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Abstract: High temperature, high pressure transcritical condensing CO2 cycle (TC-CO2) is compared with transcritical steam (TC-steam) cycle. Performance indicators such as thermal efficiency, volumetric flow rates and entropy generation are used to analyze the power cycle wherein, irreversibilities in turbo-machinery and heat exchangers are taken into account. Although, both cycles yield comparable thermal efficiencies under identical operating conditions, TC-CO2 plant is significantly compact compared to a TC- steam plant. Large specific volume of steam is responsible for a bulky system. It is also found that the performance of a TC-CO2 cycle is less sensitive to source temperature variations, which is an important requirement of a solar thermal system. In addition, issues like wet expansion in turbine and vacuum in condenser are absent in case of a TC-CO2 cycle. External heat addition to working fluid is assumed to take place through a heat transfer fluid (HTF) which receives heat from a solar receiver. A TC-CO2 system receives heat though a single HTF loop, whereas, for TC-steam cycle two HTF loops in series are proposed to avoid high temperature differential between the steam and HTF. [Copyright &y& Elsevier]

• Stack-based thermal resistance network model developed for micro channel heat exchangers. • Correlations developed with straight, sinusoidal and zigzag channels for sCO 2 recuperator. • Hybrid model demonstrates that 20 rows are sufficient to attain the correct stack performance. • Several combinations of stack dimensions generated for different channel configurations. • Sinusoidal channel offered least pressure drop and stack volume compared to straight and zigzag channels. In this work, a full-scale Thermal Resistance Network (TRN) coupled with a unit-cell-based CFD model identified as the hybrid model is developed and implemented in designing a Mini Channel Heat Exchanger (MCHE) as a recuperator used in sCO 2 Brayton power block. The TRN framework converts the MCHE domains into a thermal circuit using resistances and advection currents that can be used to model the entire stack with varying thermo-physical properties of working fluids. The results from the TRN model with CFD-based heat transfer and pressure drop correlations developed for different channel lengths, are compared with the original CFD data to ascertain the lowest possible channel length for precise correlation development. An appropriate channel length is further used for deriving correlations for sinusoidal and zigzag flow paths. The hybrid model helps determine the upper limit of the side width (fin width) of the channels based on equivalent thermal resistance between fluid and walls as the lower limit is constrained by stress criteria. The present model estimates that at least 20 rows/plates need to be modelled for the correct performance estimation of a full-scale MCHE. The minimum rate of heat loss-based stack optimization approach is implemented in estimating the optimum number of rows and stack dimensions. The channel and stack dimensions are obtained and compared for straight, sinusoidal, and zigzag flow paths for Reynolds numbers ranging from 5 × 103 to 20 × 103, ensuring a temperature pinch of 5 °C at the cold inlet. For cold inlet Reynolds number and channel diameter of 1.5 × 104 and 1.5 mm, the stack volumes for straight, zigzag, and sinusoidal channels are 1.8 m3, 1.21 m3, and 1.15 m3, respectively with sinusoidal and zigzag paths resulting in almost similar pressure drop as ∼110 Pa, 27% lower compared to the straight channel. The article presents a guideline for hybrid model implementation in designing mini/micro channel heat exchangers. [ABSTRACT FROM AUTHOR]

Supercritical carbon dioxide (sCO 2) 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 sCO 2 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 sCO 2 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 sCO 2 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 sCO 2 Brayton power plant. • Thermo-hydraulic Analysis of Compact Heat Exchanger. • Simple Recuperated sCO 2 Brayton Cycle. • 1-D model based on Thermal Resistance Network (TRN) and CFD simulation. • Thermo-physical property variations of sCO 2 along the length of the channel. • Heat loss method to optimize the overall stack dimensions. [ABSTRACT FROM AUTHOR]