Numerical Design of a Planar High-Flux Microchannel Solar Receiver

Abstract

This paper discuses the design of several micro-channel solar receiver devices. Due to enhanced heat transfer in micro-channels, these devices can achieve a higher surface efficiency than current receiver technology, leading to an increase in overall plant efficiency. The goal is to design an efficient solar receiver based on use of super-critical carbon-dioxide and molten salt as heat-transfer fluids. The super-critical Brayton cycle has shown potential for a higher efficiency than current power cycles used in CSP. Molten salt has been used in CSP applications in the past. The required inlet and outlet temperatures of the fluid are 773.15 K and 923.15 K for carbon-dioxide and 573.15 K and 873.15 K for molten salt. These temperature values are determined by the power cycles the devices are designed to operate in. The required maximum pressure drop is 0.35 bar for carbon-dioxide and 1 bar for molten salt. These pressure values are intended to be a practical goal for maximum pressure drop. The super-critical carbon-dioxide power cycle requires an operating pressure of is 120 bar. Finally, each device must withstand any mechanical and thermal stresses that may exist. Devices presented range in size from 1 cm2 to 4 cm2 and in heat transfer rates from 200 W to 400 W. The size of the device is based on the output capacity of the solar simulator which will be used for testing. For carbon-dioxide, three designs were developed with varying manufacturability. The low risk design features machined and welded parts and straight parallel channels. The medium risk design features machined and diffusion bonded parts and straight parallel channels. The high risk design features a circular micro-pin-fin array created using EDM and is constructed using diffusion bonding. The absence of high operating pressure for molten salt made structural design much easier than for carbon-dioxide. Conjugate heat-transfer simulations of each design were used to evaluate pressure drop, receiver efficiency, and flow distribution. Two and three dimensional structural analyses were used to ensure that the devices would withstand the mechanical and thermal stresses. Based on the numerical analyses, a receiver efficiency of 89.7% with a pressure drop of 0.2 bar were achieved for carbon-dioxide. The design was found to have a structural safety factor of 1.3 based maximum mechanical stress occurring in the headers. For molten salt, an efficiency of 92.1% was achieved with a pressure drop of 0.5 bar.