Design of solar district heating system with seasonal thermal energy storage in northern China and its economic analysis

District heating is particularly significant in China, accounting for approximately one-third of the total building heat consumption. Utilizing clean, renewable energy for heating is a critical strategy to transform the national energy landscape, with district solar heating playing a pivotal role. This study presents a design methodology for a solar seasonal heating system, exemplified by a project from a heating company in Xi'an. The system includes a solar flat-plate collector field area of 12,400 m2 and a seasonal thermal storage volume of 52,125 m3, achieving a solar fraction of 36.32% and an annual heat supply of 48,378 GJ. Furthermore, the economic feasibility is assessed using cash flow analysis, revealing a net present value rate of 5.37% and a static payback period of 13.0 years. Sensitivity analysis identifies the heating price as the most significant factor affecting the project's feasibility, followed by construction investment and heating costs. The study proposes optimizing economic returns by increasing the heating price by 8.63%, reducing construction investment by 15.62%, or adopting a design with a 29.10% solar fraction. The study emphasizes economic and environmental benefits, recommending tailored economic targets, especially in regions with abundant solar resources or policy subsidy support.