Comparison between ORC and CO2 power systems for the exploitation of low-medium temperature heat sources

Abstract

Low-medium temperature heat sources in the range 5–50 MWth are made available by many industrial fields but they may also be of interest for biomass, geothermal and solar energy applications. ORC has been proposed in the last 20 years as a reliable solution for the exploitation of these energy sources since the alternative represented by steam cycles leads to an inefficient conversion of such small available thermal powers. However, the use of organic fluids involves a number of safety and environmental issues, either related to fluid flammability (for hydrocarbons) or to their high Global Warming Potential (for halogenated fluids), and of limitations to the achievable cycle maximum temperature, due to fluids thermal decomposition. To overcome these limitations, in recent years CO2-based transcritical and supercritical cycles have been proposed as a viable option for this kind of applications. The present work aims to present a comparison between four CO2 cycle configurations and four ORC layouts using a working fluid selected from 47 candidates. The final result is a set of performance maps that allow for an easy selection of the best solution for applications exploiting low-medium temperature heat sources.