A non-structural fuzzy decision method developed for organic Rankine cycles used in liquid-dominated geothermal fields of medium/high temperature

Abstract A reliable decision-making method is of great importance for the designing of a practical and efficient organic Rankine cycle (ORC) system employed to exploit geothermal energy. This paper develops a three-level non-structural fuzzy decision algorithm for the comprehensive evaluation of a geo-fluid driven trans-critical ORC (TORC) system on the basis of a progressive system performance hierarchy, involving environmental characteristics, safety, thermodynamic and techno-economic performance. Two representative geothermal reservoirs with medium (GR-I) and high (GR-II) temperature are investigated to realize and validate the proposed method. Four mathematical models and six working fluids with thirteen indexes are developed to fulfill the performance evaluation and decision-making courses. Parametric analysis results of the decision criteria are conducted including specific net out power ( A P net ), thermal efficiency ( η t ), exergy efficiency ( η e ), heat transfer area per net output power (APR) and electricity production cost (EPC), and the different performance of TORC for GR-I and GR-II are fully revealed. As for the GR-I, the result of the three-level fuzzy decision ranking order is R142b, R134a, R290, R1270, R227ea and R143a. In regard to the GR-II, it’s R142b, R1270, R134a, R290, R227ea and R143a. Both show that R142b performs best. In the GR-I and GR-II, R142b obtains the maximal A P net of 110.94 kW/(kg·s−1) and 198.14 kW/(kg·s−1), the maximal η t of 14.05% and 14.43%, the maximal η e of 51.42% and 42.90%, the minimal APR of 0.262(m2/kW) and 0.185(m2/kW), the minimal EPC of 0.030($/(kW·h)) and 0.022($/(kW·h)). Summarily, this three-level fuzzy decision evaluation method can provide important guidance and decisive solution by concisely display the pros and cons for each ORC scheme of geothermal resource utilization.