Abstract
The analysis and optimization of an organic Rankine cycle (ORC) used as a bottoming cycle in the Brayton/ORC and steam Rankine/ORC combined cycle configurations is the main focus of this study. The results show that CO2 and air are the best working fluids for the topping (Brayton) cycle. Depending on the exhaust temperature of the topping cycle, Iso-butane, R11 and ethanol are the preferred working fluids for the bottoming (ORC) cycle, resulting in the highest efficiency of the combined cycle. Results of the techno-economic study show that combined Brayton/ORC cycle has significantly lower total capital investment and levelized cost of electricity (LCOE) compared to the regenerative Brayton cycle. An analysis of a combined steam Rankine/ORC cycle was performed to determine the increase in power output that would be achieved by adding a bottoming ORC to the utility-scale steam Rankine cycle, and determine the effect of ambient conditions (heat sink temperature) on power increase. For the selected power plant location, the large difference between the winter and summer temperatures has a considerable effect on the ORC power output, which varies by more than 60% from winter to summer.
Highlights
The increasing global energy demand, energy cost, and sustainability issues bring the need for waste heat recovery and use
The recovery of exhaust heat and its use through the organic Rankine cycle (ORC) is an efficient and flexible method with simpler structure, higher safety, and lower maintenance requirements compared to the conventional heat recovery methods, such as steam Rankine cycle
The results show that, while the wet and isentropic working fluids do not have a significant effect on thermal efficiency, dry fluids can improve thermal efficiency by more than 9%
Summary
The increasing global energy demand, energy cost, and sustainability issues bring the need for waste heat recovery and use. The study shows that ORC with dry working fluid requires less amount of waste heat to generate specified power output and lowers irreversibility by increasing cycle efficiency. Selection of working fluids resulting in best performance (thermodynamic efficiency or net specific work output) of the ORC for the specified operating conditions (maximum temperature and pressure, heat rejection temperature, and others) is a time-consuming and arduous task, especially when a large number of working fluids is being considered. The best (most suitable) working fluid, for use in ORC in waste heat recovery applications, was selected based on the selection procedure developed by the authors and published in the previous studies [31,32,33]. A techno-economic analysis of the ORC and combined Brayton/ORC cycles was performed to determine the total capital investment and levelized cost of electricity (LCOE), and allow economic comparison of the investigated cycles
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