Multi Objective Optimization Approach for Modified Organic Rankine Cycle

Abstract

With the continuous increase in demand of energy and depletion of fossil fuels there is need of a potential technology which can generate sustainable energy without affecting the environment. From past decade organic Rankine cycle (ORC) has gained tremendous interest and many researchers from across the globe is using this economically potential technology to recover waste heat and convert it into useful form of energy (mostly electrical energy). Many industries and few renewable sources of energy can serve the purpose of heat source for ORC and it can be built in different sizes depending on the availability of resources and power requirement. However, the thermal efficiency of the ORC is very low compared to conventional steam Rankine cycles which hold back scientists and researchers to make it widely acceptable. The objective of the current work is to study the performance of modified organic Rankine cycle on the aspects of thermodynamics and economics and recommend the optimum operating condition with the aim to obtain maximum thermal efficiency at minimum levelized energy cost (LEC). Nanoparticles seeded base fluid is considered as the heat source and heat sink used for evaporation and condensation of the working fluid in the heat exchangers. Water is used as the base fluid as it is abundantly available. Nanofluids tends to improve the thermo-physical properties of base fluids and enhance the heat transfer between two streams of fluid. CuO and Al2O3 nanoparticles are considered for the investigation because of the ease in availability at low prices. Preliminary single objective optimization is carried out for three variants of ORC viz. basic ORC, regenerative ORC and ORC with internal heat exchanger (IHE) to study the effect of design variables and subsequently multi-objective optimization is performed to obtain the Pareto optimal curve between two objective functions. Genetic algorithm (GA) and heat transfer search (HTS) algorithm is used to optimize the objective functions which is to maximize thermal efficiency and minimize LEC. Pareto front obtained between the two objective functions suggest the optimum operating conditions for different design variables. Moreover, effect of mass flow of refrigerant, concentration of nanoparticles in evaporator and condenser, pinch point temperature difference and evaporation pressure on the thermal efficiency and LEC is investigated and reported in the thesis. Heat exchanger serves a very important role in waste heat recovery systems and hence investigation is done with the systems using shell and tube heat exchangers (STHE) and plate heat xix exchangers (PHE). Former are preferred for cleaning and maintenance as nanofluid tends to get unstable over the period of time. However later is preferred for compactness and high heat transfer coefficient. Finally the thesis concludes with the suggestions and recommendations for operating ORC with modifications and improvements to obtain the maximum thermal efficiency at minimum LEC. Recommendations to extend the work in future with its scopes is presented towards the end of the thesis.