Photothermal Performance and Parameter Optimization of Dynamic Nanofluid-Based Spectral Splitting Systems on the South Facade of Greenhouses

Summer overheating severely constrains protected agriculture, especially in mid-latitude greenhouses where the south-facing facade receives the highest solar load. Existing spectral splitting studies have primarily focused on static systems or roof configurations, typically with single-parameter optimization, leaving the dynamic, multi-parameter synergy on the south facade largely unexplored. This study numerically optimizes a dynamic nanofluid-based spectral splitting (NSS) system on the greenhouse south-facing facade, and for the first time systematically investigates the coupled effects of interlayer thickness (5–20 mm), nanofluid flow rate (10–60 L/h), and filling ratio (0–100%) using a validated 3D transient model. Key results show that a 10 mm interlayer provides the best compromise between daytime cooling (4.4 °C noon reduction vs. 5 mm) and thermal stability, avoiding afternoon heat release penalties. A flow rate of 20 L/h is identified as the preferred operating condition, where the thermal gain (>360 W/m 2 ) overwhelmingly outweighs parasitic pumping power, with higher rates (40–60 L/h) yielding only marginal improvements. The filling ratio effectively regulates spectral splitting: increasing it from 0% to 100% reduces the peak indoor temperature by up to 13.3 °C, while a 60–80% filling ratio balances visible light transmittance (58.5–64.8%) and near-infrared absorptance, thereby ensuring sufficient photosynthetically active radiation under extreme summer irradiance. This work provides a theoretical basis and quantitative design criteria for multi-parameter coordination of dynamic NSS envelopes, supporting low-carbon greenhouse development in high-irradiance climates.