Mehmet Selçuk MERT, Mehmet Emre BURULDAY
Introduction: The global energy consumption increases day by day. Considering the increasing energy demand and the environmental issues, the use of renewable-energy sources and development of alternative energy systems has become important to supply energy needs. At this point, the low-temperature level power cycles that use renewable sources can be considered as a promising option for thermal plants and energy systems. The low-quality heat obtained from geothermal, solar and biomass energy can be used in these cycles. The Organic Rankine Cycle (ORC) which is regarded as one of the low-temperature level power cycles can be integrated into a power plant in order to improve the plants overall thermal efficiency and the net power output. Objective: The performance of different working fluids for an organic Rankine cycle using geothermal energy source was investigated comparatively with working fluid R134a. Scope: The simulation of the process was carried out by using Aspen Hysys simulation software, and the results were evaluated. Thus, different ORC working fluids will be compared under the specific working conditions. Limitations: Geothermal energy source was used to provide the heat required by the organic Rankine power cycle. The working conditions and power output of the ORC depends on the amount of energy transferred from the geothermal source to the working fluid of organic Rankine cycle. The working fluids that can be used in the ORC are limited depending on the operating conditions of the system. Method: Organic Rankine power cycle technologies using low-to-medium energy sources (generally between the temperatures of 90-150°C) such as geothermal and solar energy have attracted attention for electricity generation. The components of the organic Rankine cycle are the same as those of conventional Rankine cycle components that mainly a circulating pump, evaporator, condenser and turbine-generator group. The thermophysical and critical properties of the working fluids affect the operating performance of ORC systems. In this study, performance analysis of different working fluids for an ORC was carried out to determine the appropriate working fluid and to produce maximum power under specific conditions. Results: A geothermal energy source was used as the heat source for an organic Rankine power cycle where R134a was used as the working fluid in the basis system. The required mass flow of the working fluid R134a, for net power output of about 12 MW, was found to be 598 kg/s. The power consumed in the circulation pump, which supplies pressurized working fluid R134a, was determined as 1400 kWe. R134a and some other organic fluids such as R114, R152a, R143a were compared based on the ORC’s thermal energy efficiency, mass flow rate, power consumption of the circulating pump and net power output of the system. As a result, the optimum working fluids were determined under the basis operating conditions of the system. Conclusion: Geothermal energy based ORC systems are important option among the renewable energy sources in supplying energy demands due to the absence of combustion and fuel cost. Furthermore, the integration of the ORCs into the thermal power plants can improve the efficiency of the overall system. The determination of the appropriate working fluid in such systems plays an important role identifying the optimum operation conditions and lowering the environmental impacts. In this study, the performance analysis of different working fluids for an organic Rankine cycle was investigated. For this purpose, the ozone depletion potential (ODP) and global warming potential (GWP) of the refrigerants were considered in order to use the suitable working fluid. Based on this criterion, the influences of selected different working fluids on the system performance were compared with the basis system operating with R134a.

Anahtar Kelimeler: Geothermal Energy, Organic Rankine Cycle, Simulation, Performance Analysis