Analysis of 5 KW ORC system operating with R245FA and plate type evaporator and condenser

In today's world, the major part of the electrical energy is produced by consumption of hydrocarbon fuels in combined power plants. The thermal efficiency of these plants is higher, because of high source temperature that is occurred by burning of hydrocarbon fuels. Utilization of natural sources like solar and geothermal energies and industrial waste heat (flue gases, exhaust gases, etc.) had not been economically efficient because of their low temperature. However, through previous years by using hydrocarbon-based fluids instead of water, efforts are in progress on the utilization of a steam power system called ORC, which utilizes low-grade heat sources. Especially ORC systems are established for utilization of geothermal sources. In this study, a 5 kW ORC heat engine is modeled. Evaporator and condenser of the engine are chosen as plate type heat exchanger (HEX). The working fluid is selected as R245fa. This ORC system consists of evaporator, condenser, expander and the pump; in addition to these components, the expansion valve is added which is basically used in times of start-up and for adjusting the power output of the system. In this work, the heat source is specified as pressurized water of 125 C temperature based on Turkey's geothermal resources. Heat sink, however, is specified 25 C based on the climate of the country. Flow and heat transfer formulations are made for all of the units and coded in Mathcad program to parametrically perform thermal calculations. Working fluid is considered as compressed liquid at inlet, and superheated vapor at outlet in the formulation of the evaporator. Since the evaporator is considered as a single pass plate heat exchanger (PHE), starting points of boiling and superheating in the plate channels are calculated in analyzing of the evaporator. Suitable friction factor and Nusselt correlations are chosen for single and two-phase regions in the evaporator and condenser. Evaporation temperature of 75.2 C and condensation temperature of 40 C values are selected as design conditions for our system. Pump and expander efficiencies are set as 35.6% and 75% , respectively. Evaporator and condenser capacities are calculated as 80.10 kW, and 75.10 kW, respectively. The thermal efficiency of the ORC cycle is obtained as 6.242% while pump and expander powers are 0.3 kW and 5.3 kW. On the other hand, the final condenser power is calculated as 80.40 kW when the working fluid passes completely through the expansion valve. For two different chevron angles of 45 and 60 degrees, heat transfer coefficients (HTC) and pressure losses are compared with each other. Plates with 60-degree chevron angles have higher HTC and higher pressure loss then 45-degree chevron angle. Pressure loss in HEXs decreases cyle thermal effıciency by 0.444% for chevron angle of 45 degrees and 0.561% for chevron angle of 60 degrees. Evaporator and condenser calculations performed by taking into account the plate number and dimensions. Required hot water mass flow rate with 125 C temperature is 0.901 kg/s in evaporator. For condenser, however, required cold water mass flow rate with 25 C temperature is accuired as 2.55 kg/s and required cold water mass flow rate when fluid passes expansion valve is obtained as 2.62 kg/s. In design conditions, HEX efficiencies are obtained as 0.53 for preheat region, 0.3 for evaporator region, and 0.98 for superheat region. In case of the condenser, for desuperheater region efficiency is obtained as 0.85 and for condenser obtained as 0.6. Regarding the effect of cold water temperature change on condenser, results indicate that cold water temperature varies between 21-28 C. Fully condensed fluid at condenser outlet can be maintained by adjusting the mass flow rate of cold water in this range of temperature. However, extra condenser is required for water temperatures over 28 C, because of excessive required water mass flow rate to have fully condensed fluid at the condenser outlet. Increase in pump and expander isentropic efficiencies can positively affect cycle thermal efficiency. Increasing of evaporation temperature and decreasing of condensation temperature can also improve thermal efficiency. Overall area of HEXs for the same output power is decreasing by increasing of evaporation temperature until 95 C.