A study of organic Rankine cycle systems with the expansion process performed by twin screw machines
The prediction of the performance of energy systems that recover power from low grade heat is one of the most important requirements for reducing their investment cost and optimising system efficiency. The aim of this work was to study, model and analyse an Organic Rankine cycle (ORC) system using a twin screw expander to generate the power output, with HFC-245fa, as the working fluid. A software package (Power Plant Performance Prediction Program), simulating ORC system performance was therefore prepared for this purpose. Major components were represented by proper units and relations between the system’s constituents defined. The preferred analytical procedure depends on both the system complexity and the requirements of the study. In this case, the whole cycle was simulated in order to obtain a good understanding of its behaviour with the aim of estimating its optimum operating conditions. The procedure adopted was to start from a basic case and then improve it, in a realistic way, in order to evaluate the system potential. Performance indicators, like thermal efficiency, specific net output, total UA and surface of the heat exchangers, as well as the relative cost of the system all need to be taken into account but it is impossible to optimise all of them simultaneously. The design value for these parameters is therefore a matter of choice, or compromise. Efficiencies of ORC systems were calculated based on the assumption that the working fluid entered the expander as wet vapour. For the heat source and sink conditions chosen for this study, the overall cycle efficiency was estimated as approximately 6% using R245fa. This and the power output are highly dependent on the ambient air temperature when using air-cooled condensers. Allowing for a small degree of subcooling at the condenser exit, it is shown that the heat recovery should be maximised.
TA Engineering (General). Civil engineering (General)
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