Effects of evaporator pinch point temperature difference on thermo-economic performance of geothermal organic Rankine cycle systems

Organic Rankine cycle (ORC) systems are being used to convert medium-low temperature geothermal energy into electricity. However, the ORC system efficiencies need to be increased and the investment costs need to be reduced to further promote this technology. The evaporator pinch point temperature difference (PPTD) is a key parameter affecting the thermodynamic and economic performance. A lower evaporator PPTD leads to higher turbine power output; however, this also increases the heat transfer area and the investment cost. Therefore, this work optimizes the evaporation temperatures to maximize the net power outputs for evaporator PPTDs of 4-15 degrees C and brine inlet temperatures of 100-150 degrees C. The heat transfer area per unit power output, the levelized cost of electricity (LCOE) and the dynamic payback period (PBP) at the optimal conditions are also analyzed. ORCs produce 1.7-2.6% more net power with every 1 degrees C decrease of the evaporator PPTD for brine inlet temperatures higher than 130 degrees C. The total area per unit power output first decreases to a minimum at an evaporator PPTD of about 7 degrees C and then increases slightly with increasing evaporator PPTD. The LCOE and the dynamic PBP of a basic ORC reach a minimum at an evaporator PPTD of about 7 degrees C with the minimum at 5-6 degrees C for drilling costs higher than 500 $/m.