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Publication Title | PARAMETRIC ANALYSIS OF CARBON DIOXIDE TRANSCRITICAL AND SUPERCRITICAL POWER CYCLES USING LOW GRADE HEAT SOURCE

Organic Rankine Cycle

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VOL. 10, NO 21, NOVEMBER, 2015 ISSN 1819-6608

ARPN Journal of Engineering and Applied Sciences ©2006-2015 Asian Research Publishing Network (ARPN). All rights reserved.

www.arpnjournals.com

PARAMETRIC ANALYSIS OF CARBON DIOXIDE TRANSCRITICAL AND SUPERCRITICAL POWER CYCLES USING LOW GRADE HEAT SOURCE

Baheta A. T. and Mohammed A. Emad

Universiti Teknologi Petronas, Mechanical Engineering Department, Tronoh, Perak, Malaysia E-Mail: aklilu.baheta@petronas.com.my

ABSTRACT

This paper considers carbon dioxide transcritical and supercritical power cycles driven by low temperature flue gases exhaust from a gas turbine. Transcritical CO2 Rankine and supercritical CO2 Brayton cycles were studied at steady state conditions and their performance were compared. Furthermore, the study carried out parametric analysis to investigate the cycles’ performance in terms of thermal efficiency and the network output at different turbine inlet temperatures. A mathematical model was developed to carry out the analysis based on the first law of thermodynamics. In order to simulate cycle performance and generate parametric tables a simulation model was developed using Engineering Equations Solver (EES). The efficiencies of the cycles were compared and it was found that transcritical Rankine cycle generates higher efficiency and net power output compared to supercritical Braytoncycle for the same turbine inlet conditions. Parametric analysis showed that as the turbine inlet temperature increases, the gas heater pressure that gives optimum efficiency increases.

Keywords: Power cycles, carbon dioxide trans critical, engineering equation solver, parametric analysis.

INTRODUCTION

The emerging environmental problems associated with power generation by burning the fossil fuel has encouraged the researches towards green energy concept [1]. That introduces to us the likes of heat recovery, cogeneration and green energy that contribute less damage to the environment and increase the thermal efficiency of a power generation unit by burning less fossil fuel.

Carbon dioxide physical properties have been studied and found to have low critical point of 7.38MPa and 31 oC. This lower critical point allows the CO2 to change states at low pressure and temperature and favors to convert low quality heat energy into useful power. There are many other working fluids that can be alternative for steam as a working fluid like R245fa, R123, R134a and zeotropic mixtures [2]. Among all these working fluids carbon dioxide has favorable thermal properties and low environmental impact. It is also non- explosive and non-toxic[3]. Moreover, carbon dioxide allows better temperature match with the heat source temperature profile during the isobaric heat transfer with a low grade temperature heat source [4, 5].

Recently, many studies were devoted to investigate the transcritical and supercritical carbon dioxide power cycles driven by various temperature ranges along with different configurations [6]. Chen et al. investigated the carbon dioxide transcritical performance utilizing low-grade waste heat energy and it was compared to the Organic Rankine cycle (ORC) and R123 as its working fluid. It was found that carbon dioxide transcritical power cycle had a slightly higher power output than the ORC under the same conditions due to the

irreversibilities involved in the ORC power cycle [7]. Cayer et al. [2] analyzed and optimized the thermodynamic parameters of a transcritical CO2 power cycle using genetic algorithm and artificial neural network to recover low-grade heat source. Zhang et al. [5] investigated a solar energy heat source Rankine cycle using supercritical cycle and carbon dioxide as the working fluid for combined power and heat production.

Nowadays, the use of supercritical carbon dioxide power cycles for extraction of nuclear energy is increasing due to its enhanced safety, sustainability, simplicity, superior economy and compactness[8]. It’s highly expected that the supercritical CO2 would benefit the renewable, nuclear and fossil power plants for its remarkable properties as a working fluid in its supercritical state.

In this paper T-CO2 Rankine and S-CO2 Braytoncycles performance were compared in terms of the cycles’ thermal efficiencies. Wider ranges of heat source temperatures are studied by developing parametric analysis. For the considered temperature range gas turbine exhaust gas can be the heat source.

System description and assumptions

As mentioned earlier this study investigates two different CO2 cycles under the following assumptions. The system is at steady state, the kinetic and potential energies as well as the heat loss are negligible and no heat regeneration. The turbine and pump/compressor isentropic efficiencies are assumed to be 90%.

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