Evaluation of the optimal point variation of the S-CO₂ cycle while considering internal pinch in recuperator

The Supercritical CO₂ power cycle (S-CO₂ cycle) is thepower cycle that adopts CO₂ as a working fluid and is designedto have a compression process near the critical point of CO₂. Dueto the non-linearity of CO₂ pyhsical properties near the criticalpoint, the S-CO₂ cycle needs relatively less compression work.Therefore, the efficiency of the S-CO₂ cycle is higher thantraditional gas cycles. Furthermore, because of the relativelyhigh system minimum pressure (near the critical point, ∼7.39MPa), an S-CO₂ cycle can be composed of smallerturbomachines. Considering these advantages, nowadays, thereare many attempts to apply S-CO₂ cycles to various fields, suchas waste heat recovery, nuclear, coal, concentrated solar powerplant and so on. These non-linear pyhsical properties become thecause of some unique issues. One of the most significant issuesis the internal pinch point problem in a recuperator. Unlike thetraditional gas-to-gas heat exchanger, each hot and cold side ofthe S-CO₂ recuperator goes through the severe change of specificheat. This dramatic change of specific heat may cause theinternal pinch point of the recuperator. When the internal pinchpoint phenomenon occurs, the performance of the recuperatormay not able to be evaluated from the pre-fixed effectiveness.This can be an issue when the compressor inlet temperaturedecreases to transcritical or subcritical region. This may alter theoptimal point of the S-CO₂ power cycle. In this paper, optimaldesign points and optimal performance of the S-CO₂ power cycleare tracked with the consideration of the internal pinch pointphenomenon. While changing the system boundary conditions,the optimal point variation due to internal pinch pointphenomenon is evaluated and compared with a traditionalmethodology. This research is progressed with an in-houseintegrated S-CO₂ power cycle analysis code, which is namedKAIST-ESCA (Evaluator for Supercritical CO₂ Cycle based onAdjoint method). The target cycle layouts are SimpleRecuperated, Intercooling, Recompression and Recompressionwith intercooling layouts. Both of the S-CO₂ Rankine andBrayton cycles conditions are considered.