Impact of built-in and actual expansion ratio difference of expander on ORC system performance

Built-in expansion ratio is set with screw expander design, influenced by the geometric shape, size, and flow characteristic. Actual expansion ratio, which equals to the evaporation pressure divided by the back pressure of discharge line, is determined by operating conditions. In the previous ORC system studies, the efficiency of expander is usually assumed as constant. However, the built-in and actual expansion ratio difference affects the screw expander efficiency greatly in a practical application. Therefore, a correction factor is needed for characterization. The purpose of this paper is to study the impact on ORC system performance caused by the expansion ratio difference. Six working fluids (R123, R245fa, n-Pentane, Isopentane, n-Hexane and Cyclohexane) have been selected as the candidates and system net output and thermal efficiency have been chosen to evaluate the system performance. The test system is defined as a kilowatt class since the net output is about 1 kW. In the whole simulation, the heat source condition remains unchanged, the condensation temperature maintains 303.2 K and the evaporation temperature ranges from 333.2 to 393.2 K. The results show that the correction factor decreases faster in the under-expansion condition than it does in the over-expansion condition. Furthermore, the actual expansion ratio which results in the maximum net output is not equal to, but higher than the built-in one. After correction, all the optimal evaporation temperature, maximal net output and thermal efficiency have been reduced, as is meaningful in the system determination.