Potential of organic rankine cycle based heat recovery systems for power generation

Abstract

Due to intense fuel dependency on energy production in the world, cost of energy has a greater bearing on the prices of fossil fuels. Most of the countries in the world are suffering due to this and Sri Lanka is no exception. It is in this context promotion of optimize the usage of thermal power generation, is so vital to the country. Even though fossil fuel base power generation plays a greater role as a source of primary energy in the country, major portion wasted to environment. WHR systems have been already introduced, but most of them are not performing effectively and efficiently. On other hand, novel systems and technologies required to investigate, to recovery most of the wasted heat of thermal plant while increasing the system efficiency and reducing the fuel cost. Conceptual thermodynamic cycles such as Trilateral Flash cycle, Organic Rankine cycle, Kalian cycle and Gaswami cycle, can be successfully incorporate for WHR applications. Hence, purpose of this research was to assess the amount of waste heat generated by thermal plants in the country while discussing the possible technologies that can be introduce for heat recovery. Further, discuss about selection of most suitable option and carryout thermo-economic analysis as a case study. Fluid selection and system optimisation based on heat source temperature are two most critical aspect of Organic Rankine Cycle. Eleven fluids were investigated to optimize the work output by varying the evaporator temperature and varying the expander pressure ratio with theoretical model. In evaporator analysis, Heptane, Pentane and Decane shows favourable results in terms of work outputs while, in terms of efficiency, Decane and Heptane are better. Further it is recommended to use fluid Pentane, when source temperatures of WHR lies between 45 – 190 oC, while fluid Heptane is recommended when source temperature between 190 – 260 oC. Fluid Decane is recommended when temperature between 260 – 340 oC. Respective monographs were developed where one point on the graph can denote approximate work output, efficiency, pressure, temperature, etc. Based on expander analysis, Decane, Heptane and Toluene fluids have shown higher work outputs while, in terms of efficiency, Decane is better. In expander selection, when inlet/outlet pressure ratios are less than 10, fluid Decane is recommended. Further, when ratios are in between 10 – 13 and 13 – 20, fluid Heptane and fluid Toluene are recommended respectively. Refer to these 03 fluids, monographs were developed accordingly. Refer to optimum working regions of temperature analysis; fluids were selected for economic evaluation. Waste heat recovery opportunities were selected from existing thermal plants for the case study and electric outputs were obtained for each plant, based upon selected fluids from theoretical model. Then maximum work out of each opportunity was selected for further economic evaluation under 07 different scenarios. Possible future economic situations of the country were predicted under those scenarios and carryout NPV calculations for each, to evaluate the investment feasibility. Scenario 2, 3 and 7 are the most possible situations of the country in future and for those conditions, WH opportunities at Supugaskanda, Lakvijaya, Keravalapitiya and Kelanithissa are most feasible to recover waste heat with ORC system.