Two-phase expansion in turbo-expanders for organic Rankine cycle power systems

Increasing the efficiency of energy production systems from renewable energy sources and reducing the energy consumption of industrial processes is crucial to limit the global emissions of greenhouse gases. In this context, organic Rankine cycles are a key technology, as they enable the efficient and costeffective recovery and conversion of lowgrade heat into electrical power from various energy sources, including geothermal sources and industrial waste heat.

The partial evaporation organic Rankine cycle represents an alternative cycle architecture to conventional organic Rankine cycles, which can significantly increase the overall power system efficiency by improving the heat transfer between the heat source and the working fluid that realizes the thermodynamic cycle. However, the feasibility of partial evaporation organic Rankine cycle systems is highly dependent on the possibility of realizing the twophase expansion in efficient and robust machines.

This Ph.D. thesis investigates the twophase, or flashing, expansion of organic fluids in turboexpanders for partial evaporation organic Rankine cycle applications at three different levels, which reflect the main objectives of this work. The first objective consists of the evaluation of the technoeconomic feasibility of partial evaporation organic Rankine cycles and the determination of the optimal operating conditions of the expander. The second objective is the identification and  development of accurate models for the prediction of the flashing phenomenon, which characterizes the operation of turboexpanders for partial evaporation cycles. The last objective is the realization of a tool for the preliminary design of axial turbines operating in flashing conditions and the evaluation of their performance.

The first part of this work presents the results from the optimization of partial evaporation organic Rankine cycles for lowtemperature geothermal power systems based on thermodynamic and economic criteria. The partial evaporation organic Rankine cycle is found to be able to provide a substantial increase in net power output of the system with respect to conventional organic Rankine cycles for a fixed heat source, at the cost of an increase in system costs. Nevertheless, partial  evaporation cycles are found to be economically more profitable when the return of the investment along the lifetime of the system is considered. In the second part of the thesis, a onedimensional model for the prediction of flashing
flows with thermal nonequilibrium effects is presented. For this purpose, the adoption of the simple geometry of a convergingdiverging nozzle allows focusing exclusively on the thermodynamic description of the flow. The excellent agreement observed when comparing the obtained numerical results with experimental data on R134a suggests that the proposed model can be applied to organic fluids with reasonable confidence.

The last part of the thesis is dedicated to the description of twophase flow phenomena in axial cascades and of a novel meanline model developed to predict the effect of such phenomena on the performance of axial turbines. The model is then applied to a case study based on the optimal operating conditions derived in the first part of the work from the cycle analysis. The results indicate that the performance of the machine is significantly affected by the twophase operation only if the deposition of liquid droplets on the blades occurs at high rates and that the choice of an adequate geometry highly contributes to limiting twophase losses.

The work presented in this thesis shows the potential of partial evaporation organic Rankine cycles and the possibility of realizing the twophase expansion in turbomachines with a limited penalization in their efficiency. The outcome of this thesis represents a basis for the future development of the partial evaporation organic Rankine cycle technology and for its application to large size power systems.