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Publication Title | Exergy Analysis of Organic Rankine Cycle with Internal Heat Exchanger

Organic Rankine Cycle

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International Journal of Materials, Mechanics and Manufacturing, Vol. 1, No. 1, February 2013 Exergy Analysis of Organic Rankine Cycle with Internal

Heat Exchanger

Kyoung Hoon Kim, Hyung Jong Ko, and Se Woong Kim



Abstract—In recent years Organic Rankine Cycle (ORC) has become a field of intense research and appears a promising technology for conversion of heat into useful work or electricity. In this work thermodynamic performance of ORC with internal heat exchanger is comparatively assessed for various working fluids based on the second law of thermodynamics. Special attention is paid to the effect of turbine inlet pressure on the exergy destructions (anergies) at various system components and the exergy efficiency of system. Results show that for a given source the component at which the greatest anergy occurs differs with working fluid. As turbine inlet pressure increases, exergy efficiency increases for working fluid such as ammonia or R22, but decreases for working fluid with low critical pressure such as iso-pentane or n-pentane.

Index Terms—Organic rankine cycle (ORC), internal heat exchanger, exergy, anergy.

I. INTRODUCTION

resources with series and parallel circuits of an ORC. Tranche et al. [7] investigate comparatively the performance of solar organic Rankine cycle using various working fluids. Volume flow rate, mass flow rate and power ratio as well as thermal efficiency are used for comparison. Hung et al. [8] examine Rankine cycles using organic fluids which are categorized into three groups of wet, dry and isentropic fluids. They point out that dry fluids have disadvantages of reduction of net work due to superheated vapor at turbine exit, and wet fluids of the moisture content at

turbine inlet, so isentropic fluids are to be preferred.

Kim [9], [10] examines comparatively the thermodynamic performance of ORC with superheater or internal heat exchanger for various working fluids including wet, dry and isentropic fluids. He points out that in selection of working fluid it is required to consider various criteria of performance characteristics as well as thermal efficiency. Kim and Han [11] investigate the thermodynamic performance of transcritical ORC with and without internal heat exchanger for various working fluids. They point out that operation with supercritical cycles can provide better performance than that of subcritical cycles because of better thermal match between the working fluid and the sensible heat source. Kim and Ko [12] carry out exergy performance assessment of ORC with superheating comparatively for various organic fluids. They show that for a given source both the anergies and exergy efficiency may have a peak value or monotonically increase

with evaporating temperature.

In this paper, the thermodynamic exergetical performance

of the organic Rakine cycle with internal heat exchanger is comparatively and parametrically investigated based on the second law of thermodynamics for various working fluids. The exergy destructions (anergies) at various components in ORC including source exchanger, exhaust, condenser, and internal exchanger, as well as exergy efficiency are investigated in terms of the system parameters such as turbine

Statistical investigations indicate that the low-grade waste

heat accounts for 50% or more of the total heat generated in

industry. Due to the lack of efficient recovery methods, a lot

of the low-grade energy is merely discarded. Since the

worldwide energy demand has been rapidly increasing but

the fossil fuel to meet the demand is being drained, an

efficient use of the low-temperature energy source such as

geothermal energy, exhaust gas from gas turbine system,

biomass combustion, or waste heat from various industrial

processes becomes more and more important. ORC is a

Rankine cycle where an organic fluid is used instead of water

as working fluid and appears as a promising technology for

conversion of low-grade heat into useful work or electricity,

since it exhibits great flexibility, high safety and low

maintenance requirements [1]-[2]. In an ORC the saturation

vapor curve is the most crucial characteristics of a working

fluid. This characteristic affects the fluid applicability, cycle

efficiency, and arrangement of associated equipment in a inlet pressure. power generation system [3].

Drescher and Bruggemann [4] investigate the ORC in solid

biomass power and heat plants, and they propose a method to II. SYSTEM ANALYSIS

find suitable thermodynamic fluids for ORCs in biomass plants. They assert that the family of alkylbenzenes show the highest efficiency. Dai et al [5] use a generic optimization algorithm identifying isobutane and R236ea as efficient working fluids. Heberle and Brueggemann [6] investigate the combined heat and power generation for geothermal

Manuscript received December 5, 2012; revised February 6, 2013.

The authors are with the Department of Mechanical Engineering, Kumoh National Institute of Technology, Gumi, Gyeongbuk 730-701, Korea (e-mail: {khkim, kohj, ksw}@ kumoh.ac.kr).

The schematic diagram of the system is shown in Fig. 1. The system considered in this work consists of condenser, pump, turbine, regenerator, preheater, boiler, and superheater. A low-grade energy in the form of sensible energy is supplied to the system. The working fluids considered in this work are nine fluids of NH3 (ammonia), R134a, R22, iC4H10 (iso-butane), R152, R143a, C4H10 (butane), iC5H12 (iso-pentane), nC5H12 (normal pentane). The thermodynamic properties of the working fluids are calculated by Patel-Teja equation of state [12], [13].

DOI: 10.7763/IJMMM.2013.V1.9

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