Thermo-hydraulic analysis of compact heat exchanger for a simple recuperated sCO2 Brayton cycle

Abstract

Supercritical carbon dioxide (sCO2) Brayton cycle-based power plants are being extensively explored as viable alternatives to traditional Rankine based steam power plants. Higher temperatures at the turbine exit in a sCO2 cycle provide better opportunities for heat recuperation, thus improving overall cycle efficiency. Among the various heat exchanger configurations available, compact microchannel heat exchangers commonly known as Printed Circuit Heat Exchangers (PCHE's) are typically used in sCO2 power plants. In the current work, a hybrid approach comprising of a Thermal Resistance Network (TRN) model coupled with a CFD model for estimating local heat transfer and pressure drops is presented for a PCHE core with straight and zigzag channel configurations. Full-scale TRN model is used to optimize the overall stack dimensions based on minimum rate of heat loss from the external surfaces of the PCHE core. The TRN model accounts for the thermo-physical property variations of sCO2 along the channel length to effectively capture the channel pressure drop and heat transfer. This is achieved by discretizing the heat transfer domain comprising of alternatively stacked hot and cold streams into sub-heat exchangers to account for variations in thermophysical properties while calculating the nodal friction factors and local heat transfer coefficients. CFD simulations are performed for a full length of a single stack of hot and cold fluid streams to arrive at corrected heat transfer and pressure drop correlations. Thermo-hydraulic analysis is performed for a range of channel hydraulic diameters and channel mass flow rates for both straight and zig-zag configurations to deduce optimum stack dimensions. The efficacy of the hybrid model is demonstrated with a case study of a counterflow recuperator used in a simple recuperated 1 MW sCO2 Brayton power plant.