A simplified method for the thermal performance analysis of double-layer pipe-embedded external wall

Reducing the heat dissipation through the envelope is critical for heating energy savings in buildings. Double-layer pipe-embedded external wall (DPEW) has the advantage of load interception and low-temperature heating, and is a promising solution to reducing energy demand for heating. To gain a comprehensive understanding of the thermal performance of DPEW, a thermal network model is established in this study, which characterizes the steady heat transfer process of DPEW by the superposition of water temperature, outdoor temperature and indoor temperature. The accuracy of the proposed model is validated against experiment data in the literature. Evaluation indexes including equivalent overall heat transfer coefficient (U-value), equivalent outdoor temperature, and energy utilization rate of outer pipe or/and inner pipe are proposed based on the thermal network model of DPEW. A case study is conducted under typical winter conditions of Beijing to investigate the influence of water temperature and outdoor temperature on the thermal performance of DPEW. The results show that the energy utilization rate of DPEW increases with the increasing outdoor temperature and inner pipe water temperature, and the decreasing outer pipe water temperature, while the energy utilization rates of the outer pipe and inner pipe remain largely unaffected. At the outdoor temperature of −9 °C, the outer pipe water temperature of 10 °C and the inner pipe water temperature of 25 °C, the energy utilization rates of the outer pipe, inner pipe, and DPEW reach approximately 27 %, 89 % and 65 %, respectively. This study gives an insight into the steady-state thermal performance of DPEW, and provides guidance for its further research and application.