Impact of different types of channels on Thermo-hydraulic performance of a sodium-sCO2 Printed circuit heat exchanger for supercritical Brayton cycle applications

Abstract

Supercritical CO2 has been proposed as a heat transfer fluid for Brayton cycles to achieve a high effectiveness in power generation from concentrated solar power plants. A printed circuit heat exchanger (PCHE) has been considered as an alternative design for a sodium to supercritical CO2 heat exchanger operating at high pressure and temperature (220 bar and 700 °C, respectively). Computational fluid dynamics has been used to compare the hydrothermal performance of different channel types and configurations to determine the most effective design for a 30 kW lab-scale system. The model was validated with relevant experimental conditions and the model was used to simulate 18 different channel types and configurations. The thermohydraulic performance of a single module of a PCHE comprising of counter-current passages for cold and hot flow, constructed out of Inconel, with different combinations of circular, semicircular, and rounded rectangular channels was systematically assessed. While straight channels were used for sodium, the hydrothermal performance of straight and wavy channels (with bending angles of 32.5° and 40°) were examined for the sCO2 side. A parametric analysis of a PCHE module with a fixed cross section (W = 6.5, H = 2.6 mm) showed the best performance for a 60 cm length with 2 g/s mass flow rate on the sodium side and 2.87 g/s on the sCO2 side. Using this outcome and limiting the maximum pressure drop to 2 % of the operating pressure at each side, the model showed that the best thermal performance (96.7 % effectiveness) was for a module with three 32.5° semicircular wavy sCO2 channels in conjunction with a straight circular sodium channel. The next highest thermal performance (96.4 % effectiveness) was achieved using a module with a straight rectangular channel for sodium and three wavy (32.5°) semicircular channels for sCO2. Using rectangular channels on both the hot and cold sides, a 96 % effectiveness was predicted between two 32.5° wavy channels for sCO2 and one straight channel for sodium.