Analysis of the heat collection performance of a capillary solar heat collection wall structure

Solar thermal utilization in high-rise buildings is often constrained by the limited surface area of the rooftop. A novel facade-integrated capillary solar heat collection wall structure was proposed in this study, which involves embedding capillary tubes that circulate water within the cement mortar material, facilitating the integration of solar thermal collector and the exterior walls. To validate the feasibility of this new type of heat collection structure, experiments were conducted to investigate the impact of water flow rates on the heat collection performance of the proposed capillary solar heat collection wall structure under indoor solar simulator conditions, and outdoor field testing were conducted to assess its heat collection performance during winter, spring, and summer in Nanjing city. The experimental results revealed that, at water flow velocities of 0.05 m/s, 0.10 m/ s, 0.15 m/s, and 0.20 m/s, the system's heat collection reached 4721 kJ, 4833 kJ, 4872 kJ, and 5326 kJ, respectively, showing a consistent increase with higher flow velocities in the capillary tube. The heat collection efficiency also increased gradually with flow velocity, but the rate of increase was relatively slow. The instantaneous heat collection efficiency decreased gradually as the experiment progressed, with a relatively small value of 47 % at a flow rate of 0.05 m/s. Data fitting revealed a linear relationship between the instantaneous collector efficiency and the normalized temperature difference at water flow velocities of 0.05 m/s and 0.1 m/s, and a quadratic exponential relationship at water flow velocities of 0.15 m/s and 0.2 m/s. During winter, spring, and summer in Nanjing city, the effective heat collection efficiency reached 45.8 %, 40.4 %, and 39.5 %, respectively, demonstrating that the capillary solar heat collection wall structure has significant potential in subtropical regions like Nanjing.