Internal volume ratio optimization and performance analysis for single-screw expander in small-scale middle temperature ORC system

Single-screw expander (SSE) has the potential to meet the high pressure ratio condition in small-scale middle temperature ORC system. In this paper, variable internal volume ratio (from 3.00 to 8.00), new methods for calculating mass flow rate and friction power are integrated into the structure-based SSE thermodynamic model. And the maximum calculation error of mass flow rate, volume efficiency and shaft efficiency is 2.8%, 2.1% and 2.3% respectively compared with the experimental data. Herein, the influence of internal volume ratio on shaft efficiency, shaft power, volume efficiency, intake/exhaust pressure loss and friction loss are studied. The optimal internal volume ratio and shaft efficiency for five working fluids (R123, HFO-1336mzz(Z), R601, Cyclopentane, R245fa) are obtained when evaporation temperature changes from 373 K to 463 K. The results show that the optimal internal volume ratio is not the bigger the better when SSE works at high pressure ratio condition, because intake pressure loss also increases with the increase of internal volume ratio. The optimal shaft efficiency increases with the decrease of friction power, but the optimal internal volume ratio is almost unaffected. Furthermore, reducing intake pressure loss is a powerful means to improve the performance of SSE with large internal volume ratio. •A new thermodynamic model of SSE with variable internal volume ratio was established.•The optimum operating conditions for five working fluids were obtained.•Main losses sources highlighted for large internal volume ratio SSE (intake pressure loss and friction loss).•Recommendations for large internal volume ratio SSE design.