Thermodynamic and economic analysis of ORC with optimiation of system components

Abstract

Organic Rankine cycles (ORCs) are used to generate power from low temper ature heat sources. There are two main chapters in this study. In both parts, 28 different working fluids are used for three different heat source temperatures (90, 120 and 150 oC). In the first part, a preliminary radial-inflow turbine (RIT) design with loss calculations is incorporated into the model to add dynamic turbine efficiency. The turbine model is run together with the ORC, and real gas properties are used. A genetic multi-objective optimization algorithm (NSGA-II) is used to obtain an ORC and design of system components according to minimum thermal conductance per net power output and maximum performance factor (PF). The decision variables are spe cific speed, condensation pressure, pressure ratio through turbine, radius ratio at the nozzle, degree of superheating and pinch point temperature difference (PPTD) in the evaporator. Pareto frontiers are obtained as a result, and a decision-making method (TOPSIS) is used to select an optimum solution for each working fluid. R1234yf, R1234ze(e) and isobutane are found to be the optimum working fluids for 90, 120 and 150 oC respectively. For each solution, turbine geometry, fluid velocities and rotor rotational speed are calculated. In the second part, a heat transfer model is added into the model used in the first part for the evaporator preferred as a plate heat exchanger (PHE). The first objective function is redefined, and evaporator heat transfer area is used instead of minimum thermal conductance. The length, the width of the plates and the spacing between the plates in PHE are added as new decision variables to optimize evaporator geometry. R1234yf is found to be the optimum working fluid for 90 oC source temperature, and butane is obtained to be ideal for both 120 and 150 oC unlike the first chapter.