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2011 2nd International Conference on Environmental Engineering and Applications IPCBEE vol.17 (2011) © (2011) IACSIT Press, Singapore
Parametric analysis of a reheat carbon dioxide transcritical power cycle using a low temperature heat source
Hanfei Tuo 1
1 Mechanical Science and Engineering, University of Illinois at Urbana Champaign
Abstract. CO2 transcritical rankine power cycle has been widely investigated recently, because of its better temperature glide matching between sensible waste heat source and working fluid in vapor generator, and its desirable qualities, such as moderate critical point, little environment impact and low cost. A reheat CO2 transcritical power cycle with two stage expansion is presented to improve baseline cycle performance in this paper. First law analysis is carried out to investigate parametric effects on reheat cycle performance. The main results show that reheat cycle performance is sensitive to the variation of medium pressures and the optimum medium pressures exist for maximizing work output and thermal efficiency, respectively. Reheat cycle is compared to baseline cycle under the same conditions. More significant performance improvements by reheat are obtained at lower turbine inlet temperatures and higher maximum cycle pressure. Work output improvement is much higher than thermal efficiency improvement, because extra waste heat is required to reheat CO2, which reduces the thermal efficiency. It is found that reheat cycle has great potential to improve thermal efficiency and especially work output of a CO2 transcritical power cycle using a low-grade heat source.
Keywords: Transcritical cycle, Carbon dioxide, Reheat, Low-grade heat source, System optimization 1. Introduction
The utilization of low-grade industrial waste heat becomes more attractive due to the world increasing electricity demand. Supercritical power cycles show high potentials to recover such low grade heat, because working fluid temperature glide above the critical points provides a better temperature profile match in the vapor generator with less irreversibility.
Most previous studies focus on thermodynamic analysis and optimization of conventional carbon dioxide transcritical power cycles. Chen et al.  found that the carbon dioxide transcritical power cycle had a slightly higher work output than did an organic rankine cycle (ORC) with equal mean thermodynamic heat rejection temperature. Zhang et al.  studied a similar cycle powered by solar energy for both power and heat generation. They also conducted experimental study to validate the feasibility of this cycle . Cayer et al.  optimized specific work output of a transcritical cycle by investigating the turbine inlet temperature on cycle performance. Two transcritical cycles were optimized by Baik et al.  for power output through examining turbine inlet temperatures and pressures. Wang et al.  used a generic algorithm and an artificial neural network to optimize a few important parameters with exergy efficiency as an objective function.
Previous studies indicated that higher turbine inlet temperatures produced more work output as well as better thermal efficiency. This is because CO2 exhibits a steeper slope for the isobaric curve of the high- pressure region than in the low pressure region, and thus specific work output is increased when carbon dioxide is expanded at a fixed expansion ratio but a higher inlet temperature . Thus, this indicates the performance of transcritical cycle can be improved by reheat cycle where CO2 is only firstly expanded to medium pressure and then reheated to maximum temperature before 2nd stage expansion. Dostal  investigated a reheat CO2 brayton cycles and analysed its improvement over baseline cycle. However, none of the published studies on transcritical CO2 power cycles have investigated effects of reheat on cycle
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