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Publication Title | SUPERCRITICAL CARBON DIOXIDE CYCLES THERMODYNAMIC ANALYSIS AND COMPARISON

Organic Rankine Cycle

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Abstract

SUPERCRITICAL CARBON DIOXIDE CYCLES THERMODYNAMIC ANALYSIS AND COMPARISON

Ing. Martin Kulhánek, Ing. Václav Dostál Ph.D.

Ústav mechaniky tekutin a energetiky, České vysoké učení technické v Praze Technická 4, 166 07 Praha 6 martin.kulhanek@fs.cvut.cz, vaclav.dostal@fs.cvut.cz

A thermodynamic analysis and comparison of supercritical carbon dioxide cycles have been performed. Analyzed cycles were: simple brayton cycle, pre-compression cycle, recompression cycle, split expansion cycle, partial cooling cycle and partial cooling cycle with improved regeneration. A computer code was developed for each cycle to evaluate all thermodynamic states. Compressor inlet and turbine inlet temperatures were hold constant (32°C and 550°C) and other parameters such as compressor outlet pressure, turbine pressure ratio were varying. From analyses among others has emerged: The pre-compression cycle achieves thermal efficiency ~44% at compressor outlet pressure 10MPa. The re-compression cycle achieves the highest efficiency for an optimal pressure ratio (~45% for 20MPa), which corresponds to compressor inlet pressure about 7.7MPa. With a change of this ratio, the efficiency significantly decreases. The split expansion cycle behaves as the recompression cycle but reaches lower thermal efficiency due to pressure reduction in the reactor. The partial cooling cycle reaches similar efficiency such as re-compression cycle, however its decrease with change of turbine pressure ratio is less significant. The partial cooling cycle with improved regeneration has a potential for proper condition reach high thermal efficiency, but mostly suffers from pinch points within recuperators. From the cycles comparison, has emerged: For compressor outlet pressures up to 20MPa seems to be the most effective improvement to simple brayton cycle an addition of pre-compressor, while for higher pressure gives better results the re-compressor with flow dividing. Via splitting the expansion, can be sufficiently reduced pressure in the reactor, without significant reduction of cycle thermal efficiency.

Key words:

S-CO2, thermodynamic, power cycle, thermodynamic analysis

1 Introduction

1.1 Why supercritical carbon dioxide?

The supercritical power cycles are taking advantage of real gas behaving in order to achieve high thermal efficiency. There are two main types of supercritical cycles. The supercritical water cycle where a heat addition at super-critical pressures increases the turbine inlet temperature and the supercritical CO2 (S-CO2), where main improvement of cycle efficiency comes from compressor work reduction due to a properties change, when it is compressed near the critical point (30.98°C, 7.38MPa). Because low critical temperature, it is possible to use water at ambient temperatures such as a coolant.

Other benefits of CO2:

- S-CO2 cycles achieve high efficiency at low temperatures

- High operating pressure allows small size components

- More than twenty years experiences of CO2 application in nuclear reactors

(MAGNOX, AGR)

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