. This test facility provides a new test platform for the University of Queensland’s ongoing research into power conversion systems. Another major achievement was the completion of a comprehensive computer code that may be used to analyse organic Rankine cycles, calculate turbine performance and produce geometry for stator and rotor passages for an impulse turbine. The last major achievement was to conduct experimentation on a single stage axial impulse turbine and use the experimental data to calibrate a loss model that could be incorporated into cycle analyses.