Abstract
This study aims to provide a thermodynamic comparison between supercritical CO2 cycles and ORC cycles utilizing flue gases as waste heat source. Moreover, the possibility of using CO2 mixtures as working fluids in transcritical cycles to enhance the performance of the thermodynamic cycle is explored. ORCs operating with pure working fluids show higher cyclic thermal and total efficiencies compared to supercritical CO2 cycles; thus, they represent a better option for high-temperature waste heat recovery provided that the thermal stability at a higher temperature has been assessed. Based on the improved global thermodynamic performance and good thermal stability of R134a, CO2-R134a is investigated as an illustrative, promising working fluid mixture for transcritical power cycles. The results show that a total efficiency of 0.1476 is obtained for the CO2-R134a mixture (0.3 mole fraction of R134a) at a maximum cycle pressure of 200 bars, which is 15.86% higher than the supercritical carbon dioxide cycle efficiency of 0.1274, obtained at the comparatively high maximum pressure of 300 bars. Steam cycles, owing to their larger number of required turbine stages and lower power output, did not prove to be a suitable option in this application.
Highlights
The increase of conversion efficiency and promotion of energy recovery represent two effective strategies for primary energy savings
Assuming a simple recuperative supercritical carbon dioxide cycle as a reference, we developed a performance comparison of transcritical ORC cycles with cycles adopting binary mixtures of carbon dioxide as a working fluid
This study focused on the analysis of carbon dioxide mixture working fluids as an alternative to organic fluids, steam and pure carbon dioxide for high-temperature heat recovery in power plants
Summary
The increase of conversion efficiency and promotion of energy recovery represent two effective strategies for primary energy savings. GJ, with an average emission value of 0.81 kgCO2 /kgcement [7,8] It is certainly appropriate, necessary and useful to recover energy from flue gases of the cement industry for the production of electricity, given the high available temperatures. According to the analysis developed in [17], carbon dioxide cycles represent a potentially more effective solution than ORC for heat source temperatures greater than 350 ◦C; this is true if cold water is available, making condensation possible. In addition to heat recovery, supercritical and transcritical carbon dioxide cycles have already been considered for a great variety of applications; for example, for solar plants, for nuclear power plants, for the exploitation of geothermal energy, and for thermo-electric storage [18,19,20,21]
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