Influence of superheat and expansion ratio on performance of organic Rankine cycle-based combined heat and power (CHP) system

Abstract

The purpose of this study is to investigate the performance characteristics of subcritical organic Rankine cycle (ORC) based combined heat and power (CHP) system according to a variation of major operating parameters and to confirm the possibility of system performance improvement. The CHP system is designed to provide output using a 200 °C heat source, including at least 2 kW of electric power and 25 kW of thermal power with 60 °C of hot water supply. Besides examining a basic ORC-based CHP system, variations of system configurations combined with heat exchangers for additional heat recovery have been considered. The effects of changes in superheat and expander volumetric expansion ratio on system performance were evaluated based on thermodynamic analysis and optimization results for each system and working fluids.

Eight candidate working fluids selected for superior thermophysical properties and environmental impacts were classified into three groups according to the magnitude of latent heat, and four types of CHP system configuration with additional heat exchangers were considered. Group A working fluids with the largest latent heat include cyclopentane, isopentane, n-pentane, and diethyl ether, while medium latent heat group B working fluids include R-1233zd and R-245fa. The smallest latent heat group C working fluids include HFE-7000 and HFE-7100. The operating conditions and performance of the CHP systems were calculated considering the fixed heat source temperature (200 °C), the superheat amount, the volumetric expansion ratio and the isentropic efficiency of the expander for parametric analyses.

Studies have shown that the group A working fluid systems are superior in performance to other fluid systems. According to the parametric analyses, the increase in superheat has a positive effect regardless of the configuration in the system using the group B working fluids. Other working fluid systems can also be expected to enhance the performance by increasing the degree of superheat in system configurations where an internal heat exchanger (IHE) and a post-heater (PH) are employed together. Increasing the volumetric expansion ratio and the isentropic efficiency of the expander improves the performance of all working fluid systems. Especially, it has been concluded that the addition of the post-heater is essential if the CHP performance is to be improved by increasing the volumetric expansion ratio and the isentropic efficiency of the expander.