Abstract
The influences of various evaporators on the system responses of a 50 kW ORC system using R-245fa are investigated in this study. First the effect of the supplied hot water flowrate into the evaporator is examined and the exit superheat on the system performance between plate and shell-and-tube evaporator is also reported. Test results show that the effect of hot water flowrate on the evaporator imposes a negligible effect on the transient response of the ORC system. These results prevail even for a 3.5-fold increase of the hot water flowrate and the system shows barely any change subject to this drastic hot water flowrate change. The effect of exit superheat on the ORC system depends on the type of the evaporator. For the plate evaporator, an exit superheat less than 10 °C may cause ORC system instability due to considerable liquid entrainment. To maintain a stable operation, the corresponding Jakob number of the plate heat evaporator must be above 0.07. On the other hand, by employing a shell and tube heat evaporator connected to the ORC system, no unstable oscillation of the ORC system is observed for exit superheats ranging from 0 to 17 °C.
Highlights
The Organic Rankine Cycle (ORC) is similar to a conventional Rankine cycle except it uses organic fluids as a working fluid rather than water
It had been statistically estimated that the low-grade waste heat accounts for more than 50% of the total amount of heat generated in industry [2], exploitation of the ORC for harvesting low grade energy has become quite promising for the past few decades
The working fluids applicable for ORC system depend on the temperature of waste heat
Summary
The Organic Rankine Cycle (ORC) is similar to a conventional Rankine cycle except it uses organic fluids as a working fluid rather than water. A recent review by Vélez et al [6] pointed out the diversity of ORC applications, including waste heat recovery, solar energy utilization, biomass application, water desalination, geothermal systems, or engine exhaust gases, and the like. Wang et al [7,8] conducted an experimental study to investigate the performance of a low-temperature solar Rankine cycle system using R-245fa Their measured cycle efficiency was lower than the theoretical value. From the foregoing discussion of previous efforts, it appears that the dynamic characteristics of the heat exchangers have a significant impact on ORC systems It is the purpose of the present study to extend further examinations of the influences on the system performance subject to various types of evaporators. Both a plate heat exchanger and a shell-and-tube heat exchanger can be used as the evaporators, and these different evaporators may give rise to different system responses, which will be reported in this study
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