Thermal response analysis of pipeline under fluctuating heat input for solar district heating system

To address pipeline delay response and substantial heat loss caused by time-varying temperature fluctuations in solar collector fluid and step-change heating temperatures within SHDS, a pseudo-dynamic thermal numerical model was developed. The thermal response characteristics of the CLP and HDP under fluctuating heat input and temperature step-change conditions were investigated. Coupled analysis of multiple influencing factors and pipeline inlet conditions quantified the evolution patterns of delay response time, temperature drop along pipeline, and heat loss. Regression analysis incorporating external dynamic boundary conditions was conducted. The results indicated that peak heat loss per unit length in the CLP reached 58.30 W/m during daytime, with temperature drop along pipeline of approximately 0.11 °C. These dynamic characteristics were synergistically governed by solar radiation intensity and fluid flow velocity. During step-change in inlet temperature, the delay response time of outlet temperature increased significantly by 97–101 min. For each kilometer increase in pipe length, the delay response time extended by approximately 12 min, whereas a 0.3 m/s flow velocity increase reduced it by about 14 min. Furthermore, pipe length extension has minimal effect on heat loss per unit length, while every 5-mm increase in insulation thickness decreased heat loss by 1.9–3.4 W/m. The study demonstrated that the pseudo-dynamic model accurately predicted dynamic thermal behaviors in pipeline, providing a reference basis for dynamic thermal management, precise regulation, and energy-efficient design of solar district heating pipeline.