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Mixtures of SF6–CO2 as working fluids for geothermal power plants

► Thermodynamic cycles for mixtures of SF6–CO2 were investigated for geothermal power plants. ► Thermodynamic cycle was solved using a Newton–Raphson method. ► SF6 15 and 20mol% were found to yield the highest Brayton and Rankine cycle efficiencies. ► Effects of SF6 fraction in CO2 on cycle thermal...

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Bibliographic Details
Published in:Applied energy 2013-06, Vol.106 (6), p.243-253
Main Authors: Yin, Hebi, Sabau, Adrian S., Conklin, James C., McFarlane, Joanna, Qualls, A. Lou
Format: Article
Language:English
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Summary:► Thermodynamic cycles for mixtures of SF6–CO2 were investigated for geothermal power plants. ► Thermodynamic cycle was solved using a Newton–Raphson method. ► SF6 15 and 20mol% were found to yield the highest Brayton and Rankine cycle efficiencies. ► Effects of SF6 fraction in CO2 on cycle thermal efficiency and size of heat exchangers was assessed. In this paper, supercritical/transcritical thermodynamic cycles using mixtures of SF6–CO2 as working fluids were investigated for geothermal power plants. The system of equations that described the thermodynamic cycle was solved using a Newton–Raphson method. This approach allows a high computational efficiency even when thermophysical properties of the working fluid depend strongly on the temperature and pressure. The thermophysical properties of the mixtures were obtained from National Institute of Standards and Technology (NIST) REFPROP software and constituent cubic equations. The local heat transfer coefficients in the heat exchangers were calculated based on the local properties of the working fluid, geothermal brine, and cooling water. The heat exchanger areas required were calculated. Numerical simulation results presented for different cycle configurations were used to assess the effects of the SF6 fraction in CO2, brine temperature, and recuperator size on the cycle thermal efficiency, and size of heat exchangers for the evaporator and condenser. For working fluids with SF6, concentrations of 15 and 20mol% were found to yield the highest Brayton and Rankine cycle efficiencies, respectively.
ISSN:0306-2619
1872-9118
DOI:10.1016/j.apenergy.2013.01.060