Investigation of a scroll expander driven by compressed air and its potential applications to ORC

Micro combined heat and power systems using an organic Rankine cycle (ORC) as a prime mover are under development. The ORC is capable of utilizing various heat sources. The key component in the ORC is an expander to produce the electric power, yet such an expander for residential applications has, to date, not been commercially available. The present study aims to investigate the performance of a scroll expander that has been converted from a commercial refrigeration scroll compressor, as these scroll machines are generally mass-produced as compressors in refrigeration applications. The viability of converting a scroll compressor into an expander for use in an ORC was examined in this paper. A test rig was built where the expander was driven by compressed air and the scroll expander was evaluated under various operating conditions, providing fundamental data that could form the basis for ORC applications. It has been found that conversion of a scroll compressor into an expander is feasible without major modifications. Experiments showed that the power output of the expander increased significantly with increasing rotating speed. The mechanical efficiency of the expander ranged from 62.7% to 60.9% at the rotating speeds of 1600-3995 rpm and the adiabatic efficiency increased from 55.5% to 58.5% at the tested rotating speeds, whereas the total efficiency varied between 34.8% and 35.6%. Based on the expander tested, an ORC system was specified and the resulting efficiency of the ORC, through modelling, was determined to be 6.9%. It was noted that the built-in volume ratio of the compressor may need to be optimized, when operating in reverse, as an expander, in order to achieve a higher ORC efficiency.