DEVELOPMENT OF A MODEL FOR THE SIMULATION OF ORGANIC RANKINE CYCLES BASED ON GROUP CONTRIBUTION TECHNIQUES

Many industrial sectors and applications are characterized by the availability of low enthalpy thermal sources with temperatures lower than 400 °C, such as the ones deriving from both industrial processes (e.g. combustion products from gas turbines and internal combustion engines, technological processes and cooling systems) and renewable sources (e.g. solar and geothermal energy). The usual systems for the conversion of thermal energy into mechanical and/or electrical energy work due to the high temperature difference available between the source (i.e., combustion products) and the sink (i.e., the ambient). The Organic Rankine Cycle (ORC) is a promising process for conversion of heat at low and medium temperature to electricity. An ORC system works like a Clausius–Rankine steam power plant but uses an organic working fluid instead of water. A certain challenge is the choice of the organic working fluid and of the particular design of the cycle. The process should have high thermal efficiency and allow a high coefficient of utilization of the available heat source. Moreover, the working fluid should fulfill safety criteria, it should be environmentally friendly, and allow low cost for the power plant. An important aspect for the choice of the working fluid is also the temperature of the available heat source, which can range from low (about 100 °C) to medium temperatures (about 350 °C). In this paper, a model for the simulation of Organic Rankine Cycles is presented. The model is based on thermodynamics tables for the calculation of fluid properties and the Lee-Kesler method for the calculation of specific heat. Six commonly used working fluids (propane, butane, benzene, toluene, R134a and R123) are considered. Both saturated and superheated cycles are evaluated. A sensitivity analysis of the main process parameters is performed. Finally, the model is applied to a micro gas turbine/ORC combined cycle.