Off-Design Modelling of ORC Turbines for Geothermal Application

Abstract

In this study, turbine modelling of a geothermal sourced organic Rankine cycle (ORC) power plant is aimed. Thermodynamic model of the plant is constructed with the help of design and off-design plant data from an existing two-cycle power plant in southwestern Anatolia. Utilizing statistical analysis tools such as maximum likelihood estimation and probability distribution, plant variables are obtained within their standard deviations. Stodola curves and probability calculations demonstrate that both turbines are most likely to have two stages. Average losses are 2.3 MW and 1.2 MW from Turbine-I and Turbine-II respectively throughout the different seasons. After the determination of losses, overall turbine efficiencies demonstrate a reverse trend with increasing reduced mass flow rate. This may be associated with the increased choking of the turbine. Correlations estimate rather fixed efficiency values at off-design conditions (84% for Turbine-I and 77% for Turbine-II); that is an expected outcome since these correlations are influenced mainly by the design isentropic efficiency, which is a constant value. On the other hand, these correlations are most likely to be proposed for non-choking conditions which are invalid for off-design conditions of existing ORC turbines. Datapoint dispersion in Turbine-II does not demonstrate a strong correlation with physical constraints such as -pressure ratio and reduced mass flow rate- as it does for Turbine-I; this phenomenon may need further attention for future work.

Keywords

• turbine curve modelling
• orc
• statistical model
• plant variables