Modelling approaches of micro and small-scale organic Rankine cycle systems: A critical review

Organic Rankine cycle (ORC) systems are a technology capable of producing electricity and heat from a wide range of energy sources and are particularly well-suited for medium and low-temperature sources. However, an almost infinite number of technical solutions (cycle configurations, working fluids, components, etc.) can be adopted making the full experimental characterisation of ORC operations for each application unfeasible. To overcome the limitations of extensive experimental investigations, numerical tools are often adopted, thereby supporting the design and operation of these plants. Therefore, over the last two decades, many researchers have put their efforts into developing models to elucidate the design and off-design performances of ORC systems. In this paper, the different modelling approaches for the analysis of ORC systems are discussed and a conclusive review is performed concerning the micro and small-scale ORCs. In total, more than 150 works are reviewed with many of them related to models of volumetric machines and assumption-based system modelling. Semi-empirical models of expanders show good capabilities and accuracy (with errors below 5%) while spatial resolution methods for heat exchangers are used to better capture the dynamics of the system. However, only a limited number of papers (10) deal with assumption-free models of the systems to predict their performance considering the actual boundary conditions. In summary, the present review paper provides a clear overview of the advantages and disadvantages of each modelling approach at both component and system levels to provide insights for interested readers in the advanced simulation of micro and small-scale ORC systems.