Thermodynamic and economic investigation of an ionic liquid as a new proposed geothermal fluid in different organic Rankine cycles for energy production

Abstract

Organic Rankine cycle (ORC) is a promising technology for electricity generation by utilizing heat sources at low to moderate temperatures. In the present work, an ionic liquid (1-butylpyridinium tetrafluoroborate, C9H14NBF4), a known green chemical and non-volatile compound with good thermal and chemical stability, and significant heat capacity are utilized as a geothermal fluid. Simulation and optimization were conducted for a basic ORC, a regenerative organic Rankine cycle (RORC) and a two-stage evaporative organic Rankine cycle (TSORC) by maximizing exergy efficiency and minimizing specific investment cost. The operating parameters considered in the optimization exercise were evaporative and regenerative temperatures, pinch point temperature difference of evaporators and the degree of superheat. Peng Robinson equation of state was used and isobutane (R-600a) and butane (R-600) were selected as working fluids. Comparisons were made between the performance of ORCs when C9H14NBF4 and water were individually used as a geothermal fluid. Utilization of an ionic liquid improved the performance of the three ORC configurations with the best performance generated from a basic ORC with exergy efficiency of 82.35% (R-600) and 87.70% (R-600a). From an economic viewpoint, the amount of specific investment cost (SIC) is comparatively lower when ionic liquid is used as a geothermal fluid.