Test bench development, experimental analysis and modelling of micro-organic Rankine cycle for low-grade heat recovery

This thesis presents the detailed experimental and numerical analysis conducted on a prototype of micro-scale organic Rankine cycle (ORC) power system. The system is conceived for stationary co-generative applications in the residential sector, since it is characterized by a power output in the kW-scale. Suitable heat sources are low-enthalpy geothermal energy, biomass combustion, solar thermal collectors or low-temperature waste heat recovery. The system is driven by a reciprocating piston expander prototype, and uses HFC-134a as working fluid, hence it is suitable for temperature of heat source in the range 60-90 °C. The micro-ORC has been tested in a wide range of operating conditions, with the aim of deeply characterizing the system behavior and understanding the effect of each process variable on the performance of the power plant. The main objectives of this thesis are: - presenting the characteristics and performance of the prototypal micro-ORC system - assessing the dependencies of the micro-ORC system steady-state performance from the off-design operating conditions, by analyzing the behavior of each component; - estimating experimentally the system response to transient variations of the operating conditions, in order to deduce guidelines for the control system design; - developing and validating a charge-sensitive thermodynamic model to simulate the ORC system in stationary and dynamic conditions. The analysis here presented aims at providing a contribution to the research field of micro-scale and low-temperature organic Rankine cycles, especially from the experimental point of view. The improvement of micro-ORC performance and reliability is one of the key factor for the market spread of this technology, helping to enhance the global electricity generation efficiency, save primary resources and greenhouse gases emissions, and increase the exploitation of renewable thermal sources.