Thermodynamic optimization of Organic Rankine Cycles for low and ultra low grade waste heat recovery applications: influence of the working fluid on the ORC net power output

The present work is focused on the thermodynamic optimization of Organic Rankine Cycles (ORCs) for power generation from low and ultra low grade waste heat recovery. The paper is aimed at providing a preference selection order of working fluids for applications characterized by low and ultra low heat source temperatures (from 80°C to 150 °C). Among the commonly available working fluids, a selection based on environmental and technical criteria was carried out resulting in a list of 12 working fluids: R245fa, R245ca, R1234yf, R134a, R227, R236fa, RC318, Isobutane, Butane, Isopentane, Pentane. A model of a simple ORC cycle was developed and optimized by means of a recent evolution of the Particle Swarm Optimization (PSO) algorithm. The evaporation pressure and the approach and pinch point temperature differences have been chosen as decisional variables. Two refrigerants (R1234yf and R134a) were able to maintain good performance in the whole considered range of temperature, whereas due to their thermos-fluid-dynamic properties the hydrocarbons always remained near the bottom apart from the temperature. However, for heat source temperatures lower than 100 °C, the difference between the most and the less performing fluid in terms of net power output and system efficiency significantly reduces, demonstrating that for ultra low waste heat recovery applications the net power output resulted not to be a really effective selective criteria and should be combined with other environmental and/or technical criteria (minimum costs, minimum environmental impact, maximum heat exchange efficiency…).