Analysis and Design of Organic Rankine Cycle based Power Plants

Abstract

Solving the world’s energy crisis, and mitigating anthropogenic climate change, requires efficiently harnessing non-polluting renewable energy. Organic Rankine cycle (ORC) can be useful in this regard. ORC is Rankine cycle using organic compounds (pure or mixture) as working fluid. It is applicable for heat sources with temperatures of about 100 ⁰C to 200 ⁰C, such as solar thermal energy, geo-thermal energy, bio-mass energy, and industrial waste heat. This M.Sc.Engg. research project explores ORC in the following 3 parts. The first part of the thesis describes a computational analysis of working fluids for Rankine cycle. Rankine cycle converts heat energy into useful work. It consists of four basic components: pump, evaporator, expander (turbine), and condenser. An optional component is economizer: a heat exchanger to transfer heat, from low pressure vapour in section between expander and condenser, to high pressure liquid in section between pump and evaporator. This may increase efficiency, but it also increases setup cost, complexity, and maintenance. Hence, economizer’s use is justified if saving to investment ratio is high enough. Two ORC configurations are explored: without economizer, and with economizer. Based on a few inputs (working fluid, expander inlet temperature, pump outlet pressure, expander isentropic efficiency, pump isentropic efficiency, ambient temperature, and economizer’s pinch temperature difference if economizer is used), there are two outputs (net work output and overall efficiency). Using REFPROP and MATLAB, 75 pure substances that are technically feasible as working fluids for Rankine cycle are analyzed. For each working fluid, expander inlet temperature is linearly varied in desired step (example 2 ⁰C) from 100 ⁰C to 200 ⁰C, and pump outlet pressure is linearly varied in desired step (example 1 bar) from 1 bar to working fluid’s critical pressure - 5 bar. Work output and efficiency are computed for each case. Best working fluids for work output and efficiency, and corresponding temperatures and pressures, are found. Heat engine efficiency usually increases with heat source temperature. But planners of Rankine cycle setups may not have liberty to choose the expander inlet temperature, and may have to work with whatever is available. Thus, analysis is done for 75 substances as working fluids, with expander inlet’s temperatures of 200 ⁰C, 150 ⁰C, and 100 ⁰C. For 200 ⁰C expander inlet temperature, the top five for work output are water, heavy water, methanol, ethanol, and ammonia, and the top five for efficiency are methylcyclohexane, nonane, octane, n-propylcyclohexane, and heptane. For inlet temperature of 150 ⁰C, top five for work output are water, heavy water, methanol, ethanol, and ammonia, and the top five for efficiency are methylcyclohexane, benzene, isohexane, dimethyl carbonate, and cyclohexane. For 100 ⁰C, the top five for work output are methanol, ethanol, ammonia, acetone, cyclopentane, and top five for efficiency are pentane, R-365mfc, isohexane, cyclopentane, isopentane. Three dimensional graphs (work output or efficiency vs. expander inlet temperature vs. pump outlet pressure) are shown for R-245fa, toluene, cyclohexane, and benzene. Mixtures are also explored as Rankine cycle working fluids. The number of possible mixtures can be very large (for example 8,060,931 mixtures, if ten components are varied in steps of 5 %). A super-computer may be needed for systematic analysis of a large number of mixtures. Here, two sample cases are presented: one mixture (mass composition of pentane 30 %, propane 35 %, butane 15 %, hexane 20 %); and fifteen mixtures (toluene, benzene, and cyclohexane varied in steps of 25 % by mass). The second part of the thesis presents a case study on ORC at Challakere campus of Indian Institute of Science (gross power output up to about 100 kW). This ORC uses solar thermal energy and diesel combustion as heat sources. R-245fa is working fluid. There is no economizer. Thermodynamic analysis of this ORC is performed, and it is observed that the overall efficiency is about 9.6 %. We also explore what the performance would be if an economizer is used and its pinch temperature difference is 5 ⁰C. It is found that ORC efficiency increases to about 11.5 %. Analysis and Design of Organic Rankine Cycle based Power Plants. The third part of the thesis describes the design and construction of a laboratory scale ORC system at Bangalore campus of Indian Institute of Science (net power output up to around 10 kW). Thermodynamic analysis of this ORC is also performed to evaluate the performance of the system.