Thermodynamic Analysis of Organic Rankine Cycles

In this thesis, investigations are carried out in order to find the best suitable working fluid for a given temperature limits of Organic Rankine Cycles (ORCs) with the focus on thermodynamic analysis, safety and environmental aspect. Making a suitable choice of working fluid for the ORC is of vital importance for the cycle efficiency and net work output. In this study only the dry and isentropic working fluids are investigated from different perspectives. Engineering Equation Solver (EES) software package was used as the main source to find thermo-physical properties of the working fluids. The data from EES were transferred to the VisSim software package and different block models were developed for the simulation of ORCs. Five organic working fluids used for this study are Butane, Isobutane, R245fa, R236fa and R124. The cycle operating parameters are varied and the effect of these parameters on the cycle performance and net work produced are investigated. Based on the simulations utilizing VisSim software, the cycle using butane as working fluid achieved the highest thermal efficiency (21.42%). The thermal efficiencies obtained for the ORCs using R245fa, Isobutane, R236fa and R124 as working fluids are 21.09, 20.87, 20.37 and 19.6% respectively. Although the thermal performance of the ORC utilizing R245fa is higher than the cycle utilizing Isobutane, specific net work produced by the cycle using isobutane is higher than the cycle using R245fa. Butane has better thermodynamic properties but it is not environmentally friendly because it is highly flammable and highly toxic with no ozone depletion potential (ODP) but with a global warming potential (GWP) of 20. The R245fa which also has good thermodynamic properties is not flammable and nearly non-toxic with no ODP and a GWP of 950. The five working fluids are investigated and the choice of the working fluid is based on the safety and its environmental characteristics. The R245fa is found to be the most suitable organic working fluid compared with the other four working fluids. Keywords: Organic Rankine Cycle, Low grade heat, Simulation