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.