Optimization of sliding-vane expanders for a low-enthalpy ORC energy recovery system

Increasingly numerous are the experimental activities on low-grade ORC focused on expanders optimization, in order to increase the cycle net power and efficiency. In the range of small power ORCs, positive displacement expanders are more suitable than dynamic expanders thanks to lower speed, good off-design performance, high expansion ratio, low cost and simple manufacturing. This paper presents an extensive experimental analysis on a low-enthalpy ORC-based recovery system tested in different conditions: two different working fluids (R236fa and R1233zd) and two different sliding-vane expanders (ExpA and ExpB) are tested and the ORC performance are mapped in detail. A particular focus is dedicated to expander optimization: the operating conditions are deeply investigated by using piezoelectric pressure transducers to determine the expansion indicated diagram and the expander mechanical efficiency. Also the effect of the variation of both the expander and pump rotational speeds is investigated. The thermal source is the hot lubricant of a mid-size air compressor, the thermal sink is tap water.