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 the power cycle that adopts CO₂ as a working fluid and is designed to have a compression process near the critical point of CO₂. Due to the non-linearity of CO₂ pyhsical properties near the critical point, the S-CO₂ cycle needs relatively less compression work. Therefore, the efficiency of the S-CO₂ cycle is higher than traditional gas cycles. Furthermore, because of the relatively high system minimum pressure (near the critical point, ∼7.39 MPa), an S-CO₂ cycle can be composed of smaller turbomachines. Considering these advantages, nowadays, there are many attempts to apply S-CO₂ cycles to various fields, such as waste heat recovery, nuclear, coal, concentrated solar power plant and so on. These non-linear pyhsical properties become the cause of some unique issues. One of the most significant issues is the internal pinch point problem in a recuperator. Unlike the traditional gas-to-gas heat exchanger, each hot and cold side of the S-CO₂ recuperator goes through the severe change of specific heat. This dramatic change of specific heat may cause the internal pinch point of the recuperator. When the internal pinch point phenomenon occurs, the performance of the recuperator may not able to be evaluated from the pre-fixed effectiveness. This can be an issue when the compressor inlet temperature decreases to transcritical or subcritical region. This may alter the optimal point of the S-CO₂ power cycle. In this paper, optimal design points and optimal performance of the S-CO₂ power cycle are tracked with the consideration of the internal pinch point phenomenon. While changing the system boundary conditions, the optimal point variation due to internal pinch point phenomenon is evaluated and compared with a traditional methodology. This research is progressed with an in-house integrated S-CO₂ power cycle analysis code, which is named KAIST – ESCA (Evaluator for Supercritical CO₂ Cycle based on Adjoint method). The target cycle layouts are Simple Recuperated, Intercooling, Recompression and Recompression with intercooling layouts. Both of the S-CO₂ Rankine and Brayton cycles conditions are considered.