Experimental investigation and modeling of a hermetic scroll expander
Résumé
As part of the Enerco_LT (ENErgy ReCOvery from Low Temperature heat sources) project, hermetic scroll expander is candidate for small-scale ORC prototypes. The aim of this work is (i) to investigate on a test bench the performances of a small-size scroll expander which is a converted compressor and (ii) to develop, based on the experimental results, a semi-empirical steady-state model which can be easily integrated as a sub-model in a larger system simulation model.
The expander was tested by means of a superheated vapour cycle (Brayton) fed with R-245fa and providing defined ranges of supply and exhaust operating conditions. The assessment of the internal unknown variables has permitted the derivation of the model correlations. The supply pressure drop and the internal isentropic efficiency are expressed as polynomial laws of the main working conditions. The model was validated by comparing predictions to the experimental results which has shown a fairly good agreement and accuracy.
The experimental investigation has revealed promising performances of the hermetic scroll expander given at high supply conditions. The net efficiency of the combined expander-generator reaches 70% from supply pressures of 20 bar. Furthermore, the numerical investigation using the developed model has pointed out the performance deterioration when the pressure ratio deviates from the expander built-in volume ratio and indicates consequently how optimizing such machines to ensure the highest performances.