Assessment of the differential impact of scroll and sliding vane rotary expander permeability on the energy performance of a small-scale solar-ORC unit

Organic Rankine Cycle ORC-based power unit is a suitable solution for combined heat and power (CHP) generation in the residential sector currently responsible for an average 28 % energy-related CO2 emissions. The ability to tolerate seasonal daily and hourly fluctuations of the available thermal energy without significant performance compromission - i.e. efficiency of electricity generation and domestic hot water (DHW) fulfillment - is crucial. The need for the plant to work under severe off-design conditions affects the expander selection which for such plants are generally of volumetric technology. The most up-to-date literature addresses scroll-type and sliding vane rotary (SVRE) expanders as the most effective options. For such reason the present paper deals with the experimental assessment of the energy performance of an ORC-based plant equipped with both a customdesign SVRE and a scroll expander. The ORC unit assumes the same plant configuration in the two cases except for the expander layout. SVRE is connected through a mechanical joint to the electric generator whereas the scroll expander shares the same shaft and casing with the electric machine. The former approach ensures to control the expander speed which is set by the dynamic equilibrium in the latter case. The comparative analysis provides useful insights on the relative advantage of both technologies and configuration and a clear indication on the different operating strategy required in each case. SVRE leads to an electric output of the power unit in the 200-700 W range with a 2-6% efficiency whilst scroll expander is associated with a 100-500 W power range and a 2-4% efficiency partly compensated with a shorter starting time and a larger operability range (17-60 g/s). Despite SVRE is regulated in revolution speed its efficiency (20-40 %) is lower than the scroll one (40-60 %). The experimental assessment of the power unit was coupled with an in-depth modeling activity to address the impact of the expander operating conditions mainly on the inlet expander pressure which is close to the upper pressure of the thermodynamic cycle. The model derived from a theoretical analysis of the expander permeability focuses on the effect of the expander revolution speed - variable in the SVRE case constrained in the scroll case - and provides good accuracy with a Root Mean Square Error below 4 % in both cases.