Multi-objective optimization of equipment capacity and heating network design for a centralized solar district heating system

Northwest China has abundant solar energy resources and a large demand for winter heating. Using solar energy for centralized heating is a clean and effective way to solve local heating problems. While present studies usually decoupled solar heating stations and the heating network in the optimization design of centralized solar heating systems, this study developed a joint multi-objective optimization model for the equipment capacity and the diameters of the heating network pipes of a centralized solar district heating system, using minimum total life cycle cost and CO₂ emission of the system as the optimization objectives. Three typical cities in northwest China with different solar resource conditions (Lhasa, Xining, and Xi'an) were selected as cases for analysis. According to the results, the solar heating system designed using the method proposed in this study presents lower economic cost and higher environmental protection in comparison to separately optimizing the design of the solar heating station and the heating network. Furthermore, the solar fraction of the optimal systems are 90%, 70%, and 31% for Lhasa, Xining, and Xi'an, and the minimum water supply temperatures are 55 ℃, 50 ℃, and 65 ℃ for an optimal economy and 55 ℃, 45 ℃, and 45 ℃ for optimal environmental protection, respectively. It was also established that the solar collector price has a greater impact on the equipment capacity of the solar heating station than the gas boiler price.