Energy and Exergy Analyses of Geothermal Organic Rankine Cycles Considering the Effect of Brine Reinjection Temperature

Abstract

The organic Rankine cycles (ORCs) have been used to convert low-enthalpy geothermal brine into power. Parameter optimization and working fluid selection are the main approaches to enhance geothermal ORC performance. This work uses environmentally friendly fluids, including R1224yd(Z), R1233zd(E), R1336mzz(Z), R601 and R601a, as the geothermal ORC working fluids. The evaporation temperatures of the selected fluids were optimized to maximize the cycle net power outputs. The thermodynamic characteristics are investigated with the consideration of the influence of the allowed reinjection temperature (ARIT). Among the selected fluids, R1224yd(Z) has the highest optimal evaporation temperature with the maximum turbine power output for a brine inlet temperature (BIT) higher than 120 °C, especially at a lower allowed reinjection temperature. However, the parasitic power consumption by the pumps in an ORC with R1224yd(Z) is also higher than with the other four fluids. The net power output for ORC with R1336mzz(Z) is slightly more than that with R1224yd(Z). Although the optimal evaporation temperature for a RORC is lower than that for an ORC, the higher preheater inlet temperature leads to a higher geothermal heating exergy efficiency and more power output for a BIT less than 120 °C. The RORC with R1336mzz(Z) produces 2.6% more net power than an ORC for a brine inlet temperature of 100 °C. As the ARIT increases from 70 °C to 75 °C, the plant exergy efficiencies of ORCs are decreased by 6–8% for a geothermal brine inlet temperature of 100 °C.

Keywords

• geothermal power generation
• organic Rankine cycle
• exergy analysis
• working fluid
• optimization