Modelling investigation on the influence of measurement error on performance evaluation of organic Rankine cycle system

Abstract

AbstractThe evaluation of the performance of organic Rankine cycle (ORC) depends on the measured data of the operating parameters. The influences of the measurement errors during the evaluation of the system’s performance, which are derived from the measuring instruments, are rarely studied in the literature. In order to explore the effects of measuring instruments’ accuracy on the evaluation of system performance, the error propagation model of a subcritical ORC system was established. The working fluids adapting to different operating temperature range, including R134a, R245fa and R123, were selected. Temperature, pressure and flow rate of fluid were adopted as the operating parameters that were measured. The net output power and the efficiency were used as the performance parameters that were evaluated. Sensitivity analyses of the measurement errors of single parameter and multiple parameters were conducted under five designed conditions. The results show that the key positions of the measurement errors affecting the performance evaluation were at the expander’s inlet, followed by the expander’s outlet, and the inlet and outlet of the fluid pump. To keep the errors of net output power and thermal efficiency within ± 10%, the measurement errors of the temperature at the inlet and outlet of the expander and fluid pump were less than ± 2%, ± 5%, ± 9% and ± 9% in all cases, respectively. To meet the same requirements, the measurement errors of the pressure at the inlet and outlet of the expander were less than ± 10% and ± 100% in all cases, respectively. The fluid adapting to higher operating parameters allowed higher measurement errors for both the temperature and the pressure. To keep the errors of net output power and thermal efficiency within ± 10%, the allowable measurement error of the expander’s inlet temperature increased from ± 0.8% for R134a to ± 2.0% for R123. The allowable measurement errors at the expander’s inlet decreased with the increase in the operating parameters for the same fluid. In most of the operating parameters, the allowable measurement errors of R245fa were larger than R134a and less than R123 under the same operating temperature range. The importance of the accuracy of temperature’s measurement outweighed those of pressure and flow rate. Finally, recommendations were made for the selection of accuracy of the measuring instruments.