Effect of Channel Aspect Ratio on the Convective Heat Transfer of Supercritical CO₂ for Compact Solar Receivers

Supercritical carbon dioxide (sCO₂) has favorable thermophysical properties, making it suitable for a wide range of applications and operating conditions. At the near-critical point (P_c ≈ 727.3 atm and T_c ≈ 31.12oC), sCO₂ has a high density and low compressor, thereby minimizing the component size. The sCO₂ Brayton cycle has many advantages, including compactness, high efficiency, low cost, and reduced emissions. It is also flexible to integrate it with nuclear, solar, waste heat, and fossil fuels. The present work focuses on the sCO₂ Brayton cycle for solar power tower plants, a sustainable energy technology, wherein adopting sCO₂ power blocks increases the power plant’s efficiency while reducing the cost [1-3]. In direct sCO₂ solar power tower systems, the solar receiver is central for converting solar energy to thermal energy. However, there are some challenges in the safe and efficient operation of solar receivers due to the high operating pressures and temperatures of sCO₂ (working fluid) and the asymmetric heat load. The sCO₂ solar receiver can be installed in either a tubular or compact configuration. Recent advances in metal 3D printing technology enable the fabrication of compact receivers with mini-channels. The mini-channel design is considered a promising candidate for future solar receivers because of its potential to withstand extreme pressure. Haynes 230 alloy was used as the solid material owing to its ability to withstand extreme temperatures and creep-resistant properties. Previous studies have reported that compact receivers with minichannels could achieve a thermal efficiency of over 90% and offer a longer service life [4-6]. Compact receivers with mini-channels often subjected to asymmetric heat loads, resulting in strong thermal stratification, where the coolant inside the mini-channel experiences a large temperature gradient. Moreover, the occurrence of heat transfer deterioration phenomenon is evident from the variations in the thermophysical properties of sCO₂. These phenomena play a crucial role in determining the heat transfer characteristics of sCO₂ in compact receivers with minichannels. Poor heat transfer due to heat transfer deterioration prevents sCO₂ from absorbing high heat loads, causing heat to build up in the receiver. This leads to hotspots, higher thermal stress in certain areas, and greater risk of the structure failure. A mini-channel design can greatly increase the surface area available for heat exchange in the receiver, thereby enhancing the thermal performance. However, this comes with a penalty including large pressure drop, thereby increasing the pumping power and lowering the overall thermohydraulic efficiency [7, 8]. Therefore, the careful design of mini-channels such as channel aspect ratio is key to mitigating thermal stratification and improving the heat transfer of solar receivers. The present work examines the effect of channel aspect ratio on the convective heat transfer of sCO₂ in mini-channels under unilateral heating for compact solar receivers using three-dimensional numerical simulations, which has not been investigated previously. The validation study of the numerical model is also performed. The numerical analysis results revealed that the channel aspect ratio plays a key role in the performance of compact solar receivers with rectangular mini-channels.