Air Cooled Two-Stage Rankine Cycles for Large Power Plants Operating With Different Working Fluids: Performance, Size and Cost

Abstract

A two stage Rankine cycle for power generation is presented in this paper. It is made of a water steam Rankine cycle and an Organic Rankine bottoming Cycle. By using an organic working fluid with higher density than water, it is possible to reduce the installation size and to use an air-cooled condenser. Following our previous studies, 3 high critical temperature organic fluids, R245fa, R365mfc, isopentane (iC5) and ammonia are tested as potential candidates for this application. The performances of the two stage Rankine cycle operating with those different working fluids are evaluated for a nuclear plant case. The size of system components (heat exchangers and turbine) is estimated for each tested fluid. The influences of their thermodynamic and transport properties are analyzed. In addition, an estimation of the installation cost is done by introducing cost functions.

Utilizing Computational Methods to Identify Low GWP Working Fluids for ORC Systems

“…Compared with the traditional steam Rankine cycle, organic fluids used in ORCs with high critical temperature, low triple point temperature, and low condensation temperature contribute notably to the recovery of heat from low temperature sources in terms of system performance [8].…”

“…Critical temperature is considered as the primary criteria to select the working fluids by most studies. Liu et al [8] indicated that the critical temperature should be as high as possible compared with the evaporation temperature in the application of a two-stage Rankine cycle in terms of ORC efficiency.…”

“…The triple point temperature should be as low as possible [7], and the condensation temperature should be below the lowest temperature of the ORC system, in case that the working fluids might freeze at any operating temperature in the cycle. Then, a condensation pressure slightly greater than the ambient pressure is preferred to prohibit air entering the ORC system [8]. In addition, the properties of organic fluids vary with operating conditions, and the thermal stability of working fluids need to be guaranteed at the range of working pressure and temperature for fear of chemical deterioration and decomposition at high temperature [10].…”