Proposal Design Procedure and Preliminary Simulation of Turbo Expander for Small Size (2–10 kW) Organic Rankine Cycle (ORC)

Abstract

Currently, one of the leading technologies for the “energy recovery” adopting a Rankine cycle (ORC) with organic fluids. ORC system operates like a conventional Rankine cycle, but instead of steam/water, uses an organic fluid. This change allows to convert low temperature heat and generate, where required, electricity. A large amount of studies were carried out to identify the most suitable fluids, system parameters and the various configurations. In reality, most ORC systems are designed and manufactured for recovery of thermal energy from various sources but at “large power rating” (exhaust gas turbines, internal combustion engines, geothermal sources, large melting furnaces, biomass, solar, etc.) from where it is possible to produce electric energy (30kW ÷ 300kW), but for the application of this system for small nominal power, as well as the exhaust gases of internal combustion engines (car sedan or town, ships, etc.) or smaller heat exchangers, there are very few applications. The aim of this work is to design a turbo-expander that meets system requirements: low pressure, small size, low mass flow rates. The Expander must be adaptable to a small ORC system utilizing gas of a diesel engine or small gas turbine to produce 2–10 kW of electricity. The temperature and pressure of the exhaust gases, in this case study (400–600° C and at a pressure of 2 bar), imposes a limit on the use of an organic fluid and on the net power that can be produced. In addition to water, organic fluids such as CO2, R134a and R245fa have been considered. Once the fluid has been chosen operating, the turbine characteristics (dimensions, temperature, input and output pressure ratio, etc.) have been calculated and an attempt to find the “nearly-optimal” has been carried out. The detailed design of radial Expander is presented and discussed. An initial thermo-mechanical performance study is carried out to verify structural tension and possible displacement. Next step of the research here proposed will be the CFD simulation to improve or modify the chosen blade profile.